Evolution of three unconformity-bounded sandy carbonate successions in the McArthur River region of northern Australia: the Lawn, Wide and Doom Supersequences in a proximal part of the Isa Superbasin

Gamma-ray curves from surface outcrops together with U-Pb SHRIMP zircon dating are used to redefine the evolution of a Palaeoproterozoic sandy dolostone succession from northern Australia. This case history indicates that gamma-ray logging of surface sections should accompany lithostratigraphic logging or an inadequate interpretation of stratigiaphic evolution is a likely outcome. The 1200 m-thick Nathan Group from the McArthur River area had previously been interpreted as a more-or-less continuous package of carbonates deposited in lacustrine and associated shallow-water environments. Now it is seen to comprise the preserved remnants of three truncated, second-order supersequences-the Lawn, Wide and Doom Supersequences-each a few hundred metres thick and each deposited over a time period of a few million years. These supersequences are separated by major stratigraphic breaks each approaching probably 10 million years duration. Each super sequence comprises several third-order sequences which themselves contain higher-order cycles. These were deposited in a series of continental, shoreline, and inner to outer carbonate platform environments. Transgressive, high-energy, fluvial to marginal marine, mixed elastic-carbonate facies dominate most of the sequences. The middle. Wide Supersequence, however, preserves deeper water (mostly sub-storm-wave-base) stromatolitic facies in one sequence, and storm-reworked clastics in another. These are interpreted as condensed intervals deposited around their respective maximum flooding surfaces and are succeeded by regressive facies that probably represent high-stand systems tracts. New correlations between these 1615-1575 Ma sandy carbonate successions of the McArthur Basin (Amos. Balbirini and Dungaminnie Formations) and time-equivalent largely elastic successions in the Lawn Hill area (Lawn Hill and Doomadgee Formations), some 400 km to the southeast, are proposed.