Datapages, Inc.Print this page

ABSTRACT: Quaternary Sedimentation and Diagenesis in a High-Latitude Reef, Houtman Abrolhos Islands, Western Australia

M. R. Rosen, L. B. Collins, K-H. Wyrwoll, B. G. Hatcher

The Houtman Abrolhos reefs are located 80 km off the west coast of Australia between latitudes 28 and 29° south. The islands are situated on three Pleistocene carbonate reef platforms which rise above the surrounding shelf. The modern coral reefs are close to the geographic limit for coral growth in the southern hemisphere and survive due to the presence of the Leeuwin Current (a poleward-flowing warm stream). Two major shallow-water benthic communities coexist in the Abrolhos: a macroalgal-dominated community on the windward platform margins and a coral-dominated community on the leeward margins. These communities overlap--particularly in the platform lagoons, where competition between macroalgae and corals is intense. This interaction has been suggested as a major actor controlling the growth of coral reefs at high latitudes.

The Holocene carbonate sediments lack nonskeletal components and are dominated by coral and coralline algal fragments with subordinate molluskan and echinoderm debris. The accumulations can be grouped into the following major facies: (1) coral framestone and coralline algal/serpulid boundstone, (2) submarine sand sheets, (3) subaerial coral storm ridges, (4) peritidal to subtidal shingle and rubble veneers composed of dominantly coral debris, and (5) eolian dunes and beach sand. The Holocene sediment is a thin (< 2 m) veneer on the Pleistocene reef platform, which is emergent as small islands. The Pleistocene platform is composed of reef facies that can be directly related to the Holocene sediments. The platform is composed of framestone and boundstone facies (corals and coralline lgal/serpulid facies), rudstones (submarine coral rubble facies), planar-bedded skeletal grainstones dipping 12-13° (submarine sand sheet and peritidal shingle facies), and large 15-m-high eolianite dunes (eolian dune facies). A generalized stratigraphy of the Pleistocene sequence consists of an upward-shallowing sequence passing from submarine reef framestone to peritidal/subtidal rudstones and detrital grainstones which are in places overlain by subaerial eolian dune grainstones. The Pleistocene dune sequence is capped by a thick calcrete which includes root zones, insect cocoons, and a breccia/pisolite zone.

The major diagenetic events associated with Pleistocene sea level fall are the formation of the calcrete, karstification of the platform (including the formation of 30-m-deep dolinelike holes in the reef platform), and the formation of phosphate-rich soil and calcrete zones which drape over and penetrate into the reef facies through solution pit. These diagenetic features have a direct influence on the recognition of major unconformity surfaces and are therefore important in developing a time-stratigraphic sequence.

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