Shorelines 
Shorelines are our interface with the ocean, and their facies, environments, and geometry depend on interactions among storms, waves, tides, rivers, and tectonic processes. 
 
There are two basic types of shorelines: Erosional shorelines are commonly marked by cliffs and beach retreat. They often have a low sediment supply and high wave or tidal energy. Wave cut platforms are very common when bedrock is eroded, and eroded material temporarily stored at foot of cliff but is ultimately removed by waves. Constructional shorelines are sites of net sediment accumulation. Rivers provide most of the sediment, which is redistributed along the coast and offshore by waves, storms, and tides. Gravity also transports sediment off shore via debris flows and turbidites. Shorelines can switch between erosional and constructional processes rapidly in both time and space.  
 
Shoreline Geometry:  
The overall geometry and characteristics of shorelines are determined by 5 factors:  
1) Sediment supply - high=constructional, low=erosional coastlines 
2) Wave energy - generally high=erosional and low=constructional, but large waves can transport significant sediment onto beaches if there is abundant sand off shore, and small waves can produce a net transport of sediment off shore, causing erosion 
3) Tidal range - high and low tidal ranges can both be either erosional or constructional 
4) Climate - changes sediment composition, can be either erosional or constructional 
5) Tectonic activity – uplift usually results in erosion, whereas subsidence often results in deposition if there is sufficient sediment supply. 
 
Basic Shoreline Environments  
Marshy Coastlines have low wave energy and moderate to high sediment supply, with most tidal ranges and climates. They tend to erode during high energy events such as storms, when currents and waves are strong enough to break up the vegetation. They also erode or become inundated as sea level rises. 
 
Mud Flats often form along low energy coastlines with an abundant supply of fine-grained sediment. They are often dominated by tidal processes even if the tidal range is low. Mud flats require either a high sedimentation rate or high enough energy to keep marshes from developing.  
 
Beaches tend to form in areas with high sediment supply and high wave energy. Tidal range and climate can vary and will affect the geometry of a beach, but beaches can form in a full range of conditions. They are constructional features. They can develop off the main coastline, creating barrier islands. 
 
Lagoons develop shoreward of beaches in depressions that are below the high tide level. If the climate is arid, they tend to have net evaporation, leading to deposition of evaporite minerals. In more humid climates, they have the same salinity as seawater or might contain more fresh water (e.g. have brackish water). They can convert to marshes if plant growth is faster than sedimentation rates.  
 
Bays, Harbors, etc. develop in protected areas of coastlines. They can be protected by beaches or bedrock protrusions into the ocean.  
 
Sea Cliffs commonly develop along erosional shorelines.  
 
Beaches 
Wave action is critical to beaches. They come in off the ocean, and start to feel the shore, especially on the upper shore face. Wave ripples form here, as well as other cross stratification from storms and tidal channels. In shallower water, the sand and rocks are washed back and forth almost continuously which produces well rounded and well sorted grains. The grain sizes present depend on the wave energy and can range from cobbles to fine sand. In the zone where waves break, the erosional force is particularly strong due to high flow speeds and abundant sediment suspended in the water. The eroded sediment is washed up onto the foreshore and deposited as the water drains back into the ocean. If the waves break very close to the beach or the beach front is very steep, the sand gets washed back into the ocean, and there is net erosion.  
 
The deposition of sand leaves stratification parallel to the slope on the beach. As wave strength or sediment supply vary, you can get cross stratification, which often consists of planar but seaward- dipping laminae that extend for meters (parallel to the beach surface) and are truncated by planar laminae with a different dip when the slope of the beach changes. This lamination can be disrupted by burrowing organisms such as clams and crabs, and animals walking on the beach. There are some good photos of beach stratification at  
http://www.geology.ucdavis.edu/~gel109/SedStructures/Shoreline.html 
 
Berms and Eolian Dunes:  
Over time, sand builds up at the far reach of the waves forming a high called a berm. Storm surges wash over the berm (sometimes eroding it) and deposit sediment on the landward dipping surface. These laminae dip away from the beach, commonly at a gentle angle. This creates a crest on the beach called the berm.  
 
Wind commonly reworks beach sand to form coastal eolian dunes. These dunes are finer grained and better sorted than the beach sand because wind is less viscous and dense than water; it can only transport finer grains. Eolian dunes tend to have high topography, so the cross stratification that they produce tends to be similarly large if there is net deposition of sand. Although the full height of dunes is rarely preserved, very tall sets of cross stratification commonly indicate an eolian dune depositional environment.  
 
Barrier Islands are beaches that are detached from the main coast; they are ridges of sediment parallel to the coast that form most readily along constructional coasts with low tidal range and significant wave energy. Barrier islands are transient features that erode and grow depending on the ever changing conditions of the sea.  
 
Deltas and Estuaries 
Deltas form at the mouths of rivers that transport enough sediment to build outward. (Building outward is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley flow into estuaries.) Deltas require substantial accumulation of sediment, in contrast to estuaries which do not build outward. Sedimentary facies are similar to other depositional environments, but the association of subenvironments are recognizable as deltas. Some of the sub environments include: river facies with all the associated sub environments; shore line deposits including beaches, marshes/swamps, etc.; submarine shelf and slope facies, including storm deposits and turbidites; etc.  
 
I will draw cross section and map views of a delta showing the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth. 
 
Progradation - Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward over more marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine, fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds. 
 
Sediment Transport Type - All deltas (by definition) have their sediment transported to the delta by rivers. Thus, riverine deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the riverine sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced. 
 
River Dominated Deltas - River dominated deltas have very low wave energy and a very small tidal range. Delta top deposits are well developed and are very similar to meandering river deposits, including channel, levees and overbank deposits. Overbank areas are commonly heavily vegetated and result in peat and coal deposition. Channels build out into the ocean (or lake) on top of their mouth bars. This leads to a coarsening upwards of grain sizes within the mouth bars as well as a change from some marine processes to unidirectional river flow. Avulsion of the rivers is common due to low gradients on the delta plain. Lobes of the delta become abandoned creating a “bird’s foot delta”. Sheltered bays are common between the lobes, and are filled with overbank deposits from floods as well as marshy deposits. The Mississippi River Delta is a classic river dominated delta. 
 
Wave Influenced Deltas - Waves redistribute the sediment deposited by the rivers. Progradation of channels is limited because mouth bars are reworked by waves into shore parallel sand bars and beaches. Spits of sand are also common. The waves sort the sediment better than rivers and, if the grains are not already well rounded, the waves will round them. The big differences for wave influenced deltas are that beach facies are abundant and channel fill and overbank facies are less common. The Niger River Delta is a wave influenced delta. 
 
Tide Influenced Deltas - Tides rework sands into elongate bars perpendicular to shore (vs. waves). These bars are analogous to mouth bars, but they contain tidal sedimentary characteristics including bi-directional flow indicators and slack tide mud drapes. Overbank areas can include tidal flats. The Ganges-Bramhaputra delta in Bangladesh is a tide dominated delta.  
 
 
 

Posted by: Dawn Sumner

@ February 13, 2008 10:43:12 AM PST ( )

Marine Processes 
 
Marine Deposition - Most of Earth is covered with oceans, there is abundant life in the oceans, most sediments eventually get transported into the oceans, and shallow marine deposits are the most abundant in the in sedimentary record due to their large volume and the low erosion rates in shallow marine environments. You need tectonics to uplift them above sea level to get significant erosion. This happens commonly, so that we can also see them exposed.  
 
Processes - Several processes are unique to shallow marine deposition (and some large lakes): Waves, tide and storms 
 
Waves - Waves have oscillating current directions every few seconds. The flow in both directions is equal in deep water, but not necessarily near shore. Draw a picture of wave water motion. (Top of wave moves in the direction the wind is blowing.) 
 
Wave Ripples - Wave ripples are like current ripples, except that they experience transport in both directions. Draw a picture with the laminar boundary layer, etc. At low flow, the boundary layer doesn’t have enough speed or momentum to remove the crest of the ripple and deposition of the grains that are moved are deposited right on the upper part of the lee slope. Thus, crests are sharp. At higher flow, the crests erode due to the higher speeds and momentum and deposition occurs farther down the lee slope. Thus, high flow ripples have rounded crests. Wave ripples can be recognized in rocks by their symmetric shape (if flow in each direction is the same speed) and most importantly, the presence of x-laminations dipping in two directions. This is the truly distinctive feature and can be present even if the ripples are not very symmetric.  
 
In shallow water, currents along the bottom from the waves are strong enough to flatten out the ripples, but they are not consistent enough in one direction to form dunes. Thus, the sedimentary surface tends to be planar or broadly scalloped as the waves are focused into certain areas. 
 
KEY POINT FOR WAVES: Bi-directional flow every few seconds 
 
Tides - Two key characteristics that are unique to tides: 1) flow changes direction 1 or 2 times per day; and 2) The strength of flow is cyclical. Draw a tidal chart marking onshore and offshore flow and flow speeds. There is lots of variability in tides depending on geography. Flow speeds vary, producing different sedimentary structures. In the Bay of Fundy, which has the highest tides recorded in the world (up to 16m - a 5 story building), the water moves up to 15 km/hr (417 cm/sec) which is fast enough to transport boulders and is well above the upper flat lamination zone for smaller grain sizes. At the low end, tidal currents are essentially non-existent. Also, there are times of slack tides when the water is essentially still or wave-dominated. Thus, the range of sedimentary structures is wide, including dunes (often called tidal bars when very large) and ripples. The main characteristic to look for, though, is variations in flow speed and DIRECTION. 
 
Tidal sedimentary structures - Due to changing flow directions, two sediment transport directions are common, one for onshore flow and one for offshore flow. Often the onshore and offshore flows aren’t in the same location, but they shift around. This gives rise to current ripples showing transport in two directions and dune migration in two directions producing herringbone cross stratification (draw it). If the dunes are small and sedimentation rates are very high, you can get HC in one tidal cycle in a modern environment. It is usually not preserved in the geological record because it is eroded prior to lithification. It is almost always the longer term changes in current locations that gives rise to preserved HC. Dunes migrate in one direction for a while, and then currents patterns change and they migrate in the other direction. HC is almost always due to tidal processes, although it isn’t all that common in the sedimentary record. Commonly, one tidal current is much stronger than the others or the flow locations aren’t systematically shifting, so tabular cross stratification is more common. It is not unique to tidal environments, however. 
 
Reactivation Surfaces - Reactivation surfaces form when flow in one direction is stronger than the other, but the other flow is strong enough to modify the bedform shape. Draw pictures... Reactivation surfaces look like irregular surfaces that are similarly oriented to the foresets, but not quite. Also, the foresets above and below the reactivation surface commonly have a slightly different orientation. Reactivation surfaces indicate varying flow directions, which is very common in tidal environments. 
 
Mud Drapes - Flow speeds are also cyclical. During slack tides (low or especially high), fine grained sediment can fall out of suspension draping tidal bedforms with mud. Because mud is cohesive, it doesn’t necessarily erode, particularly in the separation zone where flow is slow. Thus, sand foresets coated with mud are very common in tidal environments as well. 
 
KEY POINT FOR TIDAL PROCESSES: Bi-directional flow with varying speeds over hours 
 
Storms - Storms produce both large waves and strong, irregular currents. The combination and interference of these produces some unique deposits which can be used to recognize the importance of storms in a given marine sequence. Storms generally start far from shore and can approach through time. Then they either die out or move on. Thus, deposits that storms affect, i.e. those on continental shelves, tend to start out with low energy flows, increase to erosional (if strong enough) and then decrease back to lower energy flows. For example, sharp crested wave ripples might transition into round crested wave ripples, followed by cross stratification due to large waves and strong currents, followed by erosion, deposition of the coarsest sediment, and a reverse of the sedimentary structures. However, because there is usually little sediment being deposited at the beginning of a storm because there is not much sediment in motion and because flow speeds are increasing, there is usually no record of the first half of this sequence in the rock record. It is only the second half that gets preserved.  
 
HCS - The cross stratification that is deposited as a combination of strong currents and large waves is unique to storms (and is found only in medium to fine sands). It is called hummocky cross stratification (HCS) and swaley cross stratification. When currents are washing eroded sand into an area with strong oscillatory flow, rounded mounds or hummocks of sand develop on the sea floor separated by lows (swales). These mounds are a few to 10 cm high and 10’s of cm across. Draw them. Variations in current strength cause erosion locally, and the locations of the hummocks and swales change through time. This produces erosional surfaces which truncate the older laminae (note that Fig 11.9 has the wrong laminae truncated). Draw this correctly. HCS is characterized by low angle laminae truncated by low angle surfaces. There are abundant concave and convex up laminae and many fewer flat laminae.  
 
Storm Sequence - Draw a sample cross section: Mud, scoured surface, sole marks, (gravel at base), normally graded, HCS, flat laminae or wave rippled top, return to suspension settling. Contrast this to a turbidite - I will ask you to do this! 
 
KEY POINT FOR STORMS: Multi-directional flows over seconds, low to high to low energy in deep water 
 
 

Posted by: Dawn Sumner

@ February 13, 2008 10:41:39 AM PST ( )

Rivers 
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of a channel, bank and overbank or flood plane deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds. 
 
River Types -  
Straight (rare, except for ones humans have modified) 
Meandering (high sinuosity) 
Braided (many branches within a channel) 
Anastomosing (rivers with branching and merging channels) 
 
The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, and sediment grain size, particularly the ratio of suspended versus bedload sediment. A high bedload gives rise to abundant bars, which promotes formation of braided rivers. 
 
Meandering Rivers -  
Meandering rivers have a low gradient and thus slower flow, and often have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a slightly dipping plain. The flow speed in the channel varies with the geometry of the curves. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, you should predict that there is more erosion on the outsides of bends, the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available, and the insides of bends will be were sediment is accumulating and this sediment will be finer grained. If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower.  
 
The main parts of the channel include eroding bank, the Thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The lowest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination.  
 
As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter.  
 
Watch this cartoon of a meander migration in France: 
http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm 
 
Levees and Flood Plains - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the flood plane. It slows down very quickly. Thus, it can not transport as much sediment as it caries in the channel as soon as the water leaves its banks. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out as the flood waters dry up.  
 
Watch this model of a meandering river flood: 
http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm  
 
Over time, the levees build up and provide a higher bank for the channel than the flood planes. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the flood plane. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the flood plane. This results in the downstream part of the channel being completely abandoned. 
 
Meandering River Channel Facies: 
1. Scoured base of flow  
2. Lag deposit with mud rip-up clasts  
3. Fining upward sands with trough cross stratification  
4. Rippled sands  
5. Sigmoidal cross stratification from migrating point bars  
 
Flood Plane Facies 
1. Fine sand with climbing ripples 
2. Mudstone/shale with mud cracks 
3. Soils 
4. Root casts 
 
Ox Bow Lake Facies 
1. Mudstone/shale without mud cracks 
2. Organic-rich deposits, including coal 
3. Anoxic water indicators (especially in fossils and trace fossils) 
 
 
 
Braided Rivers - Braided rivers develop when the proportion of bed load sediment is high, which produces abundant bedforms and promotes the development of bars, and thus, the braided character of the river. The sediment is commonly coarse, which requires fast flow and steep gradients for the sediment to be transported.  
 
Sediment Transport:  
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest. (Like meandering rivers, the thalweg is the deepest point in the channel.) 
2) Bars are eroded up stream where the bars deflect the flow. Sediment is deposited on downstream side of bars and some on the flanks of bars where flow is slower, particularly on the insides of bends. 
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels, as seen in meandering rivers.  
 
Sedimentary textures include: 
1) trough x-bedding in channels, due to the migration of irregular dunes 
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes 
3) edges of bars fine upwards into shallower, lower Re (less turbulent) water, and can contain anything from upper flat to ripple laminations. 
 
Channels migrate back and forth leaving a sheet of sand with abundant cross stratification. These sheets of sand tend to fine upward. General characteristics of braided river deposits include:  
1) Scoured surface at the base of a channel 
2) Gravel lag at base of channel 
3) Trough x-bedded sands deposited just off the thalweg 
4) Occasional tabular x-stratification from migrating bars 
5) Sand deposited at slower speeds, finer grained (rippled possible) 
6) Overbank deposits from floods mostly composed of sand and silt, with some mud 
 
The large scale geometry of the deposits includes sheets of sand separated by flood plane deposits.  
 
 
Differences between braided and meandering river deposits: 
 
1. Braided river deposits are commonly coarser grained  
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on flood planes. 
3. Overbank deposits are better developed and finer grained in meandering river systems. 
4. Bar migration is much more regular in direction in meandering rivers because there is a well defined, single thalweg towards which the bars migrate. In contrast, braided river bar migration occurs in multiple directions. Thus, meandering rivers produce a more regular geometry of tabular cross bedding, when preserved.  
 
General Characteristics of Fluvial Sediments: 
1) On a large scale, river deposits consist of sheets and lenses of sand associated with flat laminated shales and silts with rare rippled sand beds 
2) Fining upward sequences of beds in the sands with decreasing flow sedimentary structures 
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification 
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop 
5) Soil development in associated shales deposited in the flood plane environment. 
 
Look at pictures of fluvial rocks at http://www.geology.ucdavis.edu/~gel109/SedStructures/Fluvial.html 
 

Posted by: Dawn Sumner

@ February 6, 2008 9:56:06 AM PST ( )

Weathering: 
 
Origins of Sediment 
Sediment comes from the break down of rocks into smaller, transportable components. This occurs via two processes: physical weathering and chemical weathering. Physical weathering consists of breaking apart rocks and crystals. The results of physical weathering are smaller components of the same material that is being weathered. There is no change in composition. In contrast, chemical weathering consists of changing the composition of at least some components of the rock that is weathering. The sediment does not have the same composition as the original rock. 
 
Physical Weathering: 
Physical weathering occurs via: 
 
1) Freeze-thaw action. Water in cracks expands when it freezes, putting force on the cracks. The cracks grow, and eventually crystals and pieces of rock break off into smaller components. Obviously, this process is most important in environments where temperatures cycle across the freezing point of water.  
 
2) Salt crystal growth. When water evaporates, salts precipitate. If this happens in factures in rock, the growth of the salt crystals can put pressure on the cracks, causing them to grow. This process is most important near oceans where rocks are exposed to lots of salt water spray and in arid environments where water evaporates rapidly.  
 
3) Temperature changes. Minerals contract and expand as temperature decreases and increases, respectively. Different minerals expand and contract at different rates, producing stresses which can result in cracks and physical weathering. This process is most important when temperatures change dramatically from day to night, a characteristic of many desert environments.  
 
Physical weathering tends to produce mostly sand-sized sediment and larger grains because most of the fracturing occurs along mineral boundaries. Physical weathering of fine grained or finely crystalline rock can produce abundant very fine grains, but most of the sediment from these rock types consists of rock fragments (also called lithic clasts). 
 
Chemical Weathering: 
Chemical weathering occurs via: 
 
1) Dissolution of minerals. Some minerals like halite and other evaporites dissolve very easily in water. Other minerals, particularly silicates, do not dissolve easily. Carbonates are in between and dissolve in acidic waters. (Rain water has a pH of ~5.7 due to dissolved CO2, even without “acid rain” pollution.) The results of dissolution are ions in water that are transported downstream. Ions are not deposited until the water evaporates, they react with other minerals, or organisms use them to make shells. Often, only part of a rock dissolves, leaving sediment that can be transported by wind, water, etc. 
 
2) Alteration of minerals. Silicates do not dissolve very easily, but they do react with water to form new minerals. Feldspars react with water to form clays and ions, olivine reacts with water and O2 to form oxides, clays and ions, pyrite reacts with water and O2 to form oxides and sulfate ions. Iron oxides, such as hematite, are commonly red, giving weathered rocks a red hue to them. Alteration of minerals is one of the main sources of clay minerals and mud-sized grains. 
 
Mineralogy of Weathered Rocks 
Sediments that have been subjected primarily to physical weathering have a mineralogy that is similar to that of the parent rock. Thus, if you look at the composition of a sand eroded from a high latitude mountain range, it may have some mineralogic properties that distinctly identify is as coming from that particular mountain range. If the sediments have been subject to extensive chemical weathering, it is much harder to characterize the source rocks because the composition has changed extensively. The composition of the resulting sediment depends on the mineralogy of the rock, how it is transported, and the weathering environment. Some minerals alter more quickly than others. Quartz is difficult to dissolve and is hard, so it commonly lasts through both chemical and physical weathering and is the most common composition of sand on Earth. In contrast, minerals like Ca-feldspar and olivine react to form new minerals quickly. They are substantially less common in sediments. Thus, mafic rocks (which contain Ca-feldspars, olivine and pyroxenes) tend to alter to clay minerals very easily and produce little sand and abundant mud. In contrast, granites (quartz, K-feldspar, Na-feldspars, mica) contains minerals that react more slowly and tend to produce sand-sized grains, especially quartz.  
 
Reactive (rare) Less Reactive (common) 
Olivine Ca-feldspar Pyroxene Amphibole Na-feldspar Biotite K-feldspar Muscovite Quartz 
 
The other main control on sediment mineralogy is the hardness of the grains. During transport, grains hit each other. Softer grains tend to be damaged when they collide with harder grains, and this damage can cause them to break into smaller grains. Thus, soft grains become smaller very quickly when they are transported with hard grains. Quartz is the most common mineral in sandstones because it is hard and reactive. Clays are also very common because they are too small to damage much during collisions and they are the product of the alteration of other minerals.  
 
Controls on Weathering 
The extent and style of weathering is mainly controlled by climate. Water is extremely important, even for physical weathering. The more water present, the faster weathering occurs. Temperature is also important, as discussed for physical weathering. Warmer temperatures also promote faster reactions, so chemical weathering is more effective in warm climates. Thus, warm, humid climates tend to have the most rapid weathering (and poor outcrop). Finally, vegetation has a strong influence on weathering. Plants tend to increase the extent of chemical weathering by producing organic acids which help break down rocks into soil through both dissolution and alteration.  
 
Erosion 
Gravity: Diffusion, rock/debris falls, landslides & slumps, and debris and mud flows... 
see these videos:  
Rock Fall: http://faculty.gg.uwyo.edu/heller/SedMovs/SultanSlide.htm 
Landslide: http://youtube.com/watch?v=f19Onak6KC0 
Debris Flow with Increasing Water Content: http://faculty.gg.uwyo.edu/heller/SedMovs/FlashJerol.htm 
 
Rivers 
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of a channel, bank and overbank or flood plain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds. 
 
River Types -  
Straight (rare, except for ones humans have modified) 
Meandering (high sinuosity) 
Braided (many branches within a channel) 
Anastomosing (rivers with branching and merging channels) 
 
The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, and sediment grain size, particularly the ratio of suspended versus bedload sediment. A high bedload gives rise to abundant bars, which promotes formation of braided rivers. 
 
Meandering Rivers -  
Meandering rivers have a low gradient and thus slower flow, and often have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a slightly dipping plain. The flow speed in the channel varies with the geometry of the curves. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, you should predict that there is more erosion on the outsides of bends, the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available, and the insides of bends will be were sediment is accumulating and this sediment will be finer grained. If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower.  
 
The main parts of the channel include eroding bank, the Thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The lowest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination.  
 
As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter.  
 
Levees and Flood Plains - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the flood plane. It slows down very quickly. Thus, it can not transport as much sediment as it caries in the channel as soon as the water leaves its banks. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out as the flood waters dry up.  
 
Over time, the levees build up and provide a higher bank for the channel than the flood planes. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the flood plane. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the flood plane. This results in the downstream part of the channel being completely abandoned. 
 
Meandering River Channel Facies: 
1. Scoured base of flow  
2. Lag deposit with mud rip-up clasts  
3. Fining upward sands with trough cross stratification  
4. Rippled sands  
5. Sigmoidal cross stratification from migrating point bars  
 
Flood Plane Facies 
1. Fine sand with climbing ripples 
2. Mudstone/shale with mud cracks 
3. Soils 
4. Root casts 
 
Ox Bow Lake Facies 
1. Mudstone/shale without mud cracks 
2. Organic-rich deposits, including coal 
3. Anoxic water indicators (especially in fossils and trace fossils) 
 
 
 
 

Posted by: Dawn Sumner

@ February 6, 2008 9:46:40 AM PST ( )

 
Stratigraphy and Time: 
 
Facies 
Facies are groupings of rock types based on similar features. We use these groupings to generalize individual properties into useful, genetically related categories. Turbidite facies include: 
 
Based on sedimentary structures and grain size: 
Bouma a: Coarsest grains fining upward (graded) 
Bouma b: Upper plane bedded sandstone 
Bouma c: Rippled sandstone 
Bouma d: Faintly planar laminated siltstone 
Bouma e: Shale 
 
Based on large-scale depositional styles: 
Inner Fan 
Mid Fan 
Outer Fan 
Channels 
 
Organic Matter, Oil and CO2 
Focus the Nation – Climate Change events – Tomorrow! 
Carbon Cycle and Sedimentology – Carbon is removed for the atmosphere primarily by accumulation of organic matter and carbonate rocks. Both are part of sediments, and with weathering, volcanic activity, and metamorphism, they regulate earth’s climate.  
 
<Carbon Cycle image from: http://www.inforse.dk/europe/dieret/WHY/carbon.jpg > 
 
Organic matter accumulates with fine-grained sediment, particularly clays. It settles out from solution slowly, and it binds to the surfaces of minerals. Small minerals also settle out slowly, and they have large surface areas. Low oxygen environments favor burial rather than oxidation of the organic matter. Thus, environments that do not mix air into the water also favor organic matter accumulation. Based on the carbon fluxes, an increase in biomass, and isolation of that biomass from oxidation, can consume significant carbon.  
 
Carbonate rocks are a very big reservoir of carbon, but the chemistry of carbonate mineral precipitation makes it difficult to use carbonates to control CO2 over human time scales. Weathering converts CO2 into alkalinity by consuming acid. The resulting alkalinity forms carbonate rocks. If carbonates form without the addition of alkalinity, it can cause the waters to release MORE CO2 on short time scales.  
 
CO2 sequestration underground is an interesting option. 
 
Stratigraphy - the study of sedimentary rocks in space and time.  
 
Stratigraphy is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes.  
 
Example: Beach Facies. Beach environments grade laterally into each other. The off shore areas grade into the swash zone of the foreshore. The foreshore grades into the berm and backshore (if present). Eolian (wind) dunes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tent to change upward. They also vary laterally. See figure 18.6 on pg. 238 of Nichols. 
 
Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time.  
 
Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time. 
 
Side note: Formations - Formations are defined as lithologically distinctive stratigraphic units that are large enough to be mapped out. They are the basis for map making. By choosing formations based on lithology, a detailed understanding of depositional environments is not necessary, and it makes recognition of the formations straight forward in the field. However, there is no direct connotation of time except the law of superposition, which is that older formations underlie younger ones. Two separate formations may be deposited at the same time, however, if they formed in distinct depositional environments. Thus, formations represent the extent of a depositional environment both in space and through time. This is the basis of lithostratigraphy; it is useful for map making, understanding water or oil reservoirs, etc., and it is the easiest type of stratigraphy because you match rock types. However, to understand the landscape and the distribution of depositional environments at a given time, you need to correlate rocks of the same age rather than of the same type. End of Side Note 
 
Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments. 
 
Chronostratigraphy - Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history rather than as a set of mapable units. However, it is often much more difficult to determine which rocks were deposited at the same time. Even though I have a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate these two section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.  
 
Distribution of Rock and Time - One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in land based environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates - thickness ≠ time! 
 

Posted by: Dawn Sumner

@ January 30, 2008 12:03:30 PM PST ( )

Lorenz Equations 
You do not need to know this for class, but since I got the weather wrong on Saturday, I thought I talk a bit about weather prediction and chaos.  
 
Prof. Lorenz is an atmospheric scientist who began modeling convection in the atmosphere on early digital computers (in the 1970’s). He found that slightly different initial conditions in his models led to wildly different weather patterns. He eventually simplified his models until he described convection using three variables:  
 
 
 
 
 
 
 
These equations, particularly the non-linear terms xy and xz, make the solutions very sensitive to initial conditions in some regions. The behavior jumps from one state to another (or from one attractor to another in the parlance of chaos). When the state jumps can be calculated from the equations, but a change in a distance decimal place can dramatically change when the system jumps. I will show a demonstration of this in class. 
 
Now for the class topic: 
 
Turbidites 
Turbidites are sedimentary deposits that can be ideal for seeing flow changes and form distinctive facies. They are deposited from slurries of sediment and water in any standing body of water (lakes, oceans).  
 
1) Start: slope failure in soft sediment with lots of water still in it (places where lots of sediment is coming in from a river, etc. and slope becomes too steep; earthquakes can trigger) 
 
2) Sediment and water mix: “fluid” is denser than the surrounding water because of the entrained sediment, so if flows downhill even if the slope is very low (1° or flat) 
 
3) Base of the flow is commonly erosional on steep slopes, so more sediment is entrained in the flow. 
 
4) Enough sediment is entrained that erosion stops, and deposition begins as the slope gets shallower or the flow starts to slow down. Initially, the coarsest grains are deposited (remember the Hulstrom diagram) and then finer grains, so the sediment is “graded”. However, the sediment is usually poorly sorted because the flow is a slurry of water and sediment so hydraulic sorting is reduced. (Facies = Bouma a) 
 
5) Sediment concentration decreases so get more hydraulic sorting. The flow is very fast so the sediment has upper plane bedding. (Facies = Bouma b) 
 
6) As the flow slows more, ripples start to form. Dunes are not usually found for two reasons: a) often only fine sand and finer grains are left in the flow by this point; and b) dunes do not have time to develop. (Facies = Bouma c) 
 
7) Eventually, the flow slows to the point that bedload transport stops and deposition is mostly settling of silt and then clay. The progressive settling of coarser and then finer grains produces a faint lamination, but it is not as strong (ideally) as the planar laminations in Bouma b. (Facies = Bouma d) 
 
8) Mud settles out producing shale. This can look identical to background settling of clays brought into the lake/ocean as suspended sediment. (Facies = Bouma e) 
 
Bouma divisions a-d can take hours or a day or so to be deposited. However, division e, which is usually the thinnest, commonly accumulates over months or longer (e.g. hundreds of years) depending on how frequent turbidites are in the area.  
 
Watch these movies of turbidites in flumes: 
http://faculty.gg.uwyo.edu/heller/SedMovs/middletonturb.htm 
http://faculty.gg.uwyo.edu/heller/SedMovs/Turbidity%20ignition.html 
 
Changes in Character Downslope - The parts of turbidites that are deposited change downslope and usually only a few of the subdivisions are preserved. In the most proximal (upslope) environments, divisions a and b are most common. In the more distal areas, all of the coarser sediment has already been deposited upstream, so divisions d and e are most common. Generally, there are also channels which fan out producing variations in rock types that change in space and through time.  
 
Draw stratigraphic columns of turbidites characteristic of each environment. This is similar to the diagram in the book on page 57. We can summarize these differences in a stratigraphic column that lumps individual beds together. This is like the second half of the next homework set.  
 
Turbidite Facies Models - Over the decades, sedimentologists have described and interpreted sedimentary rocks and defined generalized facies and facies associations that are characteristic of different depositional environments. These generalized facies and associations are called Facies Models. Each depositional environment or system has its own facies model. This is a VERY powerful tool.  
 
Extra on Turbidites - Turbidite facies analysis and the resulting facies model led to the discovery of a new process. Sedimentologists had characterized turbidites all over the world. They all had the same flow characteristics consisting of a very strong erosive flow, deposition of a normally graded bed which was massive, followed by upper plane bedding, rippled finer sands, coarsely laminated silts, then shales. Comparisons with known flows showed that this sequence of deposits must come from a strong initial flow that slowed through time to still water. And this repeated again and again. The associated facies and the succession of different facies in these sequences suggested that the deposits had to be in deep water. For example, the fossils were all characteristic of deep water, shales were abundant and only settle from still water (shallow or deep), and they were sometimes associated with deep water storm deposits. Thus, the sedimentologists proposed slope failure and turbid currents flowing downslope and called them turbidity currents. A process like this had not been observed in modern depositional environments, so the idea was controversial. Many geologists didn’t believe that you could generate strong enough currents underwater to get those flow characteristics. Eventually in 1964, two geologists Heezen and Drake realized that an event in 1929 provided strong evidence for turbidity currents. In 1929, without satellites, under water telegraph cables were strung from Newfoundland to Europe. In November, about 30 cables broke in order from farthest north and shallowest to farther south and deeper water. At the time, people didn’t know why they broke, but Heezen and Drake suggested that a turbidity current was triggered by an earthquake and the cables broke as the turbidity current passed over them (they are strong flows!). Because they were continuously used for communication, the time each cable broke was very well known. Heezen and Drake calculated that the flow traveled at 250 km/h (36,000 cm/s) when it first formed and then slowed to around 20 km/h (7000 cm/s) by the time the last cables broke 500 km from the source. This was a fast, strong flow and may be typical of turbidites. These speeds are above the upper end of both the Hjulstrom diagram and are very erosive. It is only after the turbidite slows down even more that you get deposition. The characteristics of the flow seen by the breaking cables fit the flow characteristics proposed by the sedimentologists, and now turbidity currents and the facies model developed for turbidites are widely accepted and often treated as an ideal example of rocks that closely reflect flow characteristics. They are close to an ideal example of a good Facies Model. 
 
Stratigraphy and Time: 
 
Facies 
Facies are groupings of rock types based on similar features. We use these groupings to generalize individual properties into useful, genetically related categories. Some examples include: 
 
Facies based on grain size: 
Coarse-grained sandstone with 1-5% pebbles  
(suggests high flow speeds) 
Fine-grained, well-sorted sandstone  
(suggests low flow speeds with either only one size sediment source or a consistent flow speed) 
Mudstone  
(suggests standing water) 
 
Facies based on sedimentary structures: 
Fine-grained sandstone with current ripple cross lamination 
Fine-grained sandstone with upper planar lamination 
Fine-grained sandstone lacking cross stratification, but with abundant burrows 
 
Facies based on grain composition: 
Coarse-grained sandstone with 25% lithic fragments, 25% feldspar, and 50% quartz 
Coarse-grained sandstone with 80% quartz, 10% mica, and 10% feldspar 
Coarse-grained sandstone with 99% quartz and trace gold flakes 
 
Stratigraphy - the study of sedimentary rocks in space and time.  
 
Stratigraphy is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes.  
 
Example: Beach Facies. Beach environments grade laterally into each other. The off shore areas grade into the swash zone of the foreshore. The foreshore grades into the berm and backshore (if present). Eolian (wind) dunes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tent to change upward. They also vary laterally. See figure 18.6 on pg. 238 of Nichols. 
 
Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time.  
 
Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time. 
 
Side note: Formations - Formations are defined as lithologically distinctive stratigraphic units that are large enough to be mapped out. They are the basis for map making. By choosing formations based on lithology, a detailed understanding of depositional environments is not necessary, and it makes recognition of the formations straight forward in the field. However, there is no direct connotation of time except the law of superposition, which is that older formations underlie younger ones. Two separate formations may be deposited at the same time, however, if they formed in distinct depositional environments. Thus, formations represent the extent of a depositional environment both in space and through time. This is the basis of lithostratigraphy; it is useful for map making, understanding water or oil reservoirs, etc., and it is the easiest type of stratigraphy because you match rock types. However, to understand the landscape and the distribution of depositional environments at a given time, you need to correlate rocks of the same age rather than of the same type. End of Side Note 
 
Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments. 
 
Chronostratigraphy - Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history rather than as a set of mapable units. However, it is often much more difficult to determine which rocks were deposited at the same time. Even though I have a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate these two section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.  
 
Distribution of Rock and Time - One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in land based environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates - thickness ≠ time! 
 

Posted by: Dawn Sumner

@ January 28, 2008 12:03:09 PM PST ( )

Sedimentary Structures Continued 
 
Field trip this Saturday to Bodega Bay! 
 
Ripples and Dunes (A review with a bit of additional information) 
 
A sketch of a ripple or dune like the one in lecture:  
http://www.geology.ucdavis.edu/~gel109/Lectures/duneXStrat.jpg 
 
Remember where the separation point and attachment point are located. The geometry of the laminar flow layer tracks these points. Erosion can only occur where there is a bed shear stress sufficient to move sediment. In other words, the laminar sublayer for the main flow must be on the sediment surface. Deposition occurs temporarily where there is net erosion, but sediment accumulates into a deposit in the flow shadow downstream of the ripple or dune crest. In other words, sediment accumulates in the detachment zone. Laminae are visible where deposition occurs due to variations in flow speed and thus the grain sizes transported and deposited.  
 
Sediment is transported into the detachment zone in two ways. Grains saltate and roll off the crest of the ripple into the zone of deposition. If most grains are saltating and the saltation distance is about the same as a third to half the wavelength of the bedforms, deposition occurs throughout the detachment zone by grains falling from above. However, if most grains are rolling or the saltation distance tends to be less than a third of bedform wavelength, grains pile up near the top of the slope. The slope gets steeper until it avalanches, and grains are transported down the slope.  
 
Dunes and ripples behave similarly at the level of detail that I have been describing them. Their cross stratification geometries are similar. However, dunes are larger than ripples. If the distance between erosion surfaces defining cross sets is greater than a few centimeters, the cross stratification has to be from a dune. Ripples are only a few centimeters tall, and they cannot create laminae that are higher than the ripple crest-to-trough distance. Thus, if cross sets are greater than a few centimeters high, the cross stratification must be from dunes. However, if the cross sets are only one centimeter high, the cross stratification could be due to either ripples or dunes. It is possible for ALL sediment to be eroded as a dune migrates, leaving no cross stratification. If only a small amount of sediment accumulates, the cross sets might be only a centimeter high, much like ripples. In the field, grain size variations and changes in cross stratification along an outcrop can help you distinguish between ripples and dunes in a case like this. For example, you could look for an instance where the cross stratification is more than a few centimeters high. If you did not find one, that might suggest ripple cross lamination rather than dune cross stratification.  
 
Bedforms and Grain Size - Bedforms also vary with grain size (see Figure 4.19, Nichols, 1999). Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples. The erosional force at the reattachment point is not strong enough to erode the coarse grains and produce the erosional surfaces on the backs of ripples. Without this erosion, troughs do not form and without troughs, crests do not form. The sequence of structures in granules with increasing flow is: 
 
1) no transport 
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring 
3) dunes - the flow is strong enough to erode at the attachment point 
4) upper planar lamination 
5) antidunes 
 
In contrast, the sequence of structures in silt is: 
 
1) no transport 
2) ripples 
3) upper planar lamination 
4) antidunes 
 
Antidunes - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly. Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface, and the surface of the water shows the geometry of the antidunes. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases. However, we have seen them on almost every field trip to the beach for this class. 
 
A photo of antidunes from Scott Creek Beach: 
http://www-geology.ucdavis.edu/~gel109/sedstructures/Lg/ScottAntidunes.jpg 
 
To watch antidunes in a flume on YouTube: http://www.youtube.com/watch?v=lNg_woUaPgM 
 
Environments and Facies 
Look at the photo of Scott Creek Beach again. Note that the antidunes are forming in one part of a creek. The middle of the creek has upper planar lamination flow speeds, and the closest part is very shallow and has some antidunes again. (I know some of this from being there more than from looking at the photo.) Note that there is a faint lamination present in the eroding bench on the far side of the creek. This lamination mimics the beach surface. It is lamination from the waves swashing and transporting sediment on the beach. If all sediment transport stopped immediately, one would see a suite of sedimentary structures: Antidunes and upper planar laminae next to each other in the creek, an erosional surface overlying planar stratification that undulates like a beach. The association of these features would tell you that the sediment was deposited in an environment with a variety of flow conditions.  
 
The suite of structures forms a facies. A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines. Usually facies are defined based on a suite of characteristics in rocks.  
 
From Sediment Transport to Rocks - We have been talking about sediment transport and structures. These are processes that influence sedimentary rocks. What we really need is to be able to use our understanding of the processes to interpret ancient rocks when we can no longer see the processes in action. As I mentioned in the first class, we can use the modern processes as a model for interpreting past processes, which is the principle of Uniformitarianism. However, it is often very different to see a process going on than it is to look at the ultimate deposited rock and interpret the process. For example, with bed forms, the entire shape of the structure you see as it migrates is rarely preserved. Instead, you only see a small part of it, if you get any sediment accumulation at all. Thus, we can also start the interpretation from the rock end by describing the general characteristics of the rocks and interpret flow from things like grain size, preserved cross stratification, and biogenic components. Then we can evaluate which environments are consistent with those characteristics.  
 
Facies vs Environments - By grouping characteristics of the rocks into facies, the depositional environments can be more easily compared and interpreted. It is important to remember that the sedimentary environment is the combination of physical, chemical and biological processes that influence sediment deposition, whereas sedimentary facies are the characteristics of the rocks after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same.  
 
 

Posted by: Dawn Sumner

@ January 28, 2008 12:02:32 PM PST ( )

 
Key Points of Sediment Transport  
1) Stronger flows move larger grains. Flow strength is a combination of flow speed and bed shear stress (the laminar sublayer characteristics).  
2) Sediment is transported as bedload and in suspension. Bedload consists of rolling and saltating grains.  
3) Grain size and flow strength determine how grains are transported.  
4) As flow strength changes, grains are eroded or deposited. These relationships are represented in Hulstrom diagrams. 
 
A Few Definitions:  
1) "Stratification" - layers in rocks; stratified rocks are those organized into beds 
 
See Grand Canyon Beds: http://www.geology.ucdavis.edu/~gel109/Lectures/L1/12GrandCanyon.jpg 
 
2) “Beds” are separated by “bedding planes” - cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation or changes in grain size. In other words, they usually represent changes in flow characteristics. 
 
See Cache Creek Turbidite Beds and Bedding Planes: http://www.geology.ucdavis.edu/~gel109/Lectures/L1/13tiltedturbidites.jpg 
 
3) "Laminae" are color, composition, or grain size variations defining surfaces within a bed. They typically represent variations in flow velocity, sediment supply, sediment composition, etc. Planar Laminae are parallel to bedding, e.g. planar. 
 
4) "Cross Lamination”, "Cross Stratification" or "Cross Bedding" are laminations or layers that are oriented obliquely to bedding. They truncate older laminae and are truncated by younger laminae. The erosional surfaces that separate “sets” of similarly oriented laminae are called “bounding surfaces”. There are lots of subdivisions of cross stratification; different types represent different types of bedforms and different flow conditions.  
 
See Burns Cliff on Mars: http:// http://www.geology.ucdavis.edu/~gel109/Lectures/L4/3BurnsCliff.jpg The upper part of the image has planar lamination, and the lower part to the far left has cross lamination or stratification. 
See: http://www.geology.ucdavis.edu/~gel109/SedStructures/Lg/TroughXStrat3.jpg and other examples at: http://www.geology.ucdavis.edu/~gel109/SedStructures/Dunes.html 
 
See ripple cross lamination on Mars: http://www.geology.ucdavis.edu/~gel109/Lectures/L4/5MartianRipples.jpg 
 
Bedforms 
 
When sediments get deposited from turbulent flows, the sediment interacts with the geometry of the flow. Depending on the flow speed, turbulence, and sediment characteristics, different structures or bedforms develop. 
 
See: http://faculty.gg.uwyo.edu/heller/SedMovs/mcbriderips.htm 
 
Bed Geometry and Flow Separation - Until now, we have been implicitly assuming that the bases of beds are flat and smooth, but if sediment is present, they are not. If you start with a smooth bed of sand and increase water speed above it, irregularities form from irregularities in the flow and develop into ripples. First, a few grains pile up. Once the height of the pile is several grains high, there is a flow shadow down stream of them, and the viscous sublayer detaches from the base of the flow. The water has enough momentum that it does not hug the bed surface and instead, goes shooting out over the top. This point is called the separation point. The water flows forward and downward and reconnects with the bed at the attachment point. At the attachment point, water is flowing directly towards the sediment with a lot of force. This force moves the grains and causes erosion. In contrast, the area between the separation point and the attachment point has very low flow. In fact there are back eddies, where the flow is upstream. Thus, sediment transport is very irregular along the bedding surface at a local scale. 
 
Sediment Transport Over a Ripple - Sediment grains are mobilized at the attachment point - more so than in normal flow because the water is shooting directly into the sediment - and the grains are moved downstream by saltation and traction. As the flow becomes parallel to the sediment surface again, its ability to transport sediment decreases. Thus, the grains tend to pile up and a new mound forms. This gives a periodic chain of mounds - the beginnings of ripples. As flow continues, grains roll and saltate up the stoss (upcurrent) side of the ripples. Once they pass the crest, they reach the low flow on the lee side of the ripple. The larger grains settle out and roll partway down the slope; this is the site of net deposition. As the process of deposition on the lee side and erosion on the stoss side continues, the ripples migrate downstream. If there is net deposition of sediment, the ripples leave behind distinctive dipping layers between two erosional surfaces that can be preserved in the rock record. These layers slope downstream and are one type of cross lamination. 
 
A sketch of a ripple or dune like the one in lecture:  
http://www.geology.ucdavis.edu/~gel109/Lectures/duneXStrat.jpg  
 
To see the back eddy watch this video on YouTube: http://youtube.com/watch?v=FWAszNXY3Bs 
 
Watch the USGS bedform movies described at: http://www.geology.ucdavis.edu/~gel109/homework/USGSBedforms.html  
 
Bedforms and Flow Velocity- The size and shape of subaqueous bedforms depends on flow strength and grain size and can be used to interpret ancient flow characteristics in a depositional environment from looking at sedimentary rocks. See Nichols (1999, p. 51) for definitions of bedform, flow speed, and grain size relationships. 
 
Ripples (<60 cm wavelength) – The minimum flow for ripples is determined by the minimum velocity for sediment transport. Once this flow speed is reached, ripples form if the sediment is transported as bedload. The maximum flow speed for ripples depends on the location of the attachment point on the stoss side of the ripples. As flow gets faster, too much erosion occurs at the crests of the ripples - the point of attachment is too far up the stoss side of the ripple- and the ripples flatten out. Dunes develop.  
 
Dunes (60cm-100’s m wavelength) - Dunes develop as ripples flatten out because large scale irregularities start to develop. The basic ideas of dune and ripple formation are the same. The difference is that the area of flow separation is much larger (see Fig 3.3 Davis, 1992). Roller vortexes (e.g. upstream flows along the lee sides of dunes) are common, and the upstream flow can be strong enough to form ripples that migrate upstream. As flow speeds increase, the dunes start to flatten out. 
 
Planar/Flat Lamination – Planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed. This zone of planar lamination is called “upper flow regime”. (Why “upper”? - there is a zone of planar lamination in coarse grained sediment at low flow velocities.) 
 
Antidunes - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Puta Creek in flood; tidal channels; creeks flowing across beaches - photo). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases. However, we have seen them on almost every field trip to the beach for this class. 
 
Watch antidunes in a flume on YouTube: http://www.youtube.com/watch?v=lNg_woUaPgM 
 
Other Types of Flows - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at http://geology.ucdavis.edu/~gel109/sedstructures/ARipples.html Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments.  
 
 
Bedforms and Grain Size - Bedforms also vary with grain size (see Figure 4.19, Nichols, 1999). Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples. The erosional force at the reattachment point is not strong enough to erode the coarse grains and produce the erosional surfaces on the backs of ripples. Without this erosion, troughs do not form and without troughs, crests do not form. The sequence of structures in granules with increasing flow is: 
 
1) no transport 
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring 
3) dunes - the flow is strong enough to erode at the attachment point 
4) upper planar lamination 
5) antidunes 
 
In contrast, the sequence of structures in silt is: 
 
1) no transport 
2) ripples 
3) upper planar lamination 
4) antidunes 
 
Extra 
High Sediment Loads - Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher Re and more turbulent flow (Re=u*l*r/µ). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of which ever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes. 
 
If there is very little water associated with the sediment flow, the flow can be very viscous due to the charge attraction among clay particles and have laminar flow characteristics (Re=u*l*r/µ). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until they seize up and stop. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent.  
 
There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion.  
 
 

Posted by: Dawn Sumner

@ January 16, 2008 11:48:31 AM PST ( )

Lecture 3: Fluid Flow and Sediment Transport 
 
Key Concepts from Lecture 2 
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re>2000 and laminar flow has Re<500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.  
 
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer. Watch the movies on the Turbulent and Laminar Flow YouTube playlist: http://www.youtube.com/view_play_list?p=23D938B8B55F49E8 
 
Sediment Transport 
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. 
 
The Bernoulli Effect 
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect. 
 
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is a grain's position relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average. These are summarized in Shields and Hulstrom diagrams, which we’ll talk about in a few minutes. 
 
Bedload and Suspended Load Transport 
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain falls due to gravity versus how turbulent the water is (back to Re...). Upward bursts in turbulent flow can counteract gravity, particularly for small grains. Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water. 
 
Here is a good movie of bedload transport: 
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb) 
 
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow. 
 
Photo of suspended sediment in a Costa Rica River: 
http://www.geology.ucdavis.edu/~gel109/Lectures/L3/CostaRicaRiver.jpg  
and the Rhone River flowing into Lake Geneva: http://en.wikipedia.org/wiki/Image:Leman_img_0573.jpg 
Movie of suspended clay in a flume: http://www.youtube.com/watch?v=-IV5leEAPpg 
 
Hjulstrom Diagrams 
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams (and Shields diagrams). Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!  
 
A copy of the Hjulstrom Diagram from the book: http://www.geology.ucdavis.edu/~gel109/Lectures/l3/Hjulstrom.jpg 
 
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar sublayers at the base of flow are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than shown Hjulstrom diagrams which summarize a lot of characteristics into 2 axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess. 
 

 
Aside: Shields diagrams are similar to Hjulstrom diagrams, but they show sediment transport as a function of the ratio of the grain size to the size of the laminar sublayer and bed shear stress. Thus, they include more specific information about the flow characteristics. However, they are also more abstract and are harder to relate directly to a natural flow. Thus, we use a simplified Hjulstrom diagram in this course rather than Shields diagrams. If you are interested in sediment transport, however, Shields diagrams are really interesting and helpful. 

 
 
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clays tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.  
 
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).  
 
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.  
 
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb) 
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb) 
 
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and grains are left on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.  
 
Ripples and Other Bedforms 
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser lamiae that dip downstream.  
 
Next Time: Bedforms, cross lamination, and other cools stuff! 
 

Posted by: Dawn Sumner

@ January 14, 2008 10:31:21 AM PST ( )

Lecture 2: Fluid Flow 
 
Key Concepts from Lecture 1 
The Principle of Uniformitarianism – the processes that formed ancient deposits are the same as those that form modern deposits. 
 
The Principle of Original Horizontality - strata (or sedimentary rock layers) are deposited in a nearly horizontal position. If they are no longer horizontal, later deformation much have changed their orientation.  
 
The Law of Superposition - younger sediments overlie older sediments. 
 
Faunal Succession - fossil organisms succeed one another in the stratigraphic record in an orderly, recognizable fashion.  
 
Walther’s Law Depositional environments vary in space and time such that “The facies [rock types] that occur conformably next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894). 
 
Sediment Transport 
 
Most sediment transport is due to gravity. Things fall down hill in slumps, debris flows, and mudflows, and are transported downhill by fluids, like water, ice, and air. In some cases, processes like waves, currents, and wind transport sediment up a slope such as a beach or up mountain sides. This transport goes against gravity and is driven by the processes of fluid dynamics. Fluid dynamics is the main topic of today's and Monday’s lectures. We will come back to mass wasting processes when we talk about erosion. Mass wasting is important for transporting large volumes of sediment short distances, but fluid transport is required to move sediments long distances and is responsible for most sediment transport. To understand sediment transport, it is essential to understand the mechanics of fluid flow. 
 
Fluid Flow 
There are two end member ways fluids flow: 1) laminar flow and 2) turbulent flow. There is a wide gradation between these two end members, specifically flows that are called transitional flows.  
 
Laminar Flow - In laminar flow, water molecules move in straight, parallel lines down current. If you add a dye to water that is in the laminar flow regime, the dye would not mix into the water; it would streak out in an approximately straight line. Laminar flow is characteristic of very slow moving, shallow water, which is uncommon in nature. It is also characteristic of flows in “fluids” that are very viscous, like glacial ice or mud flows that have little water.  
 
Turbulent Flow - In contrast, turbulent flow is characterized by complex motion of water (or other) molecules. Molecules move in all directions in bursts of upward, downward, and forward motion, and even some backward movement. There is abundant mixing in the flow because neighboring molecules move in different directions, and an added dye would mix into the water very quickly. Most water and air flows are turbulent, at least to some degree. Turbulence is important for sediment transport because it makes grains easier to transport and tends to keep them moving longer.  
 
Transitional Flow – Transitional flows have some characteristics of laminar flow and some of turbulent flow. For example, dye may take some time to mix into the flow, but it does mix.  
 
Movies of Laminar and Turbulent Flow: http://www.youtube.com/view_play_list?p=F02560C499E2A9D4 
 
image of a glacier; image of rivers mixing 
 
Reynolds Number - The Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid.  
 

 
[Viscosity is a measure of the resistance of a material to flow, i.e. how “thick” and easily deformed it is. Viscosity is sort-of like the amount of friction within a substance. Walking through air is easy, because there is not much friction between air molecules. Air has a low viscosity. Swimming is more difficult because the water drags on your body. This is due to the “friction” between adjacent water molecules, i.e. higher viscosity. Ice is more viscous and impossible to move through because of the crystal bonds between the water molecules. It flows, but it does so slowly. ]  

 
 
Back to the Reynolds number. The variables for the Reynolds number (Re) are: flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (ρ), and fluid viscosity (µ). The book uses µ/ρ = v (kinematic viscosity). Re = (fluid inertial forces)/(fluid viscous forces) = l*u*ρ/µ. The units for this equation are typically (length)*(length/time)*(mass/length3)/(mass/(length*time)). These all cancel out to form a unitless number, if you choose the same set of units for each variable, which you should always do.  
 
Re can be viewed as inertial forces divided by viscous forces. Inertia is the resistance to change in motion, and inertial forces tend to make a bit of the fluid keep flowing in its own direction if it is misdirected from the main flow direction. Thus, high inertial forces tend to cause more turbulence. In contrast, viscous forces tend to suppress turbulence by damping out variations in motion through friction. Thus, a flow with a high viscosity (ice) tends to have less turbulence than a low viscosity flow (air).  
 
The magnitude of Re gives an idea of whether the flow is turbulent or laminar. Turbulent flow has Re>2000 and laminar flow has Re<500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well. In most cases, water and air flows have high Re because l is large, u is high and µ is low. Rivers and wind storms are good examples of turbulent flow. In contrast, ice has a large µ and flows slowly (u is low), so it is usually laminar. Also, very thin, slow flows of water, such as water flowing off a smooth cement parking lot, has a low Re because l and u are small. Thus, it can be laminar. Laminar flow also occurs locally in turbulent flows right at the contact between the fluid and a smooth surface it is flowing over because u becomes very slow. This is really important for sediment transport, and we'll talk more about it in a few minutes. 
 
It is useful to think about which variables are important for different comparisons. When comparing ice and water, the main difference is viscosity; the viscosity of ice is >10^3 kg/(m*s) and up to more than 10^20 kg/(m*s) depending on temperature. In contrast, the viscosity of water is ~10^-3 kg/(m*s). The density of both is very close to 1000 kg/m^3. Thus, ice is almost always laminar but water is usually turbulent, although it can be laminar. When considering water flows, the flow speed and water depth are both very important. The viscosity and density change a little bit with temperature, but variations in flow speed and water depth are typically much larger effects. 
 
For air, both the density and viscosity are low, so does Re tend to be high or low? The density of dry air at 1 atm at 15°C is 1.225 kg/m3, and its viscosity is 1.8x10^-5 kg/(m*s), giving p/µ=6.8x10^5 s/m2 for air versus 1.0x10^6 s/m2 for water. In a sense, the low viscosity of the air overcomes its low density to make it more likely for the flow to be turbulent. The thickness of typical air flows (meters to 100’s of meters) also promotes turbulence.  
 
Boundary Layer - There is boundary layer at the edge of every flow. Flows have an average speed in the middle, but friction with immobile surfaces slows down the speed of the flow right at the surface. This creates a boundary layer that has different flow characteristics than the rest of the flow. Right at the surface, the water does not move, but as you go higher into the flow it starts to move more like the average flow. The area of the flow that has a reduced speed is called the boundary layer. The thickness of the boundary layer depends on Re (i.e. the amount of turbulence) and the roughness of the surface the flow is moving past. If the main water flow is turbulent, it changes the velocity distribution because more of the high speed water is mixed down into the lower speed areas. Thus, the boundary layer tends to be thin. In laminar flow, there is very little mixing of high speed water into the boundary layer, so the boundary layer tends to be thicker.  
 
Viscous/Laminar Sublayer - Within the boundary layer, right next to the surface, the laminar sublayer is present. Re=u*l*ρ/µ - remember this defines the difference between laminar and turbulent flow. Because u (water speed) is very low at the base of the boundary layer, the Re is low there and the flow is laminar. The laminar flow part of the boundary layer is called the viscous or laminar sublayer, “viscous” because the viscous effects are more important than the inertial effects. (The fluid is NOT more viscous here.) Farther up in the flow, u is higher, so the flow is typically turbulent. If grains do not extend above the top of this layer, they do not “see” much turbulence, and they are less likely to be transported. If they do stick up beyond the viscous sublayer because the viscous sublayer is thin or the grains are large, the grains feel the force of the turbulent flow. 
 
Bed roughness or the characteristics of the surface also affect the boundary layer by affecting the amount of water that has to interact with the surface. A very smooth bed, say one made of mud, does not deflect the water at all, so there is less mixing and less turbulence. There is a well developed laminar sublayer. In contrast, a bed with pebbles or boulders disrupts the direction of water flow in the boundary layer. The water gets deflected around the pebbles. Water from above tends to take its place. Since it is moving faster, the average water speed in the boundary layer increases. Thus, a rough bed reduces the thickness of the boundary layer much like a more turbulent flow does. A rough bed also disrupts the laminar sublayer by forcing the flow to move around objects. The laminar sublayer is developed locally, but in general, rough beds are turbulent. 
 
Sediments and Flow  
Key Concept: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. 
 
Bed Shear Stress - Sediments are affected by the difference in flow speeds from the bottom to the top of the boundary layer, gravity, and friction with the ground. Bed shear stress is a measure of these differences; it is the differential force that a grain feels from top to bottom. In a thick boundary layer, the speed of water flow at the top of the grains is not much different from the bottom, so bed shear stress is lower, and sediment is less likely to move. In a thin boundary layer, bed shear stress is much higher, and grains are likely to roll down flow. Thus, more turbulent flow (with a thinner boundary layer) results in more sediment transport. Bed shear stress increases with increasing fluid density, slope, and turbulence (water depth and flow speed). For example, water is better at moving sediment than air because it has a higher density and exerts a larger bed shear stress than air can. Deep, fast rivers move more sediment than shallow, slow rivers because of more turbulence and higher flow speeds in the boundary layer in fast rivers. 
 
Next Time: The Bernouli Effect, which causes grains move, the Hjulstrom Diagram, and sediment transport. Read Chapter 4 again. 
 

Posted by: Dawn Sumner