Sedimentary response to a tectonically induced sea-level fall in a shallow back-arc basin: the Mulichinco Formation (Lower Cretaceous), Neuquen Basin, Argentina

This study examines the sedimentary response to a tectonically driven relative sea-level fall that occurred in the Neuquen Basin, west-central Argentina, during the late Early Valanginian (Early Cretaceous). At this time the basin lay behind the emergent Andean magmatic arc to the west. Following the relative sea-level fall, sedimentation was limited to the central part of the Neuquen Basin, with the deposition of a predominantly clastic, continental to shallow marine wedge on top of basinal black shales. This lowstand wedge is called the Mulichinco Formation and consists of a third-order sequence that lasted about 2 Myr and contains high frequency lowstand, transgressive, and highstand deposits. Significant variations in facies, depositional architecture, and internal organization of the sequence occur along depositional strike. These variations are attributed mainly to tectonic and topographic controls upon sediment flux, basin gradient, fault tilting, and shifting of the depocentre through time. These controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west. The superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the Mulichinco Formation. However, the Mulichinco sequence boundary is marked by shallow, low erosional relief and widespread fluvial deposition. The surface lacks prominent valleys traditionally associated with sequence boundaries. This non-erosive sequence boundary geometry is attributed to the ramp-type geometry of the basin and/or rapid uplift that limited stratigraphic adjustment to base-level fall. Significant along-strike facies changes and a low-relief sequence boundary are attributes that may be common in tectonically active, semi-enclosed basins (e.g. shallow back-arc basins, foreland basins).