Abstract

The extent of lithofacies variability in fine-grained sedimentary rocks is very poorly known in comparison to that present in coarser grained clastic and carbonate successions. The absence of this information means that sediments present on continental shelves are rarely considered as integrated systems because fine-grained facies (such as shales) are mostly excluded from sophisticated, regional facies models. To shed light upon lithofacies and grain size in shale-dominated successions, so that they can be incorporated into shelf-wide depositional models, the Blackhawk Formation of the Mancos Shale (Campanian age), exposed in the Book Cliffs, Utah was investigated using combined field, whole-rock geochemical, optical, and electron optical methods. These sediments show systematic grain-size variations, are intensely bioturbated, and composed predominantly of detrital clays (mainly dioctahedral micas), quartz, and feldspar with minor pyrite and organic matter. They are organized into very thin (< 10 mm), upward-fining, genetic beds; they exhibit both systematic lateral (10³ m scale), and vertical (10⁻² to 10⁰ m scales) lithofacies variability. Preferentially cemented units occur close to sequence boundaries (unconformities). These cemented units typically contain a very different detrital assemblage (including significant quantities of chlorite) from the rest of the succession and are located close to levels where there are marked stacking-pattern discontinuities.

Overall, lithofacies variability is interpreted in terms of deposition occurring on an oxic continental shelf, with the coarser-grained facies being deposited in proximal settings (offshore-transition zone), in contrast to the finer-grained units that were deposited in more distal environments (offshore zone). Storms are interpreted to have been the dominant mechanism dispersing the sediment. Once deposited, the surface layers of the sediment were intensely reworked by burrowing organisms. Preferential preservation of ichnogenera from mid- and lower tiers suggest that the depositional environment was energetic and erosion commonly removed the surface sediment layers, prior to deposition of thin (10⁻² m) storm beds. The larger scale (10⁰ m) upward-coarsening units that are composed of stacked beds, are interpreted to be parasequences.

Preferential cementation, at levels where large-scale stacking patterns change, is interpreted to occur at horizons where there were breaks in sediment accumulation, and bacterial metabolic processes were able to supply sufficient solutes to fill uncompacted pore space with cement. The presence of a very different detrital assemblage in these units suggests that winnowing occurred. Despite their relatively innocuous appearance in the field, these surfaces are likely to be significant bypass surfaces, over which sediment was delivered to the deeper parts of the basin during times of relative lowstand of sea level.