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Fluvial Architecture and the (Mis)use of Lithostatigraphy in Actively Deforming Salt Basins: Chinle Formation, Paradox Basin, Utah


Understanding how fluvial systems respond to syn-sedimentary halokinesis is important for reconstructing the palaeogeography of salt basins, determining the history of salt movement and predicting development and architecture of sandstone bodies for subsurface fluid extraction. To assess both the influence of salt movement on fluvial system development and the use of lithostratigraphic correlation schemes in salt basins we have analysed the late Triassic Chinle Formation in the Paradox Basin, Utah. Despite extensive previous work on the Chinle Formation in the Paradox Basin a number of uncertainties remain with respect to the nature of the interaction between sedimentation and salt movement. Previous studies have taken a lithostratigraphic approach suggesting that specific channel belt and floodplain packages can be correlated within and between mini-basins. Our interpretations differ, we illustrate that a lithostratigraphic approach to correlation between mini-basins cannot be supported. Key controls on fluvial sandstone body development are: 1) access of the fluvial system to mini-basins, 2) the periodic and diachronous nature of salt movement, and 3) the three dimensional topography of salt highs. The key control on channel sandstone distribution is whether fluvial channel systems were able to enter mini-basins: net channel sandstone percentage thicknesses can vary by as much as 60% between adjacent mini-basins. Where fluvial systems are present in the axis of mini-basins, conformable packages of downstream and laterally accreting fluvial channel and overbank deposits are developed. Multi-storey axial channel belts form packages 10 to 50 m thick which can be traced from 2 to 10 km parallel to mini-basin axes. Laterally, these sands thin, pinch-out or become truncated towards syn-sedimentary salt highs. Floodplain fines and associated paleosols occur both within deeper parts of mini-basins and across salt highs. Floodplain preservation is strongly influenced by proximity to axial channel belts. Mudstone units within sand-rich mini-basins are of limited lateral extent and are often eroded beneath channel belts, whereas in sand-starved mini-basins thick mudstone packages are developed often with extensive, mature paleosols. This study illustrates the complexity that relatively subtle salt movement can induce on fluvial stratigraphy and highlights how lithostratigraphic approaches to correlation in salt mini-basins are likely to be unsuccessful.