Masters Theses

Date of Award

6-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Thomas W. Broadhead, Steven G. Driese, Stephen Haase

Abstract

The Moccasin Formation is the uppermost unit of the Chickamauga Group (Middle Ordovician) and is restricted to northwestern strike belts in the Southern Appalachian Valley and Ridge Province. Most of the unit consists of roughly coequal amounts of carbonate and siliciclastic components. The Moccasin was deposited in a complex mosaic of peritidal environments which extended over a large area of the Southern Appalachians during late Medial Ordovician time.

Outcrops along U.S. 25E near Thorn Hill, Tennessee, expose a complete section of the Moccasin Formation measuring 191.50 m. These outcrops were measured and described in the field and 196 samples were taken and processed into polished slabs, acetate peels, and thin sections in order to characterize the stratigraphy of the Moccasin, to reconstruct the environments of its deposition, and to examine the assemblages of fossils which occur in the formation.

Nineteen lithotypes recognized in the Moccasin belong to four interrelated fades: Tidal Pond; Supratidal/Intertidal Flat; Tidal Channel; and Adjacent Marine. The Tidal Pond Fades varies from bioturbated, argillaceous rich, peloidal mudstone to fossil-bearing, peloidal wackestone and packstone. The Tidal Pond Fades represents shallow, quiet-water tidal ponds which were located on the interior of the Moccasin tidal flat complex. The Supratidal/Intertidal Flat Fades ranges from size-graded beds of peloidal packstone and argillaceous mudstone, to fossiliferous, intraclastic packstone. This facies represents broad, gently sloping flats which bounded subtidal tidal ponds and tidal channels of the Moccasin tidal flat complex. Evidence of desiccation (fenestral fabric, mudcrack polygons) is ubiquitous in the Supratidal/Intertidal Flat Fades, indicating that deposition was intermittent and alternated with subaerial exposure. The Tidal Channel Fades consists of peloidal packstone with small-scale currentor wave-induced sedimentary structures and rare beds of intraclastic, fossiliferous packstone. This fades represents the system of shallow channels which dissected the Moccasin tidal flat complex. The Adja cent Marine Fades is composed of relatively pure, grey limestone which ranges from laminated, peloidal wackestone to fossiliferous, Tetradium packstone. These, lithologies represent deposition in the northwest ward, most seaward areas of the Moccasin tidal flat complex.

Seven volumetrically important fossil assemblages recur in the Moccasin Formation and were analyzed within the paleoenvironmental context of Moccasin deposition. Each assemblage consists of indigeneous and exogenous components. After their inferred place of occurrence within the Moccasin tidal flat complex, the assemblages include: (1) landward supratidal flat assemblage composed of small and medium size, vertical burrows; (2) landward intertidal flat assemblage characterized by small and medium size, vertical burrows and horizontal burrows; (3) landward tidal pond assemblage dominated by horizontal burrows with small and medium size, vertical burrows, arthropods, inarticulate brachiopods, and sponges; (4) landward tidal channel assemblage with rare small, medium, and large size, vertical burrows; (5) seaward supratidal/intertidal flat assemblage consisting of blue-green algal mats, small and medium size, vertical burrows, and horizontal burrows; (6) seaward tidal channel assemblage dominated by bryozoans, molluscan bivalves, articulate brachiopods, and trilobites with moderate numbers of other groups; and (7) quiet subtidal adjacent marine assemblage dominated by horizontal burrows and Tetradium corals with other diverse groups present In low numbers.

Comparison of the mixed carbonate-siliciclastic tidal flat complex of the Moccasin with other tidal flats composed of nearly pure carbonate sediment reveals that similarities between mixed and pure carbonate tidal flats may be found In the patterns of biotic abundance and diversity, the distribution of sedimentary structures, and In the nature of the sharp and frequent vertical fades changes. Most notable differences Include the absence of penecontemporaneous dolomite and the lack of widespread cryptalgal laminations in mixed carbonate-siliciclastic tidal flat complexes. In the Moccasin Formation, these differences may result from abundant admixed argillaceous material and a humid climatic setting. Study of the Moccasin Formation has resulted In the development of a model of humid, low energy, mixed carbonate-siliciclastic tidal flat deposition at the margins of Early Paleozoic epicontinental seas.

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