Seismic scale bottomset facies of a mixed carbonate siliciclastic ramp: The Jurassic-Cretaceous Vaca Muerta Formation in the Picún Leufú area, Neuquén basin, Argentina

PAZ, M.; PONCE, J.J.; MÁNGANO, M. GABRIELA; CARMONA, N.B.; DESJARDINS, P.,

Calgary

Congreso; Geological Society of America Rocky Mountain Section Meeting; 2017

Geological Society of America

A preliminary sedimentological analysis was carried on in two stratigraphic sections of the Jurassic-Cretaceous Vaca Muerta Formation exposed in the Picún Leufú area, Neuquén basin, central-westernArgentina. The Vaca Muerta Formation comprises shallow-marine mudstone, marl and limestonelithofacies with high organic matter content. At a seismic scale, these deposits constitute aggradingbottomset reflections followed by prograding, sigmoidal foreset reflections downlapping over thebottomset top. In outcrop, deposits lacking visible downlap terminations were correlated with thebottomset reflections. These strata display important facies variations, and were subdivided into shorefaceand open bay (FAA), mixed carbonate-siliciclastic ramp (FAB) and depositional lobe (FAC) faciesassemblages, representing retrograding-prograding and retrograding successions. The first retrogradinginterval is composed by the FAA (~5.5 m thick), which is separated from the underlying eolian dunedeposits of the Quebrada del Sapo Formation by a sharp and erosive surface. The prograding interval isdeveloped above the FAA, and consists of a thick succession (~100 m) of FAB and FAC, making the bulkof the bottomset stratal thickness. The FAB is composed by outer ramp and mid ramp facies, whereas theFAC consists of distal lobe fringe, lobe fringe and lobe facies. The prograding succession starts with lobefringe and distal lobe fringe deposits, which are the product of hyperpycnal flow sedimentation in anouter ramp setting. These facies are followed by a slumped outer ramp capped by mid ramp facies and amonotonous package of outer ramp facies. The succession is interrupted by the second retrogradinginterval composed by lobe facies, which ends up with a maximum flooding surface at the top. In short, thesedimentological dataset allows facies differentiation of homogeneous seismic scale strata, due to therecognition of (1) a normal transgressive event at the base, and (2) the progradation and subsequentretrogradation of a depositional lobe system in outer ramp and mid ramp settings subject to hyperpycnalflow deposition.