THIN EOLIANITES INTERBEDDED WITHIN A FLUVIAL AND MARINE SUCCESSION - EARLY PROTEROZOIC WHITWORTH FORMATION, MOUNT ISA INLIER, AUSTRALIA

The early Proterozoic Whitworth Formation in the Mount Isa Inlier, Aus tralia contains the oldest known, thinly bedded eolian dune and amalga mated interdune deposits, and records the complex interplay of eolian, fluvial and marginal-marine depositional processes. The Whitworth For mation accumulated during the waning phases of rifting in a north-sout h trending trough that began between 1808 and 1790 Ma and ended before 1740 Ma. Three eolian facies are recognized: (1) cosets of cross stra ta, (2) sets of horizontal to low-angle strata, and (3) isolated sets of cross strata. Facies E1 records preservation of dunes with low angl es of climb without intervening interdune deposits. Sets of facies E2 are interpreted as either sand sheet, preserved dune plinths or amalga mated interdune deposits. Facies E3 is encased in facies E2 and consis ts of lenses of cross strata with irregular tops. This association of facies is comparable to recent gypsum dune and interdune deposits at W hite Sands, New Mexico. Facies E3 reflects limited dune preservation d uring migration and accumulation at or near the groundwater table. Ver tical change from amalgamated interdune deposits to dune without inter dune deposits records dunefield initiation or lateral migration from a peripheral to a more centralized position within the dunefield. Incre ased sedimentation rates promoted preservation of dune deposits repres ented by facies E1. Decreased sedimentation rate and possible limited sediment bypassing resulted in amalgamation of interdune deposits on t he dunefield periphery. Eolian facies E1 and E2 are interbedded with e phemeral-river deposits. Three ephemeral-river facies are recognized: (1) cosets of horizontal strata; 2) cosets of horizontal strata and ov erlying trough cross beds; and (3) cosets of medium-scale trough cross beds. Ephemeral-river facies F3 often are preserved in channels incis ed into eolian facies E3. Associated marginal-marine facies consist of : (1) thin-bedded arenites of intertidal to supratidal origin, and (2) trough and tabular planar cross-bed sets and cosets that resulted fro m migration of subaqueous dunes within a tide-dominated subtidal setti ng. Interbedded fluvial and eolian deposits reflect either autogenic p rocesses, such as reworking of fluvial sands by eolian processes, or i ncursion of a fluvial system into a dunefield related to allogenic pro cesses such as climatic change. Dune deposits in the Whitworth Formati on probably developed under arid to semi-arid conditions as indicated by extensive ephemeral-river deposits, presence of evaporite pseudomor phs, and absence of wet interdune deposits. Interbedded eolian, fluvia l, and marginal-marine deposits resulted from relative sea-level fluct uations related to a combination of eustasy and intrabasinal tectonics .