Date of Award
Master of Science
Kenneth R. Walker
Robert D. Hatcher, Stephen Driese
The Middle Cambrian Craig Limestone Member (Rogersville Shale) and Maryville Limestone are part of the thick Cambro-Ordovician pericratonic sedimentary package exposed in the Valley and Ridge province in East Tennessee. Exposures in the Dumplin Valley fault zone provide further details concerning the origin and development of these limestone formations.
Description of five sections, analysis of 125 thin sections and 98 slabs, and regional reconnaisance in the Dumplin Valley area reveal that the Craig Limestone Member represents "premature" demise of a carbonate shelf in that it did not develop as fully as the Maryville shelf and had no rimmed edge or widespread peritidal environments. Instead, the underlying lower Rogersville Shale represents basinal and slope deposits, and the Craig represents deposition on a gentle ramp with deposits analogous to the mid-ramp and aggrading ramp packages of the Maryville. No shoal, lagoon, or peritidal deposits were observed.
In contrast, the Maryville Limestone consists of six genetic packages, each 10s of meters thick: 1) the slope package (debris flows and turbidites deposited below storm wave base); 2) the mid-ramp (burrow mottled mudstone with very thin packstone to grainstone lenses or layers deposited below storm wave base) / aggrading ramp package (subequal amounts of mottled mudstone and packstone/grainstone representing increased wave activity); 3) the shoal package (ooid grainstones representing migrating ooid shoals); 4) the lagoon package (mudstone and packstone/grainstone deposited in protected settings behind ooid shoals); 5) the peritidal package ("cyclic" shoaling-upward sediments deposited in settings associated with tidal flats and islands); and 6) the backstepping platform/shelf package (a variety of lithologies deposited in response to relative sea-level rise).
Through its evolution, the Maryville environmental regime developed from a ramp to a rimmed platform with Renalcis/Girvanella bioherms at the platform edge and platform-interior peritidal environments. 10s-of-meters-scale stratigraphic packages were controlled by sedimentary aggradation and progradation. Controls on subtidal meter-scale interbeds of mud-dominated lithologies and grain-dominated lithologies appear to have been processes such as wave sweeping and storm activity; no evidence for regular, shallowing-upward cycles is evident. Deposition of peritidal shallowing-upward parasequences was probably controlled by a combination of autocyclic mechanisms, irregular (jerky?) tectonism, and eustatic sea-level fluctuations.
A petrographic study of the Maryville revealed four "diagenetic patterns" (DP): 1) DP 1 (marine fibrous and bladed cements and burial calcites and dolomites, all non ferroan); 2) DP 2 (extensively dolomitized sediments); 3) DP 3 (dissolution, vadose silt, equant calcite, depleted oxygen isotope ratios of -9 o/oo); 4) DP 4 (marine fibrous and bladed calcite, burial ferroan and non-ferroan calcites and dolomites). DP 1 is present in the subtidal sediments of the slope through lagoon packages. DP 2 is related to early? dolomitization of peritidal sediments. DP 3 is developed at the top of the Maryville in shelf-edge and lagoon areas, and at the top of and within the peritidal package in platform interior areas. This facies is related to platform exposure. DP 4 is present in the backstepping shelf/platform package and shows no evidence for subaerial exposure.
The results of this study suggest that the Maryville platform and the Craig Limestone Member (Rogersville Shale) ramp are capped by surfaces of subaerial exposure. Subaerial exposure and meteoric diagenesis are manifest by fabric-selective and non-fabric selective dissolution, depleted oxygen isotope ratios, and erosionally truncated equant cements. In more off-platform areas, the exposure surface is also a drowning surface overlain by shale, whereas in platform interior locations, a deepening-upward trend is present within the carbonates. Framboidal pyrite, manganese- and phosphate-coated grains, and deeper-water shales reflect relative deepening following platform exposure. Exposure provided a shutdown of carbonate production. Once re-flooded, the very slow sedimentation rates coupled with episodic pulses of subsidence resulted in an apparently "instantaneous" drowning of the platform, and the deeper-water lithologies directly overlying shallower-water lithologies.
The episodic pulses of tectonism documented herein and Middle Cambrian tectonism across much of the Iapetan "passive" margin suggest that the southern Appalachian continental margin was not completely stabilized until the Late Cambrian. The passive margin evolved in three stages: rift stage (characterized by active rifting and thick rift basin sediments; Ocoee Supergroup, parts of Chilhowee Group), immature passive margin (effects of thermal and episodic non-thermal subsidence combined; Conasauga Group), and mature passive margin (subsiding by thermal subsidence alone; Knox Group).
Rankey, Eugene Carlton, "Carbonate Platform Response to Tectonism and Eustasy: The Middle Cambrian Carbonates of the Lower and Middle Conasauga Group, East Tennessee. " Master's Thesis, University of Tennessee, 1993.