Incipiently drowned facies within a cyclic peritidal ramp sequence, Early Ordovician Chepultepec interval, Virginia Appalachians

Abstract

The Chepultepec interval (145 to 260 m thick) in Virginia, although dominated by peritidal carbonates, contains thick subtidal sequences that formed during repeated, incipient drowning of the Early Ordovician ramp. Cyclic peritidal sequences make up the lower member (up to 150 m thick) and upper member (up to 85 m thick). Subtidal limestone and bioherms make up the middle member (up to 110 m thick) and pass landward into cyclic peritidal facies. Average long-term subsidence rates were up to 4 cm/1,000 yr (mature passive margin rates), and ramp gradients commonly were extremely low (2 cm/km). Computer modeling helped define mechanisms involved in formation of the stratigraphic sequence. The peritidal sequences are upward-shallowing cycles (average period 140,000 to 200,000 yr) of pellet and skeletal limestone, pellet grainstone, thrombolites, intraclast grainstone, and laminite caps. These resulted from periodic sea-level oscillations of several metres amplitude, and possible slow, long-teim (third-order) sea-level fall superimposed on long-term subsidence. Tidal flats were shifted far westward after each sea-level rise but had sufficient time to prograde back across the ramp during sea-level fall. The middle number contains repeated sequences of subtidal skeletal limestone and pellet limestone with storm beds and burrowed-units (formed below fair-weather wave base), local rippled pellet limestone, and thrombolite bioherms (subwave-base to wave-agitated shoals) that may have erosional tops overlain by thin grainstone sheets. Tidal-flat facies are absent. These sequences formed by periodic sea-level rise (more than 12 to 17 m amplitude), which caused incipient drowning of the platform, and migration of tidal flats several hundred kilometres back onto the platform. These flats were unable to prograde onto the outer ramp during regression because of insufficient time and because seaward retreat of the shoreline outpaced progradation of tidal flats. This left them abandoned on the inner ramp. Similar sequences reflecting incipient drowning are widespread in the geological record.