Masters Theses

Date of Award

8-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Steven G. Driese, Claudia I. Mora

Abstract

The Middle to Upper Cambrian strata of east Tennessee consist of the alternating shale and carbonate formations of the Conasauga Group. The Conasauga was deposited during the final stages of evolution of the Laurentian passive margin following the Late Precambrian to Early Cambrian rifting of Rodinia. The shale and carbonate formations record the interplay between a broad, carbonate bank (Honaker/Elbrook Formations) to the present day east and a western deeper-water intrashelf basin (Conasauga Shales). This research investigates the stratigraphy and sedimentology of the first carbonate formation, the Middle Cambrian Rutledge Limestone. Three major goals were addressed: 1) build a stratigraphic framework for the Rutledge, 2) determine the depositional environments of the major lithofacies, and 3) carry out a preliminary study of diagenesis.

Detailed study of 6 stratigraphic sections, 130 thin sections, and associated polished slabs reveal that the Rutledge is from 53 to 65 meters thick. The formation consists of seven distinct lithofacies. The dominant one, burrow-mottled mudstone/wackestone, consists of a carbonate mudstone matrix with an anastomosing network of burrows. Burrow fabric is preserved through the partial to complete dolomitization of the burrow-fill and is characteristic of Thalassinoides. The burrow-mottled lithofacies, together with the ribbon- laminated mudstone lithofacies, the skeletal, oncoid wackestone/packstone lithofacies, and the nodular mudstone with shale lithofacies, comprise over ninety- five percent of the Rutledge and attest to its dominantly muddy character. The ooid, peloidal grainstone lithofacies is limited to a thin, correlatable interval in the middle of the Rutledge section. The calcareous/non-calcareous shale lithofacies and the interbedded carbonate mudstone/wackestone/packstone and shale lithofacies are limited to the lower transitional interval with the underlying Pumpkin Valley Shale and to a shale interval in the lower Rutledge.

Deposition occurred on a gently sloping carbonate ramp. The ramp is interpreted to have been a protected, homoclinal ramp facing the deeper-water intrashelf basin to the west and bordered to the east by shallow-water carbonate deposits. Deposition was characterized by the settling from suspension of shallow-water derived fine-grained carbonate sediment into environments below fair-weather wave base and below mean storm wave base. Subsequent burrowing by infaunal, soft bodied organisms produced the burrow-mottled fabric of the units. The overall homogeneity suggests that the ramp depositional system persisted throughout deposition of the Rutledge. Marked shallowing events, periodic reworking of sediments by storms, and progradation of the shallow-water environments into the basin are interpreted to have occurred during deposition, but these processes were minor in the overall history of the Rutledge in the field area.

Deposition was controlled by three factors: the relative position of sea level, the rate of clastic influx from the craton, and thermal subsidence. Sea level and the resulting water energy gradients controlled the distribution of facies. The rate of clastic influx from the craton, along with changes in the rate of sea level rise and fall, played a major role in the transition between the Rutledge and the underlying Pumpkin Valley and the overlying Rogersville Shales. Continuous, relatively high rates of thermal subsidence controlled the overall depositional stasis of the ramp system.

The Rutledge is interpreted as a highstand systems tract that, when combined with the underlying Pumpkin Valley Shale, comprises a third-order depositional sequence and defines the first grand cycle of the Cambrian grand cycles of the southern Appalachians. Carbonate deposition began as the result of a decrease in the rate of sea level rise during the initial phases of the Sauk II transgression. Increased production of carbonate sediment in shallow-water environments kept-up with increases in accommodation space allowing for the transport of carbonate sediment basinward. Carbonate deposition possibly concluded as the result of sea level fall that temporarily suppressed carbonate production. Subsequent sea level rise resulted in the onlap of terrigenous clastics of the Rogersville Shale.

The Rutledge Limestone did not evolve into a flat-topped carbonate platform as did younger, major carbonate units of the Conasauga because of higher thermal subsidence rates, higher rates of input of terrigenous clastics, and decreased carbonate production area. This suggests that thermal subsidence and the effects of siliciclastics on carbonate deposition decreased throughout deposition of the Conasauga and that deposition of the older carbonate units of the Conasauga were controlled by subsidence and "catch-up" sedimentation whereas younger units were characterized by slower rates of subsidence and "keep-up" or "prograde" sedimentation. The antecedent topography of the Rutledge provided younger carbonate units with a larger sediment source area which resulted in a faster establishment of ramp deposition in the basin and an enhanced progradation potential of the shallow-water environments. Finally, continued aggradation of early Conasauga carbonates and time equivalent shallow- water deposits to sea level, coupled with eustatic sea level rise and subsidence, led to the development of an elevated shelf edge and sediment starved conditions in the area between the bank and the craton. The combined effects of above mention processes gradually led to the deepening of the area between the bank and the craton and the development of the intrashelf basin.

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