Local-scale, along-shelf variability in depositional processes, sedimentology and stacking patterns of a mudstone-dominated shelf, Book Cliffs, Utah.
Core based mudstone studies, which form a significant part of recent research into fine-grained successions, provide limited insight into local-scale (<2km) variability down depositional dip and across depositional strike. For this reason, regional mudstone sedimentology and architectural variability is generally better understood than local variability. This outcrop study aims to characterise local-scale mudstone sedimentology, depositional process and sedimentary architecture variability within a major shallow-marine mudstone, the Mancos Shale, Utah. Extensive exposures of the Blackhawk Formation (Campanian) allow key stratal surfaces to be correlated from coastal plain through shoreface, to offshore muddy deposits over distances of 50+ km. Thorough sedimentological analysis has been completed on specific stratigraphic successions of coeval offshore deposits of the Blackhawk Fm. (Aberdeen and Grassy Members) in the Book Cliffs, Utah, USA. These successions were logged, and oriented samples of each lithofacies were collected along each logged succession at regular 2m intervals. Thin section analysis quantified the compositional variability, laminations styles and bioturbation index (BI). Petrographic analysis revealed several bed types that range from <1mm to 1cm thick but are typically ~1mm thick and display a variety of grading motifs. Normally graded deposits, exhibiting bipartite organisation, are the most abundant bed type recognised. Other bed types include tripartite normally-graded deposits, normally graded deposits without partitioning, bigradational deposits with two subdivisions (normally graded upper section an overlying inversely graded basal section) and rare laminated fine mud beds (~1cm thick). These deposits are interpreted to represent a range of depositional processes including; storm-induced and/or gravity-collapse turbidity currents (bipartite), wave-enhanced sediment-gravity flows (WESGFs) (tripartite), hyperpycnal flows (bigradational) and suspension settling (laminated). Preliminary results indicate far greater local temporal and spatial variability in sedimentology and depositional processes than the literature suggests. These local variations may be a result of local topography and/or wider palaeogeographic influences such as river discharge and delta-slope instability. The variability observed indicates local dynamics are complex within shelfal mudstones resulting in highly heterogeneous mudstone successions. Consequently, appreciating local-scale variability will expand our knowledge of reservoir quality and improve our understanding of unconventional hydrocarbon systems.
AAPG Datapages/Search and Discovery Article #90351 © 2019 AAPG Foundation 2019 Grants-in-Aid Projects