Development, evolution, and destruction of the saline mineral area of Eocene Lake Uinta, Piceance Basin, western Colorado

Abstract

Halite and the sodium bicarbonate mineral nahcolite were deposited in Eocene-age saline Lake Uinta in the Piceance Basin, northwestern Colorado. Variations in the areal extent of saline mineral deposition through time were studied using descriptions of core and outcrop. Saline minerals have been extensively leached by groundwater, and the original extent of saline deposition was determined from the distribution of empty vugs and collapse breccias. Because vugs and breccias strongly influence groundwater movement, determining where leaching has occurred is an important consideration for in-situ oil shale extraction methods currently being developed. Lake Uinta formed when two much smaller freshwater lakes, one in the Uinta Basin and the other in the Piceance Basin, expanded and coalesced across the Douglas Creek arch. Early Lake Uinta inherited much of the topography in the freshwater lake and surrounding alluvial plains that preceded it, and the deep central lake area of Lake Uinta developed largely over the offshore area of the freshwater lake. A prolonged period of infilling followed the formation of Lake Uinta, creating broad lake-margin shelves prior to the onset of saline mineral deposition. These shelves almost certainly played a critical role in the evolution of the brine layer that accumulated in the deep central lake area. Broad marginal shelves formed as well around the Uinta Basin part of Lake Uinta. Brines that formed on those shelves also probably migrated to the deep central lake area in the Piceance Basin, as no saline minerals were deposited in the Uinta Basin until late in the history of Lake Uinta. Oil shale in the deep central lake consists of interbedded laminate oil shale beds that originated within the deep lake area, and blebby and streaked oil shale beds that were transported into the deep lake area by sediment gravity flows. Blebby and streaked oil shale beds contain carbonate clasts and siliciclastic clasts similar to lithologies found on the marginal shelves. It is possible that these clasts and the highly saline brines that evolved on the marginal shelves were incorporated into the same gravity flows. Saline mineral deposition is informally subdivided into early, middle, and late phases. During the early phase, nahcolite and rich oil shale were deposited in the deep central lake area, and carbonate-rich sandstone, siltstone, and mudstone and ostracodal, oolitic, algal limestone were deposited on the marginal shelves. During the middle saline mineral phase, oil shale deposition gradually expanded across the marginal shelves, and by the end of that phase, oil shale deposition covered almost all of the former shelf areas. An increasing flow of water from Lake Gosiute to the north, as it was gradually filled in by volcaniclastics, may have caused this expansion. Saline mineral deposition also expanded during the middle saline mineral phase, reaching to near the former shelf break by the end of that phase. This suggests that the former shelf break remained a topographic feature that confined the deep saline brine layer throughout the middle phase. By the beginning of the late saline mineral phase, Lake Gosiute had been completely filled in and volcaniclastic debris reached the northern shore of Lake Uinta. This initiated a north-to-south infilling of the Piceance Basin part of Lake Uinta that progressively pushed the saline mineral area southward and ultimately onto the former marginal shelf areas in the southern part of the basin. A saline mineral area formed for the first time in the eastern part of the Uinta Basin during this infilling and, for a time, saline minerals were deposited in both basins. By the end of the late saline phase, the Piceance Basin part of Lake Uinta was filled in and saline mineral deposition shifted entirely into the Uinta Basin. Leaching of saline minerals began sometime after the Green River Formation was lithified enough to allow collapse breccias to form. Leaching is ongoing today, indicated by the discharge of highly saline water from a series of springs in the northern part of the basin. Groundwater invasion and saline mineral dissolution is commonly incomplete in areas that lack fractures, leaving behind pockets of unleached saline minerals in otherwise leached intervals. Today, the base of the leached zone slopes toward the north and toward the area where the brines are being discharged.