High-Resolution Surface and Subsurface Sequence Stratigraphy of Late Middle to Late Ordovician  (Late Mohawkian-Cincinnatian) Foreland Basin Rocks, Kentucky and Virginia

Abstract

The late Middle to Late Ordovician (late Mohawkian to Cincinnatian) supersequence in Kentucky and Virginia was deposited in a tectonically active foreland basin during a transition from the ice-free Early Ordovician to the glacial Late Ordovician. This sequence was deposited in less than 12 m.y., is 250-500 m thick, and is composed of four large third-order sequences (each 40-150 m thick) that are regionally correlative in outcrops, cores, and gamma-ray logs. Smaller scale third-order parasequence sets (up to 20 m thick) and component parasequences (1-8 m thick) make up the larger sequences, and are only locally correlative. Subtidal-dominated parasequences comprise the basal part of each sequence, whereas shallower subtidal- or peritidal-dominated parasequences compose upper parts of sequences. Each large, third-order sequence is asymmetric and marked by lowstand, transgressive, and highstand systems tracts with unique lithologic and gamma-ray log response characteristics. Ramp margin wedges (RMW) are poorly developed and consist of marine siltstone or grainstone/packstone sheets extending into deep ramp settings; these sheets have high gamma-ray values in their base and lower, blockier gamma-ray responses in their tops. Transgressive systems tracts (TST) are thin; composed of high-energy, locally phosphatic grainstone ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 7, 1996; revised manuscript received May 27, 1997; final acceptance June 16, 1997. 2Department of Geology, Virginia Tech, Blacksburg, Virginia 24061-0420. Present address: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. 3Department of Geology, Virginia Tech, Blacksburg, Virginia 24061-0420. This study was supported by NSF Grant EAR-9316057 to J. F. Read, a grant from Mobil Oil Company, and grants-in-aid of research from the Geological Society of America and Appalachian Basin Industrial Associates to Mike Pope. We thank John Haynes, Brian Keith, and Rick Major for thorough and insightful reviews of an earlier version of this manuscript.