Holocene stromatolites and microbial laminites associated with lenticular gypsum in a marine-dominated environment, Ras El Shetan area, Gulf of Aqaba, Egypt MAHMOUD A. M. AREF Department of Geology, Faculty of Science, Cairo University, Giza, Egypt [E-Mail: mahmoudaref@frcu.eun.eg] ABSTRACT Field and petrographic investigations of Holocene evaporites in the Ras El Shetan area, Gulf of Aqaba, Egypt, indicate the presence of microbial mats either in the form of laminites or stromatolites. The morphology of microbial mats and gypsum crystal size characterize the following lithofacies: (1) slump-stromatolitic gypsarenite, (2) random gypsrudite, (3) stromatolitic gypsarenite, and (4) microbially laminated gypsrudite. These evaporite lithofacies are formed above pre-evaporitic mudstones rich in disrupted cyanobacterial ®laments, burrows and cerithid gastropods. The morphology of the gypsum crystals is mainly lenticular, indicating enrichment of dissolved organic compounds in the depositional environment. The difference in size of the lenticular gypsum crystals is related to minor changes in salinity and temperature of the parent brine. Fluid inclusions in gypsum crystals indicate their formation at low temperature (<50°C) in a seawater sourced brine that evaporated to gypsum saturation or higher. The brine salinities range from 10á62 to 12á99 equivalent wt% NaCl, and the brine densities range from 1á08 to 1á11 g/cm 3 . The change in morphology of the microbial mats (stromatolites and laminites) is related mainly to changes in water depth, from a very shallow salina to a coastal sabkha. Lenticular gypsum nucleated displacively in the microbial mats from saline, oxygenated groundwater that seeped from the sea through a barrier. INTRODUCTION Microbial mats are often formed in shallow water evaporitic environments, as shown by their pres- ence in many modern and ancient evaporites (Rouchy & Monty, 1981; Krumbein, 1983). Mod- ern environments, where microbial mats are well developed and can therefore be regarded as analogues of ancient stromatolites, include hyper- saline lagoons and lakes (Walter, 1976; Bauld, 1981, 1984; Cohen et al., 1984; Friedman & Krumbein, 1985; Gerdes & Krumbein, 1987), marine intertidal and supratidal areas (Fisk, 1959; Logan, 1961; Kendall & Skipwith, 1968), and large tide-free solar salt works (Schneider & Herrmann, 1980; Orti Cabo et al., 1984; Reineck et al., 1990; Cornee et al., 1992). Well preserved cyanobacterial ®laments have been identi®ed in many ancient evaporitic- stromatolites including the Archean of Western Australia (Lowe, 1983), the Permian of northern Poland (Gasiewicz et al., 1987), the Messinian of Cyprus (Rouchy & Monty, 1981), the middle Miocene of Gebel Abu Shaar El Qibli, Red Sea (Monty et al., 1987), and the late Pleistocene of El Gharbaniyat, Egypt (Wali, 1993). In the Holocene evaporite deposits of the Ras El Shetan area (Gulf of Aqaba) microbial mats are similarly very common in evaporitic environ- ments. Evidence for this is the presence of well preserved organic or calcareous ®laments, lam- inites and stromatolites encased in gypsiferous sediments. The gypsum crystals have mainly lenticular morphology and vary in dimension Sedimentology (1998) 45, 245±262 Ó 1998 International Association of Sedimentologists 245