Gradual establishment of Iapetan "passive" margin sedimentation; stratigraphic consequences of Cambrian episodic tectonism and eustasy, Southern Appalachians

Abstract

The Middle to Upper Cambrian Conasauga Group of the southern Appalachians constitutes part of the thick pericratonic Cambro-Ordovician Sauk sequence that represents the interplay between an extensive carbonate platform to the east and a deeper-water intrashelf shale basin to the west. The Conasauga Group consists of a series of interfingering carbonate and shale formations; the shales represent deeper-water deposition (50 m+), and the carbonates show evidence for gradual shoaling (through aggradation and progradation) from deeper water to shallow water. The upper parts of the Craig Limestone Member (Rogersville Shale) and the Maryville Limestone (both Middle Cambrian) contain evidence for subaerial exposure of subtidal sediments followed by platform drowning. Following platform exposure that shot down carbonate production, a large relative sea-level rise (driven by an increase in the rate of subsidence) led to platform reinundation, but was rapid enough to drown the carbonate platform. Flooding was sufficient to allow deeper-water basinal shales to onlap the drowned platform. Changes in the rate of subsidence driven by cooling of the lithosphere, sediment loading, and/or regional extension were probably responsible for cyclic sedimentation, even though burial curves suggest gradual, thermal subsidence through this time. Lithofacies patterns and the regional tectonism reviewed herein suggests that the Iapetan margin was not fully stabilized (subsiding uniformly spatially and temporally, a true margin) until the Late Cambrian. Critical stratigraphic studies of other Cambrian (and younger) passive margins may reveal comparable anomalies related to similar, jerky subsidence patterns.