High-resolution sequence-stratigraphic correlation between shallow-marine and terrestrial strata: Examples from the Sunnyside Member of the Cretaceous Blackhawk Formation Book Cliffs eastern Utah

The Sunnyside Member of the Upper Cretaceous Blackhawk Formation in the Book Cliffs of eastern Utah provides an ideal opportunity to investigate high-resolution sequence-stratigraphic correlation between shallow-marine and terrestrial strata in an area of outstanding outcrop exposure. The thick, laterally extensive coal seam that caps the Sunnyside Member is critical for correlating between its shallow-marine and terrestrial components. Petrographic analysis of 281 samples obtained from 7 vertical sections spanning more than 30 km (18 mi) of depositional dip enabled us to recognize a series of transgressive-regressive coal facies trends in the seam. On this basis, we were able to identify a high-resolution record of accommodation change throughout the deposition of the coal, as well as a series of key sequence-stratigraphic surfaces. The stratigraphic relationships between the coal and the siliciclastic components of the Sunnyside Member enable us to correlate this record with that identified in the time-equivalent shallow-marine strata and to demonstrate that the coal spans the formation of two marine parasequences and two high-frequency, fourth-order sequence boundaries. This study has important implications for improving the understanding of sequence-stratigraphic expression in terrestrial strata and for correlating between marine and terrestrial records of base-level change. It may also have implications for improving the predictability of vertical and lateral variations in coal composition for mining and coalbed methane projects.