Thin eolianites interbedded within a fluvial and marine succession: early proterozoic whitworth formation, mount isa inlier, australia

Abstract

The early Proterozoic Whitworth Formation in the Mount Isa Inlier, Australia contains the oldest known, thinly bedded eolian dune and amalgamated interdune deposits, and records the complex interplay of eolian, fluvial and marginal-marine depositional processes. The Whitworth Formation accumulated during the waning phases of rifting in a north-south trending trough that began between 1808 and 1790 Ma and ended before 1740 Ma. Three eolian facies are recognized: (1) cosets of cross strata, (2) sets of horizontal to low-angle strata, and (3) isolated sets of cross strata. Facies El records preservation of dunes with low angles of climb without intervening interdune deposits. Sets of facies E2 are interpreted as either sand sheet, preserved dune plinths or amalgamated interdune deposits. Facies E3 is encased in facies E2 and consists of lenses of cross strata with irregular tops. This association of facies is comparable to recent gypsum dune and interdune deposits at White Sands, New Mexico. Facies E3 reflects limited dune preservation during migration and accumulation at or near the groundwater table. Vertical change from amalgamated interdune deposits to dune without interdune deposits records dunefield initiation or lateral migration from a peripheral to a more centralized position within the dunefield. Increased sedimentation rates promoted preservation of dune deposits represented by facies E1. Decreased sedimentation rate and possible limited sediment bypassing resulted in amalgamation of interdune deposits on the dunefield periphery. Eolian facies E1 and E2 are interbedded with ephemeral-river deposits. Three ephemeral-river facies are recognized: (1) cosets of horizontal strata; 2) cosets of horizontal strata and overlying trough cross beds; and (3) cosets of medium-scale trough cross beds. Ephemeral-river facies F3 often are preserved in channels incised 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 from migration of subaqueous dunes within a tide-dominated subtidal setting. Interbedded fluvial and eolian deposits reflect either autogenic processes, such as reworking of fluvial sands by eolian processes, or incursion of a fluvial system into a dunefield related to allogenic processes such as climatic change. Dune deposits in the Whitworth Formation probably developed under arid to semi-arid conditions as indicated by extensive ephemeral-river deposits, presence of evaporite pseudomorphs, and absence of wet interdune deposits. Interbedded eolian, fluvial, and marginal-marine deposits resulted from relative sea-level fluctuations related to a combination of eustasy and intrabasinal tectonics.