A closer look at mixed-process subaqueous delta platforms: proximal to distal variability along the Jurassic Lajas Formation (Neuquén Basin, Argentina)

Subaqueous deltas are common features in modern shelves, but they are not often explicitly recognized in the rock record as dynamically linked components of a single deltaic depositional system. The large-scale continuous outcrops of the Jurassic Lajas Formation in the Neuquén Basin (Argentina) offer the opportunity to study in detail the sedimentary facies and stratigraphic architecture of subaqueous delta clinothems. The Lajas Fm. deposits at Lohan Mahuida are characterized by three main facies associations that represent shallowing and progradation of subaqueous delta lobes, from subaqueous delta slope, to rollover to subaqueous platform. This is expressed as upward-coarsening parasequences. The subaqueous delta slope and distal subaqueous platform are characterized by mud-rich heterolithic parasequence deposits with a dominance of density-driven and storm-wave processes (FA1). The middle reaches of the subaqueous platform are characterized by the co-existence of cross-beds, plane-parallel, low-angle and hummocky cross laminations, indicating that transport and deposition of sediment was controlled by a mix of density-driven, tidal and storm-wave processes (FA2). The inner reaches of the subaqueous platform are dominated by tidal processes and show stacked cross-strata interpreted as tidal bars and dunes (FA3) (as with the modern tide-dominated Han River Delta). The deposits of the Lajas Fm. at Lohan Mahuida enable us to better understand facies variability, process partitioning and the architecture of subaqueous deltas in the rock record within an integrated stratigraphic framework linking subaerial and subaqueous components. Based on the data presented in this work, we propose a depositional model for mixed process (tidal, wave and fluvially influenced) subaqueous deltas, which can be applied to better understand facies distribution on (paleo)shelves and in subaqueous depocenters.