Depositional controls on mixed carbonate–siliciclastic cycles and sequences on gently inclined shelf profiles

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Abstract

The interplay between carbonate production and siliciclastic input produces mixed systems that typically contain a very high degree of lateral and vertical facies heterogeneity. This heterogeneity complicates the sequence stratigraphic analysis of mixed systems. Outcrop studies facilitate the deciphering of controls and understanding of facies distributions within sedimentary successions. The Picún Leufú Anticline in the Neuquén Basin (Argentina) offers the opportunity to integrate large-scale depositional architecture with detailed facies descriptions of the shelf to basin successions of the Upper Jurassic – Lower Cretaceous Quintuco – Picún Leufú – Vaca Muerta System. The strata in the system are mixed and range in depositional environments from shallow marine sandstones and limestones to deep basinal shales. These environments are arranged in metre-scale shallowing upward cycles and cycle sets, with increasing carbonate proportions in regressive hemicycles. Increased input of siliciclastic material from the volcanic arc area occurred during phases of relative sea-level rise and was controlled by the intensity of along-shelf currents. The shelf transport was driven by the available accommodation space on the shelf, and therefore was a function of the eustatic sea-level fluctuations. Within the studied section, a pure carbonate depositional system developed because siliciclastic input was shut down either due to long-lived highstand settings or a sudden climatic change to more arid conditions. Carbonate–siliciclastic mixing in this setting is a function of siliciclastic dilution of the carbonate sedimentation and differs from the classical reciprocal sedimentation model, which typically includes shut-off of carbonate production during lowstand periods. In the regional context, the subsurface strata of time-equivalent reservoirs in the Eastern Neuquén Embayment display strong similarities of architecture, indicating that similar mixing processes occurred along most of the Neuquén Basin.

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