Facies Architecture and Paleopedology of Ancient River Systems, Eocene Wilkins Peak Member of the Lacustrine Green River Formation, Wyoming

Kuwanna M. Dyer¹, Alan R. Carroll¹, and Jeffrey T. Pietras^2
1 University of Wisconsin-Madison, Madison, WI
² BP Exploration (Alaska) Inc, Anchorage, AK

Nine arkosic sandstone-siltstone-mudstone marker beds were deposited as a part of the Wilkins Peak Member when Lake Gosiute receded and rivers flowed across the basin. The thickest of these bedsets has been termed the D bed and contains fluvial sandstone interbedded with lacustrine siltstone and mudstone. Sandstone beds containing climbing ripples with climb angles of 10-20° are abundant throughout the study area and are indicative of waning flow conditions and rapid sediment unloading, a phenomenon typical in dryland rivers. However, the D bed also contains intervals of lateral accretion surfaces approximately one meter high and tens of meters long, suggesting more continuous channel flow. Some sandstone, siltstone, and mudstone beds show evidence of pedogenic alteration. The degree of alteration varies laterally and stratigraphically according to time and topography. Some paleosols are well developed and contain abundant mm-scale root traces, plant material, cm-scale burrowing, mm- to cm-scale concretions, mottling, and soil structures (peds, horizonation, pseudo-anticlines); others are poorly developed and show only little evidence of pedogenesis, usually in the form of mm-scale burrowing, few mm-scale rootlets, and overall thin successions. Some successions show thin A/C horizonation. While well-developed paleosols typically occur on paleo-topographic highs and are associated with nodules, paleosols tend to be less developed on paleo-topographic lows, where nodules are absent. The lateral distribution of facies within the D bed suggests that fluvial environments co-existed with aerially restricted lacustrine environments. These environments evolved through time to yield a composite bedset of fluvial, lacustrine, and pedogenic strata reflecting basin topography.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005