Masters Theses

Date of Award

12-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Thomas W. Broadhead, Kula C. Misra

Abstract

The Middle and Upper Ordovician Martinsburg Formation crops out in northeast-southwest trending belts from Southeast New York to East Tennessee. Two measured sections, located in Grainger County, Tennessee, were studied. The Highway 25E section is nearly 590 meters thick and is conformably underlain by the Middle Ordovician peritidal Moccasin Formation and is comformably overlain by the Upper Ordovician peritidal Juniata Formation. Only the upper 146 meters of the Powder Springs section are exposed.

Analysis of field observations, thin sections, and polished slabs provided the data needed to subdivide the Martinsburg Formation into three facies. The subdivisions are based primarily on the predominant lithology of each division. Each of the facies can also be assigned to one of three sub-environments of a carbonate ramp. These sub-environments are the seaward ramp exterior, the ramp margin, and the landward ramp interior. During the Upper Ordovician, the ramp was covered by a prograding clastic wedge that resulted from the Hudson Valley phase of the Taconic orogeny.

The rocks of the shale facies were deposited on the ramp exterior. The ramp exterior environment was below normal wave-base and below the photic zone; therefore, pelagic sedimentation dominated, and current action was at a minimum. The rocks of the carbonate facies were deposited at the ramp margin, where normal wave-base intersected the ramp. The presence of intraclasts indicates wave agitation, and the presence of Girvanella implies that the ramp margin was within the photic zone. The rocks of the mixed carbonate-clastic facies were deposited on the ramp Interior. This was an environment of dampened current energy that was landward of the ramp margin. Therefore, periodic storm deposited fining upward sequences are the predominant current influenced beds in the shales of the ramp interior.

Eleven intervals within the Martins burg were sampled for the paleoecology study. The various fossil assemblages reflect the shifting environmental conditions between the ramp exterior, margin, and interior. The rather homogeneous sediments of the ramp exterior contain low fossil diversities. The assemblages of the shale facies are dominated by single species of brachiopods, Zygospira or Dalmanella, along with abundant deposit feeders. The agitated, oxygenated, and nutrient rich water of the ramp margin provide stable environmental conditions. The carbonate facies is characterized by specialized niches and high biotic diversity. The Hebertella-Parvohallopora and Zygospira-Constellaria assemblages exhibit stratification of the trophic groups at the ramp margin. The low energy environment of the ramp interior contains fossil assemblages dominated by Sowerbyella and Rafinesquina. The nearshore sub-environments consist of bryozoan biostromes dominated by Heterotrypa, along with an abundance of the bivalves Ambonychia and Modiolopsis.

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