Carbonate ramp to rimmed shelf evolution: Lower to Middle Cambrian continental margin, Virginia Appalachians

Abstract

Drill cores through the Lower to Middle Cambrian Shady Dolomite carbonate platform (600 to 1,200 m thick) in the Austinville, Virginia, region have provided critical platform-margin facies relations not exposed in outcrop. The control provided by the closely spaced drill cores has allowed detailed cross sections to be constructed through the margin, which allow the evolution of the carbonate platform from a gently sloping, carbonate ramp to a high-relief, reef-rimmed shelf to be interpreted. The Shady Dolomite records the inception of carbonate sedimentation on the Lower Cambrian passive margin and forms the initial carbonate foundation for the over-lying Cambrian-Ordovician carbonate shelf sequence. Carbonate sedimentation was initiated on a submerged Lower Cambrian clastic shelf, resulting in deposition of a thick sequence (300 m) of deep-ramp, nodular limestones. This facies shallows upward into a carbonate ramp fringed by low-relief, stromatactoid mud-mound buildups (as much as 30 m thick and 3.5 km wide). Downslope these buildups pass into slope conglomerate and deep-ramp carbonate facies; landward of the buildups, cyclic peritidal sequences developed. By latest Early Cambrian time, as the ramp increased in relief, skeletal algal bioherms and lime sands were established on the high-energy margin to form a high-relief (as much as 500 to 700 m), rimmed shelf with thick (>600 m) off-platform deposits immediately seaward of the shelf edge. In the shelf interior, a major regression during Early to Middle Cambrian time resulted in development of a major unconformity and in deposition of red beds. On the more rapidly subsiding outer platform, subsidence rates exceeded sea-level-fall rates, allowing continued upbuilding of the reefal rimmed shelf. This rimmed shelf persisted for about 30 m.y. and influenced carbonate sedimentation throughout the Cambrian until it was destroyed by incipient collision during the Early Ordovician period (Read, in press).