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Nahcolite and halite deposition through time during the saline phase of Eocene Lake Uinta, Piceance Basin, western Colorado

Ronald Johnson

Halite and the sodium bicarbonate mineral nahcolite were deposited during the saline phase of sediment accumulation in Eocene Lake Uinta in the Piceance Basin, western Colorado. Variations in the extent of saline mineral deposition through time were interpreted from studies of core and outcrop. Saline minerals were leached extensively by relatively recent groundwater movement; in these areas the original extent of saline deposition was estimated from the distribution of empty vugs and collapse breccias. Because the presence of vugs and breccias strongly influence groundwater movement, determining where leaching has occurred is an important consideration to the in-situ oil shale extraction methods currently being developed. Saline deposition generally corresponded to the area where the richest oil shale was deposited throughout the history of the saline phase of Lake Uinta in the Piceance Basin. A progression from disseminated nahcolite and nahcolite aggregates to bedded nahcolite and ultimately to bedded nahcolite and halite occurred during the early stages of saline deposition. Both saline mineral and rich oil shale deposition were confined to a comparatively limited area in the north-central part of the basin during this progression. Later, as the extent of rich oil shale deposition expanded, the area of saline deposition also expanded, but only disseminated nahcolite and nahcolite aggregates appear to have been deposited. Finally, as Lake Uinta in the Piceance Basin was in-filled from north to south by volcanic-rich sediments from Wyoming, deposition of nahcolite aggregates and disseminated nahcolite was pushed progressively southward in front of this prograding deltaic system.


AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012