Recent sedimentary environments and diagenesis of marine carbonates from the Persian Gulf

Study of three long cores from an area approximately sixty kilometers offshore from the Trucial Coast, Persian Gulf, reveals that sediments deposited throughout the Flandrian transgression are essentially the same as those accumulating today in the Persian Gulf. Basal sediments from the cores are Pleistocene in age. The boundary separating Flandrian and Pre-Flandrian age sediments is marked by indications of subaerial exposure and a disconformity. The disconformity is overlain by 79.7 to 83.3 meters of sediments. These sediments show an overall transgressive sequence. The average rate of deposition of these sediments was 0.4 cm per year. X-ray diffraction analysis revealed that, in sediment fractions with grain sizes smaller than sixty-three micrometers, correlative mineralogical changes are present between cores. Mineralogical changes which can be traced between cores include zones of calcite enrichment and dolomite enrichment. Six intervals of calcite enrichment were observed. The depths below the sediment-water interface at which the intervals of calcite enrichment occur closely correspond with the depths below present sea level where Flandrian stillstands occurred. The intervals of calcite enrichment probably are due to introduction of fresh water, possibly by means of a fresh water wedge into these sediments during stillstand. Two intervals of dolomitized sediments were also observed in the Pleistocene interval. These two intervals correspond with peritidal and/or highly restricted conditions at the time those strata were deposited. Cementation has occurred in patches in many of the sediments, forming hardened nodules; three intervals were totally lithified. Radially fibrous aragonite and magnesium calcite cements occur as void fill and as fibrous rims cementing grains. These cements probably formed under submarine conditions at the sediment-water interface or after shallow burial. Micrite and equant spar also occur as grain cements. The formation of these cements probably occurred in connate waters low in Mg[raised ++] or in intervals where connate and phreatic waters mixed, where Mg[raised ++] and Na[raised +] content are both low.