Lowstand systems of the Cretaceous Blackhawk Formation, Book Cliffs, Utah: new insights on location and preservation

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 depositional sequences and constituent parasequences. A long-term point of contention has been the apparent absence of lowstand deposits down dip of regionally developed sequence boundaries and overlying fluvio-estuarine incised valley-fills. Despite the world-class exposures, the outcrop provides a restricted, largely two-dimensional view of the regional scale depositional system. Integration of a new 400 well dataset covering ca. 5000 sq. miles with the classic outcrops has allowed recognition of new subsurface units across depositional strike and down depositional dip. A new large fluvio-deltaic system here named the Bitter Creek delta is recognized to the north of the outcropping Tusher system, together with linked dual sourced lowstand facies belts for the entire Blackhawk Formation. A Storrs–Aberdeen age subaqueous thick diverted all subsequent lowstand drainage far to the north of the outcrop belt whose eastward extent transects an off-axis location. Evidence of falling trajectories, top truncated late forced regressive and lowstand shoreline deposits in the Grassy and Kenilworth record basinward shifts of between 30–50 km consistent with punctuated base level falls of 15–20 m on a low gradient ramp margin. We suggest that the combination of (1) a low angle ramp margin where small rises in sea level drove strong transgressive erosion, landward detachment and off-axis reworking of transgressive shorelines into younger highstand systems and (2) Greenhouse climate suppressing the amplitude of orbital forcing and depth of fluvial erosion on sequence boundaries, enhanced ravinement and of-axis removal of lowstand shorelines. This additional work on the Book Cliffs system provides an alternative scenario to the ‘attached lowstand’ model used to explain absence of lowstand deposits despite well documented regional up dip erosion.