Alluvial architecture of reworked pyroclastic deposits in peri-volcanic basins: Castillo Formation (Albian) of the Golfo San Jorge basin, Argentina

Abstract

Spatial distribution of lithofacies and lithofacies associations is paramount to understand the variations in fluvial styles and dimension of channels, external controls and degree of predictability of the alluvial architecture of reworked pyroclastic de posits preserved in peri-volcanic basins. These issues were addressed by the study of the Albian Castillo Formation of the Golfo San Jorge Basin in the N-S striking San Bernardo Fold Belt, a group of ranges located 190 km eastward of the Andean Volcanic Chain. A detailed stratigraphic framework was carried out, based on 19 sedimentary logs and the characterization of 284 fluvial channels, analyzing seven study locations from the northern basin-margin to the most complete sections at the southernmost part of the Fold Belt. The unit was divided in fifteen lithofacies, which were grouped in six main lithofacies associations: (i) subaerial floodplains and ponds; (ii) proximal floodplain deposits; (iii) low-sinuosity and braided fluvial channels; (iv) subaerial debris-flow deposits; (v) sheetflood deposits; (vi) reworked ash-fall beds. Spatial variation in the lithofacies association proportions and variation in the dimensions of the sandbodies are mainly related to seasonal or episodic delivery of water and volcaniclastic particles in local or regional networks of channels which were developed in areas of variable subsidence rate. In spite of the control of volcanism over the composition of volcaniclastic deposits, the main features of the fluvial system preserved in this peri-volcanic basin were mainly established by intrabasinal processes.