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Local-Scale (<2km), Sedimentary Architecture and Depositional Process Variability of a Mudstone-Dominated Shelf Succession, Book Cliffs, Utah

Abstract

Core-based mudstone studies, which form the larger part of recent mudstone research, give very limited insights into local-scale lateral variability in mudstones. So whilst regional variability in depositional processes is generally well understood, smaller scale variability is not. This study aims to characterize the local-scale along-shelf variability in facies, depositional processes and facies stacking patterns in a mudstone-dominated shelf succession. This study focused on offshore deposits of the Aberdeen Member (Blackhawk Formation) of the Book Cliffs (Utah) where excellent exposures allow key stratal surfaces to be correlated from coastal plain through shoreface deposits, to offshore muddy deposits. Comprehensive sedimentological analysis was completed on two ~45m stratigraphic successions north of Green River, approximately 1.5km apart and oriented parallel to the palaeoshoreline. The two logged successions are characterised by 0.5m to 2m thick coarsening upward bedsets that stack to form 5m - 12m parasequences. The basal parts of bedsets consist of heavily-bioturbated homogenous fine-mudstone with sporadic bed relics that thicken and coarsen upward into less-bioturbated well-preserved coarse-mudstone beds. This is interpreted to reflect a change from occasional current-deposited beds with depositional breaks allowing fauna to inhabit and disturb surface sediment to more frequent varying energetic conditions with relatively shorter to non-existent hiatuses. In some cases, bedsets are occasionally capped by small-scale (4cm) normally graded and inversely graded beds interpreted to represent deposition via turbidity currents and hyperpycnal flows respectively. The two measured sections exhibit varying facies, depositional processes and facies stacking patterns. For example, turbidite and hyperpycnite deposits are not present across the same stratigraphic intervals and bedset quantity differs across the measured sections indicating that mudstone-dominated shelf settings vary at extremely local scales. This is likely to be a result of local topography and/or wider palaeogeographic influences such as river discharge and delta-slope instability. The variability indicates that local dynamics are complex within shelfal mudstones resulting in highly heterogeneous mudstone successions. Furthermore, petrophysical properties will be affected by the heterogeneity, subsequently hindering or facilitating aspects of conventional and unconventional hydrocarbon systems.