ABSTRACT

Forereef slopes in the Red Sea of Sudan exhibit a uniform biozonation that is independent from the topography of the slopes. Below - 120 m, ledges protrude horizontally from sleep cliffs of barrier reefs and atolls as well as from patches of in situ lithified slope sediment on inclined fringing reef slopes. Free surfaces and cavities within these ledges are partly covered by laminar micrite crusts of 7-20 mm thickness. The ledges are formed by an organic framework of living azooxanthellate corals, bryozoans, serpulids and fossil red algae. They are affected by repeated episodes of boring, infilling, and cementation which obliterate much of the original fabric. Concomitant cementation and boring result in asymmetric cement linings which often show geopetal fabrics. The laminar m crite crusts, however, show no significant traces of bioerosion, which are a typical feature of fossil deep-water stromatolites. Two types of lamination have been observed: 1) vertically stacked, irregular, anastomosing laminate with ragged outlines and little lateral persistence which are often accentuated by iron staining, and 2) 0.5-mm-thick laminae of light-grey micrite, rich in fine bioclasts, showing smooth, thin coatings of dark homogenous micrite, which are weakly fluorescent in ultraviolet light. No algal films are preserved, but micritic tubes 10-30 µm in diameter, often bifurcated, form a dense network on the upper surface of the crusts by overgrowing and binding particles. Associated aragonitic rosettes 30-40 µm in diameter may represent calcified coccoid algae or bacteria. If true, these organisms suggest a formation of the latter type of crust by biogenic processes not related to the photic zone. Fenestral porosity and Frutexites textures are absent.

Radiocarbon dates from corals within the ledges provide ages of 10,500 YBP. From the time-related decrease in oxygen isotope ratios (versus PDB) of boring micrite (¹⁸O = +3.72), micrite crusts (¹⁸O = +1.69), and unlithified mud (¹⁸O = +0.47) within the youngest voids of the hard substrates, we conclude that crust formation must have taken place in shallower water when sea level was lower during the Early Holocene rise of sea level.