Mixed carbonate (transgressive) / siliciclastic (regressive) cycles deposited in a shallow-water ramp: the Upper Mulichinco Member (Valanginian), Neuquén Basin, Argentina

Abstract

This study integrates sedimentological, ichnological and palaeoecological analyses in order to better understand the origin of a mixed siliciclastic-carbonate succession deposited during the early Late Valanginian in the back-arc Neuquén Basin (Argentina). The studied succession, or Upper Mulichinco Member, accumulated within a shallow and broad marine setting (epeiric sea) under long-term highstand conditions. The unit (150 m thick) is dominated by siliciclastic sediments interbedded with sharp-based, carbonate-rich deposits representing < 25% of the total succession. The siliciclastic packages consist of meter-scale coarsening-upward successions (2-8 thick). They typically begin with laminated mudstones at the base grading upward to bioturbated muddy sandstones and lenticular, very fine-grained sandstones with HCS. Fine-grained sandstones having SCS occasionally occur at the top of the coarsening-upward units. Siliciclastic facies have trace fossils attributed to Cruziana and Skolithos Ichnofacies, but low or null macrofossil content. The carbonate-dominated packages have sharp and locally erosional basal surfaces (with up to 2 m of relief). Basal surfaces are also demarcated by the presence of robust Thalassinoides burrows penetrating into the underlying sediments attributed to represent a Glossifungites Ichnofacies. Carbonate-dominated deposits (0.30-3.4 m thick) mostly consist of massive micrite-rich floatstones characterized by gravel-size shells, poorly sorted finer bioclasts and very fine terrigenous sand (glauconite is also common). Shell concentrations are interpreted to represent parautochthonous and autochthonous fossil associations based on skeletal preservation. Epibenthic associations are common at the base of carbonate packages. Although they are typically dominated by cemented oysters and gregarious serpulids, globose corals and colonial serpulids are locally abundant. These epibenthic-dominated associations are commonly replaced upwards and laterally by endobenthic-dominated associations, composed of shallow- and deep-burrowing bivalves, and irregular echinoids. These, in turn, are capped by silty wackestones where moulds of deep-burrowing bivalves in life position are closely spaced. The siliciclastic coarsening-upward successions of the Upper Mulichinco Member are interpreted to represent progradational events on a wave- and storm-dominated offshore to shoreface system, characterized by relatively high rate of terrigenous supply. In turn, the attributes of the contact between the siliciclastic and carbonate deposits suggest extensive erosion of the sea floor that created widespread firmgrounds colonized by crustaceans and suitable for development of cemented, epibenthic macrofauna. These discontinuity surfaces bounding the carbonate deposits are believed to represent transgressive surfaces of erosion during drowning of the siliciclastic system. Low net sedimentation rates favored epifaunal communities and glauconite formation. Despite transgressive conditions, carbonate productivity and physical reworking of bioclastic material favored areas where endobenthic-dominated associations developed. Eventually, the entire carbonate system was drowned during continuous relative sea-level rise leaving behind wackestones with in situ bivalves (condensed shellbeds). The turnaround from transgressive to regressive conditions is marked by the reappearance of siliciclastic mudstones on top of carbonates. Results of this study highlight that high-frequency fluctuations in relative sea level (probably in the range of 40-100 k.y.) produced carbonate-dominated transgressive hemicycles followed by siliciclastic regressive hemicycles in a partially-restricted epeiric sea. Although similar stratigraphic patterns have been found in Neogene to Recent better calibrated successions of New Zealand and Mediterranean regions, ancient examples as this lower Cretaceous case study have not been to date widely reported.