XENOCONFORMITIES AND WATERSHED EVOLUTION, EOCENE GREEN RIVER FORMATION, WYOMING

The lake-type model of Carroll and Bohacs (1999) cast lacustrine deposits into three separate categories, based on the occurrence of distinctive sedimentary facies associations and stratal patterns that characterize each. Subsequent work has shown that rather than simply representing reference points in a sedimentary continuum, overfilled, balanced-fill, and underfilled lacustrine facies associations often constitute discrete rock bodies that are separated by abrupt, basin-wide stratigraphic transitions. These boundaries are typically conformable, and therefore do not mark significant gaps in the temporal record. They differ from other conformable stratigraphic successions, however, that have been interpreted under the rubric of “Walther’s Law” to result from progressive spatial rearrangement of coexisting depositional environments. Lake-type boundaries instead reflect wholesale modification of contemporary depositional environments across an entire basin. Because no existing stratigraphic terminology adequately describes such surfaces, the new term “xenoconformity” is proposed herein. Xenoconformities are defined as generally conformable stratigraphic transitions, which separate persistently dissimilar sedimentary facies associations above and below. Note that this definition is not meant to apply to surfaces caused by short-lived natural catastrophes, or to normal sedimentary cycles interpretable using Walther’s Law.

Recent work has shown that lacustrine xenoconformities in the Green River Formation, Wyoming, record relatively rapid reorganizations of watersheds related to Eocene Lake Gosiute. For example, the onset of underfilled conditions resulted from tectonic diversion of a major river that previously entered the lake from the north. Approximately 1.5 Ma later the basin shifted back to balanced-fill conditions, due to expansion of the lake’s watershed to the east. A third major transition to overfilled conditions occurred about 0.7 Ma later, coincident with capture of a river that drained rising volcanic topography in central Idaho. Xenoconformities within Eocene lake deposits therefore provide a unique perspective on the dynamic geomorphorphic evolution of the western U.S. Laramide foreland.