Variability of processes in the compositionally mixed Marginal-Marine System of the Agua de la Mula Member (Argentina).

The uppermost interval of the Agua de la Mula Member (Agrio Formation, Lower Cretaceous, Neuquén Basin, Argentina) comprises a siliciclastic-carbonate succession, representing a transition from distal to marginal-marine systems. The present study aims to understand the spatial and temporal variability of this system. Sedimentary logging, high-resolution 3D models, facies and architectural analysis were carried out in two outcrop sectors, 10 km apart from each other. The studied interval (80 m thick) shows a general shallowing-upward stacking composed of smaller-scale coarsening-up intervals (~10-15 m) having beds compositionally ranging from pure siliciclastic to pure carbonate. Basal cycles are composed of siliciclastic muddy facies grading to HCS-sandstones, interpreted as offshore-transition environments were settling, fair-weather and storm-wave processes prevail, with occasional interbedded floatstones interpreted as marine transgressions or carbonate-rich input streams due to storm events. Upper cycles are dominated by tabular to lenticular muddy to sandy mixed deposits mostly produced by unidirectional currents interbedded with extensive carbonate-dominated beds mostly comprising cross-stratified oolitic grainstones. These intervals were interpreted as a marginal-marine deltaic system were fluvial currents and wave reworking coexist as dominant processes, and riverine-supplied siliciclastic sands were mixed with an intermittent carbonate input or production. Furthermore, towards the top of this interval, a tidal influence is suggested due to the presence of heterolithic layers, mudstone drapes and scarce microbialites. The compositional and processes variations observed cannot be satisfactorily explained by simplistic facies models. This work discusses a new approach to explain compositionally mixed facies variations when making detailed paleoenvironmental characterizations.