Multi-scale influence of topography on depositional architecture of long-term transgressive successions (Jurassic, Neuquén Basin, Argentina)

Abstract

Shallow-marine successions deposited during long-term transgressions are considered to develop relatively thin and well-sorted deposits. Thick transgressive successions are rarely preserved in the stratigraphic record, although a few examples are described in subsurface and outcrop studies of rift basins. In these settings, reworking by active shelf processes and biogenic activity often result in partial or complete destruction of primary sedimentary structures; bed boundaries and geometries might become homogenized, and stacking patterns and stratigraphic sequences more difficult to define. In this study, an outcrop example from the Lower-Middle Jurassic of northern Neuquén Basin (Argentina) is presented, with the aim to a) refine the depositional model of a longterm transgressive succession in an early post-rift setting; and b) to constrain controls on stratigraphic architecture and lateral facies variability, and their response to postdepositional processes. To do this, a <300 m-thick succession has been studied along a>10 km continuous exposure, with detailed mapping, sedimentary logging and physical correlation of stratigraphic units, integrated with subsurface, biostratigraphic and ichnological data. The succession shows an overall retrogradational fining-upward development, with severalminor order regressive units. The lower part of the succession comprises laterallydiscontinuous (10´s m-long), coarse-grained transitional deposits, dominated by 10´s m-thick coarsening-up packages and discrete m-scale erosive conglomeratic lenses, interpreted asmouth-bars and distributary channel-fills. Above these, the middle and upper parts of thesuccession cover the entire length of the outcrop (>4.5 km) and thicken gradually southwards, although subsurface data reveals a fault-controlled, limited regional extension.They comprise laterally-continuous (>100´s m-long) fine-grained shallow-marine deposits, dominated by <1m-thick fine-grained structureless and highly bioturbated tabularmuddy sandstones and sandy mudstones, with locally-preserved HCS and bioclastic-richlevels. They are interpreted as storm-dominated lower-shoreface to upper-offshoredeposits. Internal characteristics and bed boundaries are diffuse due to pervasivebioturbation, suggesting recurrent periods of seabed exposure, oxygenation and colonization of organisms.The coarse-grained nature and lithology of the lower part of the succession are consistentwith a proximal sediment source, likely associated with erosion of intra-basinal highs. Itsvariable thickness and lateral distribution, and the marked onlap termination against theunderlying syn-rift deposits, demonstrates the partial infill of localized higheraccommodation areas. Above this, the middle and upper parts of the succession are more extensive, and display well-defined parasequences internally composed of tabular, laterallycontinuous bedsets (<5 m-thick), still preserving original stacking patterns. However, the vertical thickness variability of parasequences in their overall retrogradational pattern is interpreted to reflect the stratigraphic response of a transgression over a larger-scale rampstep and underfilled rift topography. The overall good sorting and fine-grained nature of sandstones indicates a mature, distal source of sediment, which was redistributed alongshore by storm/wave-dominated processes, and accumulated in inherited post-rift depocentres, where intensive biogenic reworking was favored. This study offers new insights in how lo interpret transgressive systems based on both primary depositional mechanisms and postdepositional processes, and provides useful tools lo understand and predicithe nature and potential preservation of these deposits in limited subsurface datasets, especially in the syn-rift lo early post-rift transition.