GRADUAL ESTABLISHMENT OF IAPETAN PASSIVE MARGIN SEDIMENTATION - STRATIGRAPHIC CONSEQUENCES OF CAMBRIAN EPISODIC TECTONISM AND EUSTASY, SOUTHERN APPALACHIANS

The Middle to Upper Cambrian Conasauga Group of the southern Appalachi ans 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 carbo nate and shale formations; the shales represent deeper-water depositio n (50 m+), and the carbonates show evidence for gradual shoaling (thro ugh 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 su baerial exposure of subtidal sediments followed by platform drowning. Following platform exposure that shut down carbonate production, a lar ge relative sea-level rise (driven by an increase in the rate of subsi dence) led to platform reinundation, but was rapid enough to drown the carbonate platform. Flooding was sufficient to allow deeper-water bas inal shales to onlap the drowned platform. Changes in the rate of subs idence driven by thermal cooling of the lithosphere, sediment loading, and/or regional extension were probably responsible for ''cyclic'' se dimentation, even though burial curves suggest gradual, ''thermal'' su bsidence through this time. Lithofacies patterns and the regional tect onism reviewed herein suggests that the Iapetan margin was not fully s tabilized (subsiding uniformly spatially and temporally, a true passiv e margin) until the Late Cambrian. Critical stratigraphic studies of o ther Cambrian (and younger) ''passive'' margins may reveal comparable ''anomalies'' related to similar, ''jerky'' subsidence patterns.