--> Compensational Stacking and Architecture of Mouth-Bar Deposits in a Mixed-Process Deltaic Environment; Mulichinco Formation, Neuquén Basin, Argentina

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Compensational Stacking and Architecture of Mouth-Bar Deposits in a Mixed-Process Deltaic Environment; Mulichinco Formation, Neuquén Basin, Argentina

Abstract

This study reports on the detailed examination of the Valanginian Mulichinco Formation outcrops in the Neuquén Basin, west-central Argentina. The treated unit was first deposited as a lowstand wedge-, followed by a transgressive systems tract in the context of a low gradient ramp. Deposits therein comprise contemporaneous continental and shallow marine strata of mixed-process coastal and inner shelf origin, which crop out along flanks of multiple eroded structural domes in the Andean fold and thrust belt. The Upper Member of the Mulichinco Formation contains mouth-bar deposits that preserve interbedded fluvial-, swash-, storm-, and tidal deposits distributed along a 12 km continuous north-south oriented outcrop. This interbedded relationship readily testifies the intrinsically complex and dynamic nature of mixed-process coastal environments and is considered key to understand the interaction of their parental processes. High resolution (down to 2 cm) photogrammetric models at outcrop scale have been coupled with measured sections to confidently trace and correlate important stratigraphic surfaces, and map spatial distribution of architectural elements. Variations in wedge-shaped parasequence geometries are attributed to compensational stacking of 4 progradational mouth-bar cycles, evidenced as the outcrop transects parallel to the palaeo-shoreline. The topmost parasequence reflects the maximum regression of mouth-bars and concludes thus the Mulichinco Formation lowstand wedge development. The overlying transgressive systems tract is characterised by retrogradational stacking. Storm- and tidal deposits occur within both distal and proximal mouth-bar deposits, both separately and interbedded. Where they occur together, storm deposits are encapsulated within tidal deposits, and are sharp-based homogeneous sandstones with lensoidal- to tabular bedforms. Furthermore, an upward gradual increase in tidal signature is common, which is attributed to gradual increase in tidal reworking capacity in the aftermath of storm events. Storm deposits increase and decrease in abundance, from proximal to intermediate and then to distal directions, respectively, indicating that the storm wave oscillation in such mixed-process environments has the strongest erosional-depositional impact at the intermediate position.