The principal structural features of the Eastern Uinta Mountains and adjacent areas are ploted on figure 25. The mountains first arose in latest Cretaceous time when the old Western Interior seaway drained for the last time. Tectonic events since that time, and corresponding geomorphic and sedimentologic events, are summarized in table 2. In a study of the sedimentary history of the Uinta Basin, Ryder and others (1976, p. 510) noted deformation of the Cretaceous coastal plain and closure of the Uinta Basin in latest Cretaceous time, accompanied by the rise of the Uinta Mountains. The easternmost Uinta Mountains region apparently was structurally low as late as Maastrichtian time, inasmuch as marine Lewis Shale has been recognized in the No. 1 Raeder-Government test well drilled on the Dry Mountain anticline in sec. 29, T. 9 N., R. 99 W., Moffat County, Colo. (Ritzma, 1965b, p. 135). The western part of the range began to rise earlier, when the Maastrichtian, orogenic Currant Creek Formation (of Walton, 1944) was deposited on the early Campanian, pre-orogenic Mesaverde Formation (Walton, 1944, p. 126). According to Bruce Bryant (written commun., 1983), the Currant Creek contains clasts from the Uinta Mountain Group. Between these two areas, strong early Laramide uplift of the east-central part of the range is indicated at Goslin Mountain, where the late Campanian Ericson Sandstone was overturned and faulted before the Fort Union Formation (Paleocene) was deposited. If the Lewis Shale was ever deposited there, it was removed by pre-Fort Union erosion. Obvious chips of Mowry Shale and probable gastroliths from the Morrison Formation or the Cloverly are preserved in the Fort Union conglomerates at Goslin Mountain. Their presence indicates 2,400-3,000 m of erosion off the Uinta anticline by Paleocene time (Hansen and Bonilla, 1954, p. 11; Hansen, 1965, p. 170). No really coarse debris was shed from the Eastern Uintas at that time, but little rock hard enough to yield coarse debris had yet been exposed.
TABLE 2.Summary of Tertiary and Quaternary
tectonic, geomorphic, and sedimentologic events in the Eastern Uinta
By Wasatch time (early Eocene) the eastern part of the range was eroded to its Precambrian core, as cobbles from the Uinta Mountain Group accumulated in the main body of the Wasatch Formation north of Clay Basin (Hansen and Bonilla, 1954, p. 11; Wiegman, 1964, p. 41) and in the Tipton Tongue of the Green River Formation southwest of Vermillion Creek (Sears and Bradley, 1925, p. 97; Ritzma, 1955, p. 38). Just north of Manila, near the Henrys Fork fault (fig. 25), the Wasatch contains very bouldery conglomerate derived from most of the resistant formations in the nearby mountains down to and including Mississippian rocks. The clasts appear in the Wasatch section in the reverse order of their original stratigraphic positions. The coarseness of these deposits suggests a nearby source higher than the present Uintas. One limestone boulder 3.3 m across must have been traveled at least 13 km. Major uplift of the Uintas during Tipton time may have coincided with movement on the Henrys Fork fault (Anderman, 1955, p. 131), with renewed movements on the Uinta fault north of Goslin Mountain, where the Fort Union is faulted against the Uinta Mountain Group (Hansen, 1965, p. 170), and with the flood of coarse debris into the Tipton near Vermillion Creek (Schultz, 1920, p. 31; Sears and Bradley, 1925; Ritzma, 1955, p. 39). The Laramide orogeny apparently ended in the Eastern Uinta Mountains after the youngest Eocene rocks had been deposited, including the bulk of the Duchesne River Formation, whose upper member, the Starr Flat Member of Andersen and Picard (1972), is conglomeratic near the mountains. The Starr Flat Member probably is Oligocene (Andersen and Picard, 1972, p. 16; Emry, 1981). The range then lapsed into a long period of quiescence, during which the Gilbert Peak erosion surface began to take form. On the south flank of the Uinta Mountains, however, the Starr Flat Member(?) rests unconformably on a truncation surface that might be the Gilbert Peak erosion surface. If so, the surface may have begun to form in late Eocene or early Oligocene time. Basinward from the mountains the Starr Flat Member apparently intertongues with the subjacent Lapoint Member of the Duchesne River Formation (of Andersen and Picard, 1972, p. 15-16). Perhaps the Starr Flat and the Bishop are isochronous.
Compressive Laramide severing of the Uinta anticline from its root has been suggested by several workers, as noted by Hamilton (1981, p. 90). Stokes (1976) and Sears and others (1982, fig. 5) have postulated a Precambrian aulacogen, which may subsequently have influenced Laramide and later Tertiary structural trends. Regional northward tilting in a similar mode in Late Proterozoic (?) time (Hansen, 1977a) and stratigraphic thickening along the Uinta Mountains through parts of Phanerozoic time (Hansen, 1965) seem to support the aulacogen concept or, at least, the probability of an east-trending structural trough bounded by faults. Bryant (1985) has summarized evidence of an east-trending continental margin along the site of the Uinta Mountains in Archean time. This margin might have affected tectonic events through much of subsequent geologic time. Bryant's concept does not contradict the evidence of a later east-west trough along the Uinta trend.
Laramide propagation of fracturing beneath the flanks of the range (fig. 26, inset) might best be visualized as underthrusting due to crowding of the thick prism of Uinta sediments between the Colorado Plateau to the south and the Wyoming Basin to the north. The Uinta fault has a pre-Laramide history, probably as a normal fault, and the Laramide arching of the Uinta anticline would have effectively reduced the dip of the fault to an attitude conducive to thrusting along the preexisting fracture (Hansen, 1984, p. 9). The asymmetry of the fold suggests that the range has translated northward relative to the Green River Basin. It verges northward, and the north limb is steeper than the south. Perhaps the Uinta Basin boundary fault is a back thrust on which the south limb has ridden passively over an underthrust Uinta Basin margin.
The long period of crustal stability needed to form the Gilbert Peak erosion surface and to deposit the overlying Bishop Conglomerate across thousands of square kilometersperhaps as much as 26,000 km2ended with renewed deformation in an extensional mode that brought about notable changes in the pattern and regimen of drainage. This deformation is well documented, but in some places it is masked by movements that followed the deposition of the Browns Park Formation. These movements may, in fact, have been part of a continuum of alternate activity and quiet, but the greater part of the deformation seems to have preceded the deposition of the Browns Park Formation.
After the Bishop was deposited the whole eastern part of the range began to founder, partly by regional warping and partly by localized displacements along faults. Figure 26 explains the observed deformation as a consequence of north-to-south rotation of the range. Rotation is expressed in tilted erosion surfaces. I suggest that the Eastern Uinta Mountains have rotated over a root severed by Laramide or earlier faultingthat a faulted, semirigid upper crust has rotated over a ductile lower crust at a depth of perhaps 20 km in response to regional north-south extension. Such rotation requires a curved fault surface and a mass imbalance. Imbalance is indicated by a relative positive Bouguer gravity anomaly over the north flank of the range (Behrendt and Thiel, 1963) and by the fact that the axis and heights of the range are closer to the north flank than the south. Rotation has not taken place under the western part of the range, where flanking subthrusts appear to be planar (Clement, 1977; Gries, 1981). The rotation and subsidence as thus visualized would be taken up partly by fault slippage and partly by monoclinal flexing under the south flank over the concealed Uinta Basin boundary fault.
Middle to late Tertiary extensional deformation is common in other ranges of the Rocky Mountain foreland. (See Ingersoll, 1982, for example.) Among other things, it is responsible for the subsidence of the south part of the Wind River Range, of the south part of the Owl Creek Mountains, and of the Granite Mountains, all in central Wyoming (Keefer, 1970; Love, 1960, 1970, 1971). In general, post-Laramide extensional deformation was more complicated along the north flank of the Eastern Uinta Mountains than along the south (not surprisingly, as Laramide uplift and faulting were greater on the north flank), but the overall effect was a lowering of the crest of the range, relative to both flanks, and inward tilting of both flanks. Maximum subsidence was along the zone of faults that trends eastward to southeastward from Bear Mountain to Browns Park, thence along the northeast border of Browns Park to and beyond the east end of the range (fig. 27).
Most of the post-Bishop deformation on the north flank was concentrated between the above-noted zone of faults and the Uinta fault a few miles north. Some faults that had Laramide or earlier movements were reactivated. A reversal of movement on the south-dipping Uinta fault has been recognized since the time of Powell. Its large compressional Laramide displacementup on the south, with a probable component of left-lateral strike slip (Hansen and Bonilla, 1954, p. 15; Hansen, 1965, p. 158)was countered by a much smaller, post-Bishop gravitative reversal. This movement, down to the south, amounted to about 520 m between Cold Spring Mountain and Diamond Peak (Bradley, 1936, fig. 18) and about 790 m at Goslin Mountain. It was accompanied by marked warping between the Uinta zone and the Bear MountainBrowns Park zone. Bear Mountain has been lowered about 975 m since Bishop time by gravitative movement on the Uinta fault. (See p. 57.) North of the Uinta fault, the Miller Mountain area (Bradley, 1936, p. 185) and the lookout Mountain area were tilted northward, increasing the northward gradient of the Gilbert Peak erosion surface.
South of the Uinta fault, O-Wi-Yu-Kuts Mountain was bowed upward relative to adjacent Cold Spring and Bender Mountainsits crestline now forms a broad east-west archbut the whole area of these mountains dropped with respect to the north side of the Uinta fault. As one crustal block, the area of Goslin, Bender, O-Wi-Yu-Kuts, and Cold Spring Mountains also was tilted southward, flattening and reversing the slope of the Gilbert Peak erosion surface and thereby redirecting the runoff into the newly formed drainage line of Browns Park.
The southerly component of tilt gradually increased westward from Cold Spring Mountain to a maximum at Goslin Mountain, where the present southward slope of the Gilbert Peak surface is about 83 m/km. Southerly tilt extended west from Goslin Mountain at a diminishing rate at least as far as Bear Mountain and probably to Phil Pico, west of Manila. The southerly slope of this mountain's flattish summit is most distinct as seen from an aircraft. Inasmuch as the initial slope of the Gilbert Peak erosion surface on Goslin Mountain was northwardprobably about 38 m/km, judging from Bradley's reconstruction of the undisturbed Hickey Mountain profile farther west (1936, table, p. 174)the total amount of southward tilting on Goslin Mountain probably was about 120 m/km.
North to south, Goslin Mountain is about 5.6 km across. If tilted southward at 120 m/km, its south end at Little Hole has been lowered about 700 m relative to its north end at the Uinta fault since Gilbert Peak time. Using those figures and relating the tilting of Goslin Mountain to the area immediately to the south, and to O-Wi-Yu-Kuts Flats and Pine Mountain to the east, I estimated (Hansen, 1965) that the total collapse of the crestline of the Uinta Mountains south of Goslin Mountain was about 1,370 m. If that figure is added to the present mean altitude of the crestline south of Goslin Mountainnow about 2,740 mthe restored altitude before collapse is 4,115 m. This figure almost exactly equals the maximum present height of the western part of the range that was unaffected by post-Gilbert Peak subsidence (Hansen, 1965, p. 172).
Comparable but more refined results can be obtained graphically by projecting the gradient of the Gilbert Peak erosion surface southward from Little Mountain in Wyoming across the Uinta fault to Goslin Mountain in Utah and assuming that the surface steepened from a measured 24 m/km at Little Mountain to an assumed 38 m/km at Goslin Mountain, as explained in the preceding paragraphs. This reconstruction (fig. 28) assumes that the Uinta fault is a downward-steepening fracture that dips about 60° southa dip indicated by the attitude of sheeting in the fault gouge just east of Goslin Mountain at Red Creek and by the attitude of large tabular slices of resistant rock caught in the fault zone. The dip may be a bit lower at Cold Spring Mountain, perhaps about 45° (Hansen, 1984, p. 17). Downward steepening, or upward flattening, is suggested by overturning in the foot wall far back from the fault trace. With that geometry, any renewed down-to-the-south movement of the Uinta fault would rotate the Gilbert Peak erosion surface southward. The observed rotation geometry of the Gilbert Peak erosion surface, in fact, demands a curved fault surface. The present southward slope of Goslin Mountain would result if the rotation were about 5° of arc, the radius of curvature were about 12 km, and the downthrow were about 790 m. This downthrow is appreciably more than what Bradley (1936) calculated (520 m) for the subsidence of Cold Spring Mountain 32 km or so to the east, but the southward rotation of Cold Spring Mountain is much less than that of Goslin Mountain, and the downfaulting, accordingly, should be less also. Cold Spring Mountain's altitudeabout 150 m higher than Goslin Mountainalso indicates less displacement. Furthermore, the Gilbert Peak erosion surface has a strong westerly tilt component between Cold Spring Mountain and Goslin Mountain (Hansen, 1965, p. 163, fig. 63), which would increase the displacement at Goslin Mountain also. Projecting the Gilbert Peak erosion surface south to Little Hole suggests that the south end of Goslin Mountain has been lowered about 1,280 m.
South of Little Hole, across the Dutch John fault, zone at the foot of Goslin Mountain, the crest of the Uinta Mountains has been lowered about 300 m more, relative to the south end of Goslin Mountain, judging from the profile, figure 28. Added to the 1,280 m of deduced subsidence at the south end of Goslin Mountain, this figure gives a total collapse of the crestline of about 1,590 m. This figure can be duplicated by simply projecting the restored Gilbert Peak erosion surface south from Goslin Mountain and measuring its height above remnants preserved south of Little Hole.
Tilting and displacement similar to that described in the preceding paragraphs can be obtained if a dip lower than that postulated here is assumed for the Uinta fault, so long as the fault surface remains convex upward. Some geologists have suggested dips as low as 10°. However, the amount of extension required to achieve the observed tilting increases rapidly as the assumed dip angle is reduced.
A northerly tilt along the south flank of the Uinta Mountains, as expressed in the attitude of the Bishop Conglomerate, is visible from many vantage points on or above the Bishop outcrop all the way from Diamond Mountain northeast of Vernal to the extreme east end of the range. This tilting reversed the drainage of several small streams along the crest of the range, redirecting them from their old south-flowing courses into the newly formed Browns Park valley. Even though the slope to the north is low, mostly only 1 or 2 degrees, the pattern of barbed drainages is obvious and remarkable. Inasmuch as the original slope of the pediment and the depositional slope of the Bishop were southward, the present slope is a minimal measure of the actual amount of tilt.
Tilt is well documented in the Stuntz Reservoir quadrangle, where the Gilbert Peak erosion surface is well preserved and is little deformed except for tilting (fig. 29). There it is partly mantled with patchy Bishop Conglomerate and hence is readily depicted in profile (Hansen and Rowley, 1980b, map and section CC'). In that area the northward slope of the surface is about 16-17 m/km, some parts being a bit steeper than others. The original slope of the erosion surface in the opposite direction, however, may have been as much as 13 m/km, perhaps appreciably more, judging again from the undeformed Hickey Mountain profile on the north slope of the range. An original slope of 13 m/km toward the south added to the present northward slope would yield a minimal northerly tilt component of 28 m/km since the Bishop Conglomerate was deposited. That order of tilt, if uniform over a distance of 56 kmthe distance from the south boundary of the Stuntz Reservoir quadrangle to the crest of the Uinta anticlinewould amount to differential subsidence of about 1,600 m at the crestline. Without a known fixed datum the absolute subsidence cannot be determined, but this figure agrees closely with the estimated subsidence farther west at Goslin Mountain.
NONUNIFORM TILTING ON THE SOUTH FLANK
Farther west on the south flank of the range, nonuniform tilting has been noted northwest of Vernal, where remnants of the Bishop Conglomerate straddle the South Flank fault zone (Kinney, 1955, p. 127). There the Gilbert Peak erosion surface has been warped into a shallow syncline whose axis is parallel to the fault zone and is about 3 miles south of it. The limbs of the syncline slope 38-57 m/km (D. M. Kinney, written commun., 1959). A synclinal structure there could result from a north-side-down movement on the subsurface Uinta Basin boundary fault of Ritzma (1969). Kinney also noted that the Gilbert Peak surface gradually decreases in altitude eastward from the Uinta River toward Diamond Mountain.
In the Diamond Mountain area, generalized structure contouring on the base of the Bishop Conglomerate indicates nonuniform warping centered on Island Park (fig. 30). This warping probably is a result of post-Bishop activity along the Island Park fault zone at the downwarped northwest border of the Split MountainRuple RidgeHarpers Corner crustal blockwhat Powell (1876, p. 177) called the Island Park sag. The Diamond Mountain side has been lowered 60-120 m relative to Harpers Corner, judging from the structure contours. This displacement is plainly discernible on the ground from distant vantage points. Quaternary faulting also cuts the Bishop on Diamond Mountain along a west-northwest fault zone just south of Diamond Gulch. Several kilometers northeast of Diamond Gulch along Pot Creek, a synclinal closure straddles a zone of very old faults that have had Quaternary movement in the Crouse Reservoir area (Hansen and others 1981). One of these faults in Lodore Canyon contains a dike of Late Cambrian or Early Ordovician age (Hansen and others, 1982, 1983). These faults also trend west-northwest.
WEST-TO-EAST TILTING ON THE SOUTH FLANK
Although west-to-east tilt is clearly visible on the south flank of the Eastern Uintas, it is difficult to quantify because the original surface sloped away from the crestline and because the surface has also been modified by the northerly tilt component. As a rough measure of the easterly tilt north of Vernal, however, the present surface descends about 900 m west to east, from about 3,050 m just west of the Uinta River to about 2,135 m at Diamond Gulcha rate of about 12 m/km. This tilt is shown graphically by figure 31.
CLIFF RIDGE AREA
In the Dinosaur area, easterly tilt is also evident from Cliff Ridge, one of the most southerly parts of the Gilbert Peak erosion surface and the highest point on that surface south of the line of the Yampa River (summit elevation about 2,524 m). From Cliff Ridge the Gilbert Peak surface and the Bishop Conglomerate slope northeast and east toward the Stuntz Reservoir area and Mud Springs Draw (a tributary of Hells Canyon). Stuntz Ridge, which is a topographic continuation of Cliff Ridge, was a monadnock that stood above the Gilbert Peak surface and received no gravel. Its smoothly graded crestline suggests that it is a remnant of the Wild Mountain upland surface. The gravel that spread out into Mud Springs Draw must have come from the north via Cliff Ridge, because it contains abundant red quartzite clasts from the core of the range to the north, but it detoured around the west side of Stuntz Ridge. Bishop Conglomerate preserved on the south side of Stuntz Ridge caps a narrow berm that slopes east toward Mud Springs Draw (fig. 32).
The northerly component of tilt from Cliff Ridge is manifest in the slope to the Yampa Plateau, whose conglomerate-capped summit is about 2,350 m in altitude, and to Split Mountain, a bit farther northwest at 2,315 m. Split Mountain's summit is a point on the Gilbert Peak erosion surface, but it lacks a gravel cap. Thirteen kilometers due north of Split Mountain, the rim of Diamond Mountain also reaches slightly above 2,315 m, but the conglomerate is at least 90 m thick there, so the elevation of the underlying Gilbert Peak surface is, accordingly, about 2,225 m.
MUD SPRINGS DRAW AND WOLF CREEK
East along Mud Springs Draw and Wolf Creek the general easterly tilt component is expressed in a rather narrow belt of Bishop Conglomerate about 32 km long. In that distance the altitude of the Bishop descends about 300 m, a rate of 10 m/km. Some of this slope may be initial dip and may represent the depositional gradient of the Bishop Conglomerate before tilting, inasmuch as the proportion and coarseness of clasts derived from the Uinta Mountain Group diminish eastward. Figure 33 shows that a shallow synclinal warp at the base of the Bishop south of Hells Canyon probably is a response to post-Bishop movement along the Mud Springs monocline. This downwarp is centered along the length of Blue Mountain, and because the Bishop Conglomerate coincides with the downwarp, the monocline appears to have helped localize the deposition of Bishop in that area. The downwarp thus had negative relief before the gravel was deposited, as well as after.
Fingers of gravel reached north from Wolf Creek up narrow tributaries that drained the heights of Round Top Mountain, Marthas Peak, Tanks Peak, and Bear Valley Ridge. Many patches of this gravel still remain, now loosely cemented into conglomerate or coarse sandstone, but the tilt component cannot be measured, owing to the scatter of the remnants and uncertainty as to their original gradients. Northerly tilt is indicated, however, by a reversal of streams that now drain north to the Yampa River but formerly flowed south into Wolf Creek. Johnson Draw (figs. 33 and 39) is a good example (Hansen and Carrara, 1980). Turner Creek and Mud Springs Draw are otherscaptured by north-draining Hells Canyon (Hansen and Rowley, 1980a).
At the southeast end of the range, the subsidence of the Uinta arch and its southeastward extension, the Axial Basin anticline (west part) lowered the Bishop Conglomerate below present drainage. Much of the deformation there is younger than the Browns Park Formation, but some of it is older, and it produced a sag into which the Browns Park Formation was deposited on or partly on the Bishop. In nearly continuous outcrop the Bishop extends from Bear Valley east to Elk Springs, declining in altitude from 2,440 m at the head of Bear Valley to 1,950 m at Elk Springs, where it passes beneath the white sands of the Browns Park Formation. East of Cross Mountain, where the Bishop is deeply buried by the Browns Park Formation, its altitude is less than 1,460 m (Dyni, 1980).
At Elk Springs, what I identify as Bishop Conglomerate is well exposed at the base of the Browns Park Formation in a large gravel pit. It contains the usual assemblage of Uinta Mountains rock typesred quartzite, gray limestone, and gray chertin cobbles 10-15 cm across and occasional boulders as large as 75 cm across (Dyni, 1980). The bedding attitude changes from a very low northeast dipalmost flatto about 20° to 25° north in response to post-Browns Park deformation. Dyni (1980) has suggested that the Bishop (which he called the lower conglomerate unit of the Browns Park Formation) may have been deformed before or during deposition of the upper sandstone unit (of the Browns Park Formation). Dyni's suggestion corroborates my view about the disparate ages of the two formations.
East from Elk Springs the conglomerate continues along strike as a narrow line of outcrop at the base of the Browns Park Formation along Elk Springs Ridge, dipping about 25° north and finally disappearing under Browns Park overlap east of Cedar Springs Draw. A deep downwarp between Elk Springs Ridge and Cross Mountain, broadening east toward Maybell, contains as much as 520 m of Browns Park rocks (Dyni, 1980). According to Dyni this trough may have formed over growth faults that were active during Browns Park deposition.
Due north from Elk Springs, the conglomerate is offset by several small faults, but it slopes gently north toward Lily Park at the confluence of the Little Snake and Yampa Rivers, forming the caprock on the high mesas there (such as Twelvemile Mesa) 300 m above river level (Dyni, 1968). Eastward on Twelvemile Mesa it passes beneath the Browns Park Formation near Cross Mountain, but to the north, correlative deposits on Klauson Pasture stand about 200 m above the Browns Park Formation in the canyon of the Little Snake River. (See p. 32.) Though it is a loosely cemented, bouldery gravel and is nearly bare of the white sandstone cover west and north of Elk Springs, geologists back to the time of A. R. Schultz have included it in the Browns Park Formation. According to Dyni (oral commun., 1981), it is discontinuous beneath the Browns Park Formation and, hence, is hardly mappable. I suspect that it was partly removed by pre-Browns Park erosion.
At the northeast end of the Uinta Mountains, between Vermillion Creek and Sand Wash, the whole flank of the Uinta anticline has been obscured by downwarping, faulting, erosion, and sedimentary overlap by the Browns Park Formation. Just west of Dry Mountain, the Browns Park Formation is as much as 600 m thick (Ritzma, 1965b, p. 131). A zone of mostly normal faults trends southeast from the Vermillion Creek area to and beyond the Little Snake River near Sunbeam. Most of these faults are downthrown toward the south. This zone is a structural continuation of the faulting along the northeast side of Browns Park, and it also merges with the Uinta-Sparks zone north of Cold Spring Mountain. It is partly overlapped by an unconformity within the Browns Park Formation (Ritzma, 1965b, p. 131), which suggests intra-Browns Park deformation. Sears (1924a, p. 296) noted a similar local unconformity northwest of the mouth of Vermillion Creek. I interpret the faulting as a gravitative response to post-Laramide extension of the northeast flank of the range.
The Browns Park Formation itself, as well as the underlying Bishopor basal Browns Park of earlier reportsis sharply tilted along the northern margin of its outcrop belt between Vermillion Creek and the Little Snake River. Dips commonly reach 20° south and, locally, are as much as 40° to 65° south (McKay, 1974). The subjacent Eocene rocks, chiefly the Bridger Formation, dip northward beneath a marked unconformity into the Sand Wash Basin. This unconformity is presumably the Gilbert Peak erosion surface. The Bishop there is discontinuous. In part it has been deleted by faulting, and its locally steep dip is surely caused by drag, but in part the Bishop either was not deposited or was removed by pre-Browns Park erosion. Abrupt changes in thickness and the absence of Bishop in some sections point toward pre-Browns Park erosion rather than nondeposition. For example, between the Little Snake River and Simsberry Draw, a distance of only about 2.4 km, the thickness diminishes abruptly from about 24 m (Izett and others, 1970) to less than 1 m (McKay, 1974), and the provenance changes from the Uinta Mountains to the Park Range. A few miles to the east, north of Sunbeam, S. J. Luft and I found the Browns Park Formation resting directly on the Wasatch, with no intervening conglomerate at all.
The main body of the Browns Park Formation through most of its exposure has the structure of a shallow syncline turned up sharply along its north and northeast margin by warping and discontinuous faulting. Mapping its western part, I called it the Browns Park syncline (Hansen, 1965, pl. 1); McKay and Bergin (1974), mapping its eastern part, called it the Lay syncline, but it is a continuous structure and its axis is one and the same. Its axis trends east-southeast through most of its length, from the head of Browns Park to Sunbeam, then veers gradually eastward (Rowley and others, 1979). S. J. Luft (written commun., 1983) has traced it nearly to Craig. Luft has also traced a secondary axis southeast from Sunbeam toward Axial Basin.
This syncline coincides with the old erosional valley of Browns Park along the breached crest of the Uinta anticline and its extension, the Axial Basin anticline (Sears, 1924a, fig. 1; Hancock, 1925, pl. 19). I visualize the valley as developing along the depressed sag in the Gilbert Peak erosion surface. At Axial Basin the Browns Park Formation fills the valley floor below cliffs and highlands eroded from the Mesaverde Group on both flanks; hence the topographic configuration of Axial Basin at the time of Browns Park deposition was not greatly different from what it is now.
Mapping in Moffat County, Sears (1924a, p. 288) attributed the form of the syncline primarily to deformation, but he recognized the effects of initial dip into the old trough. Mapping farther west, I attributed the structure to a combination of (1) initial dip on the sloping floor of the old valley, (2) valleyward thinning of coarse-grained beds, and (3) differential compaction of fine-grained sediments, the total compaction being greatest in the thickest part of the deposit over the deepest part of the buried valley (Hansen, 1965, p. 154). Steep dips along the north and northeast margins of the old valley, however, are due to drag and flexing along and near faults. Along the Mountain Home fault at Jesse Ewing Canyon (figs. 27 and 40) near the head of Browns Park, for example, the dip approaches vertical (Hansen, 1965, pl. 1 and fig. 59). Less steep inclination along the south margin of the old fill along Elk Springs Ridge east of Elk Springs may be due to resurgence of the Elk Springs anticline, whose axis is just south of the outcrop (Dyni, 1968). There, beds that originally sloped southward from the Uinta Mountains now slope north.
At the confluence of the Little Snake and Yampa Rivers, the short, south-trending Lily Park syncline (Dyni, 1968; McKay, 1974) separates the Cross Mountain anticline to the east from the east plunge of the Uinta anticline to the west. This syncline is expressed by sharp folding in the pre-Tertiary rocks and faulting on the east margin. Overlying these rocks, the Bishop (or basal Browns Park of earlier reports) forms a 60-m-thick caprock on high terrace remnants on both sides of the valley. These terraces project south across the Yampa valley to Twelvemile Mesa. If the slopes of the terraces were projected to the center of the valley, the base of the Bishop would form a gentle swale superimposed on the Lily Park syncline. This swale represents initial dip on the floor of a consequent valley eroded into the syncline, because the source of Bishop debris for the west limb was the Uinta Mountains and the source for the east limb was Cross Mountain, but the swale might have been enhanced by subsequent warping. A post-Bishop tilt to the north, in any event, is unmistakable.
The pre-Bishop Lily Park syncline plunges southward about 120 m/km (Dyni, 1968), whereas the superimposed swale structure plunges north; the north plunge is an expression of the widespread post-Bishop tilting of the region, which amounted to about 9 m/km in that area and was not sufficient, therefore, to reverse the southerly plunge in the older rocks. One may recall that the crestline of Cross Mountain, which is a relic of the pre-Gilbert Peak, Wild Mountain upland surface, also slopes southward despite the general northerly post-Gilbert Peak (post-Bishop) tilt of the region. This southward slope, like that of the Lily Park syncline, predates the Bishop Conglomerate.
Tectonic movements in the eastern Uinta Mountains involving the Browns Park Formation have already been mentioned, particularly movements in the Elk Springs area. These movements appear to have been continuations of earlier deformation; rather large-scale deformation preceded, accompanied, and followed deposition of the Browns Park Formation. Some of this deformation has been discussed by Sears (1924a, p. 291) and by Bradley (1936, p. 185), both of whom believed that the major collapse of the Uinta arch was after the deposition of the Browns Park. I believe it was before, inasmuch as it produced the broad, erosionally modified tectonic downwarp into which the Browns Park Formation was depositedthe Uinta arch was depressed hundreds of meters before the Browns Park Formation was deposited. Further deformation then followed.
DIAMOND PEAK AND BROWNS PARK
Post-Browns Park deformation involved widespread tilting and the reactivation of previously formed faults. Between Diamond Peak and Cold Spring Mountain, for example, the Browns Park Formation extends northwest up the valley of Talamantes Creek along the Uinta-Sparks fault zone, probably as an old valley fill, resting on the older rocks with a sharp angular unconformity. South-dipping drag in the Browns Park indicates renewed movement along the fault, down on the south toward the collapsed Uinta arch (Weber, 1971, fig. 4), a continuation of movements that earlier had lowered Cold Spring Mountain relative to Diamond Peak.
Similar faulting in Browns Park is better exposed, better documented, and even more pronounced. Near the head of Browns Park, movement on the Mountain Home fault, the Beaver Creek fault, and other faults reversed the dip in the Uinta Mountain Group, forming drag anticlines and sharply tilting the Browns Park Formation (Hansen, 1965, plate 1, fig. 59; 1957 a,b). Just east of Jesse Ewing Canyon, however, the Mountain Home fault passes beneath the Browns Park Formation without cutting it, showing that the fault antedates the Browns Park Formation. Renewed movement on parts of the Mountain Home fault and the Beaver Creek fault after deposition of the Browns Park Formation, south sides down, indicates further subsidence of the Uinta arch.
The Browns Park Formation is broken by displacements along these and other faults all the way from the head of Browns Park to points north and west of Craig, Colo. (Hansen, 1957a,b; Ritzma, 1959; Tweto, 1976; Rowley and others, 1979). In the Dry MountainDouglas DrawVermillion Creek area, in an added complication, the axis of the Browns Park syncline plunges gently northwestward at a rate of about 9.5 m/km. This plunge also reflects subsidence of the Uinta arch.
One of the major structural features on the south flank of the Eastern Uinta Mountains is the faulted Yampa fold bordering the north side of Blue Mountain (figs. 25 and 33). This fault is a low-angle thrust between Hells Canyon and Johnson Draw. At Hells Canyon its dip is about 22° south and its upper plate is dragged into a sharp bend, the Yampa monocline. Stated differently, the fault is the ruptured synclinal bend of the monocline. Although most of the fault's 1,500-m displacement is Laramide, Sears (1924a, p. 291) ascribed large-scale post-Browns Park movement to it and invoked this movement to explain the course of the Yampa River. I know of no clear evidence of any post-Browns Park offset; evidence to the contrary near the western end of the fault indicates little or no movement since the Bishop Conglomerate was deposited.
Dyni (1980), however, found evidence of inter- and post-Browns Park deformation east of the Yampa monocline in the Elk Springs area, some of which he attributed to recurrent movements on growth faults. Near the eastern terminus of the Yampa fault and monocline the Bishop Conglomerate has been displaced by renewed warping on the monocline (fig. 33), but whether or not this movement was post-Browns Park in age is uncertain. Structure contouring based on abundant remnants of Bishop Conglomerate indicates that post-Bishop warping is superimposed on preexisting structural features. In T. 5 N., R. 99 W. and R. 100 W., a noselike swing of the contours (fig. 33) coincides with the plunging south branch of the Yampa monocline. Just to the north, in T. 6 N., a synclinal reentrant in the contours coincides with the north branch of the Yampa fault and its synclinal extension. These features suggest resurgence of older structures. Here the Bishop and the subjacent Gilbert Peak erosion surface have been warped about 120 m. Inferred post-Browns Park warping on the nearby Deerlodge monocline (see next section) suggests possible warping of that age at the eastern end of the Yampa structure also.
West of Hells Canyon, however, the Yampa fault dies out, and its monoclinal extension is truncated by the Gilbert Peak erosion surface and the Bishop Conglomerate. (See fig. 29.) Some minimal post-Bishop distortion of the surface across the monocline is possible in this area, but it isn't obvious. At any rate, because the fault does not displace the Bishop Conglomerate, the trough-like or bench-like outer canyon of the Yampa River between Douglas Mountain on the north and Blue Mountain on the south is a product of erosion, not faulting (fig. 34). The trough resulted from the differential erosion of soft Triassic and younger rocks from the downthrown side of the Yampa fault or monocline in post-Bishop time.
Sears (1924a, p. 303; 1962, p. 23-24) visualized a new Yampa River flowing west into a drag syncline just north of the Yampa fault, but inasmuch as the Gilbert Peak surface and the Bishop Conglomerate extend unbroken across the Yampa monocline west of Hells Canyon near Harpers Corner, the fault zone and downwarp could not have been the avenue for the Yampa River. The present river is far north of the fault zone and, because of the low southerly dip of the Yampa fault along most of its length, moreover, any gravitative "collapse" of the downthrown (north) side as visualized by Sears is unlikely. Unaware of its low southerly dip, Sears assumed, as Powell had before him, that the Yampa fault was a high-angle normal fault. Following Sears' lead, other authors have similarly concluded that the Yampa fault is the southern boundary of the so-called Browns Park or Uinta Mountain graben, in the mistaken belief that the Yampa fault dips north. This graben simply does not exist.
Geomorphic relationships along the Little Snake and Yampa Rivers suggest warping on the Deerlodge monocline during or after Browns Park deposition. This monocline is the short, abrupt fold that terminates the Uinta uplift on the east (Hansen and others, 1984) and provides the dramatic entry into Yampa Canyon from Deerlodge Park (fig. 13). Just upstream from Deer lodge Park along the Little Snake River, a basal conglomerate beneath the Browns Park Formation be tween Cross Mountain and the eastern Uintas (p. 32) indicates through drainage by a vigorous stream in that area in a canyon-like setting at the onset of Browns Park deposition. Where this drainage went is problematical, but it almost certainly turned west along the present Yampa Canyon, because egress in other directions is blocked by higher ground, even though there is little likelihood that a canyon as deep and narrow as the present one could have persisted since Miocene time. A south-flowing course toward the White River looks plausible on a planimetric map, but geomorphic evidence is against it. More likely, the drainage turned west into a then-shallower, more juvenile Yampa Canyon, which deepened as the resurgent Deerlodge monocline rose beneath it (or as the connecting Lily Park syncline sank).
The rise of the monocline would have inhibited drainage upstream and would have abetted the aggradation that attended the deposition of the succeeding Browns Park Formation. This reconstruction of events accords well with Hunt's concept of anteposition (1967, 1969), whereby a drainage temporarily blocked by a rising uplift overtops the obstruction by aggrading its bed, then resumes its previously established course. The presence of loose Browns Park sandstone at rim level on Bishop Conglomerate upstream from Yampa Canyon (Dyni, 1980, map) adds credence to this hypothesis.
East Cactus Flat, which truncates the Deerlodge monocline, slopes gently southwardits rim is highest next to Yampa Canyon. The slope probably postdates the entrenchment of the river; otherwise the river should have migrated downslope to the south, away from its present position. The southerly slope probably is a manifestation of resurgence of the Deerlodge monocline during or after the deposition of the Browns Park Formation.
Last Updated: 09-Nov-2009