Early post-rift sequence architecture: evidence for physiographic and relative sea-level control (Middle Jurassic, Neuquén Basin, Argentina)

Abstract

A detailed facies and sequence-stratigraphy analysis was carried out in the early post-rift succession of central Neuquén Basin (Middle Jurassic Cuyo Group). More than three/fourths of the total post-rift strata (less than 320 m thick) comprise fine-grained sandstones to mudstones representing a storm-dominated shoreface to offshore system (Late Toarcian-Early Bathonian). The remaining stratigraphy (less than 30 m) is dominated by sand-rich deposits with clinoforms interpreted to be a fluvio-dominated deltaic system (Late Bathonian-Early Callovian). In the Late Toarcian? Early Bathonian succession nine parasequences were identified and grouped in four parasequence sets (PSS). A long term transgressive trend (about 10 ma) is defined for the basal three PSS, which are dominated by offshore-transition to upper-offshore deposits. The maximum transgression is represented by organic-rich shales. These deposits are overlain by the last PSS which represents highstand conditions. A regressive surface of erosion cuts this succession, indicating an abrupt basinward shift and the onset of the fluvio-deltaic system. The long-term transgression that dominates the early post-rift suggests a period of sustained accommodation creation. Changes in the stacking pattern of retrogradational parasequence sets are thought to be produced by changes in the bathymetry of the topography being flooded. Regional variations in thickness and timing for the early post-rift succession could be therefore the result of the relatively passive infill of an inherited topography of the syn-rift, suggesting that most of the accommodation was already created before the onset of the long-term transgression. The Cuyo Group succession reported here reflects the complexity in terms of depositional styles, sequence stratigraphic patterns, and controls that can be expected during the early-post rift infill of syn-rift structural depressions.