LACUSTRINE XENOCONFORMITIES IN THE EOCENE GREEN RIVER FORMATION (Invited Presentation)

The Green River Formation encompasses freshwater to hypersaline lacustrine strata deposited in several different basins in Colorado, Utah, and Wyoming between ~54-44 Ma. It contains the world’s largest known occurrences of both oil shale and soda ash, as well as a detailed archive of the Early Eocene climatic optimum, the warmest prolonged interval of the Cenozoic. Green River Formation strata may be subdivided into several discrete facies associations that are characterized by their distinctive lithologies, stratigraphic architecture, biota, and geochemistry. Carroll and Bohacs (1999) originally defined 3 such associations, or “lake types”, and proposed that they reflect differences in the balance between potential accommodation and water+sediment fill. Our subsequent studies suggest that the evaporative facies association may in some cases be further subdivided, and Tänavsuu-Milkeviciene and Sarg (2012) identified 6 lake“stages” in the Piceance basin in Colorado. Transitions between these associations are generally abrupt. They often superimpose the deposits of paleoenvironments that likely did not coexist, in apparent violation of Walther’s Law. Carroll (2014, 2017) proposed the new term xenoconformity for such transitions, defined as a stratigraphic surface or gradational interval that records a fundamental, abrupt, and persistent change in sedimentary facies across basinal to global scales. Although conventionally interpreted as the result of climate change, several lacustrine xenoconformities in the Green River Formation are associated with evidence for a sudden change in sediment or water provenance. This implies that they primarily reflect changes in regional drainage organization within a dynamically evolving Laramide landscape. Smith et al. (2014) interpreted these changes broadly to have resulted from a southwest-migrating wave of dynamic topography associated with rollback of the Farallon plate. This uplift is inferred to have caused stream capture, drainage reversals, and the progressive isolation of Eocene lakes. Xenoconformities also mark paleoenvironmental tipping points that signal transformations in how climate signals were transferred into the stratigraphic record, making their recognition critical to accurate regional paleoclimate reconstructions.