Sedimentology and geomorphic significance of the bishop conglomerate and the Browns Park Formation, Eastern Uinta Mountains, Utah, Colorado, And Wyoming (Thesis and maps)

Much of the later Tertiary history of the eastern Uinta Mountains is manifested within the heterogeneous and widespread strata of the Bishop Conglomerate and the Browns Park Formation. The type Bishop Conglomerate on the north flank of the Uinta Mountains typically is boulder-conglomerate near its base and grades upward into pebble- conglomerate and coarse sandstone. The Bishop Conglomerate mantles a remarkably smooth erosion surface that is interpreted to be a pediplain. The type Browns Park Formation in Browns Park is much more diverse: sandstone, siltstone, tuff, tuffaceous sediment, and conglomerate were deposited in complexly intergrading strata that represent several terrestrial depositional environments. Distribution of the Browns Park strata is related to events that modified the pediplain. A cycle of valley-cutting intervened between completion of the pediplain (with its cover of Bishop Conglomerate) and deposition of the Browns Park Formation. Browns Park strata are preserved both within the valleys and upon the uplands. Because the newly-formed Bishop Conglomerate supplied much of the detritus for the Browns Park Formation, there is gross lithological similarity between the two formations. However, the Browns Park Formation near its type area is distinguished readily from the Bishop Conglomerate by its greater heterogeneity and large proportions of volcanic material. The two formations are not readily distinguishable on the south flank of the Uinta Mountains. For example, on Diamond Mountain Plateau, Bishop-like conglomerate grades upward into Browns Park-like beds of variable lithology. The entire sequence mantles an erosion surface that correlates with the pediplain on the north flank of the range. Outcrops are not extensive and the exposed strata are poorly indurated, so an obscure unconformity between the two formations may have gone unrecognized. Alternatively, while deposition of the two formations was interrupted on the north flank by subsidence and valley cutting, deposition may have been continuous on the south flank so that no distinct break marks the boundary between them. The Browns Park Formation can be divided into four units of different dominant lithologic assemblagest (1) a homogeneous basal conglomerate unit; (2) a heterogeneous tuff, tuffaceous sediment, and sandstone unit; (3) a homogeneous cross-bedded sandstone unit; and (4) a fairly homogeneous calcareous sandstone and siltstone unit. Units (2), and (4) appear to succeed one another vertically but may be, in part, facies equivalents. The relative position of unit (3) is uncertain; probably it is partly contemporaneous with unit (2). Units (2) and (4) contain notable amounts of volcanic material. K-Ar dates have been obtained for two vitrie-crystal tuff samples from near the base of the Bishop-Browns Park sequence on the south flank of the Uinta Mountains. One sample from Yampa Plateau is reported to be 41»3 • 1»1 million years old; the second sample from Diamond Mountain Plateau is reported to be 26.2 + 0.7 m. y. The older date is difficult to assess. If it is correct, either the Bishop- Browns Park sequences have been misidentified or they are of different ages in different places. The younger date has received independent corroboration from other studies. A K-Ar date of ca. 25 m. y. has been obtained by the U. S. Geological Survey from the basal Browns iv Park Formation on the north flank of the Uinta Mountains, and fossil mammal collections from near the same locality have been assigned an early to middle Miocene age. An additional K-Ar date from vitric tuff in the medial Browns Park Formation in Browns Park is reported to be 11.8 + C.4 wti y. Fossil mammal collections from the upper Browns Park Formation nearby have been assigned a late Miocene to early Pliocene age. Thus, deposition of the Bishop Conglomerate apparently began in Oligocene, or perhaps even late Eocene, time. Deposition may have been continuous on the south flank of the range during much of Oligocene and Miocene time, but, on the north flank, subsidence of the Browns Park area and consequent dissection of the upland surface was a major interruption. Deposition of the Browns Park Formation began in late Oligocene and (or) early Miocene time and continued through early Pliocene time, when regional epeirogeny brought its accumulation to an end. The Bishop Conglomerate and the Browns Park Formation reflect geological conditions that were operative in much of"" the western interior of the United States. The Bishop Conglomerate represents the rather abrupt termination of early Tertiary aggradation that had lasted from Laramide uplift of the Uinta Mountains until exterior drainage was developed out of the flanking basins. Impetus for deposition of the Browns Park Formation was subsidence of part of the range, which resulted in a first stage of drainage changes. Ultimate burial of most of the eastern Uinta Mountains by Browns Park strata resulted in a second stage of drainage changes--most significantly, superposition of the major river systems, the Green and Yampa Rivers.