Doctoral Dissertations

Date of Award

8-1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

R. E. McLaughlin, T. W. Broadhead, E. C. Clebsch

Abstract

The Middle Ordovician carbonate rocks which are exposed along Highway 58 in southeastern Tennessee (measured stratigraphic sections are: Goodfield, Center Point, Ten Mile, and Barnardsville), were deposited in a very complex tidal flat and subtidal-lagoonal array of environments on a gently sloping platform-shelf. Fluctuation in sea level, basin subsidence, tectonic activity, and geometry of the basin caused lateral migration of the environments, resulting in a patternless series of complex fades which repeated several times during deposition of the entire Middle Ordovician in the study area.

The most volumetrically important lithofacies and the inferred depositional environments within these Middle Ordovician carbonate rocks are: (1) cherty dolostone-dolostone (supratidal dolomitic flat), (2a) green and red silty mudstone (supratidal mudflat), (2b) greenish gray micrite-biomicrite (supratidal mudflat), (3a) red silty, intrapelbiosparite (intertidal channel), (3b) green and red, loosely packed ostracod rich pelbiomicrite (intertidal pond), (3c) stromatolitic mudstone (intertidal levee), (Aa) bryozoan-ostracod-brachiopod rich pelbiomicrite (subtidal "intrabasinal lagoon" or pond, type 1), (Ab) silty, packed pelbiomicrite-pelbiosparite (subtidal "intrabasinal lagoon" or pond, type 2), (Ac) ostracod-gastropod rich, bioturbated pelbiomicrite (subtidal "intrabasinal lagoon" or pond, type 3), (Ad) bioturbated brachiopod-molluscan rich biomicrite (subtidal "intrabasinal lagoon" or pond, type A), (5a) bioturbated green and red sity mudstone-silty sparse biomicrite (subtidal quiet-protected lagoon), (5b) greenish gray-tan mudstone-sparse biomicrite (subtidal protected lagoon), (6) intrapelbiosparite-pelbiosparite-pelbiomicrlte (subtidal channel), and (7) Tetradium rich packstone (subtidal "wave baffle").

At one locality (Center Point), where 40 samples were selected for geochemical study, based on the inferred depositional environments of the lithofacies, Sr, Na, Mn, Mg, and Fe were determined for the soluble fraction of each sample by atomic absorption spectrometyr, and calcium by titration. Local variations in elemental composition showed that Na, Mn, and Fe (partially) are controlled by detrital minerals, high Mg content of some samples by stylolitization and dolomitization. The inverse relationship between Sr and insoluble residue may indicate the close association of Sr with the original mineralogy (aragonite and calcite) and the influx of terrigenous clastic materials into the depositional environment. Discriminant function analysis and Mn vs Fe plot both showed that carbonate rocks were statistically different and can be divided into two distinct groups and one transitional group. Depositional environment, sediment type, and diagenesis are believed to be responsible for differences between groups and variation in the trace element content.

Distinct diagenetic features recognized within the Middle Ordovician in the study area include: (1) biological activity (burrowing, boring, micritization), (2) crystal silt, (3) fringe cement, (4) syntaxial overgrowth cement, (5) equant-drusy calcite mosaics, (6) neomorphism, (7) physical compaction, (8) pressure solution, and (9) replacement. Primary and secondary porosity in these rocks includes: (1) inter- and intraparticle porosity, (2) moldic, (3) shelter, (4) fenestral, (5) fracture, (6) dissolution, (7) boring, (8) burrowing, and (9) shrinkage cracks. However, much of the porosity of the original sediment as well as secondary porosity types is now occluded and these rocks could not act as a reservoir for hydrocarbon (oil and gas) entrapment.

The Markov model of Gingerich (1969), and the modified Markov method of Carr (1982), were applied to test the presence or the absence of "orderly events, or cyclic pattern" within these rocks. The Gingerich method has shown the presence of a very complex fades transition or "cyclic pattern," which is very hard, if not impossible, to interpret in the context of the inferred depositional environments and may also be statistically meaningless. However, Carr's method has only shown two important fades transitions: (1) subtidal channel to intertidal channel, and (2) intertidal channel to intertidal pond. These fades transitions are not surprising because channels within this complex depositional system may have traversed subtidal to inter tidal environments and connected to intertidal ponds which were adjacent to the intertidal channels.

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