Carbonate sedimentology of the early Precambrian Hamersley Group of Western Australia


The early Precambrian Hamersley Group of Western Australia contains both major and minor occurrences of well-preserved carbonate sedimentary rock. The Carawine Dolomite and the middle or Paraburdoo Member of the Wittenoom Formation are the two most extensive occurrences. The Carawine Dolomite is restricted to the eastern part of the Hamersley Basin and contains abundant stromatolites, oncolites, and wave ripples, as well as local occurrences of evaporite crystal pseudomorphs and oolitic to pisolitic textures. These carbonate strata were deposited in a shallow-water paleoenvironment herein referred to as the Carawine Platform. In contrast, the Wittenoom Formation is restricted to the centraland western parts of the Hamersley Basin, and such shallow-water features are entirely absent from its carbonate strata. These strata consist largely of thinly laminated lutite, but also include some thin carbonate turbidites. The carbonate sedimentary rocks of the Wittenoom Formation, as well as some in the Carawine Dolomite which display similar characteristics, are therefore interpreted as sediments that were deposited off-platform in deeper-water paleoenvironments. Paleocurrent, thickness, and grain size trends of the carbonate turbidites indicate they were deposited by paleoflows moving south and west. Even though the Carawine Dolomite lies to the northeast of the Wittenoom Formation, stratigraphic correlations using a newly recognized marker bed of probable impact origin suggest that the carbonate strata of the Carawine Dolomite are younger than the Paraburdoo Member of the Wittenoom Formation. If true, the Carawine Platform could not have been a source of sediment for most of the carbonate sedimentary rocks in the Wittenoom Formation, but it may havebeen the sediment source for similar carbonate turbidites in younger formations such as the Mt. McRae Shale and the Dales Gorge Member of the Brockman Iron Formation. This, in turn, suggests that carbonate sediments were accumulating in the shallower parts of the Hamersley Basin while some of the BIFs were being deposited simultaneously in the deeper parts.



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Precambrian Research



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