Expanding the spectrum of shallow marine carbonate-siliciclastic systems: processes, facies distribution, and depositional controls of a siliciclastic-dominated example

Abstract

Most of the present knowledge of shallow-marine, mixed carbonate-siliciclastic systems relies on examples from the carbonate-dominated-end of the carbonate-siliciclastic spectrum. This contribution provides a detailed reconstruction of a siliciclastic-dominated mixed system (Pilmatué Member of the Agrio Formation, Neuquén Basin, Argentina) that explores the variability of depositional models and resulting stratigraphic units within these systems. The Pilmatué Memberregressive system comprises a storm-dominated, shoreface-to-basinal setting with three subparallel zones: a distal mixed zone, a middle siliciclastic zone, and a proximal mixed zone. In the latter, a significant proportion of ooids and bioclasts were mixed with terrigenous sediment, supplied mostly via along-shore currents. Storm-generated flows were the primary processes exporting finesand and mud to the middle zone, but were ineffective to remove coarser sediment. The distal zone received low volumes of siliciclastic mud, which mixed with planktonic-derived-carbonate material. Successive events of shoreline progradation and retrogradation of the Pilmatué system generated up to 17 parasequences, which are bounded by shell beds associated with transgressivesurfaces. The facies distribution and resulting genetic units of this iliciclastic-dominated mixed system are markedly different to the ones observed in present and ancient carbonate-dominated mixed systems, but they show strong similarities with the products of storm-dominated, pure siliciclastic shoreface-shelf systems. Basin-scale depositional controls, such as arid climaticconditions and shallow epeiric seas might aid in the development of mixed systems across the full spectrum (i.e., from carbonate- to siliciclastic-dominated end-members), but the interplay of processes supplying sand to the system, as well as processes transporting sediment across the marine environment are key controls in shaping the tridimensional facies distribution and thegenetic units of siliciclastic-dominated mixed systems. Thus, the identification of different combinations of basin-scale factors and depositional processes are key for a better prediction of conventional and unconventional reservoirs within mixed, carbonate-siliciclastic successions worldwide.