Bottomset and foreset sedimentary processes in the mixed carbonate-siliciclastic Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Picún Leufú Area, Argentina

Abstract

The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.