Controls on grain size distribution in an ancient sand sea

Grain size distribution in deserts is driven by a combination of autogenic controls such as grain abrasion and sorting due to wind transport and allogenic controls such as provenance and spatial changes in wind-direction. Downwind grain-size trends in present day sand seas display contrasting results. For example, the Namib and Hexi Corridor sand seas show broad downwind fining and sorting trends which are not present in the Ténére, Australian and Sinai sand seas. This study examines the grain size distribution along the margins of the Cretaceous Botucatu paleodesert across an area of >1,000,000 km 2 to determine the relative importance of autogenic and allogenic controls on sand distribution. A comparison between the spatial distribution of grain size metrics with detrital zircon geochronology as a provenance test, paleowind patterns as a wind-regime test and 3 transects along the basin paleomargins as a downwind abrasion test, are used to quantify the main controls on Botucatu paleodesert grain-size distribution. Grain size dispersion shows an E to W fining pattern, which agrees with provenance control - in contrast to a N-S trend expected for the predominant wind-regime control. In the NE region there is good evidence for downwind fining due to aeolian abrasion – of about 0.4 μm by km, indicating that whilst downwind fining due to abrasion does occur, but that it is limited by the rounded nature of available sands for Botucatu desert and possibly by mix of different provenance sources. The results suggests that the provenance of the available sand is the primary control on the grain-size distribution in the Botucatu sand sea and potentially in most large-scale dune fields. Pye, 2001). The dataset displays a mean grain-size ranging from 1.05 to 0.05 mm (mean 0.3 mm), sorting from 2.12 to 1.45 (mean 1.68 μm), skewness from 0.57 to -0.35 (mean -0.11 μm) and kurtosis from 1.56 to 0.74 (mean 0.99 μm). Overall, the sands are medium grained, moderately sorted, mesokurtic and symmetrical to fine skewed.