Masters Theses

Date of Award

6-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Robert E. McLaughlin, Thomas W. Broadhead

Abstract

The Benbolt, Wardell, Bowen, and Witten formations (middle Chickamauga Group) in upper East Tennessee near Thorn Hill measure 198.8m in thickness. The four formations represent a regressive carbonate sequence deposited on a broad shelf during Middle Ordovician time. Detailed petrographic analysis reveals thirteen lithologies which depict deeper subtidal deposition, ovdrlain by sediments of shallow subtidal, nearshore, and tidal flat environments.

The Benbolt Formation is irregularly bedded, cobbly weathering, fossiliferous wackestone, packstone and calcareous shale, containing diverse marine fossil assemblages, particularly echinoderms, bryozoans, brachiopods and trilobites. The Benbolt represents wholly subtidal deposition, typically below fairweather wave base, but occasionally on shallower bioclastic sand shoals. Calcareous shale near the top of the formation indicates an influx of terrigenous silt and mud, which greatly decreased carbonate production. The Wardell Formation consists of lime mudstone and highly fossiliferous wackestone and peloidal packstone deposited nearshore within fairweather and storm wave base. Wardell environments supported a rich and varied marine fauna of bryozoans, tabulate corals, calcareous algae, echinoderms, and brachiopods. The Bowen Formation is predominately reddish-brown, thinly bedded, internally laminated, sparsely fossiliferous, argillaceous lime mudstone, deposited in shallowest subtidal, intertidal, and supratidal environments. Dessication features, such as mudcracks and birdseye fenestrae are common, as are large vertical burrows. The Witten Formation consists of interbedded lime mudstone, fossiliferous wackestone, and thinly bedded, intraclastic packstone deposited in partially restricted, inner shelf, shallow subtidal, intertidal, and supratidal environments. Fossils consist predominately of molluscs, bryozoans, trilobites, ostracodes, and brachiopods. The middle Chickamauga represents a regressive sequence of carbonate deposition in middle and inner shelf environments.

Ectoproct bryozoans from the middle Chickamauga show some degree of facies specificity. The muddy subtidal environment represented by the Benbolt is dominated by very small, ramose growth forms (such as "Phylloporina", arthrostylids, and various cryptostomes), sheet-like encrusting cystoporates, and mound-shaped Monotrypa zooaria. Skeletal sand shoals in the Benbolt contain mostly robust trepostomes (e.g. Nicholsonella) and encrusters. The Wardell contains over twenty-five genera of bryozoans. This nearshore, turbulent environment, is dominated by "Phylloporina", ceramoporids, and Parvohallopora in the less agitated facies, but the ramose Amplexopora and the encrusting Constellaria overwhelmingly dominate the current-reworked skeletal sands. The restricted subtidal environment (Witten Formation) contains very few phylloporinids or ceramoporids, but the ramose trepostomes Homotrypa and Amplexopora are locally very abundant, suggesting they were adapted to conditions of varying temperature or salinity.

Some taphonomic studies have cast doubt on the preservability of community information in fossil assemblages. Although taphonomic processes destroy information, petrographic analysis suggests some syndepositional processes enhance preservation of assemblages. In muddy environments, invertebrate colonization and subsequent encrustation oduce well preserved "microbioherms" which record biotic interactions in firm-substrate-starved environments. Substrate modification by disturbance, meiobenthic interactions, invertebrate colonization, marine cementation, and early water loss results in a gradually firming substrate that resists mixing by benthic burrowers and sediment resuspension by high current energy. Rapid burial or "obrution" also serves to preserve some fossil assemblages. Coarse grained, bioclastic storm deposits not only serve to catastrophically bury an assemblage, but may also provide firm substrata for subsequent colonization. The above processes are responsible for preservation of fossil community information in the middle Chickamauga. In addition, recent studies suggest that during the Ordovician, lower rates of bioturbation and more intense marine cementation may have also enhanced preservation.

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