Abstract

The regimes of early Tertiary Lakes Gosiute and Uinta were strongly affected by pronounced imbalance between evaporation and recharge by surface and ground water. Both evaporation and recharge were strongly affected by the climate of the closed hydrographic basins where the lakes formed. However, at unique times in the histories of the Lake Gosiute and the Lake Uinta basins, the regimes of these lakes were significantly modified by enlargement of the drainage basins to include water from basins farther north.

Evidence for changes in the size of drainage basins includes southward dispersal of volcaniclastic sands containing basaltic to dacitic volcanic rock fragments from the Absaroka volcanic field in northwestern Wyoming. The southward progradation of the resulting sandstone units from the Wind River Basin into the greater Green River Basin and from there into the northern Piceance Creek Basin records the successive depositional filling of closed hydrographic basins so that water and sediment were transported by streams over the lowest divides into the adjacent basins. Filling of these basins allowed large volumes of surface and ground water to be added first to Lake Gosiute and then to Lake Uinta. The added water resulted in (1) a rise in lake level, (2) the development of nutrient-rich lakes where algal productivity led to the deposition of precursors of oil shale, and (3) in the case of Lake Uinta, a change in water chemistry such that brines precipitated sodium carbonate rather than calcium sulfate minerals.

Basin filling and enlargement of the drainage systems were probably a consequence of tectonic activity and stability of the basins and adjacent uplifts, although climatic conditions that increased sediment yield and runoff in the hydrographic basins also could have hastened their filling. However, it is difficult to explain patterns of evaporite minerals, oil shale, mudstone, and sandstone formed in Lakes Gosiute and Uinta if climate was the dominant factor.

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