Origin and evolution of shallowing-upward clastic successions: A case example from the Lower Cretaceous Agrio Formation, Neuquén Basin, Argentina

Abstract

Shallowing-upward clastic successions are very common in ancient shelfal systems. These deposits are commonly described as “parasequences” controlled by high-frequency sea-level changes. Nevertheless, the real significance of these parasequences in terms of depositional environments is controversial. This contribution discusses the origin of shallowing-upward successions from the middle-late Hauterivian Agua de la Mula Member (Agrio Formation, Neuqu´en Basin, Argentina). Detailed analysis of ten stratigraphic sections allows the recognition of six sandstone facies, three heterolithic facies, two shale facies and four calcareous facies. These facies compose four facies associations: a) prodelta/offshore, b) distal delta front, c) proximal delta front and d) carbonate ramp. In outcrop, these facies are stacked forming thickening and coarsening-upward progradational successions, grading from shales to sandstones, ending with bioclastic calcareous levels. On a regional scale, seven fourthorder depositional sequences have been identified, which in turn are composed of fifth-order elementary depositional sequences, all of allocyclic origin. Field evidences supported by facies analysis suggest that the Agua de la Mula Member was accumulated in a homoclinal shallow ramp built by very low gradient delta systems. Facies analysis suggests that these deltas resulted from a combination of dilute hyperpycnal flows and marine diffusion processes (mainly wave-action). These poorly known low-gradient delta systems correspond to hyperpycnal littoral deltas. These deltas are fed by long-lived discharges of dirty rivers, which plunge in the coastal zone due to their higher density creating dilute hyperpycnal flows. This effect would have been emphasized by the possible existence of a brackish marine environment, which would have allowed these flows to extend hundreds of kilometers basinward aided by wave action, along gentle slopes (<0.02◦). The decrease (or cessation) of dirty river discharges, controlled by allocyclic (climatic?) factors, would contribute to reestablishment of “normal” marine conditions in the basin. Consequently, the lower influx of freshwater and suspended materials would have allowed the development of calcareous (bioclastic) levels of regional extension, often recognized towards the top of the progradational sequences. These bioclastic levels represent condensed intervals, which would possibly involve a time span equal to or longer than that required for the accumulation of delta front deposits of hyperpycnal littoral deltas.