Abstract New U–Pb detrital zircon ages are presented for the Tordillo Formation. The ages indicate that the most important source region of sediment supply was the Jurassic Andean arc (peaks at c. 144, 153 and 178 Ma), although two secondary sources were defined at c. 218 and 275 Ma. Temporal variation in the provenance indicates that at the beginning of the sedimentation, Carboniferous to Lower Jurassic magmatic rocks and Lower Palaeozoic metamorphic rocks were the most important sources. Towards the top, the data suggest that the Andean arc becomes the main source region. The comparison between provenance patterns of the Tordillo Formation and of the Avilé Member (Agrio Formation) showed some differences. In the former, the arc region played a considerable role as a source region, but this is not identified in the latter. The results permit a statistically robust estimation of the maximum deposition age for the Tordillo Formation at c. 144 Ma. This younger age represents a discrepancy of at least 7 Ma from the absolute age of the Kimmeridgian and Tithonian boundary (from the chronostratigraphic timescale accepted by the International Commission of Stratigraphy, IUGS), and has strong implications for the absolute age of the Jurassic–Cretaceous boundary. Supplementary material: Sample coordinates, values of the sandstone compositional framework and U–Pb (LAM-MC-ICP-MS) age measurements of zircons grains are available at http://www.geolsoc.org.uk/SUP18718

A map of the principal grabens and half-grabens of the Neuquén Basin in the northeastern part of Neuquén Province, Argentina, was constructed based on two-dimensional (2-D) and three-dimensional (3-D) seismic information. The synrift structure of the region is characterized by high angle NW-trending normal faults. Two populations can be distinguished: one with an azimuth of 140° and the other one with an azimuth of 105°. The principal half-grabens are accommodated by NE-dipping normal faults. The principal NW-trending faults are less than 20 km long and are either crosscut by or terminate in transfer zones trending to the NE. The transfer zones are either faults or zones where the principal NW-trending faults loose slip or terminate. The principal normal faults were active until the deposition of the Lower Upper Jurassic Tordillo Formation. Subsequently, only a few faults related to differential subsidence over the half-grabens remained active. The typical structures of the region are smooth anticlines and synclines that affect the sag facies of Neuquén Basin. The anticlines developed over basement highs and the synclines over graben basins. The synclines are explained in this study as resulting from differential subsidence over the half-grabens. This differential subsidence could have been a continuous process that began in the synrift stage of the basin. A distinct element model was created to analyze the differential subsidence process and geometry. No evidence was found to support the hypothesis that tectonic inversion is an important process in this region. The main unconformities can be explained by the differential subsidence process.

ABSTRACT The Vaca Muerta Formation deposited in a retro-arc setting, favored by an under filled depocenter and uplifted peripheral zones. The onset of the deposition corresponds to a rapid flooding of oceanic water penetrated through sudden ruptures along the incipient Andean arc, as a consequence of the Late Jurassic extension during the thermal subsidence stage. The Vaca Muerta Formation forms part of the Lower Mendoza Subgroup, and is defined as the organic-rich calcareous mudstone (av. 5% TOC) deposited above the sharp contact of the continental Tordillo and the Quebrada del Sapo formations; the upper contact with the Quintuco Formation is gradational and diachronous. The Vaca Muerta Formation spans the early Tithonian Virgatosphinctes andesensis to the early Valanginian Lissonia riveroi ammonite Zones. In association with classical stratigraphic characteristics of this unit, this chapter presents a cyclostratigraphic and magnetostratigraphic study displaying an unprecedented accurate time scale for the Vaca Muerta deposition, which took place from Chron M22 to Chron M15 in a timespan of ~10 My (4.86 My within the Tithonian and 5.27 My within the Berriasian).