In the photo on the right, also from the Gamohaan Formation, the top of the ripple is coated with herringbone calcite. The ripple shows evidence for grain transport both to the right and left suggesting the influence of waves. In the layer, the abundance of calcite cement increases upward to form the dark grey later at the top which is virtually 100% "cement". The cement locked the ripple crest in place, and deposition of the following layer was influenced by the cemented in topography of the underlying bed.
The photo on the left is a thick bed of herringbone calcite from the 2520 Ma Gamohaan Formation, South Africa (Sumner and Grotzinger, 1996) with a 2 cm layer of microbial structures in the middle of it. This bed is laterally continuous for 140x50 km in the field (Sumner, 1997). The upper photo is of a polished sample from the bed on the left (scale in mm). Note the characteristic light-dark banding that is irregular, but approximately perpendicular to the growth direction (up in this figure). Banding is caused by the rotation of the crystallographic axes upward in elongate crystals. Thus, the tops of the elongate crystals are optically oriented, whereas their bases are not. The light bands consist of optically oriented areas and reflect light strongly at certain angles. The dark bands are not optically oriented and reflect poorly.
To view thin section photos of extremely well preserved herringbone calcite from caves and fissures in Middle Eocene carbonates from Egypt, download this figure (a 96k PDF file).
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616