Abstract During Aptian to Albian times a major influx of terrigenous material from the emerged Arabian Shield led to the deposition of the Nahr Umr Formation within the intracratonic Bab Basin. This argillaceous facies interfingers in northern Oman with sediments shed from reefal complexes. We studied a 100 km transect across the zone of facies intercalation extending from the margin of the Neo-Tethys (northern Jabal Akhdar) into the fringes of the Bab Basin in northern Oman (Foothills). The age of formation boundaries and key beds and surfaces in a reference section (Wadi Bani Kharus; I) was dated by use of the graphic correlation method. Graphic correlation indicates that the Nahr Umr Formation is of uppermost Aptian to Upper Albian age. Two types of stratigraphic markers were recognized: limestone marker beds, successions, and disconformity surfaces. Calcareous marker beds are probably related to increased carbonate production during sea-level lowstands. Investigations of the geochemistry (stable isotopes, fluid inclusions) and sediment petrography of limestones beneath disconformities revealed that they are of combined submarine and subaerial origin. Evidence for a subaerial exposure stage is recognized along the basin margin (northern and southern Jabal Akhdar) but is found less frequently in the basinward settings of the Foothills. Marker-bed successions and disconformity surfaces are correlated throughout the sections and indicate coeval deposition of the Nahr Umr Formation in northern Oman.

Abstract The Drakes Bay Formation is an upper Miocene sequence of siliceous mudstones containing many small dolomite nodules. The nodules probably formed without a precursor biogenic calcite supplying Ca or HCO 3 for dolomitization. Dolomite formation preferentially took place in sediment layers slightly richer in organic C than the surrounding sediments. More extensive sulfate reduction in these layers raised the porewater HCO, concentration and caused carbonate precipitation. The initial carbonate may have been dolomite or calcite, which was later converted to dolomite. Carbon and oxygen stable isotope ratios vary systematically and clearly illustrate changes in the isotopic composition of dissolved CO 2 that occurred with depth. The isotopic analyses show that dolomite formation did not begin until the pore waters were free of dissolved sulfate. The Ca contents of the dolomites decrease, and both the Mg and Fe contents increase, with depth of formation. Manganese contents correlate with Fe contents. Sodium contents of the dolomites are relatively high, probably reflecting their poor ordering and nonstoichi-ometry. Strontium contents of the dolomites are typical of those from hemipelagic sediments with moderate sedimentation rates.