REVISED CORRELATIONS IN THE EARLY PRECAMBRIAN HAMERSLEY BASIN BASED ON A HORIZON OF RESEDIMENTED IMPACT SPHERULES

The Early Precambrian Hamersley Group of Western Australia contains tw o thick packages of carbonate strata: the Paraburdoo Member of the Wit tenoom Formation and the Carawine Dolomite. The Carawine Dolomite has long been viewed as a shallow-water stratigraphic equivalent of the de eper water Wittenoom Formation because the two units occupy mutually e xclusive areas within the Hamersley Basin and the former contains some platformal deposits whereas the latter consists exclusively of basina l deposits. The Carawine Dolomite and the Wittenoom Formation each con tain a single layer partially composed of distinctive sand-size spheru les that have the characteristics of microkrystites (silicate melt dro plets ejected during large bolide impacts). In the Carawine Dolomite, the microkrystites are restricted to a dolomitic debris-flow deposit ( the dolomixtite) which is 24.7 m thick and located similar to 60 m abo ve the basal contact. In the Wittenoom Formation, the microkrystites a re restricted to a low-density turbidite known as the spherule marker bed which is <1.3 m thick and similar to 100 m above the top of the Pa raburdoo Member stratigraphically. A core drilled halfway between the closest surface exposures of the dolomixtite and the spherule marker b ed contains microkrystites in a single layer. That layer is a high-den sity turbidite 2.8 m thick located similar to 40 m below the base of t he Carawine Dolomite. We interpret all three of these microkrystite-be aring layers as part of a single bed that was deposited nearly instant aneously throughout most of the Hamersley Basin by a large sediment gr avity flow. If our interpretation is correct, it implies the basal con tacts of both the Carawine Dolomite and the Hamersley Group as a whole are time-transgressive. It also means significant thicknesses of plat formal carbonates were present elsewhere in the Hamersley Basin (proba bly on the northern Pilbara Craton) and opens the possibility that the Carawine Dolomite was deposited at the same time as some of the major banded iron-formations of the Hamersley Group.