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Periodic accumulation and destruction of aeolian erg sequences in the Permian Cedar Mesa Sandstone, White Canyon, southern Utah, USA

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Abstract

The Permian Cedar Mesa Sandstone represents the product of at least 12 separate aeolian erg sequences, each bounded by regionally extensive deflationary supersurfaces. Facies analysis of strata in the White Canyon area of southern Utah indicates that the preserved sequences represent erg-centre accumulations of mostly dry, though occasionally water table-influenced aeolian systems. Each sequence records a systematic sedimentary evolution, enabling phases of aeolian sand sea construction, accumulation, deflation and destruction to be discerned and related to a series of underlying controls. Sand sea construction is signalled by a transition from damp sandsheet, ephemeral lake and palaeosol deposition, through a phase of dry sandsheet deposition, to the development of thin, chaotically arranged aeolian dune sets. The onset of the main phase of sand sea accumulation is reflected by an upward transition to larger-scale, ordered sets which represent the preserved product of climbing trains of sinuous-crested transverse dunes with original downwind wavelengths of 300–400 m. Regularly spaced reactivation surfaces indicate periodic shifts in wind direction, which probably occurred seasonally. Compound co-sets of cross strata record the oblique migration of superimposed slipfaced dunes over larger, slipfaceless draa. Each aeolian sequence is capped by a regionally extensive supersurface characterized by abundant calcified rhizoliths and bioturbation and which represents the end product of a widespread deflation episode whereby the accumulation surface was lowered close to the level of the water table as the sand sea was progressively cannibalized by winds that were undersaturated with respect to their potential carrying capacity.

Aeolian sequence generation is considered to be directly attributable to cyclical changes in climate and related changes in sea level of probable glacio-eustatic origin that characterize many Permo-Carboniferous age successions. Sand sea construction and accumulation occurred during phases of increased aridity and lowered sea level, the main sand supply being former shallow marine shelf sediments that lay to the north-west. Sand sea deflation and destruction would have commenced at, or shortly after, the time of maximum aridity as the available sand supply became exhausted. Restricted episodes of non-aeolian accumulation would have occurred during humid (interglacial) phases, accumulation and preservation being enabled by slow rises in the relative water table. Subsidence analysis within the Paradox Basin, together with comparisons to other similar age successions suggests that the climatic cycles responsible for generating the Cedar Mesa erg sequences could be the product of 413 000 years so-called long eccentricity cycles. By contrast, annual advance cycles within the aeolian dune sets indicate that the sequences themselves could have accumulated in just a few hundred years and therefore imply that the vast majority of time represented by the Cedar Mesa succession was reserved for supersurface development.

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