STRATIGRAPHIC ARCHITECTURE OF THE UPPER TRIASSIC ISCHIGUALASTO FORMATION, ISCHIGUALASTO PROVINCIAL PARK, NORTHWESTERN ARGENTINA

The Ischigualasto Formation in the Ischigualasto Provincial Park of northwestern Argentina contains abundant fluvial-channel sandstones, overbank mudstones, and associated paleosols that were deposited in a northwest-trending back-arc rift basin during Late Triassic time. The formation progressively thins from ~700 m in the central part of the park to ~400 m in the east, over a distance of 15 km. This thinning is accompanied by a relative decrease in the abundance of preserved fluvial-channel sandstones and an increase in mud-rich overbank deposits. In addition, vertic, calcic and argillic paleosol profiles in the east comprise a larger percentage of the overall section and display a higher degree of maturity in terms of profile and horizon thickness, color, and authigenic mineral abundance.

The internal stratigraphic architecture of the Ischigualasto Formation indicates that during deposition, the central part of the basin was the location of a long-lived, N-NE flowing fluvial-channel belt that received relatively continuous channel and proximal overbank deposition. To the east, however, channel-related deposition was more infrequent and the region better drained, resulting in enhanced pedogenic modification of previously deposited alluvial sediments. The fact that the eastern parts of the basin likely occupied an "upland" position relative to areas to the west and suggests that the basin may have undergone episodic fluvial incision resulting in abandonment of the pre-existing alluvial surface. The overall thickness and facies trends observed in the Ischigualasto Formation most likely correspond to variations in fault-related accommodation development within the basin during the time of deposition.