MUD CRACKS AND DEDOLOMITIZATION IN THE WITTENOOM DOLOMITE, HAMERSLEY GROUP, WESTERN-AUSTRALIA

Several impure dolomitic limestone beds in an outcrop of the latest Ar chean Wittenoom Dolomite (Hamersley Group, Western Australia) are poly gonally cracked, The cracks appear to be sub-aerial desiccation featur es, suggesting that the known area of shallow water and locally emerge nt conditions extended from the far eastern part of the basin (the Car awine Dolomite) over 270 km farther west. This finding places shallow- water or emergent conditions either(1) near the middle of what Trendal l (1983) defined as the probable original limits of the Hamersley Basi n (Trendall's ''Pilbara Egg'') or (2) near the southern edge of what M orris (1993) thought to be a broad carbonate platform which fed a deep er water sequence to the south, In any case, the Hamersley Basin in th e area of Bee Gorge and eastward to the Carawine Dolomite may have bee n 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, Regardles s of whether shallow water was widespread or local in the Hamersley ba sin, shallow water verging on emergence is supported by evidence of di agenetic dedolomitization under conditions of low atmospheric and hydr ospheric P-O2 and precipitation of strontianite in the mud-cracked sam ple, Evidence of shallow water at Bee Gorge is consistent with Trendal l's broad evaporite-basin model and with Morris' barred-platform model for the origin of Hamersley carbonates and banded iron-formations.