Masters Theses

Date of Award

8-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Steve Driese, Claudia Mora

Abstract

The upper Honaker and lower Nolichucky Formations (Middle Cambrian, Conasauga Group) in northeastern Tennessee comprise part of a thick pericratonic Cambro-Ordovician passive margin sequence along the eastern edge of North America. Throughout the Cambrian, the interplay of autocyclic controls, including sediment supply, tectonism and accompanying subsidence, and eustasy, resulted in various sedimentary architectures, including the Conasauga platform and intrashelf basin adjacent to the craton. To date, the Middle Cambrian westerly carbonate sections adjacent to and within the intrashelf basin (near Knoxville) have been studied in considerable detail. Particularly, these studies have proposed a third-order sequence boundary near Knoxville as well as a Middle Cambrian Conasauga platform development model. Yet, the thick time-equivalent dolostones further on-platform (near Johnson City) and their relationship to overlying units, as well as the deposits further west, have only been examined in the broadest sense. These relationships are the focus for this study.

Field and stratigraphic relationships and petrographic observations reveal four depositional packages that comprise the upper Honaker and lower Nolichucky Formations, in ascending order: 1 ) peritidal; 2) ooid shoal; 3) transitional; and 4) basinal shale. The first three packages make up the upper Honaker and the fourth package comprises the lower Nolichucky Formation. The progression of these packages represents the same deepening trend that initiates a new phase of genetically related rock units further to the west during the Middle Cambrian (Srinivasan, 1993; Rankey, 1993). The change from peritidal facies to ooid shoal facies is the first indication of the drowning event, and this allows the third-order sequence boundary to be extended and placed between these shallow water facies. Hence, the extension of the sequence boundary further to the east expands the Middle Cambrian Conasauga platform development model.

The upper Honaker depositional setting consisted of subtidal thrombolite and intertidal shoal facies that created a semi-closed lagoon with hypersaline conditions further on-platform. In addition, rare evaporite molds and the abundance of penecontemporaneous dolomite imply a semi-arid climate. As the deepening event ensued and, consequently, shifted the carbonate environments towards the east, restricted areas of the platform became open to fresh seawater. Salinity levels decreased to a range more suitable for invertebrate organisms as a result of mixing during the latter part of the Middle Cambrian, as suggested by the transitional depositional package. The craton-derived basinal shale package of the lower Nolichucky Formation in more easterly areas (Johnson City, Tennessee) suggests the intrashelf basin filled, and allowed mature passive margin sedimentation to begin in the Late Cambrian (Maynardville Formation). Furthermore, the Maryville-Nolichucky (sequence boundary) contact in the west is slightly older than the upper HonakerNolichucky formational contact of this study, based on platform architecture (<1° slope) and direction of flooding, even though the carbonate units are stratigraphically equivalent to one another.

Marine, meteoric, and/or burial diagenetic calcite and dolomite phases are present in varying amounts within the upper Honaker and lower Nolichucky. Plane light microscopy, cathodoluminescence, and stable isotope compositions constrained the timing and origin of these diagenetic components. Both early and late phases of dolomite are present in this study. Evidence for early dolomite is based on its relationship to early marine components such as evaporites and fenestrae. Later replacive and authigenic dolomite phases are inferred from cross-cutting relationships and stable isotope compositions. These phases are a result of elevated temperatures and burial diagenesis associated with the Taconic Orogeny during the Middle Paleozoic that reset the δ18O isotopic signal in these rocks (average -9 0/00 PDB). Meteoric calcite (average δ13C is -8 0/00 PDB) likely precipitated late in tectonic-induced fractures related to uplift during the Late Paleozoic, based on cross-cutting relationships. Furthermore, estimated fluid compositions for the calcite (δ18O of -7 0/00 SMOW) seem to also support a late meteoric source.

This study shows that the integration of various geologic disciplines (e.g., stratigraphic relationships, petrography, and stable isotopes) is necessary in order to gain a better understanding of depositional and diagenetic relationships, as well as to constrain the timing and origin of diagenetic phases. In addition, and most importantly, this research reveals that third-order sequence boundaries are not always represented as pronounced changes in lithology (e.g., carbonate-shale). As such, sequence stratigraphy proponents may need to reconsider their longstanding concepts, particularly those related to flooding events in mixed carbonate-clastic systems.

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