Doctoral Dissertations

Date of Award

5-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Steve Driese, Claudia Mora, George K. Schweitzer

Abstract

The Middle and Upper Cambrian deposits of the southern Appalachians reveal the existence of a broad carbonate platform that was facing the Iapetus Ocean to the east and was separated from the exposed craton to the west by the Conasauga intrashelf basin. This study focuses on the Maynardville Formation, which was deposited during the early Late Cambrian along the western carbonate platform margin. As the uppermost carbonate unit of the alternating shale and carbonate units or Grand Cycles of the Conasauga Group (Middle to Upper Cambrian), the Maynardville marks a change in style of passive-margin deposition reflected in the cessation of Grand Cycle deposition.

The Maynardville is a transitional interval between the largely subtidal carbonate and siliciclastic deposits of the Conasauga Group, and the peritidal carbonate deposits of the overlying Knox Group (Upper Cambrian to Lower Ordovician). The Maynardville consists of a lower subtidal package, underlain by the Nolichucky Shale, and an upper peritidal package, overlain by the Copper Ridge Dolomite. Mixed carbonate/siliciclastic deposition took place in a deep ramp (upper Nolichucky) to shallow-ramp and lagoonal (subtidal Maynardville) setting. To the east was a broad, semi-arid carbonate tidal flat with a variety of peritidal environments (upper Maynardville/Copper Ridge). The Nolichucky represents a retrogradational depositional package that formed in response to an increase in the rate of relative sea-level rise. Deposition of a shallowing-upward succession of the Maynardville reflects carbonate platform aggradation and progradation favored by a subsequent decrease in the rate of relative sea-level rise. Stacking patterns of the Maynardville are a result of the interplay between intrinsic factors of carbonate depositional systems, the mechanisms related to the history of the adjacent siliciclastic basin, and possible eustatic sea-level changes.

The cessation of Grand Cycle deposition is a consequence of passive-margin evolution. The abrupt change from carbonate to shale deposition in the Grand Cycles may have been caused by short-term, episodic, non-thermal tectonic subsidence related to active extension and vertical readjustments, enhanced by sediment and water loading, during the immature stage of passive-margin development. These processes were superimposed on eustatic sea-level changes and thermal post-rift subsidence. The transition from an immature to a mature passive-margin setting occurred during deposition of the Maynardville Formation. The mature margin was characterized by the cessation of tectonic activity in the area. Decreased rates of thermal subsidence and the complete infilling of the Conasauga basin favored shallow-water deposition and carbonate platform progradation. The Maynardville grades conformably into the Copper Ridge Dolomite. This conformable interval is interpreted as a sequence boundary zone correlative to the Dresbachian/Franconian (Sauk II!Sauk III) unconformity. This boundary separates a third-order sequence (terminal Grand Cycle), composed of the Upper Shale Member of the Nolichucky and the Maynardville, from the thick peritidal carbonate deposits of the Knox Group, which reflect the final passive-margin stabilization.

The transition from the Conasauga into the Knox Group is characterized by a shift in diagenetic patterns. The distribution of early diagenetic phases within the Maynardville was controlled by changes in the depositional setting from a subtidal to a semi-arid tidal flat. The infilling of the Conasauga basin and carbonate platform progradation at the end of Grand Cycle deposition influenced the regional facies distribution, which consequently affected the burial diagenesis of this transitional interval.

The subtidal deposits of the Maynardville contain a variety of calcite cements that represent marine, meteoric and burial diagenetic environments. Dolomite is not abundant within these deposits, and it primarily occurs associated with argillaceous layers. The subtidal deposits contain rare ferroan saddle dolomite cement associated with Mississippi Valley Type (MVT) minerals. Pressure dissolution, diagenetic alteration of clay minerals, and pore fluids expelled from interbedded shale, provided a local source for the formation of ferroan dolomite during burial. The presence of MVT minerals suggests the involvement of externally derived diagenetic fluids.

The peritidal deposits have been extensively dolomitized. Fine-crystalline penecontemporaneous dolomite formed under sabkha-like conditions. Coarser-crystalline replacement dolomite formed from recrystallization of early dolomite and from dolomitization of limestone during burial. Fenestrae, desiccation and evaporite dissolution voids are occluded with dolomite cement, which is often complexly zoned. Zoned dolomite cement precipitated from modified marine, mixed meteoric/marine, and burial fluids. Saddle dolomite cement in pore-centers, tectonic fractures, and dissolutional voids formed during late burial from warm, basinal fluids associated with the migration of MVT mineralizing brines, and from fluids provided locally by pressure solution.

The formation and preservation of Upper Cambrian microbial deposits were controlled by the conditions within the environments of deposition, but were also biotically influenced. Digitate stromatolites and thrombolites formed by calcification of cyanobacteria in lower intertidal and upper subtidal environments, which were not primary sites for dolomitization. Early diagenetic calcification of cyanobacteria reduced the susceptibility of these deposits to dolomitization. Laterally linked hemispheroidal (LLH), vertically stacked hemispheroidal (SH), and columnar stromatolites, as well as most stratiform stromatolite laminae formed by the trapping of sediment in supratidal and intertidal environments on semi-arid tidal flats. Extensive dolomitization altered these peritidal carbonate deposits early in their diagenetic history.

The Maynardville Formation records an increase in the 13C/12C ratio of Late Cambrian sea water. Comparison with studies of time-equivalent deposits elsewhere suggests that this positive carbon-isotope excursion is secular in scope. Petrgraphic and geochemical analyses were used to evaluate the extent of variations related to the depositional and diagenetic environments, which are superimposed on the secular marine carbon-isotope trend. This approach enables carbon-isotope variations to be used as a stratigraphic tool, and as an indicator of the global cycling of carbon. The excursion is related to changes in the rate of organic-carbon burial, which can be linked to changes in ocean stratification, climate, sea-level, and paleoproductivity rate.

The excursion started during deposition of the Nolichucky Shale, and ended during the deposition of the Copper Ridge Dolomite. The maximum δ13C values (4 to 5 0/00 PDB) are associated with the sequence boundary zone at the Maynardville/Cooper Ridge Dolomite transition. Elsewhere, the excursion started at the base of the Pterocephaliid Biomere (near the base of the Aphelaspis zone). The excursion ended prior to the end of the Pterocephaliid Biomere, with the maximum excursion at the Sauk II/Sauk III unconformity.This supports the correlation between the Late Steptoean (Dresbachian/Franconian) sea-level fall and the sequence boundary at the end of the Grand Cycle deposition, and demonstrates the application of carbon-isotope stratigraphy to successions with poorly constrained biostratigraphy

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