Quantitative Paleoecologic Analysis of Fossil Communities in the Richmond Group

Abstract

The thin-bedded limestone and shale beds of the Tanners Creek Formation are divided into five assemblage zones. Within each assemblage zone, the organisms are grouped into fossil communities. The Tanners Creek Formation, which is the basal unit of the Richmond Group of Upper Ordovician age, is about 70 m. thick where it outcrops in southeastern Indiana. Time-trend curves for twenty-three fossil types are plotted to study the distribution of the organisms within the Tanners Creek Formation. A computer program was compiled to compute and plot the time-trend curves which are used to divide the formation into assemblage zones. A twenty-one-term smoothing equation is used for the major trends and a seven-term equation for the short-period fluctuations in the faunal distribution. The following assemblage zones are named for the dominant pairs of brachiopod genera: Zone A, Rafinesquina-Zygospira; Zone B, Resserella-Zygospira; Zone C, Resserella-Sowerbyella; Zone D, Platystrophia-Leptaena; and Zone E, Rhynchotrema-Plaesiomys. Within each assemblage zone, the fossils which occur most frequently together are grouped into fossil communities. Within each assemblage zone the product-moment correlation coefficients are computed between each pair of fossils. The fossils are grouped into communities, using an R-mode cluster analysis computer program. The major fossil communities which transcend assemblage zone boundaries include (1) Rafinesquina-Zygospira community, (2) crinoid-trilobite community, (3) bryozoan community, (4) Resserella community, and (5) Hebertella-Strophomena community. Field observations on the condition and orientation of the fossils are used in conjunction with point-count data on the fossils and matrix to interpret the depositional environment and its relationship to the assemblage zones and communities. A principal-components factor analysis computer program with a varimax rotation is used to study the relationship between the organisms and their environment. Based on the interpretation of field and thin-section data, a marine regression occurred from Zone A to Zone C, with a transgression in Zone D and a regression in Zone E.