Date of Award

12-1976

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

Professor Michael J. Risk

Abstract

Endolithic (boring) and chasmolithic (cavity-dwelling) marine filamentous algae are important erosive and diagenetic agents in modern carbonate environments. The rates at which endolithic algae bore into crystals of Iceland spar calcite and other substrates are geologically very rapid. Within 213 days the surface of Iceland spar monitored in the sea in Discovery Bay Jamaica is found completely infested by the algae. The curve of infestation versus time is similar to a normal population growth curve.

Variations in environmental conditions, notably light and agitation of the substrate, are important controls on rate of infestation, though agitation may not be quite as important as previously thought. The amount and composition of light available to the algae affects their activity, to the point where the algae apparently will not bore in the absence of light (this includes at depth within the sediment). This indicates that most endolithic algal infestation occurs at, or very close to, the sediment surface; as a result endolithic algae may be of value in the determination of paleophotic zones in ancient carbonate sequences.

Substrate crystallography exercises strong control over filament orientation during initial infestation; this control breaks down as infestation of the substrate proceeds.

Several processes of micritization are observed on the Iceland spar crystals, including partial boring infilling, residue micritization, and exteriour micrite envelope formation. The micrite envelopes form on the surface of the crystal by the coalescence of cemented (calcified) exposed endolithic or chasmolithic filaments. The low Mg calcite precipitated on the filaments is deposited only on dead algae and is found as early as 65 to 95 days after the crystals are placed in the sea. The broken filaments are a significant source of micrite and peloidal sediment in the reef and nearby environments. "Constructive" micrite envelopes and calcified algal filaments are found throughout the Phanerozoic as far as the Ordovician.

Constructive micrite envelopes will not likely develop in highly agitated environments where grains are constantly moved about, unless periods of quiescence in the order of weeks or months occur. During quiet water periods endolithic algae growing out of grains or micrite envelopes, or chasmolithic algae, may become calcified in intergranular pores by precipitated calcite. These intergranular calcified filaments, found as far back as the Ordovician, stabilize the sediment and may prevent remobilization. This may allow further filament calcification, micrite envelope formation, and complete to partial infilling of intergranular pores by calcified filaments and associated micrite and microspar cement.

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