An integrated approach for unravelling the sequence-stratigraphic significance of sharp-based marine sandstone bodies. A case study from the Upper Mulichinco Member (Lower Cretaceous), Neuquén Basin, Argentina

Abstract

Sharp-based shallow-marine sandstone bodies encased in offshore mudstones can develop in a wide range of sequence-stratigraphic conditions including normal regression, forced regression/lowstand, punctuated transgression or normal transgression. Yet, they can be hard to differentiate between each other having an incomplete datasets. This contribution documents sharp-based, elongated sandstone bodies occurring within the Upper Mulichinco Member (Valanginian) in central Neuquén Basin, Argentina. Sharp-based units are sandwiched between offshore to offshore transition fine-grained regressive siliciclastics beneath and transgressive skeletal carbonates on top developed within an epeiric sea. Sedimentological, architectural, ichnological, thaphonomical, and palaeocological data both from sandstone bodies and surrounding deposits was integrated in a small area (40 km2) in order to unravel the sequence-stratigraphic context and evolution of these units. Sharp-based sandstone units occur at three distinctive intervals within a 70-m thick succession. They are 3-7.5 m thick and exhibit well-defined lateral pinch-outs, outlining N-S elongated bodies, at least three times longer than wide (e.g. 3 km x 1 km). Individual bases of sandstone bodies are sharp and flat for kilometers. They are marked by oyster-dominated shell beds as well as passively-filled Thalassinoides burrows (in the underlying deposits) attributed to a Glossifungites Ichnofacies suite. Basal shell beds extend beyond sandstone bodies pinch-outs and they have bored and encrusted sandstone cobbles throughout the area. Internally, the Mulichinco bodies are composed of clean, fine-grained sandstones and are not graded or they fine slightly upwards. A vertical transition from cross-stratified, to cross-laminated, to bioturbated sandstone facies is recorded in some cases. Large-scale, low-angle cross-stratified sets (from top to bottom of body), dipping to NE and NNE are also locally observed. Bioturbation varies from moderate to very high and is dominated by Ophiomorpha burrows. Sandstone units are sharply capped by thin (< 1 m) skeletal floatstones and wackestones. This upper contact is also locally demarcated by Thalassinoides burrows attributed to a Glossifungites Ichnofacies. The evidences associated with the basal bounding surfaces of these sandstone bodies indicate that normal regressive conditions were interrupted by a period of significant erosion. Erosion of underlying siliciclastics extended far beyond the area where the sandstone bodies were formed (or finally preserved) suggesting that relative sea-level dropped even further down dip (i.e. during falling plus lowstand conditions). During the initial transgression borers and epibenthic oysters developed on sandstone cobbles exposed in the sea floor and winnowing resulted in fossil concentration. Thus, the basal shell beds are demarcating a transgressive ravinement surface. Available sand previously transported during falling/lowstand conditions was redeposited on the shallow eperic sea as north-south elongated sandstone bodies during early transgression. Bodies migrated laterally to NE and they were covered by 2D-dunes and ripples migrating both parallel and obliquely with respect to body axes. Intense bioturbation coupled with finer sands at the top of some units is suggestive of an abandonment phase. Eventually, sandstone units were covered by low-energy skeletal carbonates representing maximum transgressive conditions and minimum terrigenous supply both from the land and from palimpsest sediments. The integrated approach and findings of this study might help identifying truly transgressive sandstone bodies in ancient shallow-marine successions that are almost absent in the recent literature. Furthermore, the Mulichinco sandstone bodies are potential analogues for similar hydrocarbon-bearing deposits both in the Neuquén Basin and worldwide (e.g. Western Interior, USA; Miocene Offshore Java).