PARASEQUENCE SPATIAL AND TEMPORAL VARIABILITY IN A REGIONAL SEQUENCE STRATIGRAPHIC FRAMEWORK: GRASSY MEMBER, BLACKHAWK FORMATION, UTAH

Abstract:

The Campanian Blackhawk Formation, Book Cliffs, Utah has been studied by numerous authors over the last four decades as a field laboratory for the definition and characterization of siliciclastic parasequences as building blocks of depositional sequences and basin-fill stratigraphy. The newly expanded subsurface dataset presented in this study has allowed for a more regional characterization of the Grassy Member than possible from outcrop studies alone. The main Grassy outcrop belt exposes the Gray-Tusher axis but this study has identified new subsurface units across depositional strike and further down depositional dip to the northeast; these are interpreted as subaqueous prodelta deposits of a proposed age equivalent fluvio-deltaic Grassy system here named the Bitter Creek Delta, fed by the Alrad fluvial axis. An extended upper Blackhawk Formation correlation framework shows the underlying Sunnyside and Kenilworth Members as also thickened in this area by comparison with age equivalent strata to the east, suggesting this northern depositional axis was long lived. Newly identified subsurface lowstand deposits allow for a more complete sequence stratigraphic interpretation tied to more cryptic outcrop evidence of base level change in the upper Blackhawk Formation. Mapping of internal facies belts and new trajectory analysis of Grassy parasequences and constituent bedsets indicate a descending shoreline trajectory in the lower Grassy, interpreted as a forced regression. Upward changes in intra-parasequence bedset facies architecture suggestive of increased fluvial influence are interpreted in the context of both autogenic and allogenic processes. The position of a lowstand shoreline associated with intra-Grassy sequence boundary GSB2 has been identified. Using the erosional remnants of Grassy parasequence 3, new regional correlations indicate up to 15 m of GPS3 has been removed by subsequent transgressive ravinement. Whilst extensive work has focused over many years on characterizing the regressive record in the Blackhawk Formation, transgressive ravinement has been less discussed or quantified and may be a key component not accounted for in Blackhawk depositional models, including the removal of lowstand shorelines.