LINKS BETWEEN SEDIMENT ACCUMULATION RATES AND THE DEVELOPMENT OF ALLUVIAL ARCHITECTURE: TRIASSIC ISCHIGUALASTO FORMATION, NORTHWESTERN ARGENTINA

Recent investigations in the Late Triassic Ischigualasto Formation, located in northwestern Argentina, indicated a link between increased mean annual precipitation and sediment accumulation rates. The 700m of alluvial sandstones and mudstones of the Ischigualasto Formation were deposited within the Ischigualasto Basin, a northwest trending Triassic half-graben rift basin. Channel sandstones have northeast paleocurrent directions while the adjacent floodplain mudstones are commonly pedogenically altered. Paleosol profiles thin by 50% in the upper half of the section. Over the same interval, paleosol climatic indicators suggest there was an increase in the mean annual precipitation. Taphonomic biases indicate preferential preservation for the upper half of the formation. Avulsion frequency decreases progressively upsection. These changes through out the formation are regionally contemporaneous along 30% of the basin.

The characteristics of the upper half of the formation indicate an increase in sediment accumulation rate, which can be linked to either an increase in mean annual precipitation or tectonic activity. Decrease in the paleosol profile is a proxy for the increasing sediment accumulation rates. The increase in the sediment accumulation rates coincides with an absence of carbonate bearing paleosols, which indicates an increase in the mean annual precipitation. Taphonomic bias, found in the upper half of the Ischigualasto Formation, is an effect of increased sedimentation rates and mean annual precipitation. Avulsion frequency decreases upsection, as indicated by paleosol sequences below channel sandstones. Axial channel avulsion frequency should increase under increased tectonic activity. Evidence for tectonically mediated increase in sedimentation accumulation rates is not supported by our findings. Instead our data indicates an increase in the mean annual precipitation.