Mud cracks and dedolomitization in the Wittenoom Dolomite, Hamersley Group, Western Australia

Several impure dolomitic limestone beds in an outcrop of the latest Archean Wittenoom Dolomite (Hamersley Group, Western Australia) are polygonally cracked. The cracks appear to be sub-aerial desiccation features, suggesting that the known area of shallow water and locally emergent conditions extended from the far eastern part of the basin (the Carawine Dolomite) over 270 km farther west. This finding places shallow-water or emergent conditions either (1) near the middle of what Trendall (1983) defined as the probable original limits of the Hamersley Basin (Trendallʹs “Pilbara Egg”) or (2) near the southern edge of what Morris (1993) thought to be a broad carbonate platform which fed a deeper water sequence to the south. In any case, the Hamersley Basin in the area of Bee Gorge and eastward to the Carawine Dolomite may have been a carbonate mudflat in part with restricted circulation of sea water. The Carawine Dolomite and the Wittenoom Dolomite near Bee Gorge may have been affected by carbonate buildups along a shelf edge. Regardless of whether shallow water was widespread or local in the Hamersley basin, shallow water verging on emergence is supported by evidence of diagenetic dedolomitization under conditions of low atmospheric and hydrospheric PO2 and precipitation of strontianite in the mud-cracked sample. Evidence of shallow water at Bee Gorge is consistent with Trendallʹs broad evaporite-basin model and with Morrisʹ barred-platform model for the origin of Hamersley carbonates and banded iron-formations.