Evolution of Devonian alluvial systems in an oblique-slip mobile zone—an example from the Broken River Province, northeastern Australia

Abstract

The lower 1500 m of the Late Devonian Bulgeri Formation was deposited in a tectonically active foreland basin in response to a major Late Devonian orogeny in the Broken River Province, northeastern Australia. The basin was bounded to the east by an uplifted belt of Palaeozoic sedimentary and volcanic rocks that was deformed east of an active thrust zone (Gray Creek Fault). To the south, the basin was bounded by igneous and metamorphic basement rocks, uplifted along a major oblique-slip fault zone (Clarke River Fault). Facies analysis and mapping of alluvial stratigraphy has resulted in the recognition of compositionally distinct, coalescing, axial and transverse alluvial drainage systems.The axial drainage system flowed to the northeast, approximately parallel to the Gray Creek Fault and away from the Clarke River Fault. The system drained the cratonic basement to the south and west, and accumulated mainly feldspathic and quartzose sediments. A lower, fine-grained succession, and an upper coarse-grained succession are recognised. The lower succession (‘Rockfields’ alluvial system) was deposited in broad, sandy, low-sinuosity channels and semi-permanent floodplain lakes. These rivers drained ultimately northward into a retreating coastal plain and shallow sea that lay over the Georgetown Province. The upper succession (‘Bulgeri’ alluvial system) was deposited in gravelly and sandy braided rivers emanating from south of the Clarke River Fault. The transverse distributary system (‘Stopem Blockem’ alluvial system), accumulated mainly lithic, coarse-grained sediments deposited in gravelly braided rivers sourced from the uplifted orogenic belt to the east. These rivers flowed towards the northwest before coalescing with the axial drainage system where they changed to a northerly orientation. Uplifted fault blocks of Devonian limestone within the basin locally contributed limestone gravel to these rivers. Between the coalescing axial and transverse river systems lay extensive floodplains, which received a slow, but compositionally mixed supply of mainly fine-grained sediments. Long periods of weathering resulted in the development of reddened palaeosols, with well developed calcrete horizons. The palaeogeography and fluvial style of the lower Bulgeri Formation is compared to the Gulf of Carpentaria in northeastern Australia.Analysis of the architecture of these alluvial systems has led to recognition of at least three, subsidence-driven, tectonic cyclothems and this is supported by simple quantitative modelling. The lower part of each cycle begins with a thin conglomeratic succession above an unconformity, and is overlain by thick, relatively fine-grained, syn-tectonic alluvial facies deposited during maximum subsidence. The upper part of each cycle is marked by increasingly coarse-grained, post-tectonic alluvial facies. These were deposited by gravel progradation following cessation of uplift or thrusting in the source areas and concomitant slowing of subsidence rates.