Sedimentary Facies and Early Diagenesis of an Oolite Complex from the Silurian of West Virginia
Published:January 01, 1984
An oolite-shoal complex has been recognized from a core in the Lockport Dolomite of West Virginia. The complex consists of four different but related facies. On the windward margin of the shoal, a mobile oolite sand accumulated. Daily waves and currents kept the grains in nearly constant movement, and the sediment was almost pure oolite. At one point, a slight drop in sea level briefly exposed the oolite as an island, and lime mud was deposited on the mud flat immediately behind the island. Ooids from the shoal margin were transported bankward and deposited with peloids and skeletal grains on a mixed-oolite sand flat. The sand-flat environment was protected by the shoal margin, and the sediment was stabilized by marine cementation and perhaps by algal mats. Peloidal-skeletal sand of the platform interior formed farther bankward. The faunal community of the platform interior was restricted and not very diverse; it was dominated by ostracodes. Ooids were rarely washed into this environment.
Three important surface-related processes affected the oolite complex during early diagenesis. (1) Marine cementation on the sea floor was significant, producing abundant grapestones, flakes, and crusts. (2) Several times during sedimentation, the shallow-water deposits were subaerially exposed. At these times meteoric water invaded the sediment, and all remaining porosity was occluded by two generations of fresh-water calcite cement. (3) Near the base of the buried oolite, a zone of mixing between meteoric and marine waters is thought to have developed. The pure oolite in this lower zone was dolomitized by the resultant brackish water; secondary porosity in the oolite is related to early dolomitization.
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Carbonate Sands-A Core Workshop
Carbonate sands, both skeletal and non-skeletal, have been studied by geologists as intensely as carbonate buildups. The underlying reason for the studies is the importance of those sands as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record of carbonate sands with emphasis on lithofacies, stratigraphy of the sands and surrounding deposits, geometry of the sand deposits, diagenesis and porosity evolution, and wireline log data.