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Cretaceous Shales - Book Cliffs of Utah

Cretaceous Shales - Book Cliffs of Utah

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.

A photograph shows a large rock outcrop with different horizontal layers of shale, with a street light on the right and a road at the bottom.

For an onshore - offshore facies comparison, two well exposed sections (red bars) of the Kenilworth Member (orange shading) were studied in detail.

A presentation slide from a geological study shows a map and a stratigraphic cross-section of the Kenilworth Formation and surrounding units, illustrating the sequence stratigraphic framework with a key to show different rock types and depositional environments.

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.

A slide from the Sonyea Group Study shows a stratigraphic cross-section of various formations and their depositional environments, from coastal plain to offshore, illustrating a progradational sedimentary sequence.

In proximal settings a systematic stacking of four types of shale facies is observed. These are briefly described below.

A large geological outcrop along a highway in Kentucky shows distinct horizontal layers of the Upper Gassaway/Camp Run and Upper Huron shale formations.

see slide

A photograph of a large, layered rock outcrop in the Kenilworth Formation with two close-up samples of bioturbated gray shale, showing internal structures and a 1 cm scale bar on each.

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.

A photograph shows a geologic hammer resting on a multi-layered rock outcrop, with a red rectangle indicating a sample from the intensely bioturbated gray mudstone layer.

The next higher unit is an intensely bioturbated mudstone (burrow mottling is prevalent texture) with little if any of the primary bedding preserved.

A presentation slide shows a stratigraphic column and two photos of a rock outcrop in the Kenilworth Formation, highlighting a sample of moderately bioturbated gray shale with a 1 cm scale bar.

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.

A geological outcrop shows a vertical rock face with horizontal layers, with a person standing at the base for scale and a red arrow indicating the boundary between the Foerstia beds and the overlying Phosphatic Member.

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.

A presentation slide shows two magnified images of a geological sample, with a diagram of a plant fragment and a microscopic photograph of a large bioturbation trace fossil.

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

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.

A geological presentation slide shows two images of a rock outcrop in the Kenilworth Formation, highlighting a sample of bioturbated sandy mudstone with a 1 cm scale bar and yellow arrows indicating shale-filled scours that suggest seafloor erosion.

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.

A scientific presentation slide shows a photograph of a large, rugged cliff face in the Book Cliffs region, with text boxes summarizing the conclusion that mudstone studies provide valuable information about erosion and depositional processes.

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.