ABSTRACT: Cenozoic Cool-Water Limestones, Eucla Platform, Southern Australia

Noel P. James, Yvonne Bone

Evidence is accumulating that modern and Cenozoic cool-water (temperate water) carbonate sediments may be the best facies analogs for many open-shelf, middle to late Paleozoic carbonates; yet there are comparatively few studies of such deposits. One important example is the extensive Eucla platform, a 350,000-km² Eocene to Miocene shelf that caps the southern Australian miogeocline. Only the inner part, which lies beneath the Nullarbor Plain, has been examined in any detail.

Sediments are of the bryomol assemblage--mainly bryozoan, echinoid, mollusk, and foraminifera remains with local concentrations of brachiopods. Most deposits formed by the spontaneous postmortem disintegration of erect, flexible cellariiform cheilostome and crisiform cyclostome bryozoans, cool-water analogs of codiacean algae. Facies range from incipiently drowned deep-shelf muddy sediments to ubiquitous open-shelf skeletal wackestones and packstones to local shallow-water, high-energy sand shoals. Because of accumulation rates an order of magnitude less than tropical shelf carbollates, eustasy is expressed as hardgrounds and karst surfaces; there are no muddy tidal flats.

The cool-water platform sequence is capped by warmer water facies rich in aragonitic mollusks and calcareous algae with local concentrations of hermatypic corals. This change, which takes place across a bedding plane and reflects a shift in oceanic circulation patterns, highlights the fact that subtle changes in water temperature can result in the formation of dramatically different carbonate facies.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990