Masters Theses

Sarah R. Reid

Date of Award

6-1983

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Thomas W. Broadhead

Committee Members

R. E. McLaughlin, Kenneth R. Walker

Abstract

The Ross Formation (Rockhouse Limestone and Birdsong Shale Members) in Decatur and Benton Counties of west-central Tennessee is composed of up to 20.59 meters of Lower Devonian rocks. The unit is composed of interbedded shale and limestone, which range in lithology from bioclastic grainstone to sparsely fossiliferous wackestone.

The Ross Formation, together with the underlying Decatur Limestone, represents a marine transgressive sequence. The Decatur (of which only the uppermost 0.2 to 0.8 meters was examined in this study), represents a shallow water crinoidal/skeletal sand bank or offshore barrier. The overlying Rockhouse Limestone Member of the Ross represents an off-shore shelf environment, where sediments were commonly subjected to wave and current agitation, resulting in grainstone/packstone lithologies containing abraded and current oriented fossil allochems. The inter bedded shale and argillaceous limestone of the Birdsong Shale overlie the Rockhouse and suggest an offshore shelf environment with abundant fine-grained terrigenous influx that was only sporadically subjected to winnowing by waves and currents. Algae or algal oncolites, mudcracks, abundance of vertical burrows, and other characteristic shallow water (intertidal to shallow subtidal/lagoonal) features are conspicuously absent.

This progressively deepening sequence, superimposed stratigraphically, suggests discrete facies deposited in a transgressing shallow epeiric sea. This sequence is similar to that inferred for correlative Helderbergian formations in New York (excepting the Manlius Formation, which there represents intertidal to shallow subtidal deposition), Oklahoma, and in the central Appalachians.

Two species of Brachiopods (Atrypa tennesseensis and Leptaena acuticuspidata) were examined for epibionts as a further guide to interpreting the paleoecology of the Ross Formation, as well as the relationship between these brachiopods and their epifauna. An abundant and diverse epifauna exists, with representatives from the Bryozoa, Coelenterata, Echinodermata, Annelida, Brachiopoda, Porifera, as well as certain "uncertain" (problematic) phyla.

Both Atrypa and Leptaena are more hydrodynamically stable in a convex-up position. The position of the pedicle foramen and the subequal diversity and abundance of epibionts on brachial and pedicle valves suggests that the life position of these brachiopods was brachial valve "up" in the case of Leptaena, and Pedicle valve "up" for Atrypa, which does not display the concavo-convex shell shape of Leptaena. In both species this allows the anterior commissure to be situated up above the muddy substrate which was common during Ross, and especially Birdsong, deposition.

A moderately high energy regime, at least periodically, is suggested also by the near-equal encrustation of pedicle and brachial valves, as the shells were overturned to facilitate hydrodynamic flow, and buried in situ.

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