GABRIEL BERTOLINI, Adrian Hartley, Juliana Marques, Jhenifer Paim

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 km2 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.