Carpenter, R.M., Pandolfi, J.M., P.M. Sheehan. 1986. the Late Ordovian and Silurian of the Eastern Great
MILWAUKEE PUBLIC MUSEUM Contributions . In BIOLOGY and GEOLOGY Number 69 August 1, 1986 The Late Ordovician and Silurian of the Eastern Great Basin, Part 6: The Upper Ordovician Carbonate Ramp Roger M. Carpenter John M. Pandolfi Peter M. Sheehan MILWAUKEE PUBLIC MUSEUM Contributions . In BIOLOGY and GEOLOGY Number 69 August 1, 1986 The Late Ordovician and Silurian of the Eastern Great Basin, Part 6: The Upper Ordovician Carbonate Ramp Roger M. Carpenter, Department of Geology, Conoco Inc., 202 Rue Iberville, Lafayette, LA 70508; John M. Pandolfi, Department of Geology, University of California, Davis, California, 95616; Peter M. Sheehan, Department of Geology, Milwaukee Public Museum, 800W. Wells St., Milwaukee, Wisconsin 53233 ISBN 0-89326-122-X © 1986 Milwaukee Public Museum Abstract Two east-west transects examined in western Utah and eastern Nevada preserve Upper Ordovician-Lower Silurian lithofacies along a carbonate ramp transitional between a shelf and basin. Previous investigators have reconstructed this margin as a classic carbonate shelf with an abrupt, linear margin between shelf and slope. However, lithofacies change gradually between shelf and slope and are better explained by a carbonate ramp model. Intertidal and shallow subtidal dolomites are present to the east, with progressively deeper water limestones with increasing fine grained terrigenous content toward the west. Shelf edge reefs or shallow water carbonate margin buildups are absent. Latest Ordovician glacio-eustatic decline in sea level produced a period ofsubaerial exposure in the shallow eastern region. However, deposition continued deeper on the ramp, where shallow-water, cross laminated, massive dolomites were deposited during the glacio-eustatic regression. The carbonate ramp pattern was disrupted in the Middle or early part of the Late Llandovery, when an abrupt margin was established by listric growth faulting.
[Show full text]