Cave-filling Herringbone Calcite from a Middle Eocene Carbonate Platform, Egypt: Crystal Morphology and Geochemistry of an Unusual Carbonate Cement

by
Sarah A. Tourre and Dawn Y. Sumner

1999, GSA Annual Meeting, Abstracts

Herringbone calcite, a distinctive calcite texture with macroscopic banding and specific crystallographic properties, precipitated in fractures and caves in a Middle Eocene carbonate platform comprised of the Mokattam Formation and equivalent units, Egypt. This occurrence is unusual because herringbone calcite is abundant in Archean marine carbonates but rare in younger carbonates. Field work focused on two areas: A trio of ancient Roman quarries in Wadi Araba; and the currently active Wadi Sannur quarry, which was also quarried in ancient times. The Wadi Araba deposits consist of 20 cm-thick coatings of fibrous and herringbone calcite that precipitated on both walls of <1 m-wide fractures. The remainder of the fractures are filled with terra rosa. At the Wadi Sannur quarry, herringbone calcite composes meter-diameter botryoidal growths filling caves. Fibrous calcite is rare, but the final calcite that precipitated in some remaining voids is dog tooth spar or curved calcite rosettes.

Crystallographically, the Egyptian herringbone calcite consists of serrate bands that are 1.25-1.45 mm thick. Bands are comprised of ~ 800 micron x 120 micron crystals that are elongate perpendicular to the banding. As in other herringbone calcite, the c-axis is oriented perpendicular to crystal elongation, and the crystals are length slow. Crystal tips are either feathery or have spearhead-like crystal faces projecting at an angle out from the main direction of crystal elongation. In oblique cuts, the elongate crystals are curved up to 45¡ along their length, suggesting a complex 3-dimensional morphology for the crystals. The unstable, feathery crystal morphology of the tops of elongate crystals is not present in previously described herringbone calcite; if originally present, feathery tips may have recrystallized to the microspar observed in some older samples.

Preliminary microprobe elemental analyses suggest that the Egyptian herringbone calcite is very high in Sr. Average concentrations are 3200±600 ppm Sr in herringbone calcite and 1800±600 ppm in associated fibrous calcite. An average of 5.5±0.4 mol% MgCO3 is present in the herringbone calcite and 3.3±0.4 mol% MgCO3 in associated fibrous calcite. Fe concentrations are less than the detection limit of 690 ppm in both, and neither luminesce demonstrating very low Mn concentrations.


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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
sumner@geology.ucdavis.edu