Date of Award
Doctor of Philosophy
Kenneth R. Walker
Kula Misra, Craig Barnes, Gary Jacobs, Steve Driese
The Conasauga Group constitutes part of a thick pericratonic Cambro-Ordovician passive-margin sequence along the eastern North American continent. The Cambrian carbonate platform was flanked by a high-relief shelf margin towards the east, facing the open ocean, while to the west the carbonate platform sloped into an intrashelf basin. It is this western shelf margin that is the topic of the present study. Detailed lithofacies analysis of the Middle Cambrian Maryville Limestone along a shelf-to-basin depositional transect reveals that the shelf evolved from a gently basinward sloping ramp to a rimmed platform fringed with steeper slopes. Cyanobacterial buildups (Renalcis-Girvanella) dominated the platform margin environments. Progradation of the platform occurred towards the craton.
A process oriented approach is applied to define the sequences, sequence boundaries, and the stacking pattern of the Maryville Limestone. The Maryville Limestone sequence consists of two depositional subsequences. The boundary between the two subsequences is not a sequence boundary, because it does not separate rocks deposited in different environmental regimes. The two subsequences within the Maryville sequence consist of a combination of aggradational, retrogradational, and progradational units (with respect to the platform interior). The stacking pattern recognized is the result of variations in sedimentation rate, subsidence, and eustatic sea-level change. Each of the dominantly carbonate units within the Conasauga represents this gradual transition from a ramp-like platform to basin transition into a rimmed platform.
In the study area, the transition between the Maryville Limestone (Middle Cambrian) and the overlying Nolichucky Shale (Late Cambrian) is a sequence boundary. This sequence boundary is both an exposure surface and a drowning unconformity, and marks a distinct shift in the pattern of sedimentation.
The Maryville Limestone was subjected to a complex diagenetic history. A combined field, petrographic, and geochemical approach are applied to describe the diagenetic history of the Maryville Limestone. The stabilization history of the Maryville Limestone during early diagenesis was characterized by microscale dissolution and reprecipitation during shallow burial and fabric selective dissolution in response to subaerial exposure and influx of meteoric fluids. However, during deep burial dolomitization was the dominant diagenetic event.
Depositional components such as intraclasts and ooids and synsedimentary fibrous (marine) cements were subjected to microscale dissolution and reprecipitation during shallow burial. Depleted oxygen isotopic composition of ooids (mean δ 18O = -8.7 0/00 PDB) and fibrous cements (mean δ 18O = -8.4 0/00 PDB) relative to Cambrian marine carbonate value (δ 18O = -5.0 0/00 PDB) suggests that diagenetic alteration probably occurred during burial. Preserved ustrastructures of ooids and fibrous cements suggest that stabilization involved microscale dissolution and reprecipitation. Blocky, turbid calcite in intergranular pores is interpreted to be of shallow burial origin based on the presence of inclusions and cross cutting relationships. Blocky, turbid calcite spar is characterized by depleted oxygen isotopic composition (mean δ 18O = -8.2 0/00 PDB). Sr isotopic composition of blocky turbid calcite spar (0.7095) which is similar to Cambrian seawater values (0.7091-0.7095) suggests that shallow burial cementation occurred within a rock dominated system.
In contrast, wholesale dissolution and cementation occurred in response to the influx of freshwater during periods of subaerial exposure. Meteoric diagenesis is restricted to parts of formation near exposure surfaces. Petrographic evidence for subaerial exposure consists of planar truncation surfaces, insitu brecciation, pores partly filled with vadose silt, and fabric-selective dissolution and cementation. Fabric selective dissolution yielded biomolds. Blocky, clear calcite spar commonly occludes the moldic pores. The stable oxygen isotope composition of blocky, clear calcite spar (δ 18O = -8.0 0/00 to -9.5 0/00 PDB) is considerably depleted when compared to Cambrian marine carbonate values (-5 0/00 PDB). Depleted δ 18O values are consistent with subaerial exposure and meteroric diagenesis. However, δ 13C values show little shift. A possible reason for this lack of negative shift in carbon isotopic values is probably the absence of land plants developed on the surface during exposure. Blocky, clear calcite spar is enriched in Fe (avg. 1600 ppm). Enriched Fe values in blocky, clear calcite spar suggest that the meteoric system was relatively stagnant promoting reducing conditions. 87Sr/86Sr values of blocky, clear calcite spar which are similar to Cambrian marine carbonate composition offers supporting evidence for meteoric origin of blocky clear calcite spar.
Detailed petrographic analyses along a depositional transect from a carbonate-platform to shale basin reveal that dolomite is the principal burial diagenetic phase. Four different types of dolomite were identified based on detailed petrographic and geochemical analyses. Dolomite occurs as replacement of precursor carbonate and as inter- and intraparticle cements.
Type I dolomite occurs as small, irregular disseminations typically within mud rich facies. Type II dolomite typically occurs as inclusions of planar-e rhombs (ferroan), 5 to 300 μm in size, in blocky, clear ferroan calcite (meteorica) spar. Type II dolomite is nonluminescent. Type I and II dolomite formed during shallow-intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals approximately 10 μm to 150 μm in size. Type IV dolomite consists of baroque or saddle-shaped, 100-1500 μm crystals, and is non-luminescent. Type IV dolomite formed during maximum burial.
Types III and IV dolomite increase in volume downslope. Type III dolomite contains 1.2-2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends. Fe and Mn values decrease from 3.5-4.5 wt%Fe, and 0.1-0.3 wt% Mn in the west (slope/basin) to 1.5-2.5 wt% Fe, and less than 600ppm Mn in the east (shelf-margin), over a distance of approximately 60 km. Type III and IV dolomite have a mean δ 18O value of -7.8 0/00 and a mean δ 13C value of +1.1 0/00 (relative to the PDB standard). Based on an assumed basinal fluid composition of 5 0/00 SMOW, temperatures calculated from δ 18O values of Type III and Type IV dolomite range between 85° C to 150° C. 87Sr/86Sr compositions of Type III and Type IV dolomite are enriched with respect to Cambrian marine values and range from 0.7111-0.7139, probably indicating that the diagenetic fluid had interacted with siliciclastic sediments of basinal shales.
Based on the Fe and Mn distributions in Type IV saddle dolomite, a west-east fluid flow during late burial diagenesis is indicated. Calculated temperatures indicate that the fluids were warm. The distribution of Paleozoic facies in the southern Appalachians indicates a Cambrian shale source for these fluids, and burial curves suggest an early Ordovician age for burial fluid movement.
Srinivasan, Krishnan, "Depositional History, Sequence Stratigraphy and Diagenesis of the Maryville Limestone (Middle Cambrian) Southern Appalachians. " PhD diss., University of Tennessee, 1993.