ABSTRACT: Platform Facies of the 2.5 Ga Carawine Dolomite, Hamersley Basin, Western Australia

Bruce M. Simonson, Kathryn A. Schubel

The 2.5 Ga Carawine Dolomite of the Hamersley Group in Western Australia is among the oldest formations on earth deposited on a carbonate platform. Its primary sedimentary features are diverse and well preserved, indicating deposition via a variety of processes. LLH-type stromatolites, flat pebbles with oncolitic coatings, and strata with fabrics reminiscent of tufas in younger carbonates are all abundant, indicating that both sediment-binding and carbonate-precipitating microbial mats were probably responsible for most of the carbonate deposition. Included among these stromatolitic strata are composite domal structures with minimum diameters of 4.4 m and minimum heights of 1.2 m. Thin layers of grainstone, some with wave ripples, others rich in oolites with both

radial and concentric textures, are interbedded with the stromatolitic strata, attesting to the shallow depth of water. Some deposition in the vadose zone is suggested by a few pisolitic layers with reverse grading. Intermittent precipitation of evaporites may have also occurred, as evidenced by scattered occurrences of pseudomorphs that could have originally been aragonite, gypsum, and/or halite crystals. Some of these pseudomorphed crystals are up to 20 cm long and take the form of sprays that grew directly on, as well as close beneath, the sea floor. In contrast to these platformal rocks, the remainder of the strata in the Hamersley Group are basinal in character. Basinal and platformal strata are juxtaposed both laterally and vertically, but more work is needed to determine whethe the platform-to-basin transition was abrupt (e.g., a rimmed shelf) or more gradual (i.e., a ramp) and how it evolved through time.

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