The area of this report is the southeastern part of the Uinta Basin ancl constitutes about 2,300 square miles in northeastern Utah nncl northwestern Colorado. The major topographic feature is a north-sloping plateau that is highly dissected by steepwalled canyons. The topography along the southern margin is dominated by the rugged south-facing Roan Cliffs. Beclrocl< exposed in the area is of fluvial ancl lacustrine origin nncl of Eocene ancl Oligocene age. The oldest rocks exposed are the massive sandstone ancl variegated shale of the Wasatch I!..,ormation of Eocene age. The 'Vasatch Formation is divided into the main body ancl the Renegade Tongue (new name). The 'Vnsatch Formation interfingers with ancl is overlain by the Green River Formation, also of Eocene age. r:rhe Green H.iver Formation has been clividecl, in ascending order, into the Douglas Creek, Garden Gulch, Parachute Creek, and Evacuation Creek Members. The Douglas Creek Member is composed chiefly of sandstone, siltstone, and •algal and oolitic limestone deposited in a nearshore-lacustrine environment. It interfingers with the underlying 'Vasatch Formation toward the former shore ancl interfingers with the overlying Garden Gulch nncl Parachute Creek Members •toward the former basin. The Garden Gulch, Parachute Creek, and Evacuation Creek Members are composed chiefly of thin-becldecl marlstone, siltstone, and oil shale deposited in a lacustrine environment. The Parachute Creel< Member contains the principal oil-shale beds of the Green Uiver I!..,ormation. The richest of these beds is the Mahogany oil-shale becl, which lies within a group of rich oil-shale beds that are designated as the Mahogany ledge at the surface and as the Mahogany zone in the subsurface. The Ervacuation Creek Member of •the Green H.iver Formation is overlain by and interfingers with the Uinta Formation of Eocene age. The Uinta I!"'ormation is composed of massive sandstone and arenaceous shale deposited mainly in a fluvial environment. It contains a•bundant vertebrate fossils. The Uinta is overlain by the reel ancl gray fluvial sandstones ancl shales of the Duchesne River I!"'ormation of Eocene or Oligocene age. Most of the mapped area is on the south flank of the Uinta Bn•sin and is underlain by rocks that dip gently north. Steep dips are confined to the northeastern part of the area, where the becls are tilted southwest or west. Neither the density nor the displacements of faults are great. The Green River strata are cut by a prominent system of joints. One joint set trends northwest and the ot11er northeast. Hydrocarbons, in various forms, are abundant in the mapped area. Oil shale in the Mahogany zoi1e ancl adjacent beds that yields an average of 15 gallons of oil per ton from a continuous sequence 15 feet or more thick contains an estimated 53 billion barrels of oil. 'l'he original reserves of gilsonite in the area are estimated to be about 27 million tons. Oil and gas are produced from Jurassic, Cretaceous, and Tertiary rocks. In the southern part of the area, the Douglas Cr...

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COVER Exposures of the sandstone-dominated lower part of the tertiary Green River formation seen in the Gate Canyon area on the afternoon of day 2. Frequently fluctuating lake levels resulted in this sequence of interbedded delta plain in marginal lacustrine deposits. The pronounced lateral acretion bedding common in this part of the Green River formation formed in both fluvial and deltaic environments.

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