Sedimentology of the aeolian to alluvial transition in the Avilé Member of the Agrio Formation (Lower Cretaceous), northern Neuquén Province, Argentina.

VEIGA, G.D.; SPALLETTI, L.A.

Congreso; II Congreso Latinoamericano de Sedimentología y VIII Reunión Argentina de Sedimentología; 2000

The Avilé Member is a less than 100 m thick sandy unit intercalated between outer ramp mudstones and shales from the Lower and Upper Members of the Agrio Formation. It is widely distributed all over northern Neuquén in west-central Argentina, and it has been interpreted as the LST of a third order sequence (Mesosecuencia Mendoza Superior) produced by an important sea-level fall. In the studied area it is characterised by two different packages that were related to the lower and upper prograding complexes of a LST wedge. In the Pampa de Tril region, the upper of these packages is very well exposed, and it is composed of a 40 m thick sandstone and mudstone association. Facies and architectural analysis of these deposits allowed the identification of six different genetic units, characterising fluvial and aeolian processes. The vertical distribution of these units shows a gradual transition from aeolian arid-conditions to a more humid scenario, with the development of a more permanent fluvial system. Aeolian deposits characterise the lower and middle portion of this interval and are composed of dune units, dry interdunes, and flooded interdunes, which in terms of its grain-size and geometry could be subdivided in coarse, lenticular channelled units and heterolithic tabular ones. Dunes are composed of extensive, tabular sand bodies, of cross-stratified medium sand, that may be arranged as a simple set, up to 5 m thick, or as cosets 1 to 4 m thick. They are bounded by planar surfaces that truncate sets of cross-strata and internally show grain flow and grain fall deposits. Cross-stratification shows a nearly unimodal paleocurrent trend towards the north. Dry interdune units are composed of horizontal, trough and low-angle cross-stratified sandstones, that sharply overlie dune deposits. The basal surface is planar, with minor irregularities and may represent a deflation surface controlled by the position of the water table. Dune units are lateraly truncated by lenticular sandstones that locally scour older deposits. They are composed of coarse sands with mud intraclasts and trough cross-stratification; these deposits formed by running water represent flooded interdunes. Dune deposits are also associated with tabular heterolithic deposits probably developed during long-lived interdune flooding under lower energy conditions. These deposits are characterised by laminated mudstones with thin wavy and lenticular sandstones and abundant desiccation cracks. The wet interdune units also show some FU cycles composed of fine-grained sandstones with climbing-ripple lamination that grade upwards into mudstones that represent flashy, unconfined floods within interdune areas. The upper portion of the Avilé Member is composed of lenticular sand bodies that vertically and laterally interginge with tabular muddy deposits intercalated with fine-grained sandstones. Lenticular sandstone bodies show a concave, erosional lower boundary and also abundant mud intraclasts. The internal architecture is characterised by lateral accretion structures which suggest high sinuosity fluvial channel migration. Tabular fine-grained units are interpreted as floodplain deposits; massive and fine-grained sandstones, laterally associated with channel units, represent crevasse splay deposits. The upper Avilé represents the development of a more permanent fluvial system, formed under more humid conditions in comparison with those of the lower portion of this unit.