Discontinuity surfaces within a Lower Cretaceous mixed siliciclastic-carbonate succession (Mulichinco Formation, Neuquén Basin, Argentina): evidences for transgresive/regressive cycles in a non-tropical ramp setting

Abstract

The Upper Member of the Mulichinco Formation (150 m thick) is dominated by siliciclastic sediments that deposited in a high-latitude marine setting with a ramp-type profile within the Neuquén Basin (west-central Argentina), during the early Valanginian. Interbedded with the siliciclastics, and representing less than 25 of the total succession, are sharp-based carbonate-rich deposits with abundant macrofossils. A combined sedimentological, ichnological and palaeoecological study aiming at better understanding the origin of this mixed siliciclastic-carbonate succession is presented in this contribution. The siliciclastic intervals comprise 2- to 10-m-thick coarsening-upward successions. They typically begin with laminated mudstones at the base that grade upward to massive very fine-grained sandstones, interbedded with lenticular, decimeter-thick beds having HCS, asymmetrical ripples, and gutter casts. These facies have low to null macrofossil content. Fine-grained sandstones having SCS or large-scale high-angle cross-stratification commonly occur at the top of the coarsening-upward units. Both lithofacies have a trace fossil suite attributed to the Skolithos Ichnofacies. Macrofossils are uncommon in these facies except at their bases where thin, loosely-packed shell concentrations with variable degree of reworking occur. The carbonate-dominated intervals typically have sharp and locally erosional basal surfaces (with up to 2 m of relief), where cobble-size, carbonate-cemented sandstone clasts are common. Locally, these clasts are bored with litophagid bivalves and encrusted by oysters. Bounding surfaces of these carbonate units are alternatively demarcated by the presence of robust Thalassinoides into the underlying sandstones. The burrow systems penetrate up to 0.20 m and are filled with carbonate debris. These burrows are attributed to the Glossifungites Ichnofacies. Carbonate-rich deposits (0.30-3.4 m thick) are mainly gravel-size shell concentrations with a mixed matrix of poorly sorted bioclasts and terrigenous very fine sand (siliciclastics make up to 30% of the rock). Glauconite is also common. Macrofossil associations at the base of units are typically dominated by epibenthic cemented suspension feeders, namely oysters and gregarious serpulids. Locally, however, globose corals and colonial serpulids are dominant. Where carbonate units are thick enough, epibenthic-dominated fossil associations are replaced upwards by endobenthic-dominated associations, mainly composed of shallow- and deep-burrowing bivalves, together with irregular echinoids. These, in turn, are capped by finer-grained deposits (silty wackestones) where articulated moulds of deep-burrowing bivalves in life position dominate. In general biofabrics suggests that lateral transport was not significant; therefore shell concentrations are interpreted to be parautochthonous to autochthonous fossil associations. The siliciclastic coarsening-upward successions of the Upper Mulichinco Formation are interpreted to represent progradational events of wave- and storm-dominated offshore to shoreface system, under relatively moderate rate of terrigenous supply and high net sedimentation rate. The attributes of the contact between the siliciclastic and carbonate deposits suggest extensive erosion of the sea floor that produced exhumation of compacted sandstones and up to cobble-size sandstone clasts. This, in turn, created widespread firmgrounds colonized by crustaceans and islands of hardgrounds suitable for development of cemented, epibenthic fauna (oysters) and lithophagid bivalves. These discontinuity surfaces bounding the carbonate deposits are believed to represent transgressive surfaces of erosion during drowning of the siliciclastic system. Transgression is also supported by low net sedimentation rates that favored the development of extensive epifaunal communities immediately after deposition resumed. Despite transgressive conditions, with time, carbonate productivity and physical reworking of bioclastic material favored areas with higher net sedimentations rates where endobenthic-dominated associations developed. Eventually, the entire carbonate system was drowned during continuous relative sea-level rise leaving behind shellbeds with in situ bivalves within a fine-grained matrix (condensed shellbeds). The turnaround from transgressive to highstand conditions is marked by the reappearance of siliciclastic mudstones capping carbonates-rich units. 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 non-tropical, partially-restricted marine basin with homoclinal profile. 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.