Doctoral Dissertations

Date of Award

5-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Steven G. Driese, Otto C. Kopp, Vahid Alavian

Abstract

The basal Middle Ordovician strata in the Valley and Ridge province of eastern Tennessee signify the resumption of carbonate deposition after post-Knox exposure in depositional environments that paralleled the eastern shoreline of the North American continent. The Douglas Lake Member of the Lenoir Formation and the Blackford Member of the Five Oaks Formation are the basal stratigraphic units of this peritidal sequence, and were deposited in isolated topographic depressions on the Knox surface in a variety of isolated nearshore terrestrial and paralic environments. The Mosheim Member of the Lenoir Formation and the main body of the Five Oaks Formation comprise a complex tidal-flat fades that developed and migrated across the Knox surface during continued marine transgression after topographic depressions were filled. The Lincolnshire Formation and the main body of the Lenoir Formation overlie the tidal-flat fades, and represent deposition in the shallow subtidal marine environment that lay just offshore from the tidal flat. The vertical succession of these three facies defines a deepening-upward, transgressive sequence that resulted from the landward migration of the package of peritidal environments. The Mosheim/Five Oaks tidal-flat facies is a close ancient analogue to the Holocene tidal flat on Andros Island in the Bahamas. This tidal flat consists of three major subfacies, each of which is analogous to a subenvironment of the Andros Island tidal flat. The highly fenestral, laminated basal portion of the Mosheim/Five Oaks facies was deposited in a supratidal environment analogous to the inland algal marsh, the most landward portion of the Andros Island tidal flat complex. The majority of the Mosheim/ Five Oaks facies consists of a random arrangement of shallow subtidal, intertidal, and supratidal lithologies that represent deposition in a complex mosaic of ponds, channels, algal marshes, and levees, that collectively represent an Andros Island-type channeled belt. The uppermost layers of the sequence at many outcrops represent supratidal deposition on a beach ridge that formed the seaward margin of the tidal flat. The vertical succession of subfacies within the Mosheim/Five Oaks tidal-flat fades defines a transgressive sequence that contains no evidence of progradational, shallowing-upward cycles. The Mosheim/Five Oaks tidal-flat fades and the overlying adjacent offshore subtidal marine facies do not interfere, and the contact between the two facies signifies the final flooding of the tidal flat due to transgression by the offshore marine environment. The upper surface of the tidal-flat facies is typically sharp and erosional, and exhibits evidence of subaerial exposure, early lithification, and wave scour, and probably formed through a combination of subaerial (karst) dissolution and tidal erosion. Despite evidence of vadose alteration and erosion, paleontologic data indicate that this surface represents only a brief break in deposition, and is not a major disconformity. This erosional surface is not a depositional sequence boundary, because the vertical sequence of facies predicted by Walther's Law for transgressive superpositioning of adjacent peritidal facies is not disrupted at the surface. Also, the development of the surface is diachronous across strike belts, and therefore cannot be the result of eustatic sea level fall, but is merely the result of transgression of the subtidal offshore environment over the tidal flat. An analogous erosional surface is presently forming without sea level fall in a very similar stratigraphic setting on the northwest side of Andros Island as a result of transgressive migration of peritidal environments. Diagenetic alteration of the tidal flat facies is overwhelmingly dominated by features that indicate alteration by meteoric water in unsaturated (vadose) and saturated (phreatic) zones. Subaerial exposure and vadose alteration is indicated by several features, including: (1) abundant fenestral fabrics and rare shrinkage cracks; (2) progressive alteration of fenestral fabrics to diagenetic grainstone/pisolite fabrics; (3) exposure surfaces and thin bleached zones within and at the top of supra- and intertidal lithologies; (4) localized zones of incipient caliche development, typically just below exposure surfaces; (5) pendant and meniscus cement distributions; and (6) geopetal crystal silt in fenestrae and vugs. Additional evidence of meteoric diagenesis that may have begun in the vadose zone and continued in the phreatic realm includes dissolution features such as fossil molds and solution-enlarged molds and vugs; stabilization of lime mud to micrite; and the ubiquitous presence of clear, non-ferroan, compositionally-zoned, equant calcite cement. Stable oxygen isotope ratios for these cements show depletion in 18O relative to Ordovician seawater values. Virtually all porosity is occluded by equant calcite cement; other types of cement and other authigenic minerals are volumetrically negligible. Dolomite within the tidal-flat sequence is uncommon, and generally occurs as isolated rhombs disseminated widely through the muddy matrix. As a rule, dolomite abundance decreases upward within the tidal flat fades, and the only pervasively dolomitized lithologies occur within a meter of the base of the facies, where magnesium was available from several sources, including mobilization of Mg++ from the underlying Knox dolostones and movement of burial fluids along the Knox unconformity. Above the basal strata, dolomite abundance is related to depositional subenvironment, and is most abundant in burrowed, subtidal lithologies and least abundant in supratidal fenestral lithologies. Dolomitization of subtidal pond sediment may have occurred by interaction with mixed meteoric / marine fluids in the coastal mixing zone. Most diagenetic alteration within the tidal-flat facies occurred very early, beginning immediately upon deposition and being largely concluded before deep burial. Most diagenesis occurred in the tidal-flat depositional environment. The upper surface of the tidal-flat facies, and likely other subaerially exposed horizons within the facies, were at least partially lithified prior to deposition of the overlying strata. Evidence for early diagenetic alteration and lithification of inter- and supratidal lithologies occurs in the form of 1) paleokarst features on the upper erosion surface; 2) truncation of cements at erosional surfaces; 3) borings and vugs extending downward from erosion surfaces; and 4) encrustation of the upper erosion surface by calcareous marine organisms. The timing of diagenetic alteration of subtidal lithologies is less well constrained, but probably was largely completed during shallow burial in the phreatic environment. Hydrologically, the setting in which deposition and early diagenesis occurred was a zone of mixing of seawater and freshwater in an unconfined coastal aquifer. The composition of the fluids that covered this tidal flat and saturated the sediment beneath the water table (and mean sea level) probably fluctuated between meteoric and normal marine end members, but the diagenetic features and geochemical data indicate that the predominant water chemistry was probably closer in composition to freshwater than to seawater.

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