SEDIMENTOLOGIC COMPARISON OF TWO NEW LAVA CREEK B ASH OCCURRENCES IN WESTERN COLORADO

Remnants of the Lava Creek B (LCB) ash, which erupted from the Yellowstone volcanic center approximately 640 ka, have been characterized at two new locations (Prairie Canyon and Petrie Mesa) in western Colorado. At both locations, the ash is typically detrital, occurs in paleotopographic depressions, appears to be related to aggradational depositional episodes, and is capped by resistant Quaternary conglomerates that preserve the underlying ash. Differences between the two occurrences include the position of the ash within aggradational sequences, its thickness, geometry, degree of lithification, and overall depositional setting.

In Prairie Canyon, which is very near the Colorado-Utah border and the join between Garfield and Mesa Counties, CO, the LCB ash occurs within Pleistocene valley fills that are up to 20 m thick, resting on Cretaceous Mancos Shale. The ash occurs as white lenses of well-stratified sand-size glass shards that are 2 to 3 m thick, 100 to 200 m wide, and overlie river gravels, which constitute the base of the valley fill. The ash is buried by up to 10 m of weakly bedded beige sandy muds that encase minor gravel and sand lenses. A well-cemented fluvial conglomerate overlies the valley-fill sequence. Stratigraphic relations indicate that ash, followed by siliciclastic sediments, was reworked from hill slopes into stream bottoms, which led to stream aggradation and valley filling. These events were probably triggered by slope destabilization caused by ash deposition and the loss of vegetation from hill slopes.

Petrie Mesa, a dissected Pleistocene gravel-capped terrace resting on Mancos Shale, is located approximately 9.4 km northeast of Delta, CO, on the south flank of Grand Mesa. The ash has a maximum thickness of 2.5 m and occurs as an isolated lens approximately 150 m wide. The ash is relatively pure (less than 10 % detrital impurities), strongly cemented by calcite, faintly laminated to cross-stratified, and locally bioturbated. The ash rests on a sequence of well-stratified sand and granule-pebble gravel (5-8 m thick) deposited in a relatively low-energy alluvial setting, and is overlain by a heterogeneous sequence (15-20 m thick) of poorly sorted and stratified cobble-boulder gravel deposited in a high-energy setting (alluvial fan?). The stratigraphic relationships suggest that the ash accumulated in an abandoned channel or pond.