Masters Theses

Date of Award

12-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Abstract

The Middle Ordovician Chickamauga Group carbonates exposed near Clinton, Anderson County, Tennessee were deposited in water depths less than 20 m. Of the 450 meters of strati graphic section, the basal 200 m are composed of interbedded tidal flat and associated restricted subtidal sediments. Tidal channel deposits are comprised largely of peloids and bioclastic grains in planar laminations, starved ripples, and cross-ripple laminations. Levee crest sediments are laminated with alternating, discontinuous grainstone laminae and flat, continuous, mud-rich laminae. Relict laminae identical to that of levee crest sediments are found within bioturbated peloid packstones/grainstones; these deposits are interpreted to represent levee interiors. Thin-bedded rocks with or without internal lamina tions, mudcracks, and robust vertical burrows are inferred to be algal flat sediments somewhat analogous to Recent Andros Island inland algal marsh environments. Tidal pond sediments are completely bioturbated and possess an indigenous restricted-water biota including ctenodontid(?) clams, ostracodes, gastropods, and trilobites. Other rocks which closely resemble tidal pond deposits have a greater quantity of calcareous algae and lesser amounts of ctenodontid(?) bivalves. Tidal pond margin and/or adjacent marine margin rocks exhibit rare or absent macrofossils and common spar-filled, vertical burrows.

From 200 to 300 m in the section, a major, deepening trend culminated in the deposition of peloid-fossil packstones and grainstones with a diverse biota. These rocks are interpreted to be storm deposits since most beds display basal scour surfaces with overlying fossil lag layers which grade upwards into laminated peloid or mudstone horizons.

The upper 100 m of the section is dominated by bioturbated mudstones inferred to represent restricted lagoon deposition. This lagoon sequence is interrupted by beds containing 10 cm high, hemi spherical Tetradium heads, indigenous restricted-water fossils, and current-oriented peloids and fossil grains which were also deposited in restricted lagoon environments. Associated facies include Tetradium-bearing packstones and grainstones, and the remnants of intertidal algal mats. These deposits are inferred to represent mudbank islands and fringing environments analogous to the Porites-Goniolithon build up ringing Rodriguez Key in the present-day Horida Keys.

A holistic approach using petrographic and fabric data, vertical-contact data, recurrent strati graphic sequences, and a knowledge of processes operating in modern environments allowed the recognition of seven subenvironments within a 40 m tidal flat sequence. This approach not only aids in the recognition of subenvironments, but it also relates these subenvironments to each other as components of a depositional complex. The frequency of vertical contact between specific rock types is not random. Genetically related sediments deposited in roughly adjacent environments are more often in vertical contact with one another than sediments which were deposited in more distal environments. Recurrent stratigraphic sequences reinforce this relationship but they also record the migration of environments through time. Two idealized sequences were documented; both possess laminite caps and represent regressive cycles. Migration from tidal pond center to tidal pond margin to levee interiors to levee crest environments generated the Lithotype 4 idealized sequence. The Lithotype 5 idealized sequence was produced by migration from tidal pond center to tidal pond margin to algal flat environments. The sediments contained within the idealized sequences reflect increasing degrees of exposure as indicated by increased vertical burrow abundance, the increased occurrence of spar-fil1ings within burrows, decreased fossil diversity, the increased number of desiccation structures such as mudcracks and desiccation fenestrae, and the sur vival of lamination.

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