A High-Resolution History of Basin Fill and Spill Relationships, Laney Member, Green River Formation, Wyoming
The Eocene Green River Formation in Wyoming is well known to contain a variety of lacustrine sedimentary facies associations that reflect changes in the chemistry of the lake and the hydrology of the basin as a whole. Until recently, detailed understanding of the temporal relationships between facies within the greater Green River Basin, especially across the Rock Springs Arch, has been hampered by a lack of high-resolution time control. Through detailed field investigations, 40Ar/39Ar dating of air-fall tuffs as well as correlation of tuffs via geochemical fingerprinting of biotite crystals, a more highly-resolved history for the greater Green River Basin during the Eocene can now be developed.
During deposition of the Laney Member of the GRF the Rock Springs Arch partitioned the greater Green River Basin into eastern and western sub-basins. Each of these sub-basins preserves a unique succession of facies collectively assigned to the Green River Formation. The base of the Laney Member of the Green River Formation corresponds to the transition from a hydrologically isolated basin to a basin that was periodically connected to the regional drainage system. This transition is reflected by a change in facies as well as the reintroduction of fish into the lake. We propose that the base of the Laney Member is diachronous; deposition of the Laney east of the Rock Springs Arch predates Laney deposition west of the arch. The diachronous nature of this contact implies that the two sub-basins initially functioned as separate lakes, rather than as one unified body of water as has been previously believed. Later the two lakes likely merged across the arch, by the Upper LaClede Bed of the Laney the greater Green River Basin contained one unified lake based on known age relationships and a common δ18O history on both sides of the arch. The subsequent transition for a periodically open basin to a consistently open basin is however synchronous in the two sub-basins because clastic sediment supply outpaced accommodation through this interval.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009