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.
