Masters Theses

Ronald McComb

Date of Award

6-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Thomas W. Broadhead

Committee Members

Kenneth Walker, R. E. McLaughlin

Abstract

The Decatur Limestone and the Rockhouse Limestone Member of the Ross Formation of Decatur County, west-central Tennessee, are predominately skeletal carbonate rocks of Late Silurian-Early Devonian age. These strata represent a deepening (transgressive) marine sequence similar to the Coeymans-Kalkberg sequence of Early Devonian age of New York.

The Decatur Limestone represents an environment that was subjected to fluctuating conditions of current and wave activity and occasional storms. Four successive subtidal subenvironments interpreted to be the result of relative differences in water energy suggested by textural differences can be recognized within the Decatur. The lower Decatur represents (1) deposition below wave base, succeeded by (2) tidal channel fades draining an intertidal regime, followed by (3) a variable energy regime at or slightly below wave base. The upper Decatur represents a moderate energy shallow environment at or above wave base. The upper and lower Decatur can also be recognized by abundant echinoderms and bryozoans and a paucity of brachiopods in the lower part and abundant and diverse brachiopods in addition to echinoderms and bryozoans in the upper part.

The overlying Rockhouse Limestone was deposited in a subtidal, generally below wave base, environment that was occasionally subjected to the winnowing effects of currents and waves. The Rockhouse was deposited in deeper water than the Decatur and evidence of periods of fine-grained terrigenous influx is indicated interbedded shale.

The Decatur and Rockhouse limestones yielded an abundant and varied conodont and arenaceous foraminiferal fauna. Seventeen species of conodonts assigned to 13 multielement genera plus 5 form species were recovered. Three new genera and species were recovered: New Genus A new species a, New Genus A new species b, and New Genus B new species a. Morphologic similarities between Pedavis n. sp. E (of Broadhead and McComb, 1983) and Icriodus woschmidti woschmidti suggest the former was the phylogenetic predecessor to the latter. Twenty-seven species of arenaceous foraminifera assigned to 17 genera were collected from the Decatur-Rockhouse sequence. Ranges of arenaceous foraminifera species from the Decatur and Rockhouse extend beyond series and even system boundaries and do not aid in precise biostratigraphic definition.

By international agreement the first occurrence of the graptolite Monograptus uniformis in Czechoslovakia defines the boundary between the Silurian and Devonian systems. It has been demonstrated that the first occurrence of the conodont Icriodus woschmidti typically occurs slightly below the first occurrence of M. uniformis. The first occurrence of I. woschmidti defines the woschmidti Zone of Pridoli-Lochkov (Late Silurian-Early Devonian) age but in rocks that are devoid of graptolites, such as the Decatur and Rockhouse, an operational boundary may be utilized at the first occurrence of I. w. woschmidti. The first occurrence of I. w. woschmidti is in the upper Decatur making it and all of the Rockhouse Lochkov (Early Devonian) in age. The majority of the Decatur is of Ozarkodina remscheidensis eosteinhornensis Zone age (Pridoli, Late Silurian).

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