Syn-sedimentary salt diapirism as a control on fluvial-system evolution: an example from the proximal Permian Cutler Group, SE Utah, USA

摘要

Loading of subsurface salt during accumulation of fluvial strata can result in halokinesis and the growth of salt pillows, walls and diapirs. Such movement may eventually result in the formation of salt-walled mini-basins, whose style of architectural infill may be used to infer both the relative rates of salt-wall growth and sedimentation and the nature of the fluvial-system response to salt movement. The Salt Anticline Region of the Paradox Basin of SE Utah comprises a series of elongate salt-walled mini-basins, arranged in a NW-trending array. The bulk of salt movement occurred during deposition of the Permian Cutler Group, a wedge of predominantly quartzo-feldspathic clastic strata comprising sediment derived from the Uncompahgre Uplift to the NE. The sedimentary architecture of selected mini-basin fills has been determined at high resolution through outcrop study. Mini-basin centres are characterized by multi-storey fluvial channel elements arranged into stacked channel complexes, with only limited preservation of overbank elements. At mini-basin margins, thick successions of fluvial overbank and sheet-like elements dominate in rim-syncline depocentres adjacent to salt walls; many such accumulations are unconformably overlain by single-storey fluvial channel elements that accumulated during episodes of salt-wall breaching. The absence of gypsum clasts suggests that sediment influx was high, preventing syn-sedimentary surface exposure of salt. Instead, fluvial breaching of salt-generated topography reworked previously deposited sediments of the Cutler Group atop growing salt walls. Palaeocurrent data indicate that fluvial palaeoflow to the SW early in the history of basin infill was subsequently diverted to the W and ultimately to the NW as the salt walls grew to form topographic barriers. Late-stage retreat of the Cutler fluvial system coincided with construction and accumulation of an aeolian system, recording a period of heightened climatic aridity. Aeolian sediments are preserved in the lees of some salt walls, demonstrating that halokinesis played a complex role in the differential trapping of sediment.