Abstract
The present paper interprets and discusses the Late Triassic-Early Jurassic of the first transgression at northern Neuquén basin (Argentina) and its paleostructural control due to ancient horsts and half-grabens. Six vertical sections were chosen along two east-west and north-south transects. Depositional systems have been determined and correlated by means of ammonite, bivalve and brachiopod biostratigraphy. Two regions can be distinguished based on the areal distribution of the contemporaneous depositional systems in the studied area; one at the east and the other to the west of El Freno creek. Vertical sections in the western region record shallowingupward fluvio-dominated, transverse, normal-fault-controlled, slope-type fan deltas and intermediate shelf to Gilbert-type fan deltas. Vertical sections in the eastern region record a transgressive siliciclastic storm-dominated shelf, evolving from wave-dominated estuary to turbidity current influenced outer shelf. Accommodation vs. sedimentary supply ratio leads to identification of two stages of differing tectonic behaviour. The first one (Rhaetian-late Early Sinemurian) shows an accomodation greater than the sedimentary supply leading to the deposition of the western slope-type fan deltas. The second stage (late Early Sinemurian-Toarcian) shows a varying accommodation: during the late Early Sinemurian, accommodation was outpaced by sedimentary supply leading the intermediate-type fan delta to prograde. Later (late Early Sinemurian-Toarcian), accommodation exceeded the supply again, allowing transgression of the marine shelf and the increase in the marine depositional area. The two stages coincided with synrift and sag phases previously proposed for the southern Neuquén basin.