186-3 The Straight Cliffs Formation as a Window into the Drivers of the Cretaceous Terrestrial Revolution

Abstract:

The beginning of the Late Cretaceous (ca. 100–90 Ma) was a tipping point for terrestrial ecosystems. Life on land exceeded the diversity of the marine realm for the first time, continents assumed their modern configurations driving the emergence of mountain belts and shallow epeiric seas, and global temperature reached its highest point since the Jurassic. Global sea level reached the Phanerozoic high stand at the Cenomanian-Turonian boundary (ca. 94 Ma), with terrestrial life packed into the smallest terrestrial area since the Cambrian. During this transition, terrestrial ecosystems emerged that are more comparable to the ones we have today, characterized by flowering plants, pollinating insects, and the major groups of modern mammals. This diversification event is termed the Cretaceous Terrestrial Revolution.

Southern Utah represents the ideal global location to unravel the mechanisms driving the Cretaceous Terrestrial Revolution. In particular, the Straight Cliffs Formation (ca. 92–84 Ma) preserves a low-relief coastal record of plant and mammal fossils living along the shoreline of the Western Interior Seaway, flanked by the rising North American Cordillera. These coastal environments were constantly disturbed by sea level transgressions and regressions, occurring at multiple scales, causing these ecosystems to migrate back and forth from the alluvial plain and the mountains over an 8-million-year long period.

Despite prior work, the palynoflora and macrofossil plant records are incomplete, especially given the wide diversity of habitats of these coastal systems. The vertebrate record is more widely sampled, but taxonomic identifications remain tentative due to undersampling. The middle-Turonian Smoky Hollow Member is the temporally best dated member, constrained at its base by ammonites of Collignoniceras woolgari and a radiometric date in the top 5 m (91.87 Ma). The John Henry Member remains poorly constrained chronologically and is under-sampled for both plant and mammal records.  We have now begun to fill these gaps and is part of our ongoing research.

We suggest that the Cretaceous Terrestrial Revolution,  which stimulated the rise of modern terrestrial ecosystems,  was driven by the dynamic landscape changes—mountain uplift and associated sea-level oscillations—captured during Straight Cliffs deposition. By studying the Straight Cliffs as well as the bounding terrestrial rocks we can track changes in both plant and mammal communities from ca. 100 to 80 Ma, all within the same region.

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