Evolution of Cambrian-Ordovician Carbonate Shelf, United States Appalachians: ABSTRACT

Abstract

Cross sections and isopach maps (palinspastic) of the Cambrian-Ordovician continental shelf, United States Appalachians, show that thickness and facies trends are controlled by the Adirondack, New Jersey, and Virginia highs and depocenters in Tennessee, Pennsylvania, and by the Rome trough. Carbonate sedimentation was initiated with drowning of Early Cambrian clastics, deposition of carbonate ramp and rimmed shelf facies followed by drowning, then regional regression and deposition of Early to Middle Cambrian red beds and platform margin rimmed shelf facies. During subsequent regional transgression, the Conasauga intrashelf shale basin formed, bounded toward the shelf edge and along depositional strike by Middle to Upper Cambrian oolitic ramp facies and cyclic peritidal c rbonates. Intrashelf basin filling and regional regression caused progradation of Late Cambrian cyclic carbonates and clastics across the shelf. By this time, the margin had a relief of 2.5 km. During the Early Ordovician, incipient drowning of the shelf formed subtidal carbonates and bioherms that passed up into cyclic carbonate as sea level oscillations decreased in magnitude. Numerous unconformities interrupt this sequence in the northern Appalachians. The earlier high relief rimmed shelf was converted into a ramp, owing to uplift in the basin, heralding approaching collision. Subsidence rates on the margin were low (4 cm/1,000 yr) and typical of a mature passive margin. Shelf sedimentation in the southern Appalachians ceased with arc-continent collision and development of the Knox un onformity, which dies out into the Pennsylvania depocenter. Major exploration targets are in the Late Cambrian-Early Ordovician Knox Group. End_of_Article - Last_Page 299------------