Abstract

The Paleocene Dark Canyon sequence of dominantly matrix-supported, quartz-pebble conglomerate, conglomeratic sandstone, and sandstone forms the basal part of the Wasatch Formation in the southeasternmost part of the Uinta Basin. This sequence marks the first Tertiary depositional event after a long period of nondeposition and erosion during Maestrichtian and early to middle(?) Paleocene time and prior to rapid subsidence in late Paleocene through much of Eocene time. The predominance of chert and quartzite pebbles, the Paleozoic silicified limestone pebbles, the sandstone mineralogy, and the dominant northwest paleocurrent directions all indicate that sediment was derived mainly from Paleozoic and Mesozoic sedimentary rocks eroded from rising Laramide uplifts to the south and southeast. Massive to crudely horizontally stratified conglomerate and local thin interbeds of horizontally stratified sandstone compose the lower part of the Dark Canyon sequence. The size and abundance of pebbles decreases upward, and trough cross-stratified and horizontally stratified sandstone and pebbly sandstone are dominant in the upper part. Sedimentary structures in and geometries of the depositional units that compose this sequence indicate formation within relatively shallow, rapidly shifting channels in braided rivers that flowed on a medial to distal braidplain. The decrease in grain size vertically and from west to east in the Dark Canyon sequence indicates decreasing flow strength through time and in that direction. The San Rafael uplift appears to have restricted the western extent of deposition. Over the western flank of the Douglas Creek arch, the rivers flowed to the northeast. Both local erosion of the unit in this area and the time-transgressive, west to east onlapping of overlying beds of the Wasatch Formation over the arch suggests post-Dark Canyon movement of this structure.