Mechanisms of aggradation in fluvial systems influenced by explosive volcanism: An example from the Upper Cretaceous Bajo Barreal Formation, San Jorge Basin, Argentina

Abstract

The Late Cretaceous succession of the San Jorge Basin (Patagonia, Argentina) records different continental settings that interacted with explosive volcanism derived from a volcanic arc located in the western part of Patagonia. This paper discusses the contrasting aggradational mechanisms in fluvial systems strongly influenced by explosive volcanism which took place during sedimentation of the Bajo Barreal Formation. During deposition of the lower member of the unit, common ash-fall events and scarce sandy debris-flows occurred, indicating syn-eruptive conditions. However, the record of primary pyroclastic deposits is scarce because they were reworked by river flows. The sandy fluvial channels were braided and show evidence of important variations in water discharge. The overbank flows (sheet-floods) represent the main aggradational mechanism of the floodplain. In places, subordinate crevasse-splays and shallow lakes also contributed to the floodplain aggradation. In contrast, deposition of the upper member occurred in a fluvial-aeolian setting without input of primary volcaniclastic detritus, indicating inter-eruptive conditions. The fluvial channels were also braided and flowed across low-relief floodplains that mainly aggraded by deposition of silt-sized sediments of aeolian origin (loess) and, secondarily by sheet-floods. The Bajo Barreal Formation differs from the classic model of syn-eruptive and inter-eruptive depositional conditions in the presence of a braided fluvial pattern during inter-eruptive periods, at least at one locality. This braided fluvial pattern is attributed to the high input of fine-grained pyroclastic material that composes the loessic sediments.