Masters Theses

Date of Award

6-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Don W. Byerly

Committee Members

Kenneth R. Walker, Steven G. Driese

Abstract

The Middle Ordovician Sevier Shale basin in southwest Virginia near Avens Bridge is described and discussed in terms of its stratigraphy, sedimentology, depositional environments, and petrology. The Middle Ordovician stratigraphy includes (in ascending order); the Lenoir Formation (17 meters thick), the Blockhouse Formation (67 meters thick), the Sevier Formation (approximately 1,000 meters thick), and the Tellico Formation (465 meters thick).

These formations were deposited in an evolving foreland basin that underwent five stages of evolution (Shanmugan, 1978; Shanmugan and Walker, 1978, 1980, 1983): (1) a stable shelf edge, represented by the Lenoir; (2) a downwarping stage; (3) a starved-basin stage represented by slow pelagic fallout of Blockhouse sediments; (4) a turbidite fill stage, represented by sedimentary rocks of the Sevier and Tellico Formations; and (5) a contour-current stage, that can be seen in shale and siltstone strata in the upper part of the Tellico Formation.

The coarsest terrigenous-elastics in the Sevier basin are represented by the rocks of the Tellico Formation which consist of interbedded shale, siltstone, massive sandstone, pebbly sandstone, conglomerate, and chaotic units. The Tellico Formation was the main focus of this study. The Tellico Formation was studied in detail to determine its sedimentological characteristics and environments of deposition. Lithofacies of the Tellico including: classic turbidites, massive sandstone, pebbly sandstone, clast-supported conglomerate, debris flows. chaotic units (glide slides and slumps), and contourites in the upper part were separated into three lithofacies associations.

Lithofacies association I includes interbedded hemipelagic shales, classic turbidites, massive sandstones, and chaotic units (debris flows and glide slides) that represents deposition on the mid fan.

Lithofacies association II includes minor amounts of hemipelagic shale and classic turbidites in a sequence dominated by massive sandstones, pebbly sandstones, conglomerates, debris flows, and slumps interpreted to be deposits of an inner-fan channel.

Lithofacies association III seen as thick sequences of hemipelagic shale, thin-bedded classic turbidites, and possible contourites that are thought to represent interchannel deposits on a submarine fan.

The submarine fan is interpreted to have entered the basin from the south, prograding by lateral migration of mid-fan complexes. Lateral migration of mid-fan complexes could explain the lack of well-developed coarsening- and thickening-upward sequences in the lower fan in the study area.

Sandstone compositions in the Tellico Formation consist of quartz, rock fragments, and feldspar in a matrix of clay minerals, sericite, and chlorite. Sandstone beds in the study area may be classified as lithic graywackes (Pettijohn, 1975) and litharenites (sedarenites) (Folk, 1974). Modal analysis of the three coarse packages in the study area revealed no major compositional changes through time.

The Tellico Formation lies within the recycled orogenic provenance field of Dickinson et al. (1983). Source rocks for the terrigenous VI elastics of the Tellico Formation include the Shady Dolomite, Chilhowee Group, Knox Group, Lenoir Formation, and Fetzer Formation.

Comparison of compositional suites of the Tellico Formation with the Tourelle Formation reveal major variations in source rocks. The Tourelle contains up to 31% vs. 1.4% (for the Tellico) volcanic rock fragments. Chromite in the Tourelle Formation is thought to have been derived from ophiolites (Hiscott, 1978). No evidence for ophiolites was found in the sediments of the Tellico Formation. Variations in provenance of these Middle Ordovician foreland basins may reflect differences in the tectonic style along the Taconic orogenic belt.

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