Late Cretaceous change in fluvial style in the Neuquén Basin of Argentina: Sedimentologic and ichnologic evidence

C.A. PIRRONE; P. ARMAS

Mendoza

Congreso; International Sedimentological Congress; 2010

International Sedimentological Association. Academia Nacional de Ciencias

During the Mesozoic Era, the Neuquén Basin was a wide depositional area located at thecentral-west of Argentina. In particular, an extensive fluvial system developed all along thenorth area of the basin during the Late Cretaceous. Fluvial deposits of the Anacleto Formation(late Santonian – early Campanian) at the anticline of Ranquil-Có (southern MendozaProvince) are analyzed and compared with outcrops from other areas of the basin,highlighting differences in their dynamic and possible controls on sedimentation. This studywas based on the analysis of several sections along a 3km outcrop belt. Lithofacies andarchitectural elements were characterized in order to interpret depositional processes andsedimentary environments. Three gravel lithofacies (Gm, Gt and Gp), four sandy lithofacies(Sp, Sr and Sh), and two mudstone lithofacies (Fm and Fl) were defined. Associations ofthese lithofacies and analysis of the geometry of sedimentary bodies allows differentiatingvarious fluvial architectural elements. The intrachannel elements identified gravel channels(CH I) and sandy channels (CH II). These latter consists of multi-storey fills. Channelichnofaunas are characterized by moderate to high density and very low ichnodiversity ofvertical trace fossils probably produced by suspension feeders and emplaced at the top of thechannel-fill deposits between sedimentary events. The assemblage consists of Skolithos isp.and Diplocraterion isp., representing a non-marine occurrence of the Skolithos ichnofacies.This ichnofacies is typical of moderate-to high-energy environments. Presence of escapestructures at the base of channel-fills is consistent with rapid deposition. Transverse barsspread across gravel and sandy channels (GB and SB) aggrading and migrating in adownstream direction. Lateral accretion elements (LA), such as point bars with low angleacretional surface, are recognized also. Point bar deposits are also intensely bioturbated byelements of the Skolithos ichnofacies (Skolithos isp. and Diplocraterion isp.). These depositsare characterized by high density of trace fossils and very low ichnodiversity. Trace fossils arevisibly at the top of each surface on positive relief and due to differential erosion and filling.Presence of vertical burrows of suspension feeders is related to high oxygen levels andorganic particles in the water column in a moderate-to high-energy environment.Opportunistic producers colonized point-bar deposits during times of reduced sedimentationrates followed by pulses of erosion and rapid sedimentation. Floodplain architectural elements(OF), such as crevasse channels (CS I) and crevasse splay (CS II) have been identified. Theseelements are interpreted as evidence of strong climatic seasonality in the area. Two differentfluvial systems have been interpreted: a high sinuosity meandering system (SF II), and abraided system (SF II). The lower interval of the Anacleto Formation is represented by theFluvial System I characterized by sandy channels (CH I) with transversal sandy bars (SB) andlateral accretion elements, all of them distinctive of high sinuosity meandering rivers.Floodplain deposits (OF) are thick and interbedded with crevasse channel deposits (CS I andCS II). The upper interval is represented by the Fluvial System II which is interpreted as abraided river due to the presence of coarsening-upward packages, gravel channels (CH I) withgravel transversal bars (GB), absence of lateral accretion surfaces and paucity of floodplaindeposits. The uppermost interval of the studied succession consists of thick floodplaindeposits. Changes in the sedimentation styles are due to allocyclic controls, such us tectonicand climatic events, which affected the northern edge of the Neuquén Basin, during the UpperCretaceous. These factors were responsible for base-level changes that markedly affectedfluvial sedimentation in the area.