Abstract: Evolution of Carbonate Shelf, Silurian McKenzie Formation, West Virginia

R. A. Smosna, S. M. Warshauer

A quantified petrographic analysis of McKenzie core from Wayne County, West Virginia, defined a regressive suite of environments from shelf-edge to lagoonal to peritidal. Q-mode cluster analysis (using distance measures) of 30 variables for 80 thin sections generated nine clusters which then were characterized by calculation of constancy, fidelity, equitability, and entropy-information functions for each variable. These characterized clusters are interpreted to represent deposition in nine different environments.

Near the base of the McKenzie are rocks of a bioherm core (cluster 5), characterized by brachiopods and crinoids. As the mound grew above wave base, stromatoporoids and corals (cluster 2) capped the bioherm. The marine regression combined with growth of the mound toward sea level established a littoral environment, and sandy dolomite (cluster 7, characterized by feldspar and quartz) formed within tidal channels. The transition to an oolitic bar indicates a minor transgression. Oolitic dolomite of cluster 6 represents the fore-slope of the bar whereas oolitic limestone of cluster 1 was deposited on the crest and immediate back slope. The overlying subtidal lagoonal sediments include pelletal limestone of cluster 3 as well as cluster 4, characterized by ostracods and sparry-calcite ceme t. The regression continued; the remainder of the formation was laid down in a peritidal environment. The pelletal dolomite of cluster 8 formed on a lower intertidal mud flat, and the pyritiferous dolomite, cluster 9, is of an intertidal/supratidal origin.

R-mode cluster analysis grouped standardized variables into five genetic associations: high energy, restricted fauna; low energy, diverse fauna; peritidal; secondary; and detrital.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA