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Styles of interaction between aeolian, fluvial and shallow marine environments in the Pennsylvanian to Permian lower Cutler beds, south-east Utah, USA



The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south-east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross-bedded dune sandstone dominated by grain flow and translatent wind-ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind-ripple, wavy and horizontal planar-laminated strata resulting from accumulation on a range of dry, damp or wet substrate-types in the flats and hollows between migrating dunes; and (iii) extensive, near-flat lying wind-rippled sandsheet strata. Fluvial facies comprise channel-fill sandstones, lag conglomerates and finer-grained overbank sheet-flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south-east, whereas south-westerly flowing fluvial systems periodically punctuated the dune fields from the north-east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo-water-table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid-plains that fed sediment to a coastline that lay to the south-west and which ran approximately north-west to south-east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low-relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non-marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north-eastward directed marine transgression into the basin prior to the onset of exclusively non-marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small-scale autocyclic behaviour of competing, coeval depositional systems and larger-scale allocyclic changes that record system response to longer-term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea-level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio-eustatic driving mechanism.