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

Abstract

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.