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Shelf-margin Clinoforms of the Vaca Muerta-Picún Leufú System in the Neuquén Basin, Argentina

Abstract

The western flank of the Sierra de la Vaca Muerta (SdlVM) anticline exhibits a general NE-SW strike with E-SE dipping stratified beds with inclinations of approximately 15° to 40°. The anticline exposes a continuous record of the early to late Tithonian Vaca Muerta - Picún Leufú system. The Tithonian deposits in the SdlVM expose the stratigraphic architecture and evolution of the progradational shelf-margin clinoforms that fill the basin in a southeast-northwest direction. The clinoforms are composed of carbonate and siliciclastic sediments; the topsets contain the deposits of the Picún Leufú Formation, and the foresets and bottomsets the deposits of the Vaca Muerta Formation. During late Jurassic-early Cretaceous, low amplitude changes in relative sea level prevailed. Therefore, we look for insights into other controls beyond sea-level fluctuations that generated this mixed-system of prograding clinoforms. We studied lithological, mineralogical (near-infrared spectroscopy), and isotopic (organic δ13C and δ15N) characteristics using 733 samples from outcrops; we studied elements composition (X-ray fluorescence) using cored beds (32 1m cores from outcrop). The outcomes were integrated with field observations and satellite images to build a 3D digital elevation terrain model of the SdlVM. Additionally, a geologic transect that covers a lateral extension of ~18Km and a vertical succession of 716m was produced. We define four depositional sequences in the studied transect. The four sequences studied here, document the distal end position of the carbonate-capped clinoforms of the Vaca Muerta-Picún Leufú system at the end of the Tithonian before the system transitions into the Vaca Muerta- Quintuco system. The clinoforms prograde basinward and display a sigmoidal geometry with slope gradients that increase from 0.5° to 4°. Clinoforms with steep shelf edges are covered with carbonate-rich beds and contain large mass-transport complexes. While clinoforms with low angles are made of calcareous-mudstones and sandstones and contain small slumps. The bottomset are mostly fine-grained sediments where organic matter was preferentially preserved. The uppermost-and-younger clinoforms exhibit shoreface facies where the clastics show a detrital origin, the older clinoforms display nearshore and offshore facies.According to mineral composition, cyclic fluctuations of mineral-assemblages in relation to different positions of the stratigraphic sequences are observed. In addition, high correlations were found between clays minerals of volcanic origin and total organic carbon. Total organic carbon also shows strong positive correlations to V, Mo, Ba, and Ni.