XENOCONFORMITIES IN THE GREEN RIVER FORMATION (Invited Presentation)

Lacustrine and associated terrestrial strata commonly exhibit abrupt, basin-scale changes in lithofacies, stratigraphic packaging, biota, geochemistry, and economic resource potential. Unlike conventional stratigraphic successions that may be interpreted to record lateral migration of coexisting depositional environments (Walther’s Law), these transitions record discrete episodes of comprehensive paleoevironmental change. Carroll (2017) proposed the new term xenoconformity, defined as a stratigraphic surface or gradational interval that records a fundamental, abrupt, and persistent change in sedimentary facies across basinal to global scales. The Eocene Green River Formation (GRF) contains multiple xenoconformities, each of which is expressed as a sudden change in lacustrine sedimentary facies association (“lake type”) across an entire depositional basin. For example, the Wilkins Peak Member in Wyoming contains numerous Na-carbonate evaporite beds, has relatively low organic matter enrichment, and lacks any fossil evidence for lacustrine macrofauna. It is separated by a xenoconformity from the overlying Laney Member, which lacks bedded evaporite minerals but is relatively enriched in organic matter and contains a diverse lacustrine macrofauna. A growing body of provenance evidence has demonstrated that the GRF lakes drained much of the frontal cordillera and foreland of the western U.S., and that episodic drainage reorganizations within this large watershed exerted an important control on the balance between accommodation and sediment+water supply in downstream lakes. For example, the Wilkins Peak/Laney transition is accompanied by a dramatic increase in carbonate ⁸⁷Sr/⁸⁶Sr ratios, recording capture of new, more radiogenic river water that reduced the salinity of Eocene Lake Gosiute. Smith et al. (2014) proposed that early Eocene drainage networks evolved in response to localized thermal uplift and magmatism, caused by the progressive rollback of the Farallon plate beneath the western U.S. Many questions remain, but it has become clear that Green River Formation lakes were sensitive recorders of landscape evolution at the sub-continental scale. More broadly, watershed evolution exerts a first order control on lacustrine paleoenvironmental records and economic resources.