Studies of reefs and proximal facies in the Pleistocene of East Africa, and in the Tertiary and Miocene of Bahrain yield predictable sequences of coral sediment distribution patterns. These can be sub-divided into micro-facies ranging from the supratidal to at least -33 m. Many of these suites are comparable with facies found today around Heron Island, on the Australian Great Barrier Reef, upon which a series of facies and fabric models are reconstructed. In depths of less than -15 m five main types of facies-linked preservational styles of corals are recognised and their chemical properties are analysed experimentally: (1) essentially unaltered; (2) red-algal encrusted; (3) moldic rhodoliths after sponge borings; (4) intensely endolithically bored intertidal debris; and (5) sterile, internally zoned, leached and abraded skeletons from the strand-line. These represent a linear sequence from off-shore to the storm ridge, which are modelled into transgressive and regressive cycles. The probability of their preservation and the relative proportion of their products, from any one time interval, increase in a landward direction. Scleractinian standards are established by tracing six taxonomically distinguishable groups represented amongst Astrocoeniina, Fungiina and Faviina in the quasi-fossil strandline shingle to their origins. Each coral or coral-fragment acquires a distinct suite of epibionts on the host's death, and undergoes characteristic chemical alteration related to its micro-environment. Consequently, the same genus develops various modifications reflecting its history of postmortem transport prior to ultimate burial. Thus, the chemical integrity of the skeleton never remains completely intact, as those specimens that are not subject to encrustation and the influences of endo- and epilithic biota are the more susceptible to diagenetic exchange with percolating pore waters. Preferred pathways for interstitial trace-element enrichment are detected by means of dosing the sea-water with uranium: these are best developed at skeletal margins and along trabecular axes in sterilised specimens. The applicability of these data to bore-hole records is tested by reference to the Heron Island core log. While the implications for the analysis of diagenetically advanced material is demonstrated by Jurassic and Carboniferous examples, these data have profound implications for assessing the reliability of isotope geochemistry and radiometric dating.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes