Facies characteristics and depositional models of highly bioturbated shallow marine siliciclastic strata: an example from the Fulmar Formation (Late Jurassic), UK Central Graben

Abstract

Abstract An extensive core database from the Late Jurassic Fulmar Formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated, heterolithic, shallow marine siliciclastic strata. Twelve major sedimentary facies (1–12) are defined on the basis of lithology, grain size, visually estimated sand-silt content and the presence or absence of primary sedimentary structures. Ichnofabric characteristics are also defined and have proved to be not always facies specific. For purposes of depositional modelling the 12 facies types are resolved into six facies associations (A-F), each representative of distinct shoreline/shallow marine environments. From these associations three broad depositional models have been constructed: storm-influenced shoreface (model 1), bioturbated shoreface (model 2) and a speculative bioturbated shelf model (model 3). Stratigraphical and palaeontological criteria suggest that models 1 and 2 are shoreline-attached; sedimentological data indicate that they occur as end-members to a spectrum of shoreline settings constrained by incident wave energy ‘bands’. For shorefaces subject to significant storm influence, well-preserved event beds occur seaward of the upper shoreface. In the bioturbated shoreface model, however, the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking. The bioturbated shelf model displays no clear evidence of shoreline connection, with the resultant facies often exhibiting highly abundant siliceous sponge spicules, together with other open marine indicators such as belemnites and rare ammonites. Unlike the shoreface sand bodies, which received sediment from basin-margin fluvial systems, shelf sand bodies in the Fulmar Formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone. This sediment may have undergone a long transit/residence period on the shelf prior to eventual burial in hanging-wall depocentres strongly influenced by Zechstein salt withdrawal. This study demonstrates that an ichnofabric approach (as opposed to one of simple ichnodiversity) is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata. In the Fulmar formation, ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels, water depth, sedimentation rates, erosion rates, substrate consistency and dissolved oxygen levels. Evidence of these fluctuations is often apparent at intra-facies scale, but would largely go unnoticed using a conventional sedimentological approach to data gathering, based on ichnodiversity at best. Depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity. Ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling.