Anatomy and sedimentology of a fluvial lowstand wedge: the Avilé Member of the Agrio Formation (Hauterivian) in Central Neuquén Basin, Argentina.

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

Congreso; 23rd IAS Meeting of Sedimentology; 2004

One of the most outstanding features of the Mesozoic fill of the Neuquén basin in West Central Argentina is the development of non-marine wedges that sharply overlie shallow- and even deep-marine deposits across major erosional surfaces. These deposits have been interpreted as second order lowstands developed in response to important relative sea-level falls. Even when these wedges have similar lithology and genesis, they differ considerably in facies patterns and distribution. Therefore, it is not easy to define a common vertical evolution or external controlling factors for these deposits and detailed facies and architectural analysis is needed to improve the understanding of these lowstand wedges. The Hauterivian Avilé Member of the Agrio Formation has developed in response to one of these major relative sea level falls. It is 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 basin. Aeolian deposits characterise the eastern part of the outcrop area while the western sector is dominated by fluvial/lacustrine facies with minor aeolian intercalations. Detailed sedimentlogical logging and architectural analysis in eleven localities throughout the central part of the basin allowed the identification of 11 sedimentary bodies that record the evolution of fluvial environments trough this lowstand wedge. Large-scale channel units include complex sheets and ribbons as well as simple ribbons developed under contrasting accommodation/supply rates. Complex sheets represent the accumulation of fluvial channels, probably with a braided pattern, under low accommodation conditions. Complex ribbons might also represent low accommodation conditions related to fluvial distributaries flowing into long living ponds. Simple ribbons are closely related to fine-grained floodplain deposits and usually develop large-scale inclined surfaces. Small-scale sandy and heterolithic channels are related to fine-grained floodplain/lacustrine deposits. Also associated with these fine-grained deposits small scale bars and sandstone lobes are present 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 sands and aeolian deposits (dunes and sandsheets) are present, recording episodes of water table lowering. Sandstone channel style and distribution are linked both with regional facies distribution and with the vertical evolution within the lowstand wedge. According to palaeocurrent distribution, those localities that are situated in an upstream position (i.e. south of the study area) are characterised by a relatively small thickness and almost completely dominated by the superimposition of complex sandstone sheets. This implies low accommodation compared to a relatively high sediment supply and the development of a bedload dominated fluvial system characterised by a multiple channel pattern and with no preservation of fine-grained floodplain deposits. Towards the north of the study area, in a downstream position, the studied unit shows a much greater thickness (up to 150 metres) and a high proportion of fine-grained floodplain deposits. Yet, the basal portion of the Avilé Member in the downstream sector is also characterised by the amalgamation of complex sheets, but a gradual increase in the proportion of floodplain mudstones is recorded up in the section. This is also accompanied by the occurrence of various types of channels and bars associated with fine-grained facies and by the development of isolated simple ribbons that denote a meandering fluvial system. Even when the vertical evolution in upstream sector is obscured by reduced accommodation, in the downstream area the increase in the proportion of fine-grained facies and the change in fluvial style might reflect a gradual increase in accommodation (relative to coarse-grained sediment supply) that could be associated with an overall (second order) transgressive trend developed after the relative sea level fall that produced the onset of non-marine accumulation in the central part of the basin.