Abstract

During latest Cretaceous through middle Eocene time, over 3,000 m of siliciclastic and carbonate sediment accumulated in the Lake Uinta depocenter in northeastern Utah. Detailed stratigraphic analysis of this extensive lacustrine system, both along the western and southern outcrops and in the subsurface of the Uinta Basin, indicates three major facies: (1) open lacustrine, (2) marginal lacustrine, and (3) alluvial.

The open-lacustrine facies consists primarily of mud-supported carbonate and claystone units with minor amounts of sandstone and siltstone. Kerogen and other organic compounds produce shades of gray and brown. The open-lacustrine rock assemblages were deposited away from terrigenous clastic influxes either near the center of the lake or in nearshore settings. Rocks originating in the latter environment contain abundant fossils (mollusks and ostracodes) and scattered desiccation features.

The marginal-lacustrine facies is composed of gray-green calcareous claystone, channel-form sandstone, and grain- to mud-supported carbonate units. The dominant depositional environments are interpreted to be lake-margin carbonate flat, deltaic, and interdeltaic. Lake-margin carbonate-flat deposits consist of carbonate beds as thick as 30 m which grade lakeward from an ostracode and oolite grain-supported texture to a mixed mud- and grain-supported texture. Deltaic deposits consist of channel-form sandstone units as thick as 15 m which cut adjacent thin beds of sandstone, siltstone, and gray-green claystone. Ostracode- and oolite-bearing grainstone beds, as much as 4.5 m thick, of lake-margin carbonate-flat origin are commonly interbedded with the deltaic rocks. Interdeltaic rocks exhibit characteristics similar to the deltaic rocks but contain more carbonate beds of lake-margin carbonate-flat origin and far fewer sandstone units.

The alluvial facies, representing alluvial-fan, lower deltaic-plain, and high mud-flat environments, occupied the most proximal setting within the depositional system. The lower deltaic-plain environment is typified by 15- to 30-m-thick channel-form sandstone units and associated thin-bedded sandstone, siltstone, and red, mud-cracked claystone. The alluvial-fan environment is characterized by thick conglomerate beds with crude horizontal stratification. Red claystone, minor isolated channel-form sandstone, and thin fossiliferous gray-green claystone units characterize the high mud-flat environment. Thin mud- and grain-supported carbonate units of lake-margin carbonate-flat origin are locally interbedded with rocks of the above environments.

During most of early Tertiary time, the Lake Uinta system exhibited a northeast-trending core of open-lacustrine facies surrounded by successive halos of marginal-lacustrine and alluvial facies. The width of the open-lacustrine core continually fluctuated depending upon climatic and tectonic conditions prevailing at the time. Southerly derived feldspathic sands dominated the sediments supplied to the south flank of the basin, whereas sand contributed to the north flank was quartzose and originated from rocks exposed in the Sevier orogenic belt to the west and the Uinta uplift to the north. The north flank of the basin had a steeper depositional slope than the south flank, as manifested by the presence of coarser grained siliciclastic sediments and the narrower band of the marginal-lacustrine facies.

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