Links between sediment accumulation rates and the development of alluvial architecture: Triassic Ischigualasto Formation, northwestern Argentina

Abstract

The Late Triassic Ischigualasto Formation, located in northwestern Argentina, was deposited during the late Norian Age in a continental rift valley. Detailed field descriptions revealed a connection between sedimentation rates and a change in the mean annual precipitation rates. The mean annual precipitation in the lower half of the Ischigualasto Formation is estimated at >760 mm and the upper half is estimated at <760 mm to 1400mm. Increases in the mean annual precipitation affected the architecture of the alluvial deposits such as decreased paleosol profile thickness and increased channel interconnectedness. Avulsion frequencies were calculated from paleosol successions sandwiched between channel sandstones and showed a decrease in the avulsion frequency through time as the Ischigualasto Formation was deposited. Since these channel sandstones were deposited along the axis of the a rift valley and avulsion should increase with increases in fault movement along the main rift valley fault, it is proposed here that the avulsion frequency suggests that there is a decrease in fault activity and extension in the upper half of the Ischigualasto Formation. Initial models of the Ischigualasto Basin have been compared to field data collected and a newer model of the active rift systems, and after review, a new revised model is proposed. The new model suggests that there are three main phases of rift development that are related to the amount of crustal extension: sub-basin, single basin, and passively subsiding basin phases. Positions of lava flows within the basin fill were also considered as signs of local volcanic activity; these are absent in the upper portions of the basin fill. Chronostratigraphic frameworks developed for the entire basin fill also support a decrease in the sedimentation rate through time during the deposition of the basin fill.