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Cretaceous Shales - Book Cliffs of Utah
Kenilworth Member of the Blackhawk Formation

   
The Cretaceous Books Cliffs of Utah are best known as the Mecca of sequence stratigraphers, but besides excellent exposures that allow tracing of sandstone bodies and sequence boundaries the succession also contains abundant shales and mudstones. Because of the excellent stratigraphic control, we studied shales in the Kenilworth Member of the Blackhawk Formation to examine proximal to distal changes of shale facies.
For an onshore - offshore facies comparison, two well exposed sections (red bars) of the Kenilworth Member (orange shading) were studied in detail.
Stratigraphic sections show multiple coarsening/shallowing upwards packages, the parasequences of the Kenilworth. Although all parasequences were examined, we illustrate PS3 because of good exposures between the two sampling localities.
In proximal settings a systematic stacking of four types of shale facies is observed. These are briefly described below.
see slide
At the bottom of the "stack" we have shales with well preserved bedding and some bioturbation. Primary bedding is typically well visible in cut slabs.
The next higher unit is an intensely bioturbated mudstone (burrow mottling is prevalent texture) with little if any of the primary bedding preserved.
The following shale package shows better preservation of primary bedding and proportionally less bioturbation. Probably due to higher rates of deposition.
The topmost facies is the most sandy and more seriously bioturbated than the underlying shales/mudstones. Probably a reflection of proximity to shoreline and shallow water.
Sharp boundaries between facies packages suggest minor sea-level drops during parasequence development. Abundant thin storm produced beds (silt/sand layers, graded silt-mud couplets) in addition to hummocky cross-stratified sandstone beds of storm origin.
In distal section we see fewer facies developed within parasequences, and there is a higher proportion of mudstone in the section (sandstones are piching out in down-dip directions).
see above and left
see slide at left
see slide at left
These shales/mudstones were deposited tens of kilometers offshore, and presumably in deeper water. Yet, careful examination of hand specimens shows shale-on-shale scours and erosion surfaces.
see slide at left
The shale facies succession in proximal section reflects the sedimentary response to initial sea level rise, maximum flooding, and shoreline progradation.
Although somewhat more bioturbated than comparable Devonian mudstones, the information distilled from Cretaceous shales still enhances our understanding of the succession. We see more in detail what "background" sedimentation was like, and realize that storms of all sizes were involved in building this succession.
   
   
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© Jürgen Schieber, IU Bloomington Department of Geosciences
Last updated: June 14, 2006.