Masters Theses

Date of Award

6-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Kula Misra, Otto C. Kopp, Thomas W. Broadhead

Abstract

The Holston Limestone (Middle-Ordovician) of East Tennessee is composed of two dominant lithologies: bryozoan-micrite boundstone and flanking grainstones of ramose bryozoans and echinoderms. The Holston occupied a shelf-edge position. Three statigraphic sections were studied which ranged from off-shelf to fully on-shelf in their original environmental locations. These were chosen to examine the effect of the relation of each outcrop to the shelf-edge on post-depositional alteration.

Diagenetic studies made use of staining, cathodoluminesence and petrographic examination to determine the paragenetic sequence of cements. Allochems may be cemented initially and then broken by compactional forces, may be broken first and then cemented or, least commonly, aragonitic allochems may develop micrite envelopes followed by cementation and/or fracturing.

Paragenetic sequences indicate the following events within the marine, phreatic diagenetic environment; (1) precipitation of fringe cement (2) breakage by compaction (3) deposition of echinoderm syntaxial overgrowths (4) precipitation of isopachous radial-fibrous cement (probable predecessor to radiaxial cement) (5) stabilization of high-magnesium and aragonitic allochems. Events within the meteoric, phreatic diagenetic environment include (6) transformation of radial-fibrous to radiaxial cement (7) deposition of internal sediment and (8) precipitation of drusy sparite. The products of all of the above events are not always present and some events may have been coeval in part in the sequence. Due to the early occlusion of porosity by various cements, lithification occurred with little compaction and the resulting displacement of allochems is very minor.

Comparison of the relative abundance of cement types across the three outcrops indicates that syntaxial overgrowths and radiaxial cement are the most prevalent. Echinoderms are the most common sub-strate for syntaxial overgrowths and radiaxial cement is most commonly observed as isopachous rims around bryozoans. Variations in the amounts of these two cement types depend on the relative abundance of echino-derms and bryozoans. Neither fringe nor drusy sparite are volumetrically important. No trends concerning the relative abundance of cement types were observed across the transect from most basinward to the on-shelf localities.

Facies and allochem mineralogy are important in controlling trace and minor element distribution but atomic absorption data suggest that post-depositional alteration by meteoric waters was also an important factor. Low strontium values indicate few originally aragonitic allochems and/or post-depositional removal of strontium. Sodium values agree with previously published data from other ancient carbonate rock sequences that suggest normal, open marine conditions. Magnesium values vary but are generally quite low. Variations in magnesium may be due to differences in original allochem mineralogy or to unexplained changes within the diagenetic solutions as they passed through the pore spaces. No obvious trends in the concentrations of these elements are present from the most basinward to the on-shelf localities.

Manganese and iron values already present in the depositional environment have been enhanced as equilibration to a meteoric diagenetic environment has proceeded. These elements are potentially useful as paleobathymetric indicators, with highest values observed at Midway Road (most basinward of the three sections) and lower values at Oglesby Road and Alcoa Highway (on-shelf localities).

Comparison of chemical values across a bryozoan-micrite bound-stone reef exposed in the upper part of the Alcoa Highway section show little variation in any of the elements that can be directly attributed to the presence of the reef.

Some allochem and cement components were isolated and run as separate samples from the surrounding matrix. This was done to compare the homogenization or lack thereof, between components and matrix. Enrichment of the bryozoan allochems to matrix was less than two times. Radiaxial spar-rich material showed very similar values to that of the matrix, while a drusy spar sample showed depletion of strontium, sodium and magnesium. The drusy spar sample was enriched in manganese and iron when compared to its matrix.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS