Down-dip and along-strike characterization of shallow-marine strata in a low-gradient basin (Lower Cretaceous, Neuquén, Argentina)

SCHWARZ, E.; VEIGA, G.D.

Roma

Congreso; 35th IAS Meeting of Sedimentology; 2019

International Association of Sedimentologists

Large-scale depositional and paleogeographic reconstructions of shallow-marine tocontinental units help better understanding intrinsic/extrinsic controls such as basinphysiography and subsidence pattern, sediment delivery into the basin and dispersalprocesses within it. The Pilmatué Member was deposited in the interior sea of a back-arcbasin (Neuquén Basin), has a detailed biostratigraphic ammonoid zonation, and representsabout 2 Ma (late Valanginian-early Hauterivian). This paper, integrating outcrop andsubsurface data, documents the depositional and sequence-stratigraphic architecture of thePilmatué Member across a 60-km-long down-dip transect (E-W) and a 130-km-long alongstriketransect (S-N).In the eastern side of the down-dip transect the unit (350 m thick) comprises 25% ofcontinental deposits (delta-plain fine-grained sandstones and mudstones), 60% of shallowmarinestrata (delta-front fine to very fine-grained sandstones), and 15% of prodelta tooffshore mudstones. Six deltaic parasequence sets were recognized. Parasequence setsPSS-1 to PSS-4 show an aggradational to progradational stacking, whereas PSS-5 andPSS-6 show an aggradational to retrogradational pattern. In the western end of this downdiptransect, the unit (700 m thick) is composed of offshore mudstones with 5% of delta-frontfacies. Offshore mudstones of PSS-2 pass downdip to a carbonate-siliciclastic-sandyinterval, interpreted to represent a contemporaneous tide-dominated dune field. Individualparasequence sets double their thickness over 60 km producing a wedge with a divergenttrend and no evidence for clinoform development.The 130-km-long along-strike transect runs northward from the western end of the down-diptransect. The first 80 km show a relatively homogenous thickness but lateral facies changes.The offshore sandy interval of PSS-2 thins northward and is replaced by a highly bioturbatedpackage of sandy siltstones, which was deposited by storm-related flows distally of deltaicshorelines located to the east. Parasequence sets PSS-3 to PSS-6 in this segment arecomposed of storm-dominated, offshore-to-shoreface parasequences, without evidence ofnearby fluvial input. Together PSS-3 to PSS-5 show a long-term aggradation to progradationpattern with local progradation to the north. PSS-6 depositional architecture suggestsdeeper conditions and westward progradation. The remaining 50 km of the transect displaya significant thinning of the unit (to 300 m) and 100% of offshore-basinal facies. About 50%of these distal facies are composed of carbonate mud.The marine basin of the Pilmatué Member never developed a shelf-slope physiography,probably due to a long-term balance between sedimentation and accommodation. This low-gradientbasin was probably < 50 m deep and accommodation was larger toward themagmatic arc, developing a divergent wedge in that direction. The delivery of terrigenoussediments to the marine basin seems to have shifted laterally during the Pilmatué evolution,with a prevalent focus in the southern sector. Northern shorelines where fed by northward,along-shore currents when direct fluvial input was not present. The shallow-water conditionsfavored repeated mud re-suspension by storms that helped distributing fine-grain sedimentsover vast areas, except in the northernmost sector of the basin. Interpretation of maximumprogradation liming differs from south to north, challenging the idea that sequence stratigraphicschemes produced in small areas can be exlrapolaled lo enlire basins