Anatomy of a Fluvial Lowstand Wedge: The Avilé Member of the Agrio Formation (Hauterivian) in Central Neuquén Basin (Northwest Neuquén Province), Argentina

Abstract

The Hauterivian Avilé Member of the Agrio Formation constitutes a non marine lowstand wedge dominated by fluvial and aeolian deposits that sharply overlie deep-marine, ammonite-bearing shales of the Lower Member of the Agrio Formation in the central part of the Neuquén basin. Detailed sedimentlogical logging in twelve localities allowed the identification of eleven sedimentary bodies that record the evolution of fluvial environments through this lowstand wedge. Channel units identified include complex sheets and ribbons as well as simple ribbons developed under contrasting accommodation/supply conditions. Small-scale sandy and heterolithic channels are related to fine-grained floodplain/lacustrine deposits together with small-scale bars and sandstone lobes indicating overbank splays. In addition, large-scale lacustrine bars are also present, associated with complex ribbons, suggesting the development of distributary systems that fed relatively deep water bodies. Locally, aeolian reworking of fluvial channels and aeolian deposits (dunes and sandsheets) are present. Regional and vertical changes in fluvial style were recorded within this lowstand wedge. The up-dip area is characterised by a relatively small thickness and almost completely dominated by the superimposition of complex sandstone sheets. Towards the north of the study area, in a down-dip position, the studied unit shows a much greater thickness and a high proportion of fine-grained floodplain deposits. However the intercalation of bedload dominated and mixed load, high sinuosity fluvial intervals is recorded. This alternation represents contrasting accommodation/sediment supply conditions associated either with climatic fluctuations or with oscillations in fluvial base level that could be related to eustatic changes due to orbital processes. Although the vertical evolution in the upstream sector is obscured by reduced accommodation, in the downstream area the increase in the proportion of fine-grained facies and the gradual change to a mixed-load fluvial system reflect a gradual increase in accommodation (relative to coarse-grained sediment supply) that could be associated with an overall (low frequency) transgressive trend developed after the relative sea level fall that produced the onset of non-marine accumulation in the central part of the basin.