Masters Theses

Date of Award

12-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Kula Misra, Tom Broadhead, R. E. McLaughlin

Abstract

The Middle Ordovician Ottosee Formation outcrops in eastern Tennessee as a dominantly terrigenous-clastic unit composed of locally fossiliferous wackestones and mudstones. Isolated lenses of fairly pure limestone occur within this formation. The present study is a detailed petrographic and geochemical analysis of the environments of deposition and diagenetic history of one such lens, an oolite.

On overview, the oolite is shown to be oriented with its long axis parallel to depositional strike, and is completely surrounded by Ottosee muds. Also, the zone of occurrence of oolitic lithologies lies fairly close to the Ordovician shelf-edge, similar to modern marine sand belts.

The biota of the oolite is diverse and, in places, abundant. Ectoprocts and echinoderm fragments are the most important faunal groups; the predominance of these 2 groups implies that they were endemic to at least portions of the oolite. Other taxa present include Arthropoda, Annelida, Brachiopoda, Mollusca, Porifera, as well as various categories of algae, including Chlorophycophyta, Rhodophycophyta, Schizophyta, and probably Cyanophycophyta.

Sedimentary structures (especially cross-stratified beds) of all types are commonly observed in the oolite. Plane-bedding is evident on the bankward portion of the shoal, while cross-beds that are often bidirectional in orientation can be observed in the more basinward portions. Intraclasts are sometimes noted, as are fossil grains with mud-filled interiors or adhering mud. Silt perched on top of elongate allochems is also a common feature.

Four microfacies or lithotypes are distinguishable within the oolite complex, and are defined on the basis of their relative proportions of ooids, fossil allochems and terrigenous grains. Lithotype 1 is a very quartz-rich oolite with few, small ooids and a sparse biota. Lithotype 2 is a coarse-grained oolite that contains large, well-developed ooids and virtually no fossil allochems. Lithotype 3 is a very fossiliferous, poorly sorted oolite that contains both large and small ooids; this lithotype is the only one to contain an appreciable amount of mud. Finally, lithotype 4 is a relatively pure oolite that contains small ooids and few fossils.

By analogy with sediments currently being deposited in Recent oolitic environments, each of the lithotypes of the Ottosee oolite corresponds to a subenvironment within the oolite shoal. Lithotype 1, the quartz-rich oolite, represents a subtidal sand wave that probably provided topographic relief on the paleoshelf, and allowed ooid—forming environments to become established in this area of higher ^^ve and current agitation. Lithotype 2 corresponds to the mobile fringe environment on the basinward side of the oolite. Lithotype 3 corresponds to anastomosing tidal channels that cross-cut other subenvironments within the shoal. Finally, lithotype 4 corresponds with the bankward, stabilized oolite, where the sediment in Recent shoals is commonly bound by algae or sea grass.

Several aspects of diagenesis were investigated here, including cement types and their "paragenesis." While some variation occurs in these cement sequences (relating to lithotype), at least 2 zones of cement are observed: (1) an isopachous, bladed calcite cement, representing early marine diagenesis and (2) a blocky equigranular calcite cement representing later diagenesis in a freshwater phreatic environment.

Furthermore, these cements do not occur randomly throughout the oolite; in fact, the distribution of cements is strongly dependent on lithotype. In lithotype 2 and, to some extent, lithotype 3 extensive amounts of early cements formed; lithotypes 1 and 4 contain very little early cement, however, and apparently survived until a later stage of diagenesis before much porosity was occluded. By implication, then, it is concluded that the depositional environment was a major control of the diagenetic path taken by this sediment.

Geochemical information from the oolite seems to imply that extensive equilibration to meteoric diagenetic conditions has occurred; yet, some trace element distribution patterns have been preserved, and appear to be related to lithotype. Thus, the controlling factor influencing the final geochemical signature of the rock seems to be the primary mineralogy of the sediment. Also, very low Na values suggest that the environmental paleosalinities of the oolite were normal marine to hyposaline.

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