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Lower Cretaceous strata crop out in a series of closely spaced wadis in the Nakhl area of the northern Oman Mountains and document the evolution of the carbonate-dominated southeastern margin of the Arabian Plate. The Late Barremian to Early Aptian platform rim shows a rapid transition from bioclastic-oolitic high-energy ramp deposits to a constructed high-energy margin. The rigid construction is due to intense microbial activity rapidly binding and cementing bioclastic debris and isolated autochthonous organisms including rudists, corals and sponges. The outcrops allow investigating the geometries of these “debris reefs” and their lateral and vertical facies evolution.

Here the initiation of the constructed margin is studied in detail. This stratigraphic interval represents a shelf-margin wedge of earliest Aptian age, which is interpreted as the lowstand of the Aptian Supersequence. On the platform (the Arabian Plate) this corresponds to a stratigraphic hiatus caused by subaerial exposure. At least four phases of buildup growth are distinguished. They show a low, irregular morphology, and locally have a mound-like shape, attaining heights of up to ten meters. Sealed Neptunian dykes have also been observed. A control by water depth and different energy levels along a coastal transect is inferred. The scale and shape of the constructions resembles that of modern spur and groove structures in wave-dominated reef environments.

An important part of the binding and encrusting can be attributed to chasmolithic, cryptic microbialites and associations of Bacinella and Lithocodium s.i., respectively. Stromatolitic and thrombolitic textures are common and patches and intervals entirely formed by cryptic microbialites do occur, but quantitatively are of minor importance. Environmental factors that may trigger such a sudden “bloom” in microbial activity include elevated input of nutrients onto the platform through increased rainfall, higher water temperatures or coastal upwelling. Such conditions stress oligotrophic carbonate producers and diminish their carbonate production potential and, thus, favor the microbial community. This microbial bloom predates the widespread microbial deposition on the platform during the Early Aptian transgression, which was time-equivalent with the Early Aptian Selli event and the OAE 1a.

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