Date of Award:

5-1992

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

W. David Liddell

Committee

W. David Liddell

Committee

Peter T. Kolesar

Committee

Robert Q. Oaks Jr.

Abstract

Holocene carbonate sediments from Mexican reefs in the Caribbean and Gulf of Mexico display variations in constituent composition, texture and mineralogy which are related to their locations on the reefs. Samples were collected at barrier reef environments at Akumal and Chemuyil, on the northeast coast of the Yucatan Peninsula; at the oceanic atoll of Chinchorro, off the southeast coast of the peninsula; and at the shelf atoll of Alacranes, in the Gulf of Mexico. Samples were collected through a depth range of 0-40 m, which encompasses back reef, shallow fore reef, and deeper fore reef environments.

Constituent-particle analysis of reef sediments indicates that lagoon facies are dominated by Halimeda with lesser amounts of coral and coralline algae. In contrast, fore reef facies are dominated by coral, with lesser amounts of Halimeda and coralline algae.

Greater than 90% of the sediments (dry weight) occur in the interval 0.125-2.00 mm, with mean grain sizes (Mz) approaching 0.5 mm at most sites. Mz generally decreases with increasing depth at three of the four sites. Reef sediments are moderately to poorly sorted and typically become more poorly sorted with increasing depth. Sediments collected from reef channels (grooves) are consistently better sorted than those from the reef interstices (spurs).

Mineralogically, the sediment is predominantly aragonite (63-93%) and high-Mg calcite (3-33%), with minor amounts of low-Mg calcite (3-9%). The sediment is chiefly composed of these three polymorphs of CaC03 (96.32- 99.83%) with only a small percentage of insoluble (non-carbonate) material. The non-carbonate fraction of the sediment is dominated by organics (0.14-3.16%) with lesser amounts of clay minerals and amorphous silica (0.00-0.66%).

Quantitative analysis of Mexican reef sediments in the Caribbean and Gulf of Mexico allows the delineation of reef zonations based on constituent-particle composition. Q-mode cluster analysis of constituent-particle data from epireefal sediments enables the separation of lagoon, shallow fore reef ( > 10-15 m) and deep reef ( > 10-15 m) lithofacies. These groupings were maintained even when constituent-particle data from the barrier reefs were combined, and also when data from the atoll reefs were combined. Similar cluster groupings were obtained when constituent-particle data from a Jamaican fringing reef were combined with data from the Mexican barrier reefs. This indicates that coastal reef sediments from Jamaica and Mexico are surprisingly similar in constituent-particle composition and facies zonations. However, cluster analysis failed to produce clear associations when constituent-particle data from barrier reefs and atoll reefs were combined. The sedimentological zonations observed in the study reflect the community composition of the living reefs, and indicate that community composition, at least for calcifying organisms, is potentially preservable.

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