Formation and calcification of modern gypsum-dominated stromatolites, EMISAL, Fayium, Egypt

Recent gypsum stromatolites are forming along the margin as well as on the pond floor of the EMISAL saltworks, Fayium, Egypt. Gypsum precipitates are classified according to their morphology, fabrics, and crystal size into (1) subaqueous bottom gypsum crusts, (2) selenitic gypsum facies, (3) stromatolitic gypsum dome facies, and (4) gypsolite facies. Two types of microbial mats, lithifying and non-lithifying, can be identified. The lithifying mat is shallow and composed of an alternation of gypsum and microbial layers that are seasonally controlled. The non-lithifying mat, formed in the deeper part of the pond, is a greenish-brown slime-rich layer that exhibits a frothy macro texture and produces a firm gelatinous film covering the sediment surface. Calcium carbonate (mostly aragonite) particles, identified by light and scanning electron microscopy, and X-ray diffraction occur within the deeper part of the lithified mat and are associated with living and degrading biofilm. Precipitation of aragonite is associated with the dissolution of gypsum, which may have resulted from bacterial sulphate reduction. The latter process increases alkalinity and ultimately results in the replacement of gypsum by aragonite.