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

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

ABSTRACT

A thrust system at least 40 km long and with as much as 2,000 m of shortening has been mapped in the southern High Plateaus of Utah, partly within Bryce Canyon National Park. Seismic data clearly reveal that the thrusts are of thin-skinned, ramp-flat style, soling out in evaporite-rich layers of the Ju-rassic Carmel Formation. Mapping discloses that large portions of the thrusts terminate upward in fault-propagated folds, marked by vertical to overturned beds. Spaced cleavage reveals that some shortening is accommo-dated by pressure dissolution. Shallow drill-ing for coal provides data suggestive of more than one episode of thrusting. An older event predates deposition of the Upper Cretaceous and Paleocene(?) Canaan Peak Formation. This deformation is presumably related, in the broadest sense of the term, to the Sevier orogeny. The younger thrusting is south-vergent and involves rocks as young as the lower to middle Eocene, white limestone member of the Claron Formation, a forma-tion that is considered post-orogenic by most geologists. The upper age of the younger event is not constrained in time, and its tec-tonic significance remains poorly understood.

INTRODUCTION

Thrust structures of compressional origin were recently discovered in the Bryce Canyon area of the southern High Plateaus of Utah (Davis and Krantz, 1986; G. H. Davis and oth-ers, unpub. data). The Bryce Canyon area is located in the late Cenozoic structural and phys-iographic transition zone (High Plateaus), be-tween the Basin and Range on the west and the main part of the Colorado Plateau on the east. Major north- to northeast-striking normal faults form the boundaries of the broad and gently tilted High Plateaus. The thrusting in the Bryce Canyon area appears to be confined between the Paunsaugunt fault zone on the east and the Sev-ier fault zone on the west—two of the major late Cenozoic normal fault zones. The area is located

- 8 0 km east of the east limit of known thrusting in the Sevier orogenic belt, but it is located within the western edge of the region containing compression-related structures produced during the Late Cretaceous-early Tertiary Laramide orogeny (Fig. 1). The middle Tertiary Marys-vale volcanic field lies on the north side of the area.

The thrusts are unusual in two aspects. First, the constrictional sense is approximately north-south, which is anomalous for both the Sevier and Laramide orogenies. Second, the thrusts in-volve the Claron Formation, which generally rests with angular unconformity on rocks that are involved in compressional deformation in the region (Gregory and Moore, 1931; Gregory and Williams, 1947; Gregory, 1950, 1951; Threet, 1952,1963; Mackin, 1954,1960; Cook, 1957; Blank, 1959; Bowers, 1972; Hintze, 1983). The purpose of this study is to provide documentation of these anomalous aspects by investigating the geometry, kinematics, extent, and age of the thrusts and related structures and, if possible, to interpret their tectonic signifi-cance. To accomplish this, the area embracing the thrusts was mapped at 1:24,000 scale, well and seismic reflection data (provided by Chev-ron USA) were analyzed, stratigraphic markers needed to evaluate the amount of shortening were identified and mapped, and the thrusts and related structures were studied in detail.

STRATIGRAPHY

General Relations

Rocks within the map area consist of sedi-mentary formations of Late Cretaceous age, sed-imentary strata of the lower to middle Tertiary Claron Formation, volcanic rocks and minor in-terbedded sedimentary rocks of middle Tertiary age, and sedimentary and volcanic rocks of Quaternary age (Fig. 2).

In the map area, the Claron Formation gener-ally lies disconformably over (from youngest to oldest) the Kaiparowits, Wahweap, and Straight Cliffs Formations. Regionally, the unconformity is obvious, in places cutting out the entire Kai-

parowits and Wahweap Formations (Fig. 2). At outcrop scale, however, the unconformable rela-tionship is commonly subtle. In the northeastern part of the study area, the Claron Formation lies conformably on the Upper Cretaceous to Paleo-cene(?) Canaan Peak Formation, which in turn lies unconformably above the Kaiparowits Formation. Well data from the northeastern part of the area disclose that the Canaan Peak For-mation and another formation of local extent, the Paleocene(?) Pine Hollow Formation, lie with angular unconformity above the Upper Cretaceous Dakota Formation.

With the exception of a small outcrop of un-named conglomeratic sandstone in Little Henderson Canyon (Figs. 2, 3), tentatively as-signed to the middle Tertiary, there are no strata that depositionally overlie the Claron Formation within the map boundaries. Near the center of the map area, brecciated volcanic rocks and middle Tertiary volcanic rocks of the Mount Dutton Formation are in contact with the Claron Formation. The nature of the contact is difficult to determine, but Rowley and others (1987) interpreted it to be the eroded lower por-tion of a listric normal fault or possibly a denudation-type normal fault. The writer has adopted their interpretation but has not been able to independently confirm it. Also in fault contact against the Claron Formation are Quaternary basalts along the Sevier fault zone

(Fig. 2).

Claron Formation

Bowers (1972) subdivided the Wasatch For-mation (Claron Formation in this paper) in the Table Cliffs Plateau into three members, from oldest to youngest: the pink limestone member, the white limestone member, and the variegated sandstone member. These will be referred to here as the pink, white, and variegated Claron, respectively.

The pink Claron occurs extensively throughout the study area. It is the colorful unit responsible for most of the famous erosional fea-tures of Bryce Canyon National Park and the 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

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: BP, Bryce Point; RI, Rubys Inn; STB, eastern leading edge of Sevier thrust belt, as recognized by previous work.

cally, the member consists mostly of thick-bedded to massive, clastic limestone, inter-bedded with marl, calcareous siltstone and sandstone, and some conglomerate lenses. The base of the formation is marked in many places by a distinctive 5- to 10-m-thick pebble to cob-ble conglomerate (Gregory, 1951; Bowers, 1972). Recent investigations indicate that the pink Claron is largely a sequence of stacked paleo-soil horizons (Mullett and others, 1988). The overlying white Claron is the characteristic

cliff-forming rim cap in Bryce Canyon. It is composed mostly of massive limestone with minor interbeds of siltstone and mudstone (Bowers, 1972). The uppermost variegated Claron is an inconspicuous, slope-forming member, exposed only in Johns Valley of the map area (Fig. 2). This member is made up of mostly varicolored, fine-grained, friable sand-stone, siltstone, and mudstone (Bowers, 1972).

Of the three members, only the lower two can unequivocally be shown to be thrusted. The out-

crops of the variegated Claron do not reveal whether this member was affected by the thrust-ing, or whether it was deposited on already de-formed 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 upper-most 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 ap-pear 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

The major thrust faults in the area are the Rubys Inn and Johns Valley thrusts (Fig. 2). The east-striking Rubys Inn thrust extends from the Sevier to the Paunsaugunt fault zone, a dis-tance of ~20 km. For convenience of descrip-tion, the Rubys Inn thrust is subdivided into three segments: the Highway 12 segment, Ahl-strom Hollow segment, and Hillsdale Canyon segment (Fig. 2). On the basis of seismic pro-files, the northeast-striking Johns Valley thrust extends at least 10 km beyond the northeastern map boundary, giving it a minimum total length of 17 km.

Other faults in the area of possible compres-sional origin are the east-striking Pine Hills fault (known only from one seismic profile) and the 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

LU

I - P i " 1

T, CS

S u r f i c i a i depos i ts

Basalt

Mount Dut ton F o r m a t i o n

U n n a m e d c o n g l o m e r a t i c s a n d s t o n e

0 V a r i e g a t e d s a n d s t o n e 'Z m e m b e r re

1 W h i t e l i m e s t o n e u. m e m b e r

T~ >- Pink l i m e s t o n e _ re C p ^ member

K w

C a a n a n Peak Format ion

Kaiparowits Formation

Wahweap Format ion

Straight C l i f fs Format ion

Tropic Shale

Figure 2. ( C o n t i n u e d ) .

60 70

¿ . - A . .

30

45

^ 6 0

Contac t , dashed where a p p r o x i m a t e

Normal f a u l t , showing dip of s u r f a c e and t r e n d and p lunge of s t r i a e ; d a s h e d w h e r e a p p r o x i m a t e l y l o c a t e d ; d o t t e d w h e r e c o n c e a l e d ; ba l l on d o w n t h r o w n s ide

Thrust f a u l t , s h o w i n g d i p of s u r f a c e a n d t r e n d of s t r i a e ; d a s h e d w h e r e a p p r o x i m a t e l y l o c a t e d : d o t t e d w h e r e c o n c e a l e d

L o w - a n g l e g r a v i t y f a u l t , d a s h e d w h e r e a p p r o x i m a t e l y l o c a t e d ; d o t t e d w h e r e c o n c e a l e d

I n f e r r e d t e a r f a u l t

M o n o c l i n e

A s y m m e t r i c a n t i c l i n e s h o w i n g p l u n g e of a x i s

A s y m m e t r i c s y n c l i n e

O v e r t u r n e d s y n c l i n e

S t r i k e of v e r t i c a l b e d d i n g

S t r i k e a n d d i p of b e d d i n g

S t r i k e a n d d i p of j o i n t s

S t r i k e of v e r t i c a l c l e a v a g e

S t r i k e a n d d i p o f c l e a v a g e

Dry h o l e , d r i l l e d f o r o i l a n d g a s

S h a l l o w h o l e , d r i l l e d f o r c o a l

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 the southeast side of Johns Valley (Fig. 2). Kel-ley (1955) classified the Johns Valley anticline as a Laramide structure, and seismic profiles re-veal that this is a basement-involved, Laramide-style fold. A monoclinal flexure, referred to here as the Paunsaugunt monocline, parallels the southeast side of Johns Valley (Fig. 2). Asso-ciated with the Rubys Inn thrust and the Elbow fault are what are interpreted as fault-propa-gated folds (Suppe, 1984, 1985) (Fig. 2). The fault-propagated folds are nonplunging asym-metric synclines, with the steep limb to the north.

Bedding

Because Claron strata are essentially flat-lying in much of the region, tilted or otherwise dis-

turbed beds are an important guide to mapping the structures of the area. Generally, beds of unusual attitudes can be grouped either into areas of overturned beds that mark the northern limb of fault-propagated synclines, or into sepa-rate homoclinal domains. Examples of homo-clines are the north-dipping beds of the Pine Hills, the northwest-dipping beds of the un-named hills on the northwest side of Johns Val-ley, and the northwest-dipping beds along the flank of the Paunsaugunt monocline (Fig. 2).

Highway 12 Segment of the Rubys Inn Thrust

The best exposure of thrust faults in the study area is along Utah Highway 12, in the northern part of Bryce Canyon National Park (Figs. 2, 3, and 4), where the main fault bifurcates eastward

into two imbrications. The lower, more promi-nent fault, strand A (Figs. 3 and 4), was first mapped by Richard Hereford (U.S. Geol. Sur-vey, unpub. 1:100,000 map) as a low-angle normal fault and was subsequently described as a thrust by Davis and Krantz (1986). The upper fault, strand B (Fig. 3), was discovered during the present study. Both faults are oriented N75°E, 30°NW on the average. The thrust traces become increasingly more difficult to follow in Little Henderson Canyon (Fig. 3) as the western splay of the Paunsaugunt fault zone is approached, and it is not possible to determine whether a crosscutting relationship exists. A small outcrop of an unnamed conglomeratic sandstone, tentatively assigned to the middle Tertiary, lies with slight angular unconformity on pink Claron near the last-seen trace of strand A (Fig. 3). This sandstone does not appear to be

1042 E. R. LUNDIN

deformed and may postdate thrusting of the Claron Formation. The evidence, however, is not conclusive, and the age of the sandstone is unknown.

Prominent striae and grooves (Fig. 5) typi-cally mark thrust surfaces. These kinematic indi-cators range in direction between N10°W and N20°W along strand A, thus indicating dip-slip 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 is also mostly dip-slip except near the western end, where left-lateral strike-slip motion prevails (Fig. 3).

A zone of reddish-brown gouge, ~10 cm thick, is well developed along both strands, and cleavage within the gouge discloses thrust-slip movement (Davis and Krantz, 1986) (Fig. 6). The hanging wall of both faults consists of the same layer of pink Claron, a varicolored, mas-sive, limestone bed that is 6 to 10 m thick. The top of the layer is characterized by a gray marly

unit and is penetratively deformed by spaced place to place in the hanging wall are minor cleavage that is interpreted as a pressure solution thrust faults and conjugate fracture sets (Fig. 8). feature (Figs. 4 and 7). Cleavage planes cut bed- The varicolored unit of the hanging wall can be ding at a high angle and strike perpendicular to recognized in the local stratigraphy, and it per-the strike of fault striae. Also to be found from mits calculation of the horizontal shortening,

SYMBOLS

Contact , dotted where concea led

Normal fault showing dip: ba l l on d o w n t h r o w n s ide

Thrust fault showing dip and t r e n d and p lunge of s t r i a e

Tear fault

Overturned nonplunging syncl ine

Strike and dip of bedding; o v e r t u r n e d ; v e r t i c a l

Str ike and dip of c leavage: v e r t i c a l

" f t "

- T - j>C 3 0 6 0

r-1

7 5

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

Figure 4. View to the nor th of strand A of the Highway 12 segment of the Rubys Inn thrust . The fault surface is indicated by arrows. The hanging wall consists of a distinctive 6- to 10-m-thick, varicolored, massive limestone bed of pink G a r o n . The top of the layer is charac-terized by a gray marly unit, which is penetratively deformed by spaced cleavage.

- 3 5 0 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 over-turned. An unusually scenic expression of the folding and thrusting is preserved in the "hoo-doo" shown in Figure 9. The base of the "hoo-doo" 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 the existence of a minor thrust fault in the Bryce Point overlook cliff. A "medium"-scale thrust, named the "Bryce Point thrust," can be seen from the overlook to the east (Lindquist, 1977, 1980). Approximately 80 m of thrust slip is cal-culated for this thrust. Striae on the thrust sur-face strike due south, consistent with compres-sional movement indicators measured along Highway 12.

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 ex-tends along the entire Rubys Inn thrust. In the Ahlstrom Hollow area, the ridge is offset by in-ferred north-oriented minor tear faults.

The Pine Hills to the north (Fig. 2) are prom-inent 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 ~ 3 0 degrees from a décollement in the evaporite-rich lower part of the Carmel Formation. The massive

Navajo Sandstone, as well as underlying rocks, are not affected by thrusting (Fig. 10). The Lion Oil Company Bryce Unit No. 1 well (Fig. 2) penetrates the top of the Navajo Sandstone at a

depth of 1,845 m below the valley floor (503 m above sea level). The seismic profile indicates the existence of a smaller, south-dipping reverse fault or back-thrust named the "Pine Hills fault"

1044 E. R. LUNDIN

Figure 6. Close-up view of zone of faulting along strand A of the Highway 12 segment of the Rubys Inn thrust. View is looking toward the west. Cleavage disclose thrust-slip motion; that is, top to the south.

(Figs. 2 and 10). The fault relationship is also evident on wireline logs.

Taken together, the Ahlstrom Hollow and Pine Hills faults define a thrust wedge. Approx-

imately 1,000 m of horizontal shortening is es-timated across the thrust wedge from seismic data. The lateral extent of the Pine Hills fault is not known because it has no surface expression.

It is assumed to die out westward, before reach-ing 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 expo-sures of the relation between thrusting and fold-ing (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 slip.

A 10- to 15-cm-thick zone of reddish-brown 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 well developed in the Straight Cliffs Formation of the hanging wall (Fig. 12). The fractures are oriented N54°W, 33°SW and N85°W, 30°NE on the average and were used to calculate the maximum compressional stress orientation ac-cording to Mohr-Coulomb theory (Anderson, 1951). The derived CT, orientation is 2° S20°W.

Approximately 125 m of thrust slip is ob-served 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 do not permit tracing the thrust farther. From surface mapping, it cannot be determined whether the thrust and fold die before reaching the Sevier fault or whether they are cut by the normal fault, and seismic data are not available for Hillsdale Canyon.

Johns Valley Thrust

The N40°E-trending Johns Valley thrust is nowhere exposed at the surface, but several seismic profiles, within and beyond the map 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

Figure 7. Spaced cleavage indicative of pressure solu-tion in the gray marly unit of the pink Claron. The unit is typically found 6 to 10 m above the fault surface along the Highway 12 segment (see Fig. 4). View is toward the east. Bedding dips - 1 5 ° to the north. Cleavage planes are perpendicular to bedding as well as to the plunge of striae on fault surface.

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 explora-tion 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 be-neath a significant angular unconformity. Seis-mic profiles show that the shallow drilling was in the hanging-wall side of the thrust. This is also supported by the close similarity in attitude be-tween the Dakota Formation in the subsurface and the beds in the dip panels in Johns Valley (Fig. 2); both locations belong to the same northwest-dipping homocline.

Doelling and Davis (1978) reported Claron Formation capping the unconformity in the Widstoe Junction coal drilling wells Nos. 1 ,2 ,3 , and 4 (Fig. 2). My interpretation is instead that the capping beds are the Upper Cretaceous and Paleocene(?) Canaan Peak Formation and the Paleocene(?) Pine Hollow Formation, because lithologie descriptions from the wells correspond better to these formations (Bowers, 1972; and 1988, personal commun.). The Canaan Peak and Pine Hollow Formations are well exposed along the southeast side of Johns Valley. Both of these formations are of relatively local extent and for the most part conformable with the Claron Formation (Bowers, 1972).

Drill intercepts and wireline logging permit-ted determination of the attitude of a distinct coal bed within the Dakota Formation to N23.5°E, 20°NW by three-point solution (Doel-ling and Davis, 1978). The unconformity sur-face is essentially horizontal. The shallow drilling data are projected into the plane of the cross section C - C ' along structural strike and 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

Figure 8. View to the west of a minor thrust fault in the hanging wall of strand A of the Highway 12 segment in Little Henderson Canyon. Notice conjugate fracture sets in lower left part of photo, indicative of near-horizontal compression. Photograph courtesy of G. H. Davis.

Figure 9. View to the west of "hoodoo" located along strand A of the Highway 12 segment, just southwest of the High-way 12-thrust crossing. The lower four-fifths of the spire consists of pink Claron. Notice the steep beds near the base and the similarly steeply fold-ed beds in the background to the north of the hoo-doo (right). The middle part of the hoodoo con-sists of broken pink Clar-on, and the top part con-sists of a thrusted cap of Straight Cliffs Formation. A similar thrust relation-ship is observed in the hoodoo in the background to the south. Pink Claron strata in the distance (left) dip gently to the south.

HILLSDALE CANYON

S 2 5 0 0 m

h 2 0 0 0

y 1 5 0 0

1 0 0 0

5 0 0

AHLSTROM HOLLOW

LION, L 2 5 0 0 m

2 0 0 0

h 1 5 0 0

1 0 0 0

5 0 0

JOHNS VALLEY

WJ

1 0 0 0

1 0 0 0 2 0 0 0 m e t e r s

NO VERTICAL E X A G G E R A T I O N

THRUSTING OF THE CLARON FORMATION, UTAH 1047

Figure 10. Geologic sections. A-A': Hills-dale Canyon segment of the Rubys Inn thrust, based purely on surface data. B-B': Ahlstrom Hollow segment of the Rubys Inn thrust, constructed from surface, seismic, and petro-leum exploration-well data. C-C': Johns Val-ley thrust, based on surface, seismic, petro-leum exploration well data, and shallow coal-drilling data. The dashed horizontal fault in section C - C is an inferred structure re-sponsible for the Paunsaugunt monocline; lo-cation, geometry, and displacement of this normal fault are displayed only schematically. The question-marked arrow along the thrust in section C-C' schematically displays normal fault reactivation of the thrust fault. Forma-tion abbreviations: JN , Jurassic Navajo Sand-stone; Jc, Jurassic Carmel Formation; JE , Jurassic Entrada Sandstone; KD , Cretaceous Dakota Formation; Kx, Cretaceous Tropic Shale; K s , Cretaceous Straight Cliffs Forma-tion; K\y, Cretaceous Wahweap Formation; TK, Cretaceous-Tertiary Canaan Peak and Pine Hollow Formations undifferentiated; T cp, Tertiary pink Claron Formation; T c w, Tertiary white Claron Formation; T c v, Ter-tiary variegated Claron Formation; Q, undif-ferentiated Quaternary deposits. Well abbre-viations: Lion, Lion Oil Company Bryce Unit No. 1 well (TD 3,400 m); CP, Champlin Pe-troleum 1132 Clay Creek Federal well (TD 2,678 m); SC, SLAPCO Celeron Clay Creek Federal well (TD 3,930 m); WJ, Widstoe Junction shallow coal-drilling wells Nos. 1-4 (TD 210 m). Wells are projected along struc-tural strike into the planes of the geologic sections.

the thrust plane all the way up to the angular unconformity at 110-m depth below the valley floor. It cannot be determined whether this also is the case for the older Entrada Sandstone and Carmel Formation. On the basis of the seismic and shallow drilling data, it is possible to esti-mate the minimum amount of shortening across the Johns Valley thrust to -2 ,000 m. Because of poor outcrop exposures and valley fill, it is not possible to estimate shortening from surface mapping alone.

Minor structures are not nearly as abundant in the dip panels above the Johns Valley thrust as they are in the Pine Hills. Within the Claron Formation, spaced cleavage is occasionally pres-ent, oriented N68°E, 76°SE on the average. One conjugate fracture set was discovered, oriented N75°E, 39°NW and N30°E, 10°SE. The de-rived a] orientation is 15° N23°W according to Mohr-Coulomb theory (Anderson, 1951). Be-

cause the main fault surface is concealed and no minor fault surfaces were found, it is difficult to assess the significance of these sparse kinematic indicators.

Elbow Fault

The Elbow fault is an inferred structure. Based on the close similarity of topographic ex-pressions to the Pine Hills, it is assumed that an analogous structure exists here. Northwest of the inferred fault, dip panels of pink Claron, Wah-weap Formation, and Straight Cliffs Formation strike N65°E (Fig. 2). Within the pink Claron southeast of the inferred fault, an asymmetric syncline parallels the dip panels. This fold re-sembles the fault-propagated fold along the south side of the Rubys Inn thrust.

The dip panel and the fold terminate abruptly against relatively flat-lying beds of the pink Claron to the east (Fig. 2). This sharp, north-trending boundary is interpreted to be a tear fault. Near the middle of the dip panel, the pink Claron-Wahweap Formation contact is shifted - 1 2 0 m in a left-lateral sense along another, north-striking inferred tear fault (Fig. 2). Out-crops of the dip panel disappear beneath the alluvium to the west (Fig. 2), and it cannot be determined if or how the fault connects with the Rubys Inn thrust.

Kinematic Interpretation of the Thrusts

Based on seismic and well data, the style of thrusting in the area is thin-skinned, involving formations only above the Jurassic Navajo Sandstone. The zone of mechanical weakness that the thrusts have utilized is confined to evap-orites in the lower part of the Jurassic Carmel Formation. Fault and fold exposures along the entire Rubys Inn thrust are judged to indicate that this thrust terminates upward in a fault-propagated fold. Similar observations along the inferred Elbow fault suggest that this may be a common surface expression of the thrusts in the area. Spaced cleavage within the pink Claron indicates that some shortening was accommo-dated by pressure solution.

It appears as if the Rubys Inn thrust has expe-rienced most of its shortening across the Ahl-strom Hollow segment, -1 ,000 m on the basis of seismic data. The measured thrust shortening of the Claron Formation at the surface is only 300 and 350 m in Hillsdale Canyon and along Highway 12, respectively. Possibly the thrust dies out westward toward the Sevier fault, and the eastward decrease in shortening may be re-lated to a transfer to the Johns Valley thrust. More likely, however, the area has experienced more than one episode of thrusting, and only

where seismic data are available can the accu-mulative shortening be observed. Construction of cross sections is based on the latter interpretation.

At any one segment of the Rubys Inn thrust, the kinematic indicators are internally consist-ent; striae and grooves are oriented perpendicu-lar to the spaced cleavage planes and parallel to the maximum stress orientations calculated from conjugate fracture sets. When separate segments are compared, some variations are noticed. The movement directions along the Rubys Inn thrust are south-southwesterly in Hillsdale Canyon, southerly in the Ahlstrom Hollow segment, and south-southeasterly in the Highway 12 area. Al-though the individual movement directions de-fine a slight fan shape, the over-all pattern is one of south-directed transport.

Seismic and shallow-drilling data taken to-gether indicate a minimum of 2,000 m of short-ening across the Johns Valley thrust in the subsurface. Because of erosion and valley fill, surface shortening cannot be estimated. If the sparse kinematic indicators observed in the Claron Formation are representative, they indi-cate a south-southeasterly transport direction and consequently thrusting with a component of left-lateral slip.

The inferred tear faults that offset the dip pan-els by the Elbow fault indicate a due southerly transport direction. If this is applied to the Johns Valley thrust, the thrusting would have had a major component of left-lateral slip.

Regardless of transport direction, the Johns Valley thrust evidently experienced more short-ening than did the Rubys Inn thrust. One explanation for this difference is that more than one episode of compression has affected the re-gion; two or more compressional events with contrasting shortening directions could produce an apparent shortening disparity because the trend of the Rubys Inn and Johns Valley thrusts differ by 45 degrees. The strongest evidence for two constrictional episodes is derived from the shallow-drilling data, which suggest that an older thrusting event preceded deposition of the Canaan Peak, Pine Hollow, and Claron Forma-tions. Geologic mapping clearly shows that the Claron Formation has been thrusted.

Paunsaugunt Fault Zone

The Paunsaugunt fault and monocline (Fig. 2) are believed to be different surface expres-sions of the same structure, and therefore they are described together. The high-angle Paunsau-gunt fault maintains its regional trend of ~N10°E from the northern Kanab Plateau through the Vermillion and White Cliffs and northward along the eastern side of the Paun-

1048 E. R. LUNDIN

Figure 11. View to the east of the Hillsdale Canyon segment of the Rubys Inn thrust, displaying the thrust and fault-propagated fold. The Claron contact is offset by - 1 2 5 m of thrust slip. Straight Cliffs Formation is placed over Claron and W a h w e a p Formations. Loca-tion of photo coincides with the thrust trace across cross section line A - A ' in Hillsdale Canyon.

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 south-east side of Johns Valley, parallel to the thrust system. The flexure separates the higher Table Cliffs Plateau to the east from the lower south-ern Sevier Plateau to the west and accounts for a structural relief of - 5 0 0 m. Just beyond the northeastern edge of the map area, the Paunsau-gunt monocline appears to change its expression, and a fault again cuts the surface. A north-northeast-striking, west-side-down fault zone continues along the east side of Johns Valley 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). These structures are considered to belong to the Paunsaugunt fault zone.

The monoclinal folding in the map area is clear at the surface, but the structure is not re-vealed by seismic profiles, and there is no indica-tion of a basement-cutting, high-angle fault beneath the flexure. Despite detailed mapping efforts, these contrasting observations are not well understood.

The Paunsaugunt fault zone cannot be seen to cut the thrust system, and thrusting is not known east of the Paunsaugunt fault/monocline. The change in strike and expression of the normal fault near Shakespear Point suggest that this fault took advantage of an already existing me-chanical weakness. This relationship is schemat-ically displayed in cross section C - C ' by the dashed detachment fault (Fig. 3). An alternative to this interpretation, although not proposed here, is that the curvature of the structures to-ward each other in map view indicates that they are structurally interrelated.

In Grass Valley and on the Awapa Plateau 6.4- to 5.0-m.y.-old basalts predate the uplift of the Awapa and Aquarius Plateaus along what is here considered the Paunsaugunt fault zone (Rowley and others, 1981). Thus, most normal faulting along the Paunsaugunt fault zone oc-curred in late Cenozoic time (after 6.4 to 5.0 Ma), but the structure may have begun to form

in the early Miocene (Rowley and others, 1978a). Low scarps in pediment deposits in Bryce Canyon indicate Recent activity along the Paunsaugunt fault zone (W. E. Bowers, U.S. Geol. Survey, 1:24,000 map, unpub. mapping).

Sevier Fault Zone

The Sevier fault zone is parallel in strike to the Paunsaugunt fault zone. The throw on the Sevier fault zone is also down to the west, but its displacement is substantially larger than that of the Paunsaugunt fault zone. Seismic profiles in-dicate that offset of basement rocks by the Sevier fault zone near Red Canyon (Fig. 2) is - 9 0 0 m (assuming an average seismic velocity of 4,500 m/s) . Striated surfaces were not observed along the Sevier fault zone in the area, but Rowley (1968) reported striae indicative of pure dip-slip northwest of the study area.

The Sevier fault zone may cut the Rubys Inn thrust, but the location of the western extension of the thrust, if present, is unknown. Middle Ter-tiary to Quaternary volcanic rocks cover the Claron Formation on the west side of the Sevier Valley where the thrust conceivably could be located.

The Sevier fault zone appears to have been active slightly earlier than the Paunsaugunt fault. The main phase of faulting along the zone in the Kingston Canyon area to the north is estimated

to have occurred between extrusions of rhyolites at 7.6 Ma and rhyolites at 5.4 Ma (late Miocene) (Rowley and others, 1981), but it is possible that this structure also began to form in early Mio-cene time (Rowley and others, 1978a). Evi-dence of Quaternary displacement of the Sevier fault can be seen at the mouth of Red Canyon, near the western map boundary, where 0.5-m.y.-old basalts (Best and others, 1980) are faulted against the Claron Formation (Fig. 2).

D I S C U S S I O N

The Bryce Canyon thrusts are clearly of thin-skinned, ramp-flat style and definitely involve the white Claron, which is of presumed early to middle Eocene age. Shallow coal drilling in Johns Valley provides strong evidence for an older compressional event that preceded deposi-tion of the Upper Cretaceous and Paleocene(?) Canaan Peak Formation. If this suggestion of an older event is correct, then it is possible, and even likely, that the most recent compressional event utilized the same mechanically weak strat-igraphic layer as the first event did. Even the younger Paunsaugunt fault appears to have partly taken advantage of this weakness. A sim-ilar but more complex scenario is reported in central Utah, 150 km due north of Bryce Can-yon. There, Standlee (1982) interpreted that evaporite-rich layers of the Jurassic Arapien

THRUSTING OF THE CLARON FORMATION, UTAH 1049

Figure 12. Conjugate fracture sets in the Straight Cliffs Formation in Hillsdale Canyon indicative of near-horizontal compression.

Formation (correlated with the Carmel Forma-tion of southwestern Utah) have been utilized by a series of compressional and extensional events between Mesozoic and Recent time.

The style of thrusting, the upper time con-straint of the proposed older event, and the strat-igraphie position of the décollement in the Bryce region all resemble the same characteristics in structures of established Sevier origin in Iron Springs Gap near Cedar City (Mackin, 1954. 1960; Armstrong, 1968; Van Kooten, 1988). On the basis of drill-hole data, Standlee (1982) interpreted that Sevier thrusting extends beneath the Sevier Valley of central Utah and probably beneath the northernmost Sevier Plateau. K. R. Neuhauser (unpub. data) mapped thrusts of proposed Sevier age in the San Rafael Swell. The front of the Sevier thrust belt may therefore extend as far east as Bryce Canyon.

Sevier deformation in the Utah region is in-terpreted to have occurred between middle and latest Cretaceous (Armstrong, 1968; Heller and others, 1986). Timing of the here proposed older compressional event in the Bryce Canyon area would correspond well with this age of the Sev-ier orogeny, but the here proposed younger thrusting event involving the Claron Formation can almost certainly be excluded from classic Sevier thrusting despite the poor age control of the formation. Another argument against a Sev-ier age for thrusting of the Claron Formation is

the observed general southerly transport direc-tion for the Bryce Canyon thrusts. This direction contrasts significantly with the reported easterly to east-southeasterly transport direction for Sev-ier thrusts of southern Utah (for example, Chris-tiansen, 1952; Threet, 1963; Miller, 1966; Armstrong, 1968).

The age constraints of thrusting of the Claron Formation in the Bryce Canyon area are too poor to include or exclude the Late Cretaceous to Eocene Laramide orogeny (for example, Tweto, 1975; Dickinson and others, 1988) as the cause of the event. Because the thin-skinned style of the Claron Formation thrusting is inter-preted to be inherited from an older event, one cannot use this style as a discriminator against the Laramide orogeny. On the other hand, the approximately north-south stress field of the Claron Formation thrusting would be unusual for Laramide deformation in this region; the generally proposed compressional direction var-ies between east-northeast and northeast (for ex-ample, Kelley, 1955; Chapin and Cather, 1981).

Compressional deformation of the Claron Formation has not been reported elsewhere. It is therefore possible that the described thrust faults are local and completely unrelated to large-scale regional tectonics. The main difficulty with proposing post-Laramide timing lies in finding local forces large enough to produce the de-scribed thrusts.

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 un-derstood structures were presumably induced by gravity-sliding during the Miocene. Where pres-ent, striae on the low-angle fault surfaces plunge uniformly north-northeast (Sable and Anderson, 1986). Outside of the kinematic movement di-rection, 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-sional origin which appear related to large-scale, 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 relationship between the Bryce Canyon thrust-ing and the normal faulting along the Sevier and Paunsaugunt fault zones. The amount of strike-slip movement along the Paunsaugunt fault zone, judged from striae, is insufficient to pro-duce more than a small part of the thrusting, however. Data from the Sevier fault zone (northwest of the study area) indicate pure dip slip (Rowley, 1968). Thus, at present there seems to be no reasonable post-Laramide cause, other than shouldering by Tertiary plutonism, to explain the thrusting of the Claron Formation.

C O N C L U S I O N S

The style of thrust deformation in the Bryce Canyon area, as revealed by seismic profiles and well data, is classical thin-skinned. Shallow drill-ing for coal in Johns Valley indicates that at least two compressional episodes affected the Bryce Canyon region; one event preceded depo-sition of the Upper Cretaceous and Paleocene(?) Canaan Peak Formation, and the other event followed deposition of the lower to middle Eo-cene white member of the Claron Formation. Both compressional events seem guided by evaporitic layers of the Jurassic Carmel Forma-tion, and there are indications that the Paunsau-gunt normal fault utilized this weakness in part.

Comparison of style, timing, and stratigraphie position of the décollement of the older thrusts

1050 E. R. LUNDIN

in Bryce Canyon with those of established Sevier thrusts to the west suggests that the Sevier thrust belt extends as far east as the Bryce Can-yon region.

Field evidence indicates general north-south compression of the Claron Formation. Because of the poor age control of the Claron Formation and a lack of upper time constraint for the younger thrusting episode involving this forma-tion, one cannot rule out the Laramide orogeny as the cause of this deformation. The direction of constriction would be unusual for a Laramide event, however. At the same time, there is no known post-Laramide cause to suggest. Pres-ently it is only possible to report the existence of the enigmatic thrusts involving the Claron For-mation and to leave the question of their origin unanswered.

ACKNOWLEDGMENTS

I greatly appreciate the grants from the Geological Society of America, National Sci-ence Foundation, and Sigma Xi, which made the study possible. I also express my gratitude to the Bryce Canyon National Park Service for providing free camping, work space, other valu-able assistance, and friendliness in general. I was greatly helped by William Bowers of the U.S. Geological Survey, who gladly shared his exper-tise of the region as well as his unpublished Park map and other field sheets with me. Access to Chevron's subsurface data enhanced my under-standing of the structures immensely, and stimu-lating discussions with Frank Royse, Jr. gave me a much deeper insight to the structural prob-lems. Peter Rowley of the U.S. Geological Sur-vey provided various maps and papers, and furnished helpful information regarding critical regional relationships. I acknowledge Ernest Anderson's and William Bowers' careful re-views. Thanks to all of the above for your help

and encouragement. Finally, I express my grati-tude to George Davis for excellent advice in the field, academically, and personally.

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