Facies Analysis and Depositional Environments of Upper Part of Richmond Group (Upper Ordovician), Richmond, Indiana, to Xenia, Ohio

Christopher E. Betz, Wayne D. Martin

Rock sections of the Drakes, Elkhorn, and Whitewater Formations were studied along an east-west-trending line in order to distinguish facies changes in a slope direction across the paleodepositional basin. The Richmond limestones, shales, and dolostones formed from fine-grained, terrigenous and carbonate sediments deposited on a shallow marine ramp within the humid, tropical, low latitudes of the Southern Hemisphere.

Depositional environments on the ramp are represented by five main facies types. (1) Silty dolostone facies are characterized by evenly laminated and mud-cracked dolostones that lack fossil evidence except for stromatolites and burrows. These units developed from algal marsh sediments in the supratidal zone (Drakes Formation). (2) Dolomitic mudstone facies are variegated red and green, structureless mudstones that contain concentrations of fossil allochems representing channel deposits. These units formed from muddy sediments deposited on intertidal mud flats (Elkhorn Formation). (3) Pelletiferous wackestone facies contain a restricted leperditiopid ostracod fauna. The facies developed within shallow, semirestricted portions of the inner ramp (Whitewater Formation). (4) Skeletal packs one facies are blue-gray and contain abundant, well-rounded and well-sorted diverse invertebrate fossil fragments. These rocks were formed from carbonate sands deposited in tidal channels (Whitewater Formation). (5) Skeletal wackestone facies are poorly segregated, impure wackestones and dolomitic shales containing an abundant and diverse fossil assemblage. These strata formed from sediments derived from subtidal faunal communities and terrigenous muds (Whitewater Formation).

The five Richmond facies form a subtidal to supratidal shallowing-upward sequence. This progressive shallowing during the Late Ordovician resulted from the westward regional progradation of Queenston deltaic facies.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.