Jeffrey T. Pietras¹, A. R. Carroll¹, M. K. Rhodes¹
(1) UW-Madison, Madison, WI

Abstract: Lacustrine Sequence Stratigraphy: Example from the Green River Formation of Southwestern Wyoming

Field studies indicate that the Tipton Member of the Green River Formation locally represents one complete lacustrine sequence. As transgression commenced, increased stream flow eroded an unconformity with meter-scale relief and subsequently deposited fluvial conglomerate. Fine-to medium-grained, planar laminated sandstone with very-coarse sand and pebbles overlie the basal lag and are interpreted as a fan delta deposit. Accommodation rate continued to outpace the rate of sediment supply, deepening the lake and depositing silty-muds with gastropods, bivalves, burrows, and rare plant material. Beds of very-fine sandstone with abundant gastropods signal the maximum flood, when the rate of sediment supply began to exceed the accommodation rate. These sands are the down-dip equivalents to decameter-scale clinoforms of a southwestward prograding delta composed of fine-to medium-grained sandstone. The middle portion of the delta contains thin pebble lags that represent increased runoff during floods. As basin filling continued, planar laminated foreshore fine-grained sandstone and delta plain muddy-sandstone completes the sequence. Tipton sedimentation was followed by fluvial incision, formation of the sequence bounding unconformity, and southward progradation of the Cathedral Bluffs alluvial fan.

The fifty-meter thick Tipton stratigraphic sequence may reflect a cycle of tectonic accommodation related to movement on the Continental Fault located to the north. Alternatively, a climatic maximum may have contributed to transgression. A third scenario invokes a joint tectonic-climatic forcing mechanism. Future work will aim at resolving this dilemma through continued field correlation and the use of radiogenic isotope provenance studies.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana