Date of Award:

1981

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Richard R. Alexander

Abstract

Morphology of corals of the Late Ordovician through mid-Silurian Fish Haven and Laketown Formations were analyzed to document adaptations to inferred enviromental conditions, i.e., (bathymetry, illumination, sedimentation, currents, and energy), under which their enclosing sediment was deposited. Specimens and data were collected from six sites. Insoluble-residue tests were performed on the corals' matrices.

Individual corallites of radial-lensoidal corals radiate in all directions. Vertical growth, however, was restricted in turbulent conditions. Umbrellic, radial-lensoidal corals feature downward facing corallites, considered to be an adaptation to a well illuminated environment.

On tabular lensoidal corals, constituent corallites faces exclusively upward, an orientation needed under relatively poor illumination. Orientation of these coralla at oblique angles to bedding planes and to other coralla signify soft-sediment slumping or edge-first sinking into a semi-fluid substrate.

Hemispherical coralla prevailed under moderately high energy condition and negligible sedimentation. Conic coralla, formed during prodigious sediment accumulation, are extreme modifications of hemispherical coralla. The lower the rate of sediment accumulation, the less acute the angle of the apex (budding center of the 11cone").

Circumrotary corals, or those that are distinguished by corallites radiating in virtually all directions, are considered to have dwelled in heavy surf, by analogy to certain modern corals.

Composite corals, those that exhibit more than one morphotype or a repetition of the same morphotype, reflect a change in growth during the life of the colony.

Internal features of individual corallites were examined and measured. Most of the corals in this study feature corallites suggestive of low-sediment rejection capability. Corallite packing arrangements, as well as indicating cleansing ability, are interpreted as adaptations to different levels of hydraulic stress.

Several explanations are given to explain the lack of correlation between corallite variability and external morphology. Genetic differences may have resulted in differing variabilities of calical widths of different species under comparable environmental conditions (as indicated by intramorphotypic analyses of variance).

Differences in variability of tabulae spacing between tabular and radial coralla are attributed to the preponderance of lateral budding in the former mrophotype. Frequent budding is considered to have been concurrent with slow upward growth, reflected by narrow tabulae spacings.

A high energy fauna is represented by the abundance of tabulate and compound rugose, radial-lensoidal, and circumrotary coralla. These corals and their associated organisms in the conununity seemingly dwelled on a carbonate bank.

A favositid-dominated community provides evidence of variable sedimentation rates by the presence of diverse morphologies. Hemispherical coralla predominate indicating generally high energy and low to negligible sedimentation rates.

Two of the sites were dominated faunally by hemispherical, halysitid corals featuring cone-shaped bases. They are considered reflective of moderate energy and moderate sediment accumulation.

Tabular halysitid and favositid corals whose basal dimensions are oriented obliquely to one another and/or bedding planes signify negligible sedimentation and placid, relatively deep water conditions. These corals dominated two collecting sites.

Based on the patchiness of the occurrence of the coral assemblages, the Late Ordovician and Silurian sea floor is considered to have been an uneven, heterogeneous surface. Shallow water shelf conditions are inferred. Fluctuations in sea level elicited the observed variability in coral morphology.

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