The Sunnyside Member of the Upper Cretaceous Blackhawk Formation in the Book Cliffs of eastern Utah is composed of coal-bearing coastal-plain strata and wave-dominated shoreface deposits. The member includes a thick (up to 6 m), laterally extensive coal seam, which formed in a large ombrotrophic raised mire, parallel to the shoreline of the Western Interior Seaway. The petrographic composition of the Sunnyside coal was investigated by means of maceral analysis and telovitrinite reflectance determinations of closely spaced samples taken from seven vertical sections through the seam. Sampling was carried out in a combination of outcrop and underground mine sites, using lithotype logging to determine sample spacing. The excellent exposure in the study area enables accurate stratigraphic correlation between sampling localities, which are spread over more than 30 km of depositional dip and 50 km of depositional strike. The correlation of petrographic trends between the seven sampled sections demonstrates the reproducibility of the results and suggests that they represent regional-scale accommodation changes, as opposed to localised variation in the mire. On this basis, we are able to identify a high-resolution record of accommodation change throughout the deposition of the Sunnyside coal, spanning two cycles of increasing and decreasing accommodation. We are also able to identify a marine flooding surface within the coal, which can be traced down depositional dip into the time-equivalent shallow-marine strata, where it represents a parasequence boundary. The proportion of detrital minerals is used as the main discriminator of accommodation trends within the coal. Other useful indicators of conditions in the mire include semifusinite, pyrite, and isometamorphic variations in telovitrinite reflectance.
Bibliographical noteFunding Information:
The funding for this research was provided by The United Kingdom Natural Environment Research Council, Rio Tinto Technology, The Australian Research Council, AAPG Grants in Aid, and The BSRG Steve Farrell Memorial Fund. We also wish to acknowledge Dave Tabet of the Utah Geological Survey, and Mike Glasson, Laine Adair, and Gary Gray of Andalex Resources for help with mine access and sampling. David Large and Jennifer Wadsworth are thanked for useful suggestions and stimulating discussion of the work, as is Chris Fielding for his thoughtful and constructive review of the manuscript.
- Cretaceous coals
- Maceral analysis
- Organic petrography
- Palaeoenvironmental indicators
- Vitrinite reflectance