Thrusting of the Claron Formation, the Bryce Canyon Region, Utah

Home , Aquarius Plateau, Carmel Formation, Pink Cliffs

Thrusting of the Claron Formation, the Bryce Canyon region, Utah

ERIK R. LUNDIN Conoco Norway, Inc., P.O. Box 488, 4001 Stavanger, Norway

ABSTRACT -80 km east of the east limit of known thrusting parowits and Wahweap Formations (Fig. 2). At in the Sevier orogenic belt, but it is located outcrop scale, however, the unconformable rela- A thrust system at least 40 km long and within the western edge of the region containing tionship is commonly subtle. In the northeastern with as much as 2,000 m of shortening has compression-related structures produced during part of the study area, the Claron Formation lies been mapped in the southern High Plateaus the Late Cretaceous-early Tertiary Laramide conformably on the Upper Cretaceous to Paleo- of Utah, partly within Bryce Canyon National orogeny (Fig. 1). The middle Tertiary Marys- cene(?) Canaan Peak Formation, which in turn Park. Seismic data clearly reveal that the vale volcanic field lies on the north side of the lies unconformably above the Kaiparowits thrusts are of thin-skinned, ramp-flat style, area. Formation. Well data from the northeastern part soling out in evaporite-rich layers of the Ju- The thrusts are unusual in two aspects. First, of the area disclose that the Canaan Peak For- rassic Carmel Formation. Mapping discloses the constrictional sense is approximately north- mation and another formation of local extent, that large portions of the thrusts terminate south, which is anomalous for both the Sevier the Paleocene(?) Pine Hollow Formation, lie upward in fault-propagated folds, marked by and Laramide orogenies. Second, the thrusts in- with angular unconformity above the Upper vertical to overturned beds. Spaced cleavage volve the Claron Formation, which generally Cretaceous Dakota Formation. reveals that some shortening is accommo- rests with angular unconformity on rocks that With the exception of a small outcrop of un- dated by pressure dissolution. Shallow drill- are involved in compressional deformation in named conglomeratic sandstone in Little ing for coal provides data suggestive of more the region (Gregory and Moore, 1931; Gregory Henderson Canyon (Figs. 2, 3), tentatively as- than one episode of thrusting. An older event and Williams, 1947; Gregory, 1950, 1951; signed to the middle Tertiary, there are no strata predates deposition of the Upper Cretaceous Threet, 1952,1963; Mackin, 1954,1960; Cook, that depositionally overlie the Claron Formation and Paleocene(?) Canaan Peak Formation. 1957; Blank, 1959; Bowers, 1972; Hintze, within the map boundaries. Near the center of This deformation is presumably related, in the 1983). The purpose of this study is to provide the map area, brecciated volcanic rocks and broadest sense of the term, to the Sevier documentation of these anomalous aspects by middle Tertiary volcanic rocks of the Mount orogeny. The younger thrusting is south- investigating the geometry, kinematics, extent, Dutton Formation are in contact with the vergent and involves rocks as young as the and age of the thrusts and related structures and, Claron Formation. The nature of the contact is lower to middle Eocene, white limestone if possible, to interpret their tectonic signifi- difficult to determine, but Rowley and others member of the Claron Formation, a forma- cance. To accomplish this, the area embracing (1987) interpreted it to be the eroded lower portion that is considered post-orogenic by most the thrusts was mapped at 1:24,000 scale, well tion of a listric normal fault or possibly a geologists. The upper age of the younger and seismic reflection data (provided by Chev- denudation-type normal fault. The writer has event is not constrained in time, and its tec- ron USA) were analyzed, stratigraphic markers adopted their interpretation but has not been tonic significance remains poorly understood. needed to evaluate the amount of shortening able to independently confirm it. Also in fault were identified and mapped, and the thrusts and contact against the Claron Formation are INTRODUCTION related structures were studied in detail. Quaternary basalts along the Sevier fault zone (Fig. 2). Thrust structures of compressional origin STRATIGRAPHY were recently discovered in the Bryce Canyon Claron Formation area of the southern High Plateaus of Utah General Relations (Davis and Krantz, 1986; G. H. Davis and oth- Bowers (1972) subdivided the Wasatch For- ers, unpub. data). The Bryce Canyon area is Rocks within the map area consist of sedi- mation (Claron Formation in this paper) in the located in the late Cenozoic structural and phys- mentary formations of Late Cretaceous age, sed- Table Cliffs Plateau into three members, from iographic transition zone (High Plateaus), be- imentary strata of the lower to middle Tertiary oldest to youngest: the pink limestone member, tween the Basin and Range on the west and the Claron Formation, volcanic rocks and minor in- the white limestone member, and the variegated main part of the Colorado Plateau on the east. terbedded sedimentary rocks of middle Tertiary sandstone member. These will be referred to Major north- to northeast-striking normal faults age, and sedimentary and volcanic rocks of here as the pink, white, and variegated Claron, form the boundaries of the broad and gently Quaternary age (Fig. 2). respectively. tilted High Plateaus. The thrusting in the Bryce In the map area, the Claron Formation gener- The pink Claron occurs extensively Canyon area appears to be confined between the ally lies disconformably over (from youngest to throughout the study area. It is the colorful unit Paunsaugunt fault zone on the east and the Sev- oldest) the Kaiparowits, Wahweap, and Straight responsible for most of the famous erosional fea- ier fault zone on the west—two of the major late Cliffs Formations. Regionally, the unconformity tures of Bryce Canyon National Park and the Cenozoic normal fault zones. The area is located is obvious, in places cutting out the entire Kai- distinctive Pink Cliffs of the region. Lithologi-

Geological Society of America Bulletin, v. 101, p. 1038-1050, 12 figs., August 1989.

1038 THRUSTING OF THE CLARON FORMATION, UTAH 1039

crops of the variegated Claron do not reveal whether this member was affected by the thrusting, or whether it was deposited on already deformed pink and white Claron.

Age of the Claron Formation

The age of the Claron Formation is poorly known, and thus the age of the thrust-related deformation to be described cannot be firmly established. The pink Claron was considered to be Eocene by Gregory (1951). Subsequent to Gregory's work, however, other workers have suggested that the member may be Paleocene to lower Eocene(?) (Bowers, 1972), or perhaps even as old as latest Cretaceous (Anderson and Rowley, 1975; Rowley and others, 1978b; Rowley and others, 1979). The age of the white Claron would appear to be more tightly constrained. Gregory (1950) reported upper Eocene fossils from the uppermost part of the Claron Formation at Cedar Breaks National Monument. Fresh-water mol- lusks collected by Bowers from the white Claron in the Table Cliffs Plateau yielded an age of "early to middle Eocene but not earliest Eo- cene" (Bowers, 1972, p. 26). The variegated Claron contains no diagnostic age indicators, but Bowers (1972) reported that some of the sandstone beds of the member appear to be tuffaceous. This member is overlain, just north of the study area, by ash-flow tuff of the Wah Wah Springs Formation (Rowley, 1968; Rowley and others, 1987), dated by Best and Grant (1987) at 30 m.y. (middle Oligo- cene). Presently, the lower to middle Eocene white Claron is the youngest known formation involved in the Bryce Canyon thrusting.

STRUCTURAL GEOLOGY

Overview Figure 1. Index map of the southern High Plateaus of Utah. Shaded portion indicates area of study. Insert map displays generalized regional geologic relationships. Abbreviations: The major thrust faults in the area are the BP, Bryce Point; RI, Rubys Inn; STB, eastern leading edge of Sevier thrust belt, as recognized Rubys Inn and Johns Valley thrusts (Fig. 2). by previous work. The east-striking Rubys Inn thrust extends from the Sevier to the Paunsaugunt fault zone, a distance of ~20 km. For convenience of descrip- tion, the Rubys Inn thrust is subdivided into cally, the member consists mostly of thick- cliff-forming rim cap in Bryce Canyon. It is three segments: the Highway 12 segment, Ahl- bedded to massive, clastic limestone, inter- composed mostly of massive limestone with strom Hollow segment, and Hillsdale Canyon bedded with marl, calcareous siltstone and minor interbeds of siltstone and mudstone segment (Fig. 2). On the basis of seismic pro- sandstone, and some conglomerate lenses. The (Bowers, 1972). The uppermost variegated files, the northeast-striking Johns Valley thrust base of the formation is marked in many places Claron is an inconspicuous, slope-forming extends at least 10 km beyond the northeastern by a distinctive 5- to 10-m-thick pebble to cob- member, exposed only in Johns Valley of the map boundary, giving it a minimum total length ble conglomerate (Gregory, 1951; Bowers, map area (Fig. 2). This member is made up of of 17 km. 1972). Recent investigations indicate that the mostly varicolored, fine-grained, friable sand- Other faults in the area of possible compres- pink Claron is largely a sequence of stacked stone, siltstone, and mudstone (Bowers, 1972). sional origin are the east-striking Pine Hills fault paleo-soil horizons (Mullett and others, 1988). Of the three members, only the lower two can (known only from one seismic profile) and the The overlying white Claron is the characteristic unequivocally be shown to be thrusted. The out- inferred northeast-striking Elbow fault (Fig. 2). Figure 2. Generalized geologic map of the Bryce Canyon region. Rubys Inn thrust is subdivided into the Hillsdale Canyon (HC), the Ahlstrom Hollow (AH), and the Highway 12 (H-12) segments. Abbreviations: CP, Champlin Petroleum 1132 Clay Creek Federal exploration well; HP, Henderson Point; LI, Lion Oil Company Bryce Unit No. 1 exploration well; SC, SLAPCO Celeron Gay Creek Federal exploration well; SP, Shakespear Point; WJ, Widstoe Junction Shallow Coal Drilling wells Nos. 1-4; WP. Wilson Peak.

37°45

KILOMETERS 1 0 1 10 15 20

MILES 1 0 1 10 THRUSTING OF THE CLARON FORMATION, UTAH 1041

EXPLANATION SYMBOLS

Surficiai deposits Contact, dashed where approximate

60 70 Normal fault, showing dip of surface and trend and Basalt plunge of striae; dashed where approximately located; dotted where concealed; ball on downthrown side

1 I-Pi" Mount Dutton Formation Thrust fault, showing dip of surface and trend of striae; dashed where approximately located: Unnamed conglomeratic sandstone dotted where concealed T,C S Low-angle gravity fault, dashed where 0 Variegated sandstone ¿.-A .. approximately located; dotted where concealed 'Z member re 1 White limestone Inferred tear fault u. member Monocline ~ >- Pink limestone T _ re Cp ^ member Asymmetric anticline showing plunge of axis

Caanan Peak Formation Asymmetric syncline

Kaiparowits Formation Overturned syncline

Strike of vertical bedding K Wahweap Formation w 30 Strike and dip of bedding Straight Cliffs Formation 45 LU Strike and dip of joints Tropic Shale Strike of vertical cleavage ^60 Figure 2. (Continued). Strike and dip of cleavage

Dry hole, drilled for oil and gas

Shallow hole, drilled for coal

In addition to normal faults and thrusts, pre- and syn-thrusting folds are present in the area. The northern part of the north-northwest- striking Johns Valley anticline is exposed along turbed beds are an important guide to mapping into two imbrications. The lower, more promi- the southeast side of Johns Valley (Fig. 2). Kel- the structures of the area. Generally, beds of nent fault, strand A (Figs. 3 and 4), was first ley (1955) classified the Johns Valley anticline unusual attitudes can be grouped either into mapped by Richard Hereford (U.S. Geol. Sur- as a Laramide structure, and seismic profiles re- areas of overturned beds that mark the northern vey, unpub. 1:100,000 map) as a low-angle veal that this is a basement-involved, Laramide- limb of fault-propagated synclines, or into sepa- normal fault and was subsequently described as style fold. A monoclinal flexure, referred to here rate homoclinal domains. Examples of homo- a thrust by Davis and Krantz (1986). The upper as the Paunsaugunt monocline, parallels the clines are the north-dipping beds of the Pine fault, strand B (Fig. 3), was discovered during southeast side of Johns Valley (Fig. 2). Asso- Hills, the northwest-dipping beds of the un- the present study. Both faults are oriented ciated with the Rubys Inn thrust and the Elbow named hills on the northwest side of Johns Val- N75°E, 30°NW on the average. The thrust fault are what are interpreted as fault-propa- ley, and the northwest-dipping beds along the traces become increasingly more difficult to gated folds (Suppe, 1984, 1985) (Fig. 2). The flank of the Paunsaugunt monocline (Fig. 2). follow in Little Henderson Canyon (Fig. 3) as fault-propagated folds are nonplunging asym- the western splay of the Paunsaugunt fault zone metric synclines, with the steep limb to the Highway 12 Segment of the is approached, and it is not possible to determine north. Rubys Inn Thrust whether a crosscutting relationship exists. A small outcrop of an unnamed conglomeratic Bedding The best exposure of thrust faults in the study sandstone, tentatively assigned to the middle area is along Utah Highway 12, in the northern Tertiary, lies with slight angular unconformity Because Claron strata are essentially flat-lying part of Bryce Canyon National Park (Figs. 2, 3, on pink Claron near the last-seen trace of strand in much of the region, tilted or otherwise dis- and 4), where the main fault bifurcates eastward A (Fig. 3). This sandstone does not appear to be 1042 E. R. LUNDIN deformed and may postdate thrusting of the unit and is penetratively deformed by spaced place to place in the hanging wall are minor Claron Formation. The evidence, however, is cleavage that is interpreted as a pressure solution thrust faults and conjugate fracture sets (Fig. 8). not conclusive, and the age of the sandstone is feature (Figs. 4 and 7). Cleavage planes cut bed- The varicolored unit of the hanging wall can be unknown. ding at a high angle and strike perpendicular to recognized in the local stratigraphy, and it per- Prominent striae and grooves (Fig. 5) typi- the strike of fault striae. Also to be found from mits calculation of the horizontal shortening, cally mark thrust surfaces. These kinematic indicators range in direction between N10°W and N20°W along strand A, thus indicating dip-slip SYMBOLS motion. Striae and grooves of strand B show a larger spread in direction, ranging between N0°E and N40°W. The motion along this strand Contact, dotted where concealed is also mostly dip-slip except near the western end, where left-lateral strike-slip motion prevails Normal fault showing dip: ball on downthrown side (Fig. 3). Thrust fault showing dip and trend and plunge of striae A zone of reddish-brown gouge, ~10 cm thick, is well developed along both strands, and Tear fault cleavage within the gouge discloses thrust-slip movement (Davis and Krantz, 1986) (Fig. 6). Overturned nonplunging syncline "ft" The hanging wall of both faults consists of the same layer of pink Claron, a varicolored, mas- -T- j>C Strike and dip of bedding; overturned; vertical 30 60 sive, limestone bed that is 6 to 10 m thick. The r-1 Strike and dip of cleavage: vertical top of the layer is characterized by a gray marly 75

Figure 3. Detailed geologic map of the Highway 12 segment of the Rubys Inn thrust. The klippen are displayed in Figure 9. THRUSTING OF THE CLARON FORMATION, UTAH 1043

-350 m perpendicular to the thrust trend. Seis- mic profiles are not available for the Highway 12 segment. The two strands merge near where the fault crosses the highway, west of which the trace is concealed by alluvium (Fig. 3). Its presence there is disclosed in the footwall, where the pink Claron has been folded to vertical and overturned. An unusually scenic expression of the folding and thrusting is preserved in the "hoodoo" shown in Figure 9. The base of the "hoodoo" contains steeply folded beds of pink Claron, and the lighter colored block on top is a thrusted cap of Straight Cliffs Formation. The folding is very sharp; as little as 10 m to the south, the bedding is gently dipping. Compressional deformation in the Claron Formation is known to extend at least as far south as Bryce Point in the Park (Fig. 1). Davis and Krantz (1986) described penetrative small- scale thrust-slip and strike-slip surfaces along the trail network of the Park, and they pointed out Figure 4. View to the north of strand A of the Highway 12 segment of the Rubys Inn thrust. the existence of a minor thrust fault in the Bryce The fault surface is indicated by arrows. The hanging wall consists of a distinctive 6- to Point overlook cliff. A "medium"-scale thrust, 10-m-thick, varicolored, massive limestone bed of pink Garon. The top of the layer is charac- named the "Bryce Point thrust," can be seen terized by a gray marly unit, which is penetratively deformed by spaced cleavage. from the overlook to the east (Lindquist, 1977, 1980). Approximately 80 m of thrust slip is calculated for this thrust. Striae on the thrust surface strike due south, consistent with compres- Navajo Sandstone, as well as underlying rocks, depth of 1,845 m below the valley floor (503 m sional movement indicators measured along are not affected by thrusting (Fig. 10). The Lion above sea level). The seismic profile indicates Highway 12. Oil Company Bryce Unit No. 1 well (Fig. 2) the existence of a smaller, south-dipping reverse penetrates the top of the Navajo Sandstone at a fault or back-thrust named the "Pine Hills fault" Ahlstrom Hollow Segment of the Rubys Inn Thrust and the Pine Hills Fault

The Ahlstrom Hollow segment, at Ahlstrom Hollow, places strata of Straight Cliffs Forma- tion over pink Claron Formation along a N88°W, 28°N thrust surface. Only 5 to 10 m of the fault surface is exposed, and striae are not displayed. Beds of the footwall are folded to near-vertical attitudes, analogous to those at the base of the "hoodoo" (Fig. 9) near Highway 12. These steep beds form a resistant ridge that extends along the entire Rubys Inn thrust. In the Ahlstrom Hollow area, the ridge is offset by inferred north-oriented minor tear faults. The Pine Hills to the north (Fig. 2) are prominent topographic features that reflect north- dipping beds of the pink Claron. Small-scale, east-striking, north-dipping thrusts are common in the Pine Hills. Striae strike due north on these minor thrust surfaces, indicating dip-slip motion. The only seismic profile available along the Rubys Inn thrust crosses the Ahlstrom Hollow segment and clearly reveals a north-dipping thrust of ramp-flat style. The thrust ramps up through the section at an angle of ~30 degrees from a décollement in the evaporite-rich lower part of the Carmel Formation. The massive 1044 E. R. LUNDIN

It is assumed to die out westward, before reaching the ridge leading up to Wilson Peak, because the contact between the Claron and Wahweap Formations is normal there (Fig. 2). The Pine Hills fault appears to root in the Rubys Inn thrust, and therefore it is assumed to have formed during the late stages of thrusting.

Hillsdale Canyon Segment of the Rubys Inn Thrust

Hillsdale Canyon provides excellent exposures of the relation between thrusting and folding (Figs. 2 and 11). In the canyon, the Straight Cliffs Formation is placed over the pink Claron and Wahweap Formation along the N80°W, 32°N-trending thrust. In general, the thrust trace can be mapped with confidence, but only at the location of cross section A is the fault surface clearly expressed (Fig. 11). Striae on the fault surface plunge N32°E, indicating that thrusting was accompanied by a component of left-lateral Figure 6. Close-up view of zone of faulting along strand A of the Highway 12 segment of the slip. Rubys Inn thrust. View is looking toward the west. Cleavage disclose thrust-slip motion; that A 10- to 15-cm-thick zone of reddish-brown is, top to the south. gouge is present along the fault. Cleavage within the gouge is analogous to cleavage seen along the Highway 12 segment and thus indicates thrust-slip motion. Conjugate fracture sets are (Figs. 2 and 10). The fault relationship is also imately 1,000 m of horizontal shortening is es- well developed in the Straight Cliffs Formation evident on wireline logs. timated across the thrust wedge from seismic of the hanging wall (Fig. 12). The fractures are Taken together, the Ahlstrom Hollow and data. The lateral extent of the Pine Hills fault is oriented N54°W, 33°SW and N85°W, 30°NE Pine Hills faults define a thrust wedge. Approx- not known because it has no surface expression. on the average and were used to calculate the maximum compressional stress orientation according to Mohr-Coulomb theory (Anderson, 1951). The derived CT,orientatio n is 2° S20°W. Approximately 125 m of thrust slip is observed at the location of Figure 11, which lies along cross section A-A' (Fig. 10). Together with the folding, the horizontal shortening of the Claron Formation is estimated to be 300 m. The thrust and fold can be mapped to within 300 m of the Sevier fault (Fig. 2), but exposures Figure 7. Spaced cleavage do not permit tracing the thrust farther. From indicative of pressure solu- surface mapping, it cannot be determined tion in the gray marly unit of whether the thrust and fold die before reaching the pink Claron. The unit is the Sevier fault or whether they are cut by the typically found 6 to 10 m normal fault, and seismic data are not available above the fault surface along for Hillsdale Canyon. the Highway 12 segment (see Fig. 4). View is toward the Johns Valley Thrust east. Bedding dips -15° to the north. Cleavage planes The N40°E-trending Johns Valley thrust is are perpendicular to bedding nowhere exposed at the surface, but several as well as to the plunge of seismic profiles, within and beyond the map striae on fault surface. area, clearly indicate a major thrust fault in the subsurface. Although this fault has not pre- viously been reported in the literature, it has been known in the petroleum industry for more than 20 years (Frank Royse, Jr., 1986, oral commun.). The style of the thrust is the same as THRUSTING OF THE CLARON FORMATION, UTAH 1045 that of the Rubys Inn thrust. Seismic profiles clearly indicate the point of "lift-off' from the décollement in the evaporite-rich Carmel For- mation, and they display the thrust ramping up through the section at an angle of -22° (Fig. 10). Hanging-wall beds form good reflectors projecting toward the surface parallel to the ramp. The reflectors are lost near the surface, however, because of a low signal-to-noise ratio. It is therefore not possible to define any cutoff points in the hanging wall or to determine whether the hanging-wall beds are folded near the surface. The top of the Navajo Sandstone is inter- cepted at 1,833-m depth below the valley floor (440 m above sea level) in the SLAPCO Celeron Clay Creek Federal and the Champlin Petroleum 1132 Clay Creek Federal exploration wells. Dip panels analogous to those making up the Pine Hills occur on the northwestern side of the valley, and they mirror the strike and dip of the fault surface. Shallow drilling for coal in Johns Valley, 2.5 km northeast of the study area (Fig. 2), demonstrates that the Cretaceous Dakota Formation and Tropic Shale are truncated beneath a significant angular unconformity. Seis- Figure 8. View to the west of a minor thrust fault in the hanging wall of strand A of the mic profiles show that the shallow drilling was Highway 12 segment in Little Henderson Canyon. Notice conjugate fracture sets in lower left in the hanging-wall side of the thrust. This is also part of photo, indicative of near-horizontal compression. Photograph courtesy of G. H. Davis. supported by the close similarity in attitude between the Dakota Formation in the subsurface and the beds in the dip panels in Johns Valley (Fig. 2); both locations belong to the same Figure 9. View to the northwest-dipping homocline. west of "hoodoo" located Doelling and Davis (1978) reported Claron along strand A of the Formation capping the unconformity in the Highway 12 segment, just Widstoe Junction coal drilling wells Nos. 1,2,3, southwest of the High- and 4 (Fig. 2). My interpretation is instead that way 12-thrust crossing. the capping beds are the Upper Cretaceous and The lower four-fifths of Paleocene(?) Canaan Peak Formation and the the spire consists of pink Paleocene(?) Pine Hollow Formation, because Claron. Notice the steep lithologie descriptions from the wells correspond beds near the base and better to these formations (Bowers, 1972; and the similarly steeply fold- 1988, personal commun.). The Canaan Peak ed beds in the background and Pine Hollow Formations are well exposed to the north of the hoo- along the southeast side of Johns Valley. Both of doo (right). The middle these formations are of relatively local extent part of the hoodoo con- and for the most part conformable with the sists of broken pink Clar- Claron Formation (Bowers, 1972). on, and the top part con- Drill intercepts and wireline logging permit- sists of a thrusted cap of ted determination of the attitude of a distinct Straight Cliffs Formation. coal bed within the Dakota Formation to A similar thrust relation- N23.5°E, 20°NW by three-point solution (Doel- ship is observed in the ling and Davis, 1978). The unconformity sur- hoodoo in the background face is essentially horizontal. The shallow to the south. Pink Claron drilling data are projected into the plane of the strata in the distance (left) cross section C-C' along structural strike and dip gently to the south. are used to complete the seismic data near the surface. It is concluded that beds as old as the Dakota Formation, in the hanging wall, parallel HILLSDALE CANYON

S 2500 m

h 2000

y 1500

1000

500

AHLSTROM HOLLOW

LION, L 2500 m

2000

h 1500

1000

500

JOHNS VALLEY

1000

1000 2000 meters

NO VERTICAL EXAGGERATION THRUSTING OF THE CLARON FORMATION, UTAH 1047

Figure 10. Geologic sections. A-A': Hills- cause the main fault surface is concealed and no where seismic data are available can the accu- dale Canyon segment of the Rubys Inn thrust, minor fault surfaces were found, it is difficult to mulative shortening be observed. Construction based purely on surface data. B-B': Ahlstrom assess the significance of these sparse kinematic of cross sections is based on the latter Hollow segment of the Rubys Inn thrust, indicators. interpretation. constructed from surface, seismic, and petro- At any one segment of the Rubys Inn thrust, leum exploration-well data. C-C': Johns Val- Elbow Fault the kinematic indicators are internally consist- ley thrust, based on surface, seismic, petro- ent; striae and grooves are oriented perpendicu- leum exploration well data, and shallow The Elbow fault is an inferred structure. lar to the spaced cleavage planes and parallel to coal-drilling data. The dashed horizontal fault Based on the close similarity of topographic ex- the maximum stress orientations calculated from in section C-C is an inferred structure re- pressions to the Pine Hills, it is assumed that an conjugate fracture sets. When separate segments sponsible for the Paunsaugunt monocline; lo- analogous structure exists here. Northwest of the are compared, some variations are noticed. The cation, geometry, and displacement of this inferred fault, dip panels of pink Claron, Wah- movement directions along the Rubys Inn thrust normal fault are displayed only schematically. weap Formation, and Straight Cliffs Formation are south-southwesterly in Hillsdale Canyon, The question-marked arrow along the thrust strike N65°E (Fig. 2). Within the pink Claron southerly in the Ahlstrom Hollow segment, and in section C-C' schematically displays normal southeast of the inferred fault, an asymmetric south-southeasterly in the Highway 12 area. Al- fault reactivation of the thrust fault. Forma- syncline parallels the dip panels. This fold re- though the individual movement directions de- tion abbreviations: JN, Jurassic Navajo Sand- sembles the fault-propagated fold along the fine a slight fan shape, the over-all pattern is one stone; Jc, Jurassic Carmel Formation; JE, south side of the Rubys Inn thrust. of south-directed transport. Jurassic Entrada Sandstone; KD, Cretaceous The dip panel and the fold terminate abruptly Seismic and shallow-drilling data taken to- Dakota Formation; Kx, Cretaceous Tropic against relatively flat-lying beds of the pink gether indicate a minimum of 2,000 m of short- Shale; Ks, Cretaceous Straight Cliffs Forma- Claron to the east (Fig. 2). This sharp, north- ening across the Johns Valley thrust in the tion; K\y, Cretaceous Wahweap Formation; trending boundary is interpreted to be a tear subsurface. Because of erosion and valley fill, TK, Cretaceous-Tertiary Canaan Peak and fault. Near the middle of the dip panel, the pink surface shortening cannot be estimated. If the Pine Hollow Formations undifferentiated; Claron-Wahweap Formation contact is shifted sparse kinematic indicators observed in the Tcp, Tertiary pink Claron Formation; Tcw, -120 m in a left-lateral sense along another, Claron Formation are representative, they indi- Tertiary white Claron Formation; Tcv, Ter- north-striking inferred tear fault (Fig. 2). Out- cate a south-southeasterly transport direction tiary variegated Claron Formation; Q, undif- crops of the dip panel disappear beneath the and consequently thrusting with a component of ferentiated Quaternary deposits. Well abbre- alluvium to the west (Fig. 2), and it cannot be left-lateral slip. viations: Lion, Lion Oil Company Bryce Unit determined if or how the fault connects with the The inferred tear faults that offset the dip pan- No. 1 well (TD 3,400 m); CP, Champlin Pe- Rubys Inn thrust. els by the Elbow fault indicate a due southerly troleum 1132 Clay Creek Federal well (TD transport direction. If this is applied to the Johns 2,678 m); SC, SLAPCO Celeron Clay Creek Kinematic Interpretation of the Thrusts Valley thrust, the thrusting would have had a Federal well (TD 3,930 m); WJ, Widstoe major component of left-lateral slip. Junction shallow coal-drilling wells Nos. 1-4 (TD 210 m). Wells are projected along struc- Based on seismic and well data, the style of Regardless of transport direction, the Johns tural strike into the planes of the geologic thrusting in the area is thin-skinned, involving Valley thrust evidently experienced more short- sections. formations only above the Jurassic Navajo ening than did the Rubys Inn thrust. One Sandstone. The zone of mechanical weakness explanation for this difference is that more than that the thrusts have utilized is confined to evap- one episode of compression has affected the re- orites in the lower part of the Jurassic Carmel gion; two or more compressional events with Formation. Fault and fold exposures along the contrasting shortening directions could produce the thrust plane all the way up to the angular entire Rubys Inn thrust are judged to indicate an apparent shortening disparity because the unconformity at 110-m depth below the valley that this thrust terminates upward in a fault- trend of the Rubys Inn and Johns Valley thrusts floor. It cannot be determined whether this also propagated fold. Similar observations along the differ by 45 degrees. The strongest evidence for is the case for the older Entrada Sandstone and inferred Elbow fault suggest that this may be a two constrictional episodes is derived from the Carmel Formation. On the basis of the seismic common surface expression of the thrusts in the shallow-drilling data, which suggest that an and shallow drilling data, it is possible to esti- area. Spaced cleavage within the pink Claron older thrusting event preceded deposition of the mate the minimum amount of shortening across indicates that some shortening was accommo- Canaan Peak, Pine Hollow, and Claron Forma- the Johns Valley thrust to -2,000 m. Because of dated by pressure solution. tions. Geologic mapping clearly shows that the poor outcrop exposures and valley fill, it is not It appears as if the Rubys Inn thrust has expe- Claron Formation has been thrusted. possible to estimate shortening from surface rienced most of its shortening across the Ahl- mapping alone. strom Hollow segment, -1,000 m on the basis Paunsaugunt Fault Zone Minor structures are not nearly as abundant of seismic data. The measured thrust shortening in the dip panels above the Johns Valley thrust of the Claron Formation at the surface is only The Paunsaugunt fault and monocline (Fig. as they are in the Pine Hills. Within the Claron 300 and 350 m in Hillsdale Canyon and along 2) are believed to be different surface expres- Formation, spaced cleavage is occasionally pres- Highway 12, respectively. Possibly the thrust sions of the same structure, and therefore they ent, oriented N68°E, 76°SE on the average. One dies out westward toward the Sevier fault, and are described together. The high-angle Paunsau- conjugate fracture set was discovered, oriented the eastward decrease in shortening may be re- gunt fault maintains its regional trend of N75°E, 39°NW and N30°E, 10°SE. The de- lated to a transfer to the Johns Valley thrust. ~N10°E from the northern Kanab Plateau rived a] orientation is 15° N23°W according to More likely, however, the area has experienced through the Vermillion and White Cliffs and Mohr-Coulomb theory (Anderson, 1951). Be- more than one episode of thrusting, and only northward along the eastern side of the Paun- 1048 E. R. LUNDIN saugunt Plateau to Shakespear Point in the map area (Fig. 1). West-side-down throw of the fault ranges between 180 and 450 m, and the dip ranges from 65°W to vertical (Gregory, 1951). Within the map area, the fault dips are typically 65°W. Striated surfaces are sparse, but south of Shakespear Point, some striations rake ~70°SW, indicating that normal faulting was accompanied by a component of left-lateral slip (Davis and Krantz, 1986 and unpub. data). At Shakespear Point, the fault splays (Fig. 2). The western splay dies out in Little Henderson Canyon, and the eastern strand veers off toward Henderson Point, to be substituted along strike by the Paunsaugunt monocline. The monocline extends ~ 17 km northeastward along the southeast side of Johns Valley, parallel to the thrust system. The flexure separates the higher Table Cliffs Plateau to the east from the lower southern Sevier Plateau to the west and accounts for a structural relief of -500 m. Just beyond the northeastern edge of the map area, the Paunsau- gunt monocline appears to change its expression, Figure 11. View to the east of the Hillsdale Canyon segment of the Rubys Inn thrust, and a fault again cuts the surface. A north- displaying the thrust and fault-propagated fold. The Claron contact is offset by -125 m of northeast-striking, west-side-down fault zone thrust slip. Straight Cliffs Formation is placed over Claron and Wahweap Formations. Loca- continues along the east side of Johns Valley tion of photo coincides with the thrust trace across cross section line A-A' in Hillsdale Canyon. and into Grass Valley farther north, separating the Aquarius Plateau on the east from the Sevier Plateau on the west (Williams and Hackman, 1971; Bowers, 1972; Rowley and others, 1981). in the early Miocene (Rowley and others, to have occurred between extrusions of rhyolites These structures are considered to belong to the 1978a). Low scarps in pediment deposits in at 7.6 Ma and rhyolites at 5.4 Ma (late Miocene) Paunsaugunt fault zone. Bryce Canyon indicate Recent activity along the (Rowley and others, 1981), but it is possible that The monoclinal folding in the map area is Paunsaugunt fault zone (W. E. Bowers, U.S. this structure also began to form in early Mio- clear at the surface, but the structure is not re- Geol. Survey, 1:24,000 map, unpub. mapping). cene time (Rowley and others, 1978a). Evi- vealed by seismic profiles, and there is no indica- dence of Quaternary displacement of the Sevier tion of a basement-cutting, high-angle fault Sevier Fault Zone fault can be seen at the mouth of Red Canyon, beneath the flexure. Despite detailed mapping near the western map boundary, where 0.5- efforts, these contrasting observations are not The Sevier fault zone is parallel in strike to m.y.-old basalts (Best and others, 1980) are well understood. the Paunsaugunt fault zone. The throw on the faulted against the Claron Formation (Fig. 2). The Paunsaugunt fault zone cannot be seen to Sevier fault zone is also down to the west, but its cut the thrust system, and thrusting is not known displacement is substantially larger than that of DISCUSSION east of the Paunsaugunt fault/monocline. The the Paunsaugunt fault zone. Seismic profiles in- change in strike and expression of the normal dicate that offset of basement rocks by the Sevier The Bryce Canyon thrusts are clearly of thin- fault near Shakespear Point suggest that this fault zone near Red Canyon (Fig. 2) is -900 m skinned, ramp-flat style and definitely involve fault took advantage of an already existing me- (assuming an average seismic velocity of 4,500 the white Claron, which is of presumed early to chanical weakness. This relationship is schemat- m/s). Striated surfaces were not observed along middle Eocene age. Shallow coal drilling in ically displayed in cross section C-C' by the the Sevier fault zone in the area, but Rowley Johns Valley provides strong evidence for an dashed detachment fault (Fig. 3). An alternative (1968) reported striae indicative of pure dip-slip older compressional event that preceded deposi- to this interpretation, although not proposed northwest of the study area. tion of the Upper Cretaceous and Paleocene(?) here, is that the curvature of the structures to- The Sevier fault zone may cut the Rubys Inn Canaan Peak Formation. If this suggestion of an ward each other in map view indicates that they thrust, but the location of the western extension older event is correct, then it is possible, and are structurally interrelated. of the thrust, if present, is unknown. Middle Ter- even likely, that the most recent compressional In Grass Valley and on the Awapa Plateau tiary to Quaternary volcanic rocks cover the event utilized the same mechanically weak strat- 6.4- to 5.0-m.y.-old basalts predate the uplift of Claron Formation on the west side of the Sevier igraphic layer as the first event did. Even the the Awapa and Aquarius Plateaus along what is Valley where the thrust conceivably could be younger Paunsaugunt fault appears to have here considered the Paunsaugunt fault zone located. partly taken advantage of this weakness. A sim- (Rowley and others, 1981). Thus, most normal The Sevier fault zone appears to have been ilar but more complex scenario is reported in faulting along the Paunsaugunt fault zone oc- active slightly earlier than the Paunsaugunt fault. central Utah, 150 km due north of Bryce Can- curred in late Cenozoic time (after 6.4 to 5.0 The main phase of faulting along the zone in the yon. There, Standlee (1982) interpreted that Ma), but the structure may have begun to form Kingston Canyon area to the north is estimated evaporite-rich layers of the Jurassic Arapien THRUSTING OF THE CLARON FORMATION, UTAH 1049

The Marysvale volcanic field, whose center is located —60 km north of the study area, and whose southern edge fringes the area, was em- placed between 31 and 20 Ma (middle Oligo- cene to early Miocene) (Rowley and others, 1978a). Whether emplacement of batholiths under this igneous center could have exerted forces large enough to create or reactivate thrusts near Bryce Canyon is not known. Northwest of the study area, in the northern Markagunt Plateau, Sable and Anderson (1986) described chaotic structures, or megabreccias, over a l,500-km2-large area. These poorly understood structures were presumably induced by gravity-sliding during the Miocene. Where present, striae on the low-angle fault surfaces plunge uniformly north-northeast (Sable and Anderson, 1986). Outside of the kinematic movement direction, however, these structures have very little in common with the thrusts of the Bryce Canyon region. In the central Sevier Valley, Anderson and Barnhard (in press) have recorded late Tertiary low-angle structures of north-south compres- Figure 12. Conjugate fracture sets in the Straight Cliffs Formation in Hillsdale Canyon sional origin which appear related to large-scale, indicative of near-horizontal compression. left-lateral movements along normal faults of the transition zone. The tectonic significance of these structures is not clear at the moment. Davis and Krantz (1986) proposed a similar Formation (correlated with the Carmel Forma- the observed general southerly transport direc- relationship between the Bryce Canyon thrust- tion of southwestern Utah) have been utilized by tion for the Bryce Canyon thrusts. This direction ing and the normal faulting along the Sevier and a series of compressional and extensional events contrasts significantly with the reported easterly Paunsaugunt fault zones. The amount of strike- between Mesozoic and Recent time. to east-southeasterly transport direction for Sev- slip movement along the Paunsaugunt fault The style of thrusting, the upper time con- ier thrusts of southern Utah (for example, Chris- zone, judged from striae, is insufficient to pro- straint of the proposed older event, and the strat- tiansen, 1952; Threet, 1963; Miller, 1966; duce more than a small part of the thrusting, igraphie position of the décollement in the Bryce Armstrong, 1968). however. Data from the Sevier fault zone region all resemble the same characteristics in The age constraints of thrusting of the Claron (northwest of the study area) indicate pure dip structures of established Sevier origin in Iron Formation in the Bryce Canyon area are too slip (Rowley, 1968). Thus, at present there Springs Gap near Cedar City (Mackin, 1954. poor to include or exclude the Late Cretaceous seems to be no reasonable post-Laramide cause, 1960; Armstrong, 1968; Van Kooten, 1988). to Eocene Laramide orogeny (for example, other than shouldering by Tertiary plutonism, to On the basis of drill-hole data, Standlee (1982) Tweto, 1975; Dickinson and others, 1988) as explain the thrusting of the Claron Formation. interpreted that Sevier thrusting extends beneath the cause of the event. Because the thin-skinned the Sevier Valley of central Utah and probably style of the Claron Formation thrusting is inter- CONCLUSIONS beneath the northernmost Sevier Plateau. K. R. preted to be inherited from an older event, one Neuhauser (unpub. data) mapped thrusts of cannot use this style as a discriminator against The style of thrust deformation in the Bryce proposed Sevier age in the San Rafael Swell. the Laramide orogeny. On the other hand, the Canyon area, as revealed by seismic profiles and The front of the Sevier thrust belt may therefore approximately north-south stress field of the well data, is classical thin-skinned. Shallow drill- extend as far east as Bryce Canyon. Claron Formation thrusting would be unusual ing for coal in Johns Valley indicates that at Sevier deformation in the Utah region is in- for Laramide deformation in this region; the least two compressional episodes affected the terpreted to have occurred between middle and generally proposed compressional direction var- Bryce Canyon region; one event preceded depo- latest Cretaceous (Armstrong, 1968; Heller and ies between east-northeast and northeast (for ex- sition of the Upper Cretaceous and Paleocene(?) others, 1986). Timing of the here proposed older ample, Kelley, 1955; Chapin and Cather, 1981). Canaan Peak Formation, and the other event compressional event in the Bryce Canyon area Compressional deformation of the Claron followed deposition of the lower to middle Eo- would correspond well with this age of the Sev- Formation has not been reported elsewhere. It is cene white member of the Claron Formation. ier orogeny, but the here proposed younger therefore possible that the described thrust faults Both compressional events seem guided by thrusting event involving the Claron Formation are local and completely unrelated to large-scale evaporitic layers of the Jurassic Carmel Forma- can almost certainly be excluded from classic regional tectonics. The main difficulty with tion, and there are indications that the Paunsau- Sevier thrusting despite the poor age control of proposing post-Laramide timing lies in finding gunt normal fault utilized this weakness in part. the formation. Another argument against a Sev- local forces large enough to produce the de- Comparison of style, timing, and stratigraphie ier age for thrusting of the Claron Formation is scribed thrusts. position of the décollement of the older thrusts 1050 E. R. LUNDIN

Lindquist, R. C., 1977, The geology of Bryce Canyon National Park, Salt Lake in Bryce Canyon with those of established and encouragement. Finally, I express my grati- City: Bryce Canyon Natural History Association, Bryce Canyon, Utah, Sevier thrusts to the west suggests that the Sevier tude to George Davis for excellent advice in the 53 p. 1980, Slope processes and forms at Bryce Canyon National Park [Ph.D. thrust belt extends as far east as the Bryce Can- field, academically, and personally. thesis]: Salt Lake City, Utah, University of Utah, 134 p. Mackin, J. H., 1954, Geology and iron deposits of the Granite Mountain area, yon region. Iron County, Utah: U.S. Geological Survey Mineral Investigation Series Field evidence indicates general north-south REFERENCES CITED Map, MF-14. 1960, Structural significance of Tertiary volcanic rocks in southwestern compression of the Claron Formation. Because Anderson, E. M., 1951, The dynamics of faulting: London, England, Oliver and Utah: American Journal of Science, v. 258, p. 81-131. Boyd, 206 p. Miller, G. M., 1966, Structure and stratigraphy of southern part of Wah Wah of the poor age control of the Claron Formation Anderson, E. R., and Bamhard, T. P., 1989, Neotectonic framework of the Mountains, southwest Utah: American Association of Petroleum Geol- and a lack of upper time constraint for the central Sevier Valley area, Utah, and its relations to seismicity: U.S. ogists Bulletin, v. 50, p. 858-900. Geological Survey Bulletin (in press). Mullett, D. J., Wells, N. A., and Anderson, J. J., 1988, Early Cenozoic deposi- younger thrusting episode involving this forma- Anderson, J. J., and Rowley, P. D., 1975, Cenozoic stratigraphy of southwest- tion in Cedar-Bryce depocenter: Certainties, uncertainties, and compari- ern High Plateaus of Utah, in Anderson, J. J., Rowley, P. D., Fleck, sons with other Flagstaff-Green River basins: Geological Society of tion, one cannot rule out the Laramide orogeny R. J., and Nairn, A.E.M., Cenozoic geology of southwestern High Pla- America Abstracts with Programs, v. 20, no. 3, p. 217. as the cause of this deformation. The direction of teaus of Utah: Geological Society of America Special Paper 160, Rowley, P. D., 1968, Geology of the southern Sevier Plateau, Utah [Ph.D. p. 1-52. thesis}: Austin, Texas, University of Texas, 385 p. constriction would be unusual for a Laramide Armstrong, R. L., 1968, Sevier orogenic belt in Nevada and Utah: Geological Rowley, P. D., Anderson, J. J., Williams, P. L., and Fleck, R. J., 1978a, Age of Society of America Bulletin, v. 79, p. 429-458. structural differentiation between the Colorado Plateaus and Basin and event, however. At the same time, there is no Best, M. G., and Grant, S. K., 1987, Stratigraphy of the volcanic Oligocene Range provinces of southwestern Utah: Geology, v. 6, p. 51-55. known post-Laramide cause to suggest. Pres- Needles Range Group in southwestern Utah and eastern Nevada: U.S. 1978b, Reply to "Age of structural differentiation between the Colo- Geological Survey Professional Paper 1433-A, p. 1-28. rado Plateaus and the Basin and Range provinces of southwestern ently it is only possible to report the existence of Best, M. G., McKee, E. H., and Damon, P. E., 1980, Space-time-composition Utah": Geology, v. 6, p. 572-574. patterns of late Cenozoic mafic volcanism, southwestern Utah and ad- Rowley, P. D., Steven, T. A., Anderson, J. J., and Cunningham, C. G., 1979, the enigmatic thrusts involving the Claron For- joining areas: American Journal of Science, v. 280, p. 1035-1050. Cenozoic stratigraphic and structural framework of southwestern Utah: mation and to leave the question of their origin Blank, H. R„ Jr., 1959, Geology of the Bull Valley district, Washington Coun- U.S. Geological Survey Professional Paper 1149,22 p. ty, Utah [Ph.D. thesis]: Seattle, Washington, University of Washington, Rowley, P. D., Steven, T. A., and Mehnert, H. H., 1981, Origin and structural unanswered. 177 p. implications of upper Miocene rhyolites in Kingston Canyon, Piute Bowers, W. E., 1972, The Canaan Peak, Pine Hollow, and Wasatch Forma- County, Utah: Geological Society of America Bulletin, Part I, v. 92, tions in the Table Cliff region, Garfield County, Utah: U.S. Geological p. 590-602. Survey Bulletin 1331-B, 39 p. Rowley, P. D., Hereford, R., and Williams, V. S., 1987, Geologic map of the ACKNOWLEDGMENTS Chapin, C. E., and Cather, S. M., 1981, Eocene tectonics and sedimentation in Adams Head-Johns Valley area, southern Sevier Plateau, Garfield the Colorado Plateau-Rocky Mountain area, in Dickinson, W. R., and County, Utah: U.S. Geological Survey Miscellaneous Geological Inves- Payne, W. D., eds., Relations of tectonics to ore deposits in the southern tigations Map 1-1798, scale 1:50,000. I greatly appreciate the grants from the Cordillera: Arizona Geological Society Digest, v. 14, p. 173-198. Sable, E. G., and Anderson, J. J., 1986, Tertiary tectonic slide megabreccias, Christiansen, F. W., 1952, Structure and stratigraphy of the Canyon Range, Markagunt Plateau, southwestern Utah: Geological Society of America Geological Society of America, National Sci- central Utah: American Association of Petroleum Geologists Bulletin, Abstracts with Programs, v. 17, p. 263. ence Foundation, and Sigma Xi, which made v. 63, p. 717-740. Standiee, L. A., 1982, Structure and stratigraphy of Jurassic rocks in central Cook, E. F., 1957, Geology of the Pine Valley Mountains, Utah: Utah Geologi- Utah: Their influence on tectonic development of the Cordiileran fore- the study possible. I also express my gratitude to cal and Mineralogical Survey Bulletin 58, 111 p. land thrust belt, in Guidebook, Geologic Studies of the Cordiileran Davis, G. H., and Krantz, R. W., 1986, Post-"Laramide" thrust faults in the thrust belt: Denver, Colorado, Rocky Mountain Association of Geolo- the Bryce Canyon National Park Service for Claron Formation, Bryce Canyon National Park, Utah: Geological So- gists, 1983, v. I, p. 357-382. providing free camping, work space, other valu- ciety of America Abstracts with Programs, v. 18, p. 98. Suppe, J., 1984, Fault-propagation folding: Geological Society of America Dickinson, W. R„ Klute, M. A., Hayes, M. J., Janecke, S. U, Lundin, E. R., Abstracts with Programs, v. 16, p. 670. able assistance, and friendliness in general. I was McKittrick, M. A., and Olivares, M. D., 1988, Paleogeographic and 1985, Principles of structural geology: Englewood Cliffs, New Jersey, paleotectonic setting of Laramide sedimentary basins in the central Prentice-Hall, 537 p. greatly helped by William Bowers of the U.S. Rocky Mountain region: Geological Society of America Bulletin, Threet, R. L., 1952, Some problems of the Brian Head Formation in southwest- Geological Survey, who gladly shared his exper- v. 100, p. 1023-1039. em Utah [abs.]: Geological Society of America Bulletin, v. 63, p. 1386. Doelling, H. H., and Davis, F. D, 1978, Coal drilling, Johns Valley, Garfield 1963, Structure of the Colorado Plateau margin near Cedar City, Utah, tise of the region as well as his unpublished Park County, Utah: Utah Geology, v. 5, no. 2, p. 113-124. in Intermountain Association of Petroleum Geologists Guidebook to the Gregory, H. E., 1950, Geology of eastern Iron County, Utah: Utah Geological geology of southwestern Utah, 12th Annual Field Conference, 1963: map and other field sheets with me. Access to and Mineralogical Survey Bulletin 37,155 p. Salt Lake City, Utah Geological and Mineralogical Survey, p. 104-117. Chevron's subsurface data enhanced my under- 1951, The geology and geography of the Paunsaugunt region, Utah: Tweto, O., 1975, Laramide (Late Cretaceous-early Tertiary) orogeny in the U.S. Geological Survey Professional Paper 226, 116 p. Southern Rocky Mountains: Geological Society of America Memoir standing of the structures immensely, and stimu- Gregory, H. E., and Moore, R. E., 1931, The Kaiparowits region; A geographic 144, p. 1-44. and geologic reconnaissance of parts of Utah and Arizona: U.S. Geolog- Van Kooten, G. K., 1988, Structure and hydrocarbon potential beneath the lating discussions with Frank Royse, Jr. gave me ical Survey Professional Paper 164,161 p. Iron Springs laccolith, southwestern Utah: Geological Society of Amer- a much deeper insight to the structural prob- Gregory, H. E., and Williams, N. C., 1947, Zion National Monument, Utah: ica Bulletin, v. 100, p. 1533-1540. Geological Society of America Bulletin, v. 58, p. 211-244. Williams, P. L., and Hackman, R. J., 1971, Geology, structure, and uranium lems. Peter Rowley of the U.S. Geological Sur- Heller, P. L„ Bowdler, S. S„ Chambers, H. P„ Coogan, J. C„ Hagen, E. S„ deposits of the Salina quadrangle, Utah: U.S. Geological Survey Miscel- Shuster, M. W., and Winslow, N. S-, 1986, Time of initial thrusting in laneous Geological Investigations Map 1-591, scale 1:250,000. vey provided various maps and papers, and the Sevier orogenic belt, Idaho-Wyoming and Utah: Geology, v. 14, furnished helpful information regarding critical p. 388-391. Hintze, L. F., 1983, Structures in the Beaver Dam Mountains, southwestern regional relationships. I acknowledge Ernest Utah: Geological Society of America Abstracts with Programs, v. 15, no. 5, p. 379. Anderson's and William Bowers' careful re- Kelley, V. C., 1955, Regional tectonics of the Colorado Plateau and relation- MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 2,1987 views. Thanks to all of the above for your help ship to the origin and distribution of uranium: University of New Mex- REVISED MANUSCRIPT RECEIVED DECEMBER 16,1988 ico Publications in Geology, no. 5, 120 p. MANUSCRIPT ACCEPTED DECEMBER 23,1988

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