Date of Award:

1985

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

W. David Liddell

Abstract

Recent carbonate sediments from Jamaican north coast fringing reefs display variation in constituent composition, texture, and mineralogy related to their location on the reef. Samples were collected along lines which traversed the back reef and fore reef (0.5m to 70m).

The sediment is dominated by highly comminuted coral fragments, plates of the calcareous green alga, Halimeda, coralline algae, and the encrusting Foraminifera, Homotrema rubrum, with lesser amounts of other taxonomic groups (Foraminifera; molluscs; echinoderms). Relative abundances of these biotic components vary between sites. Q-mode cluster analysis indicates that constituent composition can be used to delineate the different reef zones which have been described from analyses of the macrofauna.

For most sediment, grain-size frequency distributions indicate that greater than 90% (by weight) of the sample is contained in the interval of 0.125mm to 2.000mm. Mean grain size approaches 0.5mm for most sites with little depth related variation. Sorting, however, becomes progressively poorer from the shallow (5m) fore reef terrace to the upper deep fore reef (70m).

X-ray diffraction and insoluble residue analyses indicate that total CaC03 in these sediments is generally greater than 95% by weight. Aragonite is the most abundant carbonate phase, followed by high-Mg calcite , and low-Mg cal cite. Amorphous siliceous sponge spicules and organic matter comprise the remaining non-carbonate fraction of these sands. Significant differences in the proportions of aragonite and high-Mg calcite between fore reef terrace sediments and sediments from other reef zones results from the contribution of high-Mg calcite to fore reef terrace sediments by coralline algae, Foraminifera (principally Homotrema rubrum) and echinoderms, which are relatively less abundant sediment constituents elsewhere on the reef.

The 120,000y.b.p.(Sangamon) Falmouth Formation along the north coast of Jamaica displays variability in sedimentological and faunal components analogous to that of back reef and shallow fore reef environments of the modern Jamaican fringing reef system.

X-ray analysis of the mineralogy of Falmouth limestones reveals that surface exposures of fore reef grainstones exhibit greater diagenetic alteration than surface exposures of back reef packstones. This indicates variability in diagenetic processes most likely related to original sediment textural characteristics.

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