Masters Theses

Date of Award

3-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Thomas W. Broadhead

Committee Members

Kenneth R. Walker, Robert E. McLaughlin

Abstract

The Ross Formation (Upper Silurian-Lower Devonian) is exposed in the western valley of the Tennessee River in west-central Tennessee and consists of five members that grade laterally into each other from north to south. To the north, the Rockhouse Limestone Member, a fossiliferous grainstone to packstone with minor shale partings underlies the Birdsong Shale Member, a fossiliferous packstone to wackestone interbedded with fossiliferous shale. To the south, in northern Hardin County, the Rockhouse Limestone Member underlies the sparsely fossiliferous, dense, recrystallized lime mudstone of the Ross Limestone Member, whereas in southern Hardin County, the Rockhouse Shale Member is below the Ross Limestone. The Bear Branch facies, found within the Ross Limestone in northern Hardin County, is a cross-bedded, hematitic grainstone with hematitic superficial ooids (less than 5 laminations).

The Ross Formation was deposited in a shallow epeiric sea, and represents environments similar to those of the Helderbergian sequences of New York and Oklahoma. In Late Silurian time, the Rockhouse Shale was deposited to the south, while farther north, in shallower, subtidal waters, the Rockhouse Limestone was deposited with only a minor amount of shale present. The water shallowed even more northward, where a high-energy, crinoidal sandbank environment (Decatur Limestone) had persisted during much of the Late Silurian. During the Early Devonian, the area to the north deepened, and the sandbanks of the Decatur were overlain by interbedded skeletal limestone and fossiliferous shale of the Rockhouse Limestone, which in turn was overlain by the quiet, offshore, muddy shelf deposits represented by fossiliferous shale and skeletal packstone of the Birdsong Shale. Simultaneously, to the south, shallowing occurred reflected by the quiet, restricted shallow subtidal to lagoonal deposits of the Ross Limestone. In a localized area of the Ross Limestone, shoaling occured along with an increase in water energy and an influx of clays causing a concentration of iron oxides. Hematitic ooids and bioclastic debris were deposited in cross-stratified beds of this shallow, subtidal environment of the Bear Branch facies.

The Silurian-Devonian boundary in the study area was determined by the first appearance of the conodont Icriodus woschmidti (hesperius-woschmidti Zone), which occurs approximately 2.0m below the Ross-Decatur contact at Parsons Quarry (northern facies) and 2.2m above the contact at Olive Hill (southern facies). Although most Icriodus platform elements are fragmental, a few were identified as I. postwoschmidti, which correlate with the eurekaensis Zone or the die gamma Zone of Nevada and Spain respectively.

Although relatively abundant in the shallower environments of the Rockhouse and Decatur Limestones, conodonts, especially Icriodus, were scarce in the deeper sublittoral environments of the Birdsong Shale. Conodont elements were not observed in the lagoonal environments of the Ross Limestone, and in the hematitic Bear Branch facies.

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