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

Abstract

Large-scale depositional and paleogeographic reconstructions of shallow-marine to continental units help better understanding intrinsic/extrinsic controls such as basin physiography and subsidence pattern, sediment delivery into the basin and dispersal processes within it. The Pilmatué Member was deposited in the interior sea of a back-arc basin (Neuquén Basin), has a detailed biostratigraphic ammonoid zonation, and represents about 2 Ma (late Valanginian-early Hauterivian). This paper, integrating outcrop and subsurface data, documents the depositional and sequence-stratigraphic architecture of the Pilmatué Member across a 60-km-long down-dip transect (E-W) and a 130-km-long along-strike transect (S-N). In the eastern side of the down-dip transect the unit (350 m thick) comprises 25% of continental deposits (delta-plain fine-grained sandstones and mudstones), 60% of shallow-marine strata (delta-front fine to very fine-grained sandstones), and 15% of prodelta to offshore mudstones. Six deltaic parasequence sets were recognized. Parasequence sets PSS-1 to PSS-4 show an aggradational to progradational stacking, whereas PSS-5 and PSS-6 show an aggradational to retrogradational pattern. In the western end of this down-dip transect, the unit (700 m thick) is composed of offshore mudstones with 5% of delta-front facies. Offshore mudstones of PSS-2 pass downdip to a carbonate-siliciclastic-sandy interval, interpreted to represent a contemporaneous tide-dominated dune field. Individual parasequence sets double their thickness over 60 km producing a wedge with a divergent trend and no evidence for clinoform development. The 130-km-long along-strike transect runs northward from the western end of the down-dip transect. 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 bioturbated package of sandy siltstones, which was deposited by storm-related flows distally of deltaic shorelines located to the east. Parasequence sets PSS-3 to PSS-6 in this segment are composed of storm-dominated, offshore-to-shoreface parasequences, without evidence of nearby fluvial input. Together PSS-3 to PSS-5 show a long-term aggradation to progradation pattern with local progradation to the north. PSS-6 depositional architecture suggests deeper conditions and westward progradation. The remaining 50 km of the transect display a 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-gradient basin was probably < 50 m deep and accommodation was larger toward the magmatic arc, developing a divergent wedge in that direction. The delivery of terrigenous sediments 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 conditions favored repeated mud re-suspension by storms that helped distributing fine-grain sediments over vast areas, except in the northernmost sector of the basin. Interpretation of maximum progradation timing differs from south to north, challenging the idea that sequence-stratigraphic schemes produced in small areas can be extrapolated to entire basins.