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Home , Geology of Colorado, Minturn Formation, Paradox Basin, Paradox Formation, Uinta Basin, Uncompahgre Plateau

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Tectonic evolution of western and eastern D. L. Baars and G. M. Stevenson, 1981, pp. 105-112 in: Western Slope (Western Colorado), Epis, R. C.; Callender, J. F.; [eds.], Geological Society 32nd Annual Fall Field Conference Guidebook, 337 p.

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D. L. BAARS Consulting Geologist Evergreen, Colorado 80439

and

G. M. STEVENSON Tierra Corp. , Colorado 80200

INTRODUCTION Uplift on the east. The generally northwest-trending The boundary between the and Southern structural depression is a diminutive copy of the . Rocky Mountain provinces lies somewhere within the western The Eagle Valley in the central part of the basin include Colorado-eastern Utah region, and is arbitrary in the geological gypsum and interbedded with drab-colored clastic rocks sense, because the stratigraphy and tectonic relationships remain (Lovering and Mallory, 1962) that interfinger eastward with shelf relatively constant throughout the area. carbonates and clastics of the Minturn Formation and arkose of The area of this report lies astride the Uncompahgre Uplift seg- the . As in the Paradox basin, salt diapirs are ment of the Ancestral Rockies orogenic system (see fig. 2). To the present at a reduced scale near Glenwood Springs and Eagle, Col- west of the ancient Uncompahgre fault block lies the northwest- orado (see also Campbell, this guidebook). trending Paradox basin and to the east is the strikingly similar Eagle basin, both of late age. Bounding the region BASEMENT FRAMEWORK on the east are the marginal Gore and Mosquito faults of the Front Although much has been said about the Laramide deformation Range segment of the Ancestral Rockies. Within this vast area the of the Colorado Plateau and Southern Rocky Mountain provinces, early Paleozoic stratigraphy is similar and essentially uniform the fact is that the structural fabric of the region was fixed by Late within the paleotectonic basins, suggesting that the two down- time and repeated rejuvenations of the basement warps were not so distinctly separated until after structure, including the Ancestral Rockies and Laramide episodes, time. However, in time the main uplift of fault only modified the original framework. blocks of the Ancestral Rockies abruptly separated and accentu- It was sometime around the summer of 1,700 m.y.b.p. that activ- ated the Paradox and Eagle evaporite basins, although the stratig- ity got underway on two major shear systems that transect the raphy of the two depressions remained remarkably similar. Both Paradox basin and Uncompahgre Uplift as we know them today the Uncompahgre and Front Range uplifts became high topo- (fig. 1). One, the dominant northwest-trending swarm of faults that graphic features and shed untold cubic kilometers of coarse, passes through the of southwestern Colorado arkosic into the eastern reaches of the structural basins. and on into the subsurface of the eastern Paradox basin, may ex- Whether the two salt basins were interconnected in a common tend as far to the northwest as Vancouver Island, B.C. and toward sea is still a matter of personal interpretation. the southeast into Oklahoma's Wichita aulacogen (Baars, 1976). The margins of the Paradox basin usually are defined by the geo- The subordinate northeasterly swarm of faults forming the conju- graphic extent of salt deposited during Middle Pennsylvanian time gate set of fractures extends from Grand Canyon, through in the . Consequently, there is little or no reflec- the to Superior (Warner, 1978). The tion of the buried basin at the surface, except for the salt diapirs in northwest-trending set, called the Olympic-Wichita lineament by the "Paradox fold and fault belt." The basin is bounded on the Baars (1976), appears to have had right-lateral strike-slip displace- northeast and east by the Uncompahgre Uplift and is surrounded ment; movement took place between 1,720 m.y. and 1,460 m.y. elsewhere by paleotectonically controlled shallow-water shoals. in the San Juan Mountains. The northeast-trending Colorado linea- The ovate basin has a northwesterly orientation, extending from ment, dated at 1,700 m.y. by Warner (1978), displaced the base- Durango, Colorado and Farmington, New Mexico on the south- ment rocks in a left-lateral sense. The two continental-scale shear east to Green River, Utah on the northwest. systems form a conjugate set that intersect in the vicinity of Moab, The Uncompahgre Uplift, which is best known for its exposures Utah. on the Uncompahgre Plateau and Black Canyon of the Gunnison The basic fracture pattern outlined above could be formed when River, extends for some 700 km northwestward from near Santa compressive forces were directed from the north (or south) in Pre- Fe, New Mexico almost to Provo, Utah. It is complexly fault- time. Perhaps a more reasonable solution would be a bounded on its southwestern margin and tilts gently eastward into right-lateral wrenching stress imposed on the western United the Eagle basin. Where exposed, the crest of the tilted fault block States that Wise (1963) proposed as his "outrageous hypothesis." is Precambrian metamorphic and igneous rocks, overlain by vari- Whether the underlying mechanism is the "outrageous hypothe- ous formations. Although it has a complex growth his- sis" of Wise or north-south compression as proposed by Moody tory, the Uncompahgre Uplift was a dominant feature throughout and Hill (1956) is irrelevant for this discussion. Recognition of the most of Permo-Pennsylvanian time. basic tectonic fabric provides a basis for understanding the The Eagle evaporite basin lies generally between the tilted "peculiar" structural features of the Colorado Plateau and South- eastern margin of the Uncompahgre Uplift and the fault-bounded ern Rockies.

A review of the strain ellipsoid oriented to properly represent the very large Uinta Mountain arch had a Precambrian origin and the conjugate fractures described above (fig. 2) indicates that fits the stress field described. Thus, the tectonic stage was set long northerly-trending normal faults (east-west extension) should oc- before the Ancestral Rockies and Laramide , and most cur in conjunction with the wrench faults. It has been well estab- likely by about 1,700 m.y.b.p. lished that the large monoclinal folds of the Colorado Plateau are drape structures over normal basement faults of Precambrian age; FAULT REJUVENATIONS The Colorado Plateau and Southern Rockies were relatively thus the originated as third-order features of the stress quiescent during early Paleozoic time; however, Baars (1966) and field. Furthermore, the large bounding faults of the Colorado Baars and See (1968) presented evidence that minor rejuvenations Rockies (the Gore and Mosquito faults along the Front Range Uplift along the Olympic-Wichita lineament in the Paradox basin oc- and the Cotopaxi fault that bounds the Wet Mountains Uplift far- curred during Cambrian, and Mississippian times. Baars ther south) are north-northwest normal faults that originated in (1972, 1975) described similar rejuvenations of tectonic activity Precambrian time (Tweto, 1980a, 1980b) and appear to have re- along major structures in the vicinity of the Eagle basin and Front sulted from the same basic mechanism. Baars (1972, 1975) demon- Range and Wet Mountains uplifts. Although early Paleozoic dis- strated early Paleozoic rejuvenation of these features. We should placement on the faults was minor, sufficient vertical movement also expect to have easterly-trending folding and thrusting (north- south compression) which readily explains the "maverick" east- west trending thrusted of the . At least

108 BAARS and STEVENSON occurred to create local shoaling conditions and alter sedimentary been segmented by the coupling of the Colorado and Olympic- on a local scale. This structural activity was responsible for Wichita lineaments. isolating local reservoir facies in Late Devonian and Early Mississip- The northwestern element of the Uncompahgre Uplift, perhaps pian rocks of the central Paradox basin. Similar potential reservoir best termed the segment, extends from about Cisco to facies in Mississippian strata of the southern Eagle basin have been the overthrust belt in the Wasatch Mountains east of Provo, Utah reported by Nadeau (1972) in close proximity to basement struc- in the subsurface. Its existence is known from geophysical and sub- tures. It appears that repeated paleotectonic rejuvenation on a surface data underlying the southern flank of the Uinta Basin be- minor scale is favorable to the development of petroleum reser- neath the Book Cliffs. Thus, the rigid basement block provides a voirs prior to Pennsylvanian time. sill over which the sagged, creating the peculiar triangular shape of the basin. It did not shed significant amounts of arkosic debris until Early time. UPRISING OF ANCESTRAL ROCKIES The relatively mild nature of the early Paleozoic tectonic activity Front Range- Wet Mountains Uplifts took a dramatic change by the beginning of Middle Pennsylvanian The complex growth history of the numerous individual seg- (Atokan) time. Vertical displacement along the basement faults of ments of the Front Range-Wet Mountains uplifts of the Ancestral the Paradox basin and Southern Rockies began slowly, but picked Rockies has been thoroughly documented and summarized by De up momentum as Middle Pennsylvania (Desmoines) time pro- Voto (1972). It will suffice here to reiterate that the history of the gressed. The major uplifts of the Ancestral Rockies developed dur- tectonic activity east of the Uncompahgre Uplift is similarly seg- ing this time as tilted fault blocks rising from Precambrian wrench mented and variably dated as that described for the Uncompahgre faults and major normal faults became major source areas for Uplift (see De Voto, 1972). coarse, arkosic clastic sediments. The Uncompahgre, Front Range and Wet Mountains uplifts arose to considerable heights, shedding Complimentary Basins hundreds of cubic kilometers of coarse clastics to the adjacent Paradox Basin basins. More than a thousand meters of arkosic debris accumu- The Paradox basin formed adjacent to the southwestern bound- lated along the eastern flanks of the Paradox and Eagle (Maroon) ing faults of the Uncompahgre Uplift as a complimentary faulted basins in Permo-Pennsylvanian time. Although vertical displace- depression (fig. 3). The deepest part of the basin lies immediately ment definitely dominated the earlier wrenching stress field, the adjacent to the uplift, having stepped down structurally in a series structural fabric of the basement remained essentially unchanged, of half-grabens from the western and southwestern shelves, or and east-west extension of the crust was greatly enhanced. Kluth hingeline (fig. 4). Restricted marine circulation resulted in evaporite and Coney (1981) have postulated that the increased tectonic ac- sedimentation throughout most of Desmoinesian (Middle Pennsyl- tivity resulted from the collision of the North American and South vanian) time. Salt deposition began in early Desmoinesian time in American-African plates at this time. However, the basic fabric re- the deeper faulted troughs (Hite, 1960) and slowly filled the basin, mained essentially unchanged from the Precambrian pattern. burying the basement faults by the close of the epoch. Simulta- neously, the Uncompahgre Uplift was supplying ever increasing Uplifts amounts of arkosic sediments to the northeastern margin of the basin. The highland was continuously, or episodically, rising Uncompahgre-San Luis Uplift throughout the remainder of Pennsylvanian time, as the basin sank The Uncompahgre Uplift had a complex history that is not com- along the continuously active basement faults. pletely understood because of poor exposures and limited subsur- Most, if not all, of the smaller structural features of the Paradox face data. Major uplift of the faulted highland began in Atokan basin were in place and growing during Paradox time. These folds time in a southern segment that extends from just east of Santa Fe, exist at the surface today, having been enhanced by Laramide New Mexico northwestward to Ouray, Colorado. Relatively thick compression, but they were sufficiently prominent in Middle Penn- Atokan clastics in north-central New Mexico and southwestern sylvanian time to significantly impede open circulation of marine Colorado attest to the early growth. This segment, the San Luis waters and to control salt thickness. Figure 3 shows the general- Uplift of early workers, continued its relatively modest rate of ized surface structure of the region from Kelley (1955) overlain uplift well into Permian time, supplying clastic sediments to the with Paradox salt isoliths. It is clear that the modern surface struc- Sangre de Cristo, Hermosa and Abo formations along the adjacent tures were actively developing around the periphery of the salt lowlands. basin during the deposition of the salt and were actively affecting A pronounced westerly arc, or flex, occurs in the highland just salt depositional patterns, as well as restricting marine circulation. east of Ouray, paralleling a similar westward flex in the adjacent These are all basement-related structures, and pre-date the Lara- Sneffels and Grenadier grabens of the San Juan Mountains (Baars mide . and See, 1968). This arcuate bend in the basement faults is inter- Perhaps the most obvious and certainly the most interesting preted to be a left-lateral drag fold of enormous proportion structural features of the Paradox basin are the large salt diapirs of caused by the Colorado lineament. It also appears to occur at the east-central Utah and west- (fig. 3). This broad northwestern termination of the San Luis (Atokan) positive feature. region, termed the "Paradox fold and fault belt" by Kelley (1955), From Ouray northwestward to the northwestern plunge of the sur- overlies the deeper structural depression of the Paradox basin face Uncompahgre Plateau near Cisco, Utah, the middle (or Un- where depositional salt thickness may have reached 1500 to 2400 compahgre) segment of the uplift did not begin shedding massive m. The oldest and thickest salt deposits lie in the basement-con- amounts of arkose until Desmoinesian (Middle Pennsylvanian) trolled half grabens described above, but massive salt flowage has time, but continued its positive tendencies well into Permian time. totally obscured other depositional characteristics. This northwesterly termination of the Uncompahgre Plateau also The salt anticlines are strongly elongated northwesterly, parallel- marks the emergence of the Colorado lineament into the Paradox ing the Uncompahgre frontal faults and overlying deep-seated basin. In other words, the Uncompahgre Uplift appears to have basement faults (Cater and Elston, 1963; Joesting and Byerly, 1958;

Baars, 1966). As salt thickness reached about a thousand meters sic sedimentation as the supply of salt diminished, marking the over the half-grabens, thick wedges of arkose were being depos- close of the diapiric activity. ited along the transition between the Uncompahgre highlands and Eagle Evaporite Basin the low-lying evaporite basin. The directional nature of the clastic The poorly known Eagle evaporite basin, lying between the Un- overburden was a large factor in the initiation of salt flowage toward the southwest and the large basement faults, which are compahgre and Front Range uplifts, has a similar history of sub- known to have vertical displacements of over 1800 m in Paradox sidence and sedimentation as the Paradox basin. A paucity of Valley (fig. 5). Thus, the deep-seated faults were major buttresses subsurface data and limited exposures of Middle Pennsylvanian to lateral salt movement. As the mobilized salt encountered the deposits provide only limited data, and interpretations are there- fault "scarps," flowage was directed upward and diapirism was ini- fore highly generalized. tiated. Continued deposition of clastic overburden to the north- As previously outlined, basement faulting with a history of re- east caused salt to flow continuously or episodically until the sup- peated rejuvenation is similar, if not identical, to the Paradox ply was depleted in Late time, as shown by repeated basin. Coarse clastic deposits were derived from the Front Range angular unconformities and pinchouts along the flanks of the Uplift, and perhaps others, to the east and southeast, and interfin- structures. The source of the ever-thickening clastic overburden gered with marine clastics and carbonates of the Minturn Forma- on the Uncompahgre Uplift was buried by Late and Juras- tion of the Central Colorado Trough and the Eagle Valley Evapo-

TECTONIC EVOLUTION 111 rites in the northwestern Trough (Lovering and Mallory, 1962). As structural geologists in recent years, the easterly trending fold was in the Paradox basin, algal bioherms of Middle Pennsylvanian age present in essentially its present configuration prior to Cambrian grew along the margins of the evaporite basin (Walker, 1972). The time, as shown by angular unconformities at that horizon. The Uin- extent and thickness of salt deposits in the restricted basin are only ta- is a negative fold, draping off the Uinta Moun- poorly known, and the nature of sub-salt structure is unknown due tain arch on the north and the Uncompahgre Uplift under its to the limited number of deep wells. southwest flank. Two northwesterly trends of Eagle evaporite exposures in the The multitude of problems encountered when the structural vicinity of Glenwood Springs and the Eagle-Gypsum area have the configuration of the Colorado Plateau and Southern Rockies is in- characteristics of salt diapirs similar to the Paradox structures. Drill- terpreted on the basis of Laramide stress fields are essentially ing indicates that salt is present in the structures, but their size and eliminated when it is realized that these structures originated in a growth histories have not been determined. These salt anticlines totally different stress regime in Precambrian time. The basement may be more similar to the Paradox structures than is generally tectonic fabric of the region was fixed before Phanerozoic time realized, and further deep exploration of the pre-salt section is cer- and repeatedly rejuvenated during the various ensuring tectonic tainly warranted. events, including the Ancestral Rockies and Laramide episodes. Bodily clockwise rotation of the provinces invoked by many DEMISE OF THE ANCESTRAL ROCKIES authors to explain the observed relationships is unnecessary and The Ancestral Rockies had probably reached their zenith by Early unjustified by field relationships, except along basement shear Permian time; they underwent denudation for about the first half zones. of the period, supplying great quantities of arkosic sediments to the lowlands during their demise. The Permian red beds from the AFTERMATH uplifts finally buried many of the long-existing positive features of Tectonic events following the Laramide disturbance have more the region, such as the Zuni, Defiance, Kaibab, and Nacimiento up- effect on the than anything else. Beginning lifts of the Colorado Plateau. The Monument Upwarp remained shortly after the termination of Laramide time (probably during the sufficiently positive to cause major facies changes along its flanks ) the entire Colorado Plateau Province was gently, but and crests, while marine Permian deposits buried the Late Penn- bodily, uplifted and tilted toward the north. A regional vertical sylvanian erosional surface on the Emery uplift underlying the San doming of central Arizona seems to have been the cause of the Rafael Swell. tilting. The end result was that surface drainage gained potential Fine-grained red beds and eolian dominated the Tri- energy and surface erosion was accelerated. The early denudation assic and Jurassic systems, with little evidence of structural growth of the Province provided considerable amounts of sediments that having played a significant role. By Late time, the intra- were carried northward into a large lake (Lake Uinta) that formed cratonic seaways shifted eastward, effectively burying the present between the structural buttresses of the northwestern Uncompah- day Colorado Plateau and Southern Rockies in marine muds and gre Uplift and the Uinta Mountain arch on the north. The Paradox sands. basin region was merely subjected to surface erosion. The most prominent topographic features on the Plateau are the LARAMIDE DISTURBANCE so-called laccolithic ranges that stand to elevations exceeding The is generally considered to have been the 3,900 m. There are eight in all, and they appear to have been em- wave of compressional tectonism that reshaped the western placed along basement lineaments (Kelley, 1955) or more likely at North American continent from the west toward the east during intersections of northwest and northeast basement lineaments latest Cretaceous through early Eocene time. During this event, (Witkind, 1975). Within the individual ranges, intrusive igneous the Colorado Plateau and Southern Rockies, as a whole, resisted rocks are exposed in the form of stock, dikes, sills, laccoliths and the extensive thrust faulting that occurred to the west and south. cactoliths. They appear to have been emplaced at two distinct The Colorado Plateau acted as a semi-rigid "mini-plate," rejecting times. One group, including the Ute, Carrizo, La Plata and Rico intensive deformation along its western margin, the Cordilleran ranges, have been dated at between 61 and 84 m.y., or about Lara- hingeline or Wasatch line, and along its southern border at the mide time. The others, including the Henry (48 m.y.), LaSal (24 Texas-Walker Lane lineament. Pre-existing basement-related struc- m.y.) and Abajo (28 m.y.) ranges were intruded around middle Eo- tures of the Plateau and Southern Rockies invariably pre-date the cene and early times (Witkind, 1975). It is interesting that Laramide in origin, but were somewhat modified by the westerly the LaSal Mountains east of Moab were intruded along the base- compressive forces. ment faults underlying Paradox and Castle Valley salt structures The primary effects of the Laramide on the Colorado Plateau and and actually pierced the heart of the salt . The resulting ig- Southern Rockies were the enhancement of pre-existing structures neous rocks are peculiar aegirine-augite-rich syenite prophyries and the overturning of drape folds, generally toward the east. The (soda-enriched) as a result of intrusion through halite. The range amplitude of virtually all of the folded structures was increased was named LaSal (salt) Mountains by the early Spanish explorers, considerably by westerly compressive stress (cr,), changing minor, because they found salt water springs in the mountains. low amplitude folds into major structures. Major monoclines Late in the episode of regional tilting during the intrusion of the formed over the larger north-oriented basement normal faults on younger laccoliths, general elevation of the Plateau and Southern the Plateau, and vertical throw was increased on similar faults in Rockies occurred. Surface and ground water drainage intensity in- the Rockies. The net effect was to overturn the basement faults creased and was gradually diverted toward the southwest along toward the east, and to create east-facing monoclines. There are a the system, perhaps by a series of stream piracies number of west-facing monoclines on the eastern Plateau and along the route. Ground waters began to remove near-surface salt White River uplift (Kelley, 1955), but most of these occur as drape by solution on the larger salt diapirs; consequently, solution col- folds over the primary basement wrench faults rather than third- lapse of the crests of the anticlines began. Overlying strata of order normal faults. In spite of a flurry of extraordinary explana tions of the Laramide origin of the Uinta Mountain arch by various 112 BAARS and STEVENSON all ages from Permian to Cretaceous slumped into the weakened REFERENCES crests of the salt anticlines, leaving them with gravity faults along Baars, D. L., 1966, Pre-Pennsylvanian paleotectonics—Key to basin evolu- the high flanks and jumbled slump folds depressed into the termi- tion and petroleum occurrences in Paradox basin, Utah and Colorado: nations of the anticlines as we see them today. Superimposed American Association of Petroleum Geologists Bulletin, v. 50, p. 2082- streams cut downward into the structures, sometimes at right 2111. , 1972, Pre-Pennsylvanian paleotectonic framework of Ancestral angles to the anticlines, creating "paradoxical" river courses that Rockies of Colorado (abs.): Colorado School of Mines Quarterly, v. 67, cut across the structures rather than flow along the axial valleys. n. 4, p. 137. Thus, the name was derived for , from which the , 1975, Pre-Pennsylvanian reservoir rocks of the eastern Colorado name was derived for the Paradox Formation and subsequently Plateau and Southern , in P. Bolyard, ed., Deep Drilling the Paradox basin. Frontiers of the Central Rocky Mountains: Denver, Colorado, Rocky Mountain Association of Geologists, p. 71-74. , 1976, The Colorado Plateau aulacogen—Key to continental-scale basement rifting: Proceedings of Second International Conference on Basement Tectonics, p. 157-164. Baars, D. L. and See, P. D., 1968, Pre-Pennsylvanian stratigraphy and paleo- CONCLUSIONS tectonics of the San Juan Mountains, southwestern Colorado: Geological Society of America Bulletin, v. 79, p. 333-350. The tectonic evolution of the Paradox basin and Southern Rock- Baars, D. L. and Stevenson, G. M., 1981 (in press), Subtle stratigraphic ies may be summarized as follows: traps in Paleozoic rocks of the Paradox basin: American Association of 1. A conjugate set of shear zones transected the region at about Petroleum Geologists Memoir. 1,700 m.y.b.p. The northwesterly shear has been named the Cater, F. W. and Elston, D. P., 1963, Structural development of salt anti- clines of Colorado and Utah: American Association of Petroleum Geolo- Olympic-Wichita Lineament; the northeasterly set the Colorado gists Memoir 2, Backbone of the Americas, p. 152-159. Lineament. They probably resulted from relative compression De Voto, R. H., 1972, Pennsylvanian and Permian stratigraphy and tec- from the north. tonism in central Colorado: Colorado School of Mines Quarterly, v. 67, 2. Related basement fractures were northerly oriented normal n. 4, p. 139-186. Hite, R. J., 1960, Stratigraphy of the saline facies of the Paradox Member of faults (extension) and easterly oriented folds (compression). the of southeastern Utah and southwestern Colo- 3. Small-scale vertical relief along the fracture system was re- rado: Geological Society Guidebook, 3rd Field Conference, juvenated throughout Cambrian, Devonian and Mississippian time, p. 86-89. creating localized reservoir facies on the high, shoaling fault Joesting, H. R. and Byerly, P. E., 1958, Regional geophysical investigations blocks. of the Uravan area, Colorado: U.S. Geological Survey Professional Paper 316-A, p. 1-17. 4. The Uncompahgre and Front Range Uplifts and the compen- Kelley, V. C., 1955, Regional tectonics of the Colorado Plateau and relation- satory Paradox and Eagle basins formed along the basement frac- ships to the origin and distribution of uranium: Albuquerque, University tures in Pennsylvanian time from extreme east-west extension of of New Mexico Press, 120 p. the crust. Kluth, C. F. and Coney, P. J., 1981, Plate tectonics of the Ancestral Rocky Mountains, Geology, v. 9, p. 10-15. 5. Salt thickened and diapiric structures formed in Pennsylvanian Lovering, T. S. and Mallory, W. W., 1962, The Eagle Valley Evaporite and its through Jurassic time from clastic overburden on the eastern mar- relation to the Minturn and Maroon formations, northwest Colorado: gins of the basins, causing the salt to flow against and upward U.S. Geological Survey Professional Paper 450-D, p. 45-48. along the basement faults. Moody, W. D. and Hill, M. J., 1956, Wrench fault tectonics: Geological Soci- 6. Laramide compressional forces from the west in Late Creta- ety of America Bulletin, v. 67, p. 1207-46. Nadeau, J. E., 1972, Mississippian stratigraphy of central Colorado: Colo- ceous to early Eocene time enhanced the existing structures and rado School of Mines Quarterly, v. 67, n. 4, p. 77-102. overturned them toward the east in most cases. The salt structures Tweto, Ogden, 1980a, Tectonic , in Colorado Geology, were little affected. Monoclines were formed over the northerly H. C. Kent and K. W. Porter, eds.: Denver, Colorado, Rocky Mountain As- oriented normal faults in the basement, and the easterly trending sociation of Geologists, p. 5-9. Uinta Mountain arch was enhanced. , 1980b, Precambrian geology of Colorado: in Colorado Geology, H. C. Kent and K. W. Porter, eds.: Denver, Colorado, Rocky Mountain Asso- 7. Regional tilting of the Plateau province toward the north ciation of Geologists, p. 37-46. caused surface drainage to fill Lake Uinta with sediments stripped Walker, T. R., 1972, Bioherms in the Minturn Formation (Des Moinesian from the south in early Tertiary time. Age), Vail-Minturn Area, Eagle , Colorado: Colorado School of Mines Quarterly, v. 67, n. 4, p. 249-278. 8. Intrusive igneous laccolithic magmas were implaced during Warner, L. A., 1978, The Colorado Lineament: a middle Precambrian Laramide and middle Tertiary times at intersections of basement wrench fault system: Geological Society of America Bulletin, v. 89, p. lineaments. 161-171. 9. Major surface structures of the Colorado Plateau and South- Wise, D. U., 1963, An outrageous hypothesis for the tectonic pattern of the ern Rockies were not formed in Laramide times, only enhanced North American cordillera: Geological Society of America Bulletin, v. 74, p. 357-362. from rejuvenation of basement structure. They resulted from a Witkind, I. J., 1975, The Abajo Mountains: an example of the laccolithic continuum of tectonic activity originating about 1,700 m.y.b.p. groups on the Colorado Plateau: Four Corners Geological Society, 8th and continuing to the present. Field Conference, p. 245-52.