Masters Theses

Date of Award

8-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Richard W. Arnseth, Thomas W. Broadhead

Abstract

The Holston and Rockdell formations (Middle Ordovician) in northeastern Tennessee represent two lithologically similar, largely coeval carbonate bu5_ldup complexes which were deposited at different positions on a carbonate shelf. The Rockdell was deposited in a stable platform setting while the Holston Formation was deposited at the shelf margin (Ruppel and Walker, 1982). Each formation may be divided into three intergenerational lithofacies. The Rockdell consists of: (1) bryozoan-echinoderm grainstone-packstones; (2) bryozoan-echinoderm packstones; and (3) bryozoan wakestones. The Rockdell bryozoan echinoderm packstone lithofacies occurs predominantly in the northwestern part of the study area whereas bryozoan-echinoderm grainstone packstone and bryozoan wakestones are best developed in the central and southeastern parts. The Holston consists of: (1) echinoderm bryozoan grainitone-packstones; (2) bryozoan wackestone-boundstones (lime-mud mounds); and (3) echinoderm-bryozoan packs tones. Rocks of the Holston Formation are limited to the southeastern part of the study area. Petrographic · and cathodoluminescence analyses allow distinction of eight cement types in the Holston and Rockdell formations: (1) radiaxial-fibrous calcite and associated cement; (2) bladed/prismatic cement; '(3) turbid (inclusion-rich) syntaxial (rim) cement; (4) cryptocrystalline (micrite) cement; (5) equant (drusy) calcite; (6) clear syntaxial (rim) cement; (7) ferroan calcite; and (8) ferroan (baroque) dolomite. Evidence suggests that radiaxial-fibrous calcite and associated cement, bladed/prismatic cement, turbid syntaxial cement, and cryptocrystalline (micrite) cement were precipitated early, in the marine diagenetic environment. The lines of evidence include: (1) high levels of trace Sr and Mg in radiaxial-fibrous calcite; (2) cement development both before and after deposition of internal marine sediment; (3) superposition within pores and cavlties (cement stratigraphy); (4) microdolomite inclusions within cement crystals (suggesting a magnesian calcite precursor); and (5) comparison of cement textures, fabrics and occurrences with similar, previously studied examples. Equant (drusy) calcite and clear syntaxial cement are interpreted as meteoric in origin because they: (1) always postdate marine cement; (2) occur as pore central intrablotic cement (equant calcite); (3) are devoid of inclusions (suggesting they were deposited as low-magnesian calcite); and (4) are similar to other previously described meteoric cement. Ferroan calcite (including the ferroan part of clear syntaxial cement) and ferroan (baroque) dolomite formed after burial under reducing conditions in the stagnant meteoric phreatic zone or much later after deep burial. A burial origin is supported by the elevated temperature of formation of baroque dolomite (60°-150°C) and the mutual occurrence of these cement phases. Marine cement and burial cement are best developed at the shelf margin (Red House Branch and Poor Valley Creek) and, in samples from these locales, represent the predominant cementing phases. Meteoric cement is best developed at shelfward localities of the Rockdell (Cedar Springs and Evans Ferry) where marine cement is much less abundant. Thus, two inverse cement gradients exist; marine and burial cement abundances increase basinward whereas meteoric cement abundance decreases basinward. Depositional units at each of the study localities were dominated by either marine, meteoric, or burial cementation. Preliminary analyses of this pattern suggest that cement dominated units do not occur in random stacking order. Instead, the zones commonly occur in triplets. Each triplet consists of: (1) a lower marine cemented zone, followed by (2) a middle meteoric zone, which is capped by (3) a zone dominated by burial cement. Some cement zones may be correlative between shelf marginal and on-shelf localities. Although their timing overlaps, the paragenetic sequence of marine cement is: (1) microcrystalline (micrite) cement; (2) bladed/prismatic cement; and (3) turbid syntaxial (rim) cement and radiaxial-fibrous calcite and associated cement. The cementation history of the Holston and Rockdell formations largely controlled other diagenetic processes. Diagenetic processes which are coeval with marine cementation, in order of earliest occurrence are: (1) macro boring and micritization of skeletal grains; (2) stromatactoid and irregular cavity formation; (3) internal sedimentation, and meteoric diagenetic environment, (4) mechanical equant (drusy) compaction. In calcite cement the and clear syntaxial (rim) cement are coeval. Diagenetic processes accompanying these cement phases, in order of their earliest occurrence, are: (1) mineralogic stabilization (magnesian calcite to low-magnesian calcite); (2) dolomitization; (3) solution of aragonite and neomorphism; and (4) silicification (replacement by chalcedony and authigenic quartz). In the burial diagenetic environment, ferroan calcite (including ferroan zones of clear syntaxial cement) and ferroan (baroque) dolomite formed simultaneously. Other diagenetic processes which developed after burial, in temporal order, were: (1) fracturing; (2) silicification (replacement by chalcedony and authigenic quartz); and (3) chemical compaction.

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