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 two thick packages of carbonate strata: the Paraburdoo Member of the Wittenoom Formation and the Carawine Dolomite. The Carawine Dolomite has long been viewed as a shallow-water stratigraphic equivalent of the deeper water Wittenoom Formation because the two units occupy mutually exclusive areas within the Hamersley Basin and the former contains some platformal deposits whereas the latter consists exclusively of basinal deposits. The Carawine Dolomite and the Wittenoom Formation each contain a single layer partially composed of distinctive sand-size spherules that have the characteristics of microkrystites (silicate melt droplets 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 ∼60 m above the basal contact. In the Wittenoom Formation, the microkrystites are restricted to a low-density turbidite known as the spherule marker bed which is \u3c 1.3 m thick and ∼100 m above the top of the Paraburdoo Member stratigraphically. A core drilled halfway between the closest surface exposures of the dolomixtite and the spherule marker bed contains microkrystites in a single layer. That layer is a high-density turbidite 2.8 m thick located ∼40 m below the base of the Carawine Dolomite. We interpret all three of these microkrystite-bearing layers as part of a single bed that was deposited nearly instantaneously throughout most of the Hamersley Basin by a large sediment gravity flow. If our interpretation is correct, it implies the basal contacts of both the Carawine Dolomite and the Hamersley Group as a whole are time-transgressive. It also means significant thicknesses of platformal carbonates were present elsewhere in the Hamersley Basin (probably 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