Masters Theses

Eric A. Weiss

Date of Award

3-1981

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Thomas W. Broadhead, Fred B. Keller

Abstract

Approximately 313 meters of Middle Ordovician rocks are exposed at Solway, Tennessee. This sequence is composed of seven formations, in ascending order: Blackford Formation, Lincolnshire Formation (Eidson Member and Hogskin Member, respectively), Benbolt Formation, Wardell Formation, Bowen Formation, Witten Formation, and Moccasin Formation. Detailed field examination and analysis of more than 220 thin sections has permitted interpretation of the environments of deposition for these rocks. With this environmental scenario in mind, a portion of the Witten Formation (unit 19) was selected for paleoecological analysis.

The Blackford Formation represents closely associated sub-environments in a coastal, tidal flat setting. These sediments v/ere deposited in the shallow, subtidal zone or by algal activity in the intertidal to supratidal zone. The Lincolnshire Formation (Eidson Member and Hogskin Member) was deposited under normal marine, subtidal conditions due to the continued transgression of the Middle Ordovician sea. The lower portion of the Eidson Member was deposited in a quiet, comparatively deep environment; whereas, the upper portion represents a slightly more agitated, shallower environment, A still shallower, more agitated environment is evidenced by sediments of the Hogskin Member. The lower portion of the Benbolt Formation represents sub-environments of a quiet, protected, lagoon and were often disturbed by influxes of fine-grained terrigenous elastics. The upper Benbolt represents a return to peritidal deposition in a restricted subtidal environment and intertidal to supratidal environments. The former may represent a nearshore lagoon, and the latter may have been tidal islands. Most of the Wardell Formation was deposited in a shallow, quiet, level-bottom, subtidal environment which was occasionally disturbed by storms. In addition, a small interval in the middle of the Wardell suggests intertidal to supratidal sedimentation—possibly representing another tidal island. The Bowen Formation is inferred to have been deposited in the intertidal to supratidal zones of a coastal, tidal flat environment. These sediments may have originated from a short regression in a generally transgressive sequence. A return to subtidal deposition is implied by the Witten Formation. These rocks represent nearshore and offshore sub-environments within a lagoon with variable water circulation. Frequent influxes of terrigenous mud are evidenced in some of these sediments, and a progressive shallowing occurred during the deposition of the upper half of the Witten. These shoaling-upward conditions resulted in the intertidal to supratidal sediments in the top of this formation. The overlying Moccasin Formation is characterized by storm deposits; semi-isolated, subtidal, low-lying interbank deposits: and tidal island or mudbank deposits. Therefore, the Solway Bridge section represents a complex array of shallow, marine shelf environments which were probably susceptable to minor environmental fluctuations.

The paleoecologic study of unit 19 of the Witten Formation revealed six benthic paleocommunities from an ancient, shallowing lagoon environment which was frequented by sporadic influxes of terrigenous mud. Each paleocommunity represents a different stage of an offshore lagoonal sere or a more nearshore lagoonal sere. In order of increasingly more mature stages, the paleocommunities of the offshore successional seguence are: (1) the Stictopora—Strophomena community, (2) the Stictopora—Rostricellula community, (3) the Bythopora—Rostricellula community and (4) the Rostricellula—Heterotrypa community. The nearshore successional seguence is only represented by an immature stage (the Stictopora—Bivalve A community) and a relatively mature stage (the Bivalve A—Heterotrypa community). Both types of seres colonized a soft substratum and were later demolished by an influx of terrigenous mud. Between times of mud influx, environmental conditions remained relatively stable in the lagoon—allowing colonization of the soft substrate left by the proceeding influx of mud. The duration of stability determined the stage (indicated by a particular paleocommunity) that either the offshore lagoonal sere or the more nearshore lagoonal sere could achieve between mud influxes. As the seres matured, the initially soft substrate became more stabilized by earlier stages of succession resulting in an increase of species variety and equitability. The high frequency of the nearshore sere in the upper portion of unit 19, suggests an encroaching shoreline which probably resulted from a regression of sea level or progradation of shoreline environments.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS