The Cretaceous palaeodesert of the Sanfranciscana Basin (SE Brazil): A key record to track dissolved evaporites in the West Gondwana

Abstract

Extensive palaeodeserts are known from the intracontinental basins of South American Platform, simultaneously to volcanic activity and thick salt beds developed during the rifting phase of Brazilian coastal basins. Based on palaeoenvironmental conditions and modern analogues, it would be expected that Cretaceous evaporites were frequently described in the intracontinental basins of West Gondwana, however, such deposits are rarely mentioned. This work evaluated the recognition and role of evaporitic-related features as palaeoclimatic/palaeoenvironmental proxies, through outcrop-based facies, X-ray diffraction and petrographic analyses of the Areado Group (Lower Cretaceous), Sanfranciscana Basin, southeastern Brazil. The Areado Group stands out as a remarkable record of evaporitic-siliciclastic interactions, consisting of alluvial, lacustrine and aeolian deposits. The stacking pattern reflects the progressive drying-up of the Sanfranciscana Basin during Cretaceous times. Regardless salt dissolution, the Areado Group comprises pseudomorphs after evaporites, evaporitic moulds, sand patch-fabric, and replication of the microtopography of efflorescent crusts. Preservation of evaporitic features in the Areado Group was controlled by the unique geological context of the Sanfranciscana Basin, which was submitted to syndepositional/eodiagenetic cementation and pseudomorphism, along with rapid burial caused by the progradation of aeolian dunes. Equivalent sedimentary structures and morphological aspects also occur in other Cretaceous palaeodeserts of intracontinental basins throughout Brazil. Timing of salt dissolution and isovolumetric/non-isolumetric replacement are important controlling factors on the geological record of evaporitic-siliciclastic systems that undergo post-depositional modifications. The database provided here indicates that playa lake deposits are commonest than traditionally represented in the non-marine Cretaceous record of the West Gondwana and other similar settings worldwide.