Biogenic reworking patterns in highly bioturbated shallow-marine transgressive successions (Jurassic, Neuquén Basin, Argentina)

Thick (10s of m) successions of highly bioturbated, storm-influenced shallow-marine deposits are relatively uncommon in the rock record. Some examples have been reported from retrogradational successions associated with long-term transgressive periods, but also from strongly progradational systems with high sedimentation rates. Therefore, it remains unclear what ultimately controls the persistent and intense biogenic reworking often observed in these storm-influenced systems.Here we present a detailed facies andichnological study of an outcrop example from the Lower-Middle Jurassic of northern Neuquén Basin (Argentina) with the following objectives: a) to describe and interpret anintensively bioturbated succession deposited in proximal-offshore to shoreface settings, b)to record the transitions from fully-preserved storm beds to pervasively bioturbatedexamples, and c) to discuss the main syn- and post-depositional controls on thedestruction/preservation of the event beds. The studied 150 m-thick succession contains four parasequences showing an overall aggradational stacking developed during long-term transgressive conditions. Parasequences are mostly composed of upward-coarsening packages including three main facies associations: a) upper-offshore sandy to silty mudstones, b) offshore-transition muddy sandstones, and c) lower-shoreface very finegrained sandstones. Primary physical structures (mostly HCS and SCS) are commonly preserved in the latter, but are rarely observed in the other two associations, due to extensive biogenic reworking. These bioturbated deposits are the focus of this paper. Muddy sandstone beds (0.30-0.90 m-thick) in the offshore-transition association display a distinct tabular geometry and they alternate with subordinated sandy mudstones (<0.30m-thick). In the muddy sandstones bioturbation is typically high (BI 4-6) and the trace fossil suite is dominated by an archetypal Cruziana ichnofacies (Teichichnus, Asterosoma,Rosselia, Chondrites, Planolites, Thalassinoides, Rhizocorallium, Palaeophycus, Phycosiphon). Beds with preserved physical structures invariably show HCS, lower bioturbation (BI 1-3) and range in thickness from 0.15-0.40 m. A complete vertical and lateral transition occurs between these storm-related beds and the fully bioturbated muddy sandstones. In the proximal-offshore association, highly bioturbated (BI 5-6), tabular sandy and silty mudstones dominate. Bed contacts are diffuse due to intense biogenic reworking and low grain-size contrast. The trace fossil suite includes a distal Cruziana assemblage (Phycosiphon, Chondrites, Phycosiphon, Planolites, Teichichnus, Helminthopsis, Thalassinoides, Rhizocorallium, Zoophycos). Sandy beds (<0.10 m-thick) with planar lamination and wavy tops are sporadically preserved within this association; bioturbation is moderate to low (BI 1-4) and typically decreases from top to bottom. These sandy beds are interpreted to represent the distal expression of storm events. Systems like this could be preliminarily classified as low-energy and storm-influenced, in which low frequency of storm events would result in their low preservation. However, similarly, it could be attributed to a system of rapid and extensive, post-storm re-colonization. Results of this study suggest that it is important to understand the main controls on the long-term ability of deposit-feeder trace makers to fully occupy sandy substrates in effective accommodation spaces, obliterating most of the sandy storm beds. Significantly, the physiography and size of the basin, as well as the distance from a coeval riverine input could be the ultimate controlling factors for these unusually thick, highly bioturbated shallow-marine succession.Fil: Schwarz, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Poyatos Moré, Miquel. University of Oslo; NoruegaFil: Boya, Salvador. Universitat Autònoma de Barcelona; EspañaFil: Gomis Cartesio, Luz Elena. No especifíca;34th International Meeting of SedimentologyRomaItaliaInternational Association of Sedimentolog

Multi-scale influence of topography on shallow-marine successions associated with long-term transgressions: from Argentinean outcrops to the Norwegian shelf subsurface

Thick shallow-marine successions associated with longterm transgressions are less well known than their thin, well-sorted counterparts, more widely studied due to their potential to form good reservoirs. In these successions, particularly in storm-dominated examples, bioturbation can obliterate primary sedimentary characteristics, making stacking patterns and sequences difficult to define, and challenging our understanding of the main controls in their resulting depositional architecture.Fil: Poyatos Moré, Miquel. University of Oslo; NoruegaFil: Schwarz, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Boya, Salvador. Universidad de Barcelona; EspañaFil: Gomis Cartesio, Luz Elena. No especifíca;Fil: Midtkandal, Ivar. University of Oslo; Noruega34th Geological Winter MeetingTrondheimNoruegaGeological Society of Norwa

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

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 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.Fil: Poyatos Moré, Miquel. University of Oslo; NoruegaFil: Schwarz, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Boya, Salvador. Universidad de Barcelona; EspañaFil: Gomis Cartesio, Luz Elena. No especifíca;Fil: Midtkandal, Ivar. University of Oslo; Noruega34th International Meeting of SedimentologyRomaItaliaInternational Association of Sedimentolog

Architecture and controls of thick, intensely bioturbated, storm-influenced shallow-marine successions: An example from the Jurassic Neuquén Basin (Argentina)

Thick (>100 m-thick), highly bioturbated storm-influenced shallow-marine deposits are not frequent in the stratigraphic record, but they tend to be common in aggradational to retrogradational successions. Individual storm-event beds have typically low preservation potential in these successions, yet depositional settings are characterized on the basis of storms processes. Here we present a sedimentological study of a thick, bioturbated exhumed succession deposited during the early post-rift stage of the Neuquén Basin (Argentina) and compare its stratigraphic record with examples worldwide, in order to discuss the potential factors controlling the total overprint of storm-event beds during several million years. The Bardas Blancas Formation being 170–220 m thick in the study area is dominated by muddy sandstones and sandy mudstones, and it also includes subordinate proportions of clean sandstones and pure mudstones, collectively representing different environments of a storm-influenced shoreface-offshore system. The offshore transition and proximal offshore strata invariably comprise intensely bioturbated deposits, with only a few preserved HCS-sandstone beds. The unit shows for most of its thickness a long-term aggradational pattern spanning 7–10 Myr and is associated with low riverine influence. By combining the observations and interpretations of the Bardas Blancas Formation with other subsurface and exhumed intensely bioturbated, shallow-marine successions, we dispute the general assumption that these are associated with low frequency or low magnitude of storms. Alternatively, we argue that the long-lived efficiency of benthic fauna on overprinting most if not all the storm-event beds that reached the offshore-transition sector, results from the combination of several factors: deposition in relatively confined marine depocentres, persistent low riverine influence, and long-term aggradational stacking pattern. As these conditions can develop in a variety of basin styles, such as rift, early post-rift, and foreland settings, the recognition of thick, bioturbated successions as the ones discussed here can be used to infer more realistic constrains for depositional models and better predict facies distribution in such storm-influenced systems