The Upper Jurassic – Lower Cretaceous formations in the Neuquén Basin – an accommodation controlled mixed carbonate – siliciclastic system

Abstract

The Quintuco - Vaca Muerta formation in the Neuquén Basin in Argentina is a mixed carbonate-siliciclastic system with a high degree of lateral and vertical heterogeneities. Integration of outcrop and core-based subsurface high-resolution sequence stratigraphy with the analysis of outcrop and seismic architecture offers new insights into the controlling parameters on carbonate-siliciclastic sedimentation. The system is an interplay between shore-parallel transported clastics and fluctuating accommodation space that control both the transport of clastics and the onset of carbonates. Seismic data of the Quintuco Formation in the Loma La Lata Field and outcrops of the time-equivalent Picún Leufú Formation in southern Neuquén Basin document a similar stratal architecture consisting of 1) a lower prograding clinoform unit dominantly composed of siliciclastics with upwards- increasing carbonate content, 2) an aggrading middle unit that can be subdivided into a lower siliciclastic dominated interval capped by a clean carbonate package, and 3) an aggrading upper unit in which siltstones and sandstones alternate with minor carbonates. In outcrop the low-angle clinoforms of the lower unit contain facies transitions from quartz sandstone in the topsets with high- energy bidirectional bedded sandstones along the shelf edge to siltstone and shale in the bottomsets. The time equivalent clinoforms in the subsurface are imaged on seismic time-slices and characterized by shelf breaks that are paleo-shoreline parallel over hundreds of kilometers. In the middle and the upper unit the carbonates are represented by ooid-skeletal grainstone intervals and oyster to coral floatstone beds. In each unit, transitions between siliciclastic and carbonate sedimentation occur rapidly at every scale. At a meter-scale clastic sandstones alternate rapidly with clean ooid- skeletal grainstones while on a decameter scale siliciclastic silt- and sandstone dominated packages alternate with pure carbonate in - tervals. The similarities of the stratigraphic architecture of the two areas, which are about 150 kilometers apart, indicate a basin-wide eustatic sea-level control on sedimentation and stacking of sequences rather than a local sediment supply driven process. The consistent clinoform breaks in the subsurface point towards longshore currents as the main mechanism for siliciclastic input. This interpretation is supported by the general fining trend of the clastic grainsize from south to north, which is in a more distal position from the continental input in the south as corroborated by provenance studies. Moreover the rapid transitions between siliciclastic and carbonate sedi - mentation are largely controlled by the available accommodation space on the shelf. High accommodation allows the development of strong longshore currents, which can carry sand and silt material for hundreds of kilometers across the shelf and thereby shut off the carbonate factory. During times of low accommodation, the cur - rent cannot reach across the shelf environment and high carbonate production leads to mixed and pure carbonate deposits.