Masters Theses

Date of Award

6-1983

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Robert E. McLaughlin

Committee Members

Thomas W. Broadhead, Kenneth R. Walker

Abstract

The primary purpose of this investigation was to make a comprehensive collection of the Middle Ordovician graptolite fauna from an area in Greene County, Tennessee, embraced by the Mosheim (TVA 181-NW) and Mohawk (TVA 172-NE) quadrangles and a portion of the Greeneville (TVA 181—NE) quadrangle north of 36°10', in an attempt to delineate the biostratigraphic zones represented by the fossils. Most previous reports on graptolites from East Tennessee contain imprecise geographic location, uncertain stratigraphic position, and/or questionable identifications. A recent detailed study by Finney (1977) of the Athens Shale graptolites in Alabama, which included a review of available Tennessee collections, has provided the present investigation a reference standard not previously available. Dr. Finney, Oklahoma State University, also furnished valuable identifications and confirmation of specimens sent to him.

From 37 sites among all the accessible exposure in the thesis area without severe deformational effects, a graptolite fauna of 22 named taxa has been assembled from the Blockhouse and Sevier formations. These are:

Amphigraptus divergens Dicellograptus bispiralis

Climacograptus bicornis Dicellograptus divaricatus

Climacograptus meridionalis bicurvatus

Cryptograptus marcidus Dicellograptus gurleyi

Dicaulograptus sp. Dicranograptus ramosus

Dicellograptus alabamensis Dicranograptus nicholsoni

Dicranograptus nicholsoni Isograptus lyra

parvangulus Laslograptussp.

Didymograptus sp. Nemagraptus gracllis

Glossograptus ciliatus Orthograptus calcaratus acutus

Glyptograptus teretiusculus Pseudoclimacograptus modestus

Glyptograptus teretiusculus, type A Reteograptus geinitzianus

Using the biostratigraphic zonation scheme devised by Berry (1960) in the Marathon Basin of southwest Texas and subsequently applied to North American graptolite faunas of Ordovician age elsewhere, three biostratigraphic zones, with some overlap, are found to be represented in the study area. Zonal marker species Glyptograptus teretiusculus (Zone 10), Nemagraptus gracilis (Zone 11), and Climacograptus bicornis(Zone 12), with associated taxa occur in ascending stratigraphic order.

Except for a possibly significant variant of Glyptograptus teretiusculus (type A), no new taxa were found in the study area. However, the discovery of Climacograptus bicornis in the stratigraphically highest beds makes it possible to identify the name-bearing zone or, at least, an overlap interval with the Nemagraptus gracilis zone. Climacograptus bicorniszone has not been recognized in the Athens Shale sections in Alabama and Tennessee, possibly due to erosional truncation; otherwise, the Blockhouse/Sevier and Athens graptolite faunas show many species in common. A similar comparison can be made with the graptolite faunas recorded elsewhere in East Tennessee. Of special interest are the taxa shared in common with sites southwest of the thesis area included in the study of the Sevier Basin by Shanmugam (1978).

A comparison of taxa collected at each of the 37 sites shows an areal distribution pattern that is believed to be significant. Species diversity decreases in opposite directions from a central area of highest diversity. Further, certain taxa are areally restricted either to one or another of the flanking areas or are linked to the central area. High graptolite species diversity has been associated with depositional environments of seaward slopes bordering tectonic basins whereas low species diversity has been interpreted as indicating shallower and deeper environments (Kaljo, 1978). Erdtmann (1976) has suggested that the degree of species diversity is a measure of the number of bathymetrically-disposed graptolite taxa (biocoenoses) represented in a given death assemblage (taphocoenosis). The presence of fully mature rhabdosomes, bedding plane preservation, and wide nongraptoliferous intervals may reflect periodic mass mortality possibly related to the filling of the Sevier Basin. In this regard, the low-high-low species diversity gradient shown to correspond to the biostratigraphic (but not necessarily stratigraphic) order of the strata in the thesis area is noteworthy.

The structural interpretation of the Mosheim Anticline in the thesis area by Tegland (1978) based on seismic lines is supported by the biostratigraphic conclusions of this thesis; the interpretation of Boeder et al. (1978) is not.

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