Structure and Evolution of Upheaval Dome: a Pinched-Off Salt Diapir

Structure and evolution of Upheaval Dome: A pinched-off salt diapir

M. P.A. Jackson* Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 D. D. Schultz-Ela } M. R. Hudec† I. A. Watson Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252 M. L. Porter }

ABSTRACT

Upheaval Dome (Canyonlands National Park, Utah) is an enigmatic structure previ- ously attributed to underlying salt doming, cryptovolcanic explosion, fluid escape, or meteoritic impact. We propose that an overhanging diapir of partly extrusive salt was pinched off from its stem and subsequently eroded. Many features support this inference, especially synsedimentary structures that indicate Jurassic growth of the dome over at least 20 m.y. Conversely, evidence favoring other hypotheses seems sparse and equivocal. In the rim syncline, strata were thinned by circumferentially striking, low-angle extensional faults verging both inward (toward the center of the dome) and outward. Near the dome’s core, radial shortening produced constrictional bulk strain, forming an inward- verging thrust duplex and tight to isoclinal, circumferentially trending folds. Farther inward, circumferential shortening predomi- nated: Radially trending growth folds and im- Figure 1. Location map of northern Paradox basin (Utah and Colorado), showing Upheaval bricate thrusts pass inward into steep clastic Dome, known salt structures, and Tertiary intrusions (after Doelling, 1985). dikes in the dome’s core. We infer that abortive salt glaciers spread from a passive salt stock during Late Triassic is inferred to be the toe of the convergent gin of Upheaval Dome—the pinched-off salt dia- and Early Jurassic time. During Middle gravity spreading system. pir hypothesis. Jurassic time, the allochthonous salt spread Upheaval Dome forms a mound that has into a pancake-shaped glacier inferred to be INTRODUCTION strongly deformed uplifted beds in the center of 3 km in diameter. Diapiric pinch-off may the dome. Strata have been elevated 120–250 m have involved inward gravitational collapse Upheaval Dome is located in Canyonlands above their regional levels. Beds near the center of of the country rocks, which intensely con- National Park (Island in the Sky district), in the the dome are tightly folded and cut by faults hav- stricted the center of the dome. Sediments in western part of the Colorado Plateau, southeast- ing generally radial strikes. The central uplift the axial shear zone beneath the glacier ern Utah (Fig. 1). The dome is one of the most passes outward into a prominent rim syncline, then steepened to near vertical. The central uplift enigmatic and controversial geologic structures into a rim monocline, outside of which strata show in North America. Hypotheses for its origin in- merely gentle regional tilting (Fig. 2). The center *E-mail: [email protected]. clude cryptovolcanic explosion, meteoritic im- of the dome is a topographic depression eroded †Present address: Department of Geology, Baylor pact, fluid escape, and subsurface salt diapirism. 350 m below the surrounding escarpment, which University, Waco, Texas 76798. We document here a new explanation for the ori- is breached by a canyon cut through its west wall

Data Repository item 9891 contains additional material related to this article.

GSA Bulletin; December 1998; v. 110; no. 12; p. 1547–1573; 23 figures; 1 table.

1547 JACKSON ET AL.

Figure 2. Structure contour map of Upheaval Dome and surroundings showing the base of the Wingate Sandstone. The circumferential traces of the rim monocline and rim syncline are shown with dashed lines. L and H refer to structural lows and highs, respectively. The Buck Mesa well is in the southeast quadrant of the dome. Elevations (in feet [1 ft = 0.3408 m]) were measured by us within the rim monocline and taken from the map of Huntoon et al. (1982) in the surrounding region. The structural high west of the dome could extend farther north, but the gentle dips there preclude accurate contouring of the outcrop data.

(Fig. 3). The dramatic canyon relief and generally accommodated downward bending of the crust anticlines (Fig. 1), but small diapiric stocks are superb exposures make Upheaval Dome a leading into the axis of the Paradox trough. locally exposed. attraction for tourists and geologists. Salt accumulated in the deepest part of the The Paradox basin has been affected by at least Paradox basin shortly after the Uncompahgre three more tectonic episodes since Late Creta- Tectonic Setting thrust fault became active. Prograding sedi- ceous time. (1) Crustal shortening related to the ments shed from the Uncompahgre uplift ex- Laramide orogeny (Late Cretaceous–early Ter- In Middle Pennsylvanian time, the Paradox pelled the salt southwestward, forming a series tiary) reactivated the Uncompahgre fault and basin formed as a northwest-trending foreland of northwest-trending salt walls and anticlines may also have reactivated some of the smaller basin to the basement-cored Uncompahgre uplift (Baars, 1966; Ge et al., 1994a, 1997). Salt basement faults within the basin. (2) Oligocene (Ohlen and McIntyre, 1965), which was thrust structures decrease in amplitude and age of ini- laccoliths built the La Sal, Abajo, and Henry over the northeast edge of the basin along the Un- tiation away from the Uncompahgre uplift. Mountains. (3) The basin may have been slightly compahgre fault (Fig. 1; Frahme and Vaughn, Truncations and lateral thickness changes indi- stretched in Cenozoic time (Ge and Jackson, 1983; Ge et al., 1994b; Huffman and Taylor, cate that diapirs grew mostly during Permian 1994; Hudec, 1995; Ge et al., 1996). These 1994). The deepest part of the trough adjoined time, but salt continued to flow until at least events had negligible effects on the flat-lying the Uncompahgre uplift, and the basin thinned 100 m.y. ago (Doelling, 1988). Near Upheaval strata around Upheaval Dome. Finally, the Col- toward gentle crustal upwarps that bounded the Dome, roughly 75 km southwest of the Un- orado Plateau was greatly uplifted after mid- basin to the south and west. Folding and faulting compahgre fault, most salt structures are gentle Miocene time (e.g., Thompson and Zoback,

1548 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

which are strata-parallel and continue at least 10 km from the dome margins in basins to the south and west. The lower Moenkopi siltstone weathers brownish-red to grayish-red. The lithologic variation and weathering character define structures well. The middle Moenkopi Formation comprises reddish shale containing layers of fine-grained, yellow, flaggy, calcareous sandstone. This member separates the lower reddish Moenkopi member from the upper Moenkopi siltstones that weather to buff-gray or olive-gray featureless masses. After another major hiatus (TR-3 from 240 to 227 Ma) in Late Triassic time, the continental Chinle Formation (Fig. 4) spread across the top of the Uncompahgre uplift, eliminating the Paradox basin as a physiographic feature. We divided the Chinle Formation into four map units, from bottom to top, (1) basal Moss Back Member sandstone and conglomerate, which generally crop out as prominent gray ledges or dip slopes; (2) Petri- fied Forest and Church Rock Member shales (la- beled “Lower Chinle” in subsequent figures), which weather gray, blue-gray, or lavender, as dis- tinct from the yellowish-gray upper Moenkopi shales; (3) another prominent sandstone about one-third up from the Chinle Formation base; this finer-matrix pebble conglomerate is probably the Figure 3. Oblique air photo looking west. From foreground to background are Trail Canyon, Black Ledge sandstone of the Church Rock Mem- Upheaval Dome, Upheaval Canyon, and the Green River. In the extreme foreground, alcoves A ber (Stewart et al., 1959); (4) shale and siltstone (containing Alcove Spring) through D appear counterclockwise from bottom left in Trail weathering brick, brown, or orange-red are inter- Canyon. The rim syncline passes through the Navajo Sandstone in the far wall of alcove D. preted to be the upper part of the Church Rock Photograph by John S. Shelton. Member (combined with the Black Ledge member as “Upper Chinle” in subsequent figures). After a brief hiatus (J-0, from 210 to 206 Ma), 1979), resulting in at least 1600 m of erosion over shed a wedge of marginal marine and continental sedimentation remained continental, with deposi- Upheaval Dome. clastic sediments (Cutler Group, Fig. 4) south- tion of the Jurassic Wingate Sandstone, a wet erg westward across the basin. In the area of Up- (the lower Wingate strata representing central erg Stratigraphic Setting heaval Dome, the Cutler Group comprises three facies, and the upper Wingate strata representing formations; from bottom to top, these are the distal, less arid, back erg facies containing silty in- The oldest known sedimentary rocks beneath Cedar Mesa Sandstone Member, the Organ Rock terbeds). The mainly fluvial Kayenta Formation Upheaval Dome are Cambrian-Mississippian Member, and the White Rim Sandstone. Only the and the dry erg Navajo Sandstone then accumu- clastic and carbonate rocks (Fig. 4). Passive-mar- upper two units may be exposed in the center of lated. The Jurassic Navajo Sandstone is the high- gin sedimentation in the area ceased with the on- Upheaval Dome (extreme deformation there hin- est stratigraphic level exposed in Upheaval Dome. set of Ancestral Rockies deformation and led to ders stratigraphic correlation). The Organ Rock Using regional thicknesses of higher units derived restricted-marine conditions in Middle and Late Member comprises fluvial dark-reddish-brown from the Book Cliffs area to the north (Molenaar, Pennsylvanian time. A carbonate shelf rimmed siltstones and mudstones. The overlying White 1981), we estimate that an additional 1600 to the basin’s southern and western margins as Rim Sandstone is a clean white sandstone de- 2200 m of section were deposited above the saline facies accumulated in the axis of the basin posited in coastal eolian environments (Steele, dome. That upper limit corresponds with the near the Uncompahgre uplift. As much as 2 km 1987). Sandwiched between finer-grained units, thickness of missing strata required to encase the of Paradox Formation evaporites collected here the resistant White Rim Sandstone weathers into currently unroofed La Sal laccoliths intruded in (Hite, 1960) and thin toward the basin margins. prominent ledges and benches. A major hiatus middle Cenozoic time some 55 km east of Up- Borehole data suggest that roughly 720 m of (TR-1 from 264 to 247 Ma) followed deposition heaval Dome. These Cretaceous and Tertiary evaporites were deposited in the vicinity of Up- of the White Rim Sandstone. units were eroded during Miocene–Holocene upheaval Dome (Woodward-Clyde Consultants, The Moenkopi Formation (Fig. 4) records a ma- lift of the Colorado Plateau. 1983). The evaporitic facies consist of 70%–90% rine transgression from the west that created a va- halite in most places and minor amounts of lime- riety of fluvial to shallow-marine environments Hypotheses for Origin of Upheaval Dome stone, dolomite, gypsum, anhydrite, black shale, dominated by fine-grained sediments (O’Sullivan and siltstone. and MacLachlan, 1975; Molenaar, 1981). To map Upheaval Dome was first described by Restricted marine conditions ended in Early the center of Upheaval Dome, we divided the Harrison (1927) as “Christmas Canyon Dome.” Permian time as the Uncompahgre highlands Moenkopi Formation into three informal units, He suggested that salt flowed into the dome be-

Geological Society of America Bulletin, December 1998 1549 JACKSON ET AL. cause of unloading produced by canyon erosion in a former meander in the Green River. We know of no such evidence for such a meander. McKnight (1940), Fiero (1958), and Mattox (1968) also postulated an underlying salt dome. Given the strong evidence for lateral constriction rather than lateral extension in the dome center (presented in the following), we reject the notion of a buried salt dome below Up- heaval Dome. Moreover, a preliminary report by Louie et al. (1995) concluded from a seismic refraction line across the dome that there is no large salt plug near the surface. Similarly, collapse due to salt dissolution, as envisaged by Stokes (1948), is equally implausible because strata in the center of the dome rise more than 100 m above their regional elevation (Fig. 2). Bucher (1936) noted similarities between the structures at Upheaval Dome and the Steinheim Basin, Germany, and proposed that Upheaval Dome was cryptovolcanic. Structures in the central depression were interpreted to be a result of ex- plosive release of gases trapped near a shallow igneous intrusion. No igneous rocks have been found within 55 km of the dome center, although geophysical anomalies (described in the following) around Upheaval Dome suggest that igneous rocks may underlie the Paradox salt. Kopf (1982) also interpreted deformation in the dome center to result from sudden upward release of a fluid slurry tectonically overpressured by a deep fault system. Boone and Albritton (1938) were the first to suggest that Upheaval Dome might have been produced by meteoritic impact. They noted that Upheaval Dome contained many features typical of impact, including circular shape, central dome, peripheral folds, radial faults, and intense deformation. Shoemaker and Herkenhoff (1983, 1984) and Shoemaker et al. (1993) argued, for similar reasons, that there was an impact in Late Creta- ceous or Paleogene time (Fig. 5A). They interpreted the thinning of Mesozoic strata observed by McKnight (1940) and Fiero (1958) to be due to normal faulting rather than depositional onlap. They explained the lack of primary physical evidence for impact by suggesting that the crater and Figure 4. Stratigraphic section through formations at or below the surface of Upheaval Dome much of its floor had been deeply eroded. Kriens and younger eroded units. Data from the Buck Mesa #1 well log, Fiero (1958), Mattox (1968), et al. (1997) interpreted a lag deposit of eroded and Woodward-Clyde Consultants (1983). cobbles as impactite ejecta indicating that Up- heaval Dome formed possibly as late as a few million years ago. Huntoon and Shoemaker trusion (Fig. 5B; Schultz-Ela et al., 1994a, 1994b). large lateral variations in seismic velocity, and (1995) proposed impact as the cause of Roberts The central part of the structure contains overbur- structural complexity. To our knowledge, no stems rift, a breccia-filled fissure 10 km long located den strata pinched in around what we infer to be of entirely pinched-off diapirs are exposed at the more than 20 km away from the dome. the necked-off stem of a broadly overhanging, surface, except where greatly squeezed by oro- We present a new alternative to the impact hy- partly extrusive pancake of salt that was subse- genic contraction. If our hypothesis is correct, the potheses. We propose that the structural and strati- quently removed by erosion. Pinched-off diapirs superb three-dimensional exposure and accessibil- graphic relations at Upheaval Dome—especially are common below the sea floor in the Gulf of ity of Upheaval Dome make it a prime candidate those indicating protracted growth of the struc- Mexico and in other salt basins (e.g., Jackson to examine the response of country rock to the ture—can best be explained as the result of stem et al., 1995), but their stems are difficult to image necking and pinching off of a diapir in an area rel- pinch-off below an overhanging diapiric salt ex- with seismic data because of the steep stratal dips, atively unaffected by orogeny.

1550 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

Transient crater Overturned flap A Central uplift Ejecta floor Mixed breccia Fault terraces Airfall breccia Maximum possible sediment thickness

Inward-verging Present topography normal faults

Inward-verging thrust faults Lower boundary of in-situ breccia

B B Before pinch-off After pinch-off Representative thrust wedge

Map view beneath salt extrusion Future faults

Cross section Rim Rim syncline Salt extrusion syncline

Figure 5. Schematic illustrations of hypotheses for meteoritic impact and salt dome pinch-off for the evolution of Upheaval Dome. (A) Impact hypothesis modified from Shoemaker and Herkenhoff (1984) by adding a specific present topographic profile, transient crater profile (dotted), typical lower limit of pervasive fracturing (shaded), and other features typical of impact craters. (B) Maps and cross sections illustrating the diapiric pinch-off inferred in the text.

CIRCUMFERENTIAL FOLD SYSTEM ward” and “inside-outside” are relative to the cen- Navajo Sandstone—the youngest unit present—is ter of Upheaval Dome. “Circumferential” is a syn- preserved within this syncline (Fig. 6). Navajo The geology of Upheaval Dome is shown in onym for the concentric or tangential direction. Sandstone dip slopes define most of the rim syn- maps (Figs. 6, 7, and 8) and two cross sections. A The rim monocline defines the perimeter of cline, but several erosional alcoves expose cross conventional linear section trends roughly west- Upheaval Dome, separating the regional homo- sections through the fold (Figs. 3 and 6). northwest across the entire dome and its sur- cline from the rim syncline (Fig. 2). The rim Successively older units crop out toward the roundings (Fig. 15), and a circular section encir- monocline trace is mostly outside the mesa con- core of Upheaval Dome (Fig. 6). Stratal dips be- cles the dome core to illustrate the roughly radial taining the dome center (Fig. 6). The monoclinal come subvertical to overturned in the central up- structures there (Fig. 16). bend is broad and gentle where it involves mas- lift; there, Moenkopi strata have been elevated The largest and most continuous structures in sive Wingate Sandstone (northwest of the dome). perhaps as much as 250 m above their regional Upheaval Dome are three circular folds having ax- Conversely, the monocline is a sharp kink where datum outside the rim monocline, and surround- ial traces concentric about the core of the dome it is defined by thinly interbedded Chinle clastic ing Wingate strata have been lifted 120 m. The (Figs. 6 and 8). From the surrounding undeformed rocks (east of the dome). inward increase in dip is disrupted in two con- strata toward the core of the dome, this circumfer- Inward from the rim monocline, stratal dips centric zones of steep dip, separated by zones of ential fold system comprises an outer rim mono- measured increase to a mean of 15°–20° and a gentle dip. A discontinuously preserved, inner cline, a rim syncline, and an inner dome. In the maximum of 73° then decrease to zero as strata steep zone 0.7–0.8 km from the dome center af- structural descriptions that follow, “inward-out- flatten in the floor of the rim syncline. Most of the fects the upper Wingate Sandstone and lower

Geological Society of America Bulletin, December 1998 1551 1 km 0.5 mi Wingate Fm. (Jw) Upper Moenkopi Fm. ( ) Lower Moenkopi Fm. ( ) White Rim sandstone dike Bedding attitude Normal fault trace Thrust fault trace Rim monocline trace Rim syncline trace Abrupt steepening trace Dog tongue Canyon Trail Alcoves in Holeman Spring alcove Panorama number and limits of view Cross section trace Central map corners Paved road Foot trail to overlooks UTM grid locations of map corners Navajo Fm. (Jn) Kayenta Fm. (Jk) Chinle Fm. ( ) (Black Ledge cg. dashed)

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13 25 W W 18 W 14 12 42 68 23 28 24 X 18 Jn 29 marked by L brackets. Locations and view directions of panoramas in Figures 11, of panoramas in Figures Locations and view directions by L brackets. marked 13, and intersection point marks the ob- numbers and pairs of arrows; with large marked 14 are position. server’s 65 29 23 50 s 20 65 60 32

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Jk 15 Upheaval Canyon 13 N Steer Mesa Steer 12SWT9152 12SWT9157 Figure 6. Geologic map of Upheaval Dome. The Chinle Formation is undifferentiated The Chinle Formation Dome. 6. Geologic map of Upheaval Figure except for the Black Ledge member,except for are intervals and the middle upper Moenkopi are 7; corners appear in Figure combined. Geologic details in the central depression

1552 Geological Society of America Bulletin, December 1998 A work, 15 and 16. lines of sections in Figures and the linear circular Figure 7. Geologic maps of the central depression,Figure (A) stratigraphic units and showing faults, and (B) stratigraphic contacts, bedding attitudes, faults, axes, fold drainage net-

Geological Society of America Bulletin, December 1998 1553 B ). Continued Figure 7. ( Figure

1554 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

Figure 8. Summary map of Upheaval Dome outer structures. Circumferential fold traces are dotted. Transport directions (large black arrows) of hanging walls of extensional faults are dominantly inward in the south and outward in the north.

Kayenta Formation (described in Inner Contrac- forms the southern limb of the gentle Grays Pas- (Fig. 4). Deeper stratigraphic units (Moenkopi tional Systems section). A virtually continuous ture syncline (Huntoon, 1988). In the rim syn- and Cutler Formations) are exposed only in the outer steep zone 1.2–1.6 km from the dome cen- cline is a crescentic low (Fig. 2), which is a typi- core of the dome, where they are too deformed to ter affects the inner margin of the lower Navajo cal fold interference pattern where a rim syncline reveal lateral stratigraphic variations. The upper Sandstone. There, dips steepen markedly from is superposed on a regional homocline (Ritz, contact of the highest unit in the dome (Navajo 20°–30° to 40° and commonly higher (Fig. 8). 1936). Two structural highs (stippled) are pre- Sandstone) is not exposed and is generally too Bounding surfaces of dune-scale Navajo Sand- sent: one is the inner circular dome described massive and coated with desert varnish to expose stone cross beds become vertical to overturned above; the other is a low, elongated periclinal large-scale internal geometries. Thus, although locally (e.g., near the Syncline Loop Trail in the dome adjoining the western flank of Upheaval we describe evidence of synsedimentary defor- southwest). Southeast of the dome, at Whale Dome, also noted by Fiero (1958). mation for only three units, we do not exclude the Rock, the innermost Navajo strata dip steepens to possibility of similar features in other formations. a subvertical attitude within ~100 m of the inner SYNSEDIMENTARY STRUCTURES Navajo contact (Fig. 9). Inward from the verti- Deformation During Deposition cal Navajo strata, however, dips in the basal A wide variety of synsedimentary features are of the Chinle Formation Navajo and underlying Kayenta strata flatten present in Upheaval Dome but have not been ob- (around 40°). served elsewhere in the region. These features The upper Chinle Formation contains angular The circumferential fold system of Upheaval provide evidence for deformation of Upheaval discordances of as much as 27° in alcoves B, C, Dome perturbs a regional homocline dipping Dome over at least 20 m.y., during deposition of and D on the east of the dome (Fig. 6), in the ex- 1.2° to the north-northwest. This homocline the Chinle, Wingate, and Kayenta Formations treme south, and in Syncline Valley in the north-

Geological Society of America Bulletin, December 1998 1555 JACKSON ET AL. west. If the truncating surfaces are restored to horizontal, truncated strata dip consistently outward from the dome. This apparent radial sym- metry eliminates regional tilting as the cause. Be- cause of poor exposure, we cannot determine the origin of all the observed truncations. If they are primary, the truncated strata may have been foresets deposited by rivers draining outward from a gentle dome. Alternatively, erosion may have cut across gently domed strata. Either way, the truncations suggest doming late in the time of Chinle Formation deposition. These truncations are below the commonly discordant, faulted base of the Figure 9. Cross section through Whale Rock (Fig. 6), in the southeast part of the dome, Wingate Sandstone. showing the abrupt, anomalous steepening of Navajo Sandstone dips on the inner limb of the withdrawal syncline. This steepening is interpreted as due to the overriding front of a spread- Deformation During Deposition ing salt glacier. of the Wingate Sandstone

Wingate Internal Diastems. Panorama 9 Herkenhoff, 1983, 1984). They are best exposed were unlithified. Kriens et al. (1997, p. 23) con- (Fig. 15, A and B), west of the Holeman Springs on the northeast and southwest walls of the inte- cluded that local deformation of the Wingate alcove, extends from outside the rim monocline rior depression (Figs. 6 and 8). Fiero (1958) as- Sandstone may have been due to “the presence of to the rim syncline. Here, the Wingate Sandstone serted that the fold axes parallel the dominant fluids and/or a low degree of lithification,” re- thickens from ~50 m on the west edge of the pro- joint system, and that slip on closely spaced sembling “soft-sediment landslides.” file to more than 75 m to the east. No faults offset joint planes caused the folding, whereas other continuously traceable bedding in the top and workers reported radial axes (Shoemaker, 1954; Deformation During Deposition bottom of the Wingate Sandstone. Instead, the Shoemaker and Herkenhoff, 1983, 1984) and of the Kayenta Formation thickness changes by a combination of onlap a lack of shattering in the folded sandstones against the base of the Wingate strata and trunca- (Mattox, 1968, Plate 4, Fig. 2). From the limited Basal Kayenta Channels. Detachment faults tion beneath a correlatable mid-Wingate surface. exposure in the third dimension and their distri- (described in the section on Outer Extensional From a uniform thickness outside the rim mono- bution around most of the inner cliffs except for Systems) transect or follow much of the Wingate- cline in the west, the Wingate Sandstone gradu- Upheaval Canyon, we conclude that these folds Kayenta formation contact, which creates an ir- ally thins inward toward the center of the profile trend radially and probably plunge outward. regular, wavy surface. However, this contact is by excision of ~20 m of section beneath the mid- The folds seem to have formed by circumfer- also wavy because Kayenta formation fluvial Wingate erosional truncation surface. From the ential shortening of the basal Wingate Sandstone sand channels to 5–6 m deep were eroded into the other end of the section, the Wingate Sandstone (see section on Inner Contractional Systems). top of the Wingate back erg dunes and interdunes. also thins westward because ~40 m of lowermost Spacing between the antiforms generally ranges Away from each channel, the sand packages thin, Wingate strata onlap against the top of the Chinle from 150 to 190 m. Given a typical Wingate pinch out, and grade into shale drapes above ad- Formation (the J-0 unconformity of Pipiringos Sandstone undeformed thickness of about 100 m, joining paleohighs. These large channels and pa- and O’Sullivan, 1978). the wavelength to thickness ratio of these folds is leohighs are well exposed in panoramas 9 (Fig. Figure 11C is a schematic reconstruction of far smaller than values observed elsewhere in na- 11) and 12 (Fig. 14). In panorama 9, the upper the inferred deformation. Stage 1 shows the ture, theory, or experiment for a vast variety of Wingate contact is fairly smooth outside the rim lower Wingate strata thickening inward (right- layer and matrix properties (reviewed by Price monocline of Upheaval Dome but deeply chan- ward) from point A, which we interpret as the and Cosgrove, 1990). Accordingly, it is improba- neled inside the monocline, indicating that chan- margin of the shifting peripheral sink around the ble that the Wingate Sandstone buckled as a sin- neling was enhanced by increased fluvial gradi- dome. Stage 2 shows a bulge forming in the gle layer of massive, lithified sandstone. ents related to deepening of the rim syncline at the lower Wingate strata over the flexed margin of Further evidence corroborates the unusual start of deposition of the Kayenta Formation. the sink. The uplifted crest of the bulge between folding behavior of the Wingate Sandstone. The Kayenta Formation Internal Diastems and points C and D is then eroded. Truncations on the top contact of the Wingate Sandstone is also Inferred Shifting Rim Synclines. Repeated eastern (inward) limb of the eroded bulge are not folded, but not in harmony with the bottom con- and variable tilting is indicated by bounding visible in this panorama, but are exposed farther tact. The folds visible within the formation surfaces at the base of and within the Kayenta east in the Holeman Springs alcove. Stage 3 are also disharmonic with regard to the top and Formation in spurs south of Upheaval Canyon shows deposition of the upper Wingate Sand- bottom contacts. near the present axis of the rim syncline stone across the erosional surface. In the northeast wall of the interior depression (panoramas 12–14; Figs. 13 and 14). These sur- Wingate Sandstone Growth Folds. Wingate are the two best-exposed radial folds deforming faces divide the Kayenta Formation into lower, Sandstone cliffs ring the depression eroded into the base of the Wingate Sandstone. There, lower middle, and upper packages. the center of Upheaval Dome. In these cliffs, the Wingate strata thin upward over the antiforms The lowest truncation surface, EB0, is the ero- basal Wingate Sandstone contact is deformed and thicken above the adjacent synforms, indica- sional base of the Kayenta Formation referred to into a train of prominent, mostly upright folds tive of growth folds (Fig. 12). in the preceding section. The truncation sense in- (Fig. 12), as noted by others (Shoemaker, 1954; These data suggest that folding began during dicates that upper Wingate strata were tilted in- Fiero, 1958; Mattox, 1968; Shoemaker and Wingate Sandstone deposition, when the sands ward before they were obliquely planed off as

1556 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

Butte). This scarcity is partly because many Kayenta faults are cut by the present-day erosional surface before reaching the Navajo Sandstone. However, in the few places that Navajo Sandstone erosional remnants still rest across faulted strata (e.g., north side of alcove B), the well-bedded Navajo basal strata are un- faulted. Another explanation for the upward ter- mination of faults within the Kayenta Forma- tion is that the faults could dissipate upward among many small slip surfaces within the Kayenta strata. However, in widespread areas where the Kayenta Formation is well exposed, we see no evidence for such upward splaying of faults. Instead, the Kayenta faults die out upward with an abruptness typical of growth faults. For example, stratigraphic separations in faults at the base of the Kayenta Formation are as follows: 30 m of separation dying out 20 m above (panorama 14); 25 m of separation dying out 20 m above (panorama 12). This upward decrease of throw is accommodated by 25– 30 m of stratigraphic thickening of the lower Kayenta Formation in the hanging wall of the Figure 10. Schematic restoration of the intra-Kayenta Formation geometries exposed in spurs fault. We have not correlated meter-scale indi- south of Upheaval Canyon. The changing patterns of subsidence and tilting suggest syndeposi- vidual packages across faults to confirm thick- tional deformation, which we ascribe to the shifting peripheral sinks during diapiric growth. ening by growth faulting, but the impression of syndepositional faulting on several panoramas is striking. lower Kayenta strata began to accumulate from the site of early Kayenta Formation subsid- (panorama 14 and Fig. 16, early Kayenta). The ence (Fig. 10). OUTER EXTENSIONAL SYSTEMS next-higher diastem, the intra-Kayenta EB1 Onlapping the EB2 truncation surface in the (panorama 13), also cuts downsection outward. upper Kayenta Formation on panorama 12, McKnight (1940) and Fiero (1958) both de- The EB1 truncation surface is the base of an ero- four flat-topped fluvial channel complexes scribed major thickness variations in the Wingate sional channel complex and is marked by a chan- were stacked imbricately as they migrated out- Sandstone and Kayenta Formation within the rim nel sandstone. This sense of truncation for both ward (Fig. 10, end Kayenta). That geometry in- syncline of Upheaval Dome. Some variations are the EB0 and EB1 surfaces suggests that subsi- dicates creation of accommodation space by produced by the stratigraphic variations de- dence was localized inboard (eastward) of the gradual subsidence of a truncation surface scribed here. However, as noted by Shoemaker end of panorama 13 during deposition of the tilted inward as part of the rim syncline. Late in and Herkenhoff (1984), most thinning is due to lower Kayenta Formation (Fig. 10, mid–early the time of Kayenta Formation deposition, slip on ramp-flat extensional faults surrounding Kayenta). therefore, the rim syncline had already mi- the dome. All these circumferential faults are ex- The upper truncation surface (EB2, panorama grated inward to its present position (Fig. 10D), posed only inside the trace of the rim monocline, 14 and Fig. 10, mid-Kayenta) cuts downsection widening or renewed outward shift of the rim and so are inferred to be related to the formation inward, opposite to the truncation sense of EB0 syncline during deposition of the four channel of Upheaval Dome. Figures 14, 17, and 19 illus- and EB1. The EB2 surface is exposed in several complexes followed. trate the structural style of these faults (pano- ridges on both sides of Upheaval Canyon and is The simplest explanation for these shifting ramas 4 and 5 in Fig. 17). marked by a pale, upward-fining sandy bar that locations is that the rim syncline migrated as Three-dimensional exposures reveal the fol- toplaps meter-scale foresets. The EB2 surface is Upheaval Dome evolved over a lengthy period lowing characteristics. The faults are either sim- the preserved erosional base of an upper Kayenta (Fig. 10). These shifting depocenters are typical ple listric faults or a series of ramps and flats. Formation channel-fill complex overlying a ge- of salt tectonics, where lateral salt flow and Even the ramps are low-angle faults, cutting bed- netically unrelated middle Kayenta Formation sedimentary accommodation space are inti- ding at a mean apparent angle of ~13° and rang- channel-fill complex. Both complexes are sand- mately linked. ing from 9° to 25° (standard deviation). Very lo- prone, but comparison of cross-stratal styles, pa- Kayenta Growth Faults. Incomplete evi- cally, however, ramp dips are near vertical over leocurrents, and set thicknesses suggests that a dence suggests that many of the normal faults short segments. The flats form detachments diastem exists between the two fluvial packages. offsetting Wingate and Kayenta strata (de- mostly at the base of the Kayenta Formation or During the diastem, beds in the lower package scribed in the next section) may be the age of Wingate Sandstone. Extensional ramps between were tilted outward by as much as 20°, then the Kayenta Formation. Several faults die out these two levels are responsible for thinning the planed off. That indicates that strata outboard of upward into the Kayenta Formation, and faults Wingate Sandstone (panoramas 2, 4, 5, 6, 11, and panorama 14 were subsiding during deposition offsetting the Navajo Sandstone base are rare 14 in Figs. 14 and 17). Less commonly, ramps of the middle Kayenta Formation, very different (we saw only one, at the west end of Syncline thin the Kayenta Formation as well (panorama 3,

Geological Society of America Bulletin, December 1998 1557 JACKSON ET AL.

West Cliff bend Northeast

Rim monocline Holeman Springs Alcove

C E cave B A D

Navajo (Jn) Kayenta (Jk) AA Wingate (Jw) Chinle (TRC )

C Figure 11. (A) Panorama 9 traced from photographs of Holeman Spring alcove and adjoining exposures farther west on the south side of Up- heaval Dome. Panorama location and view direction are shown in Figure 6. (C) Schematic reconstruction of synsedimentary deformation of the Wingate Sandstone in panorama 9, exaggerated vertically by 2×.

Fig. 17). In addition, local flats are common mean stratigraphic separation is 25 ± 11 m, and ward-dipping extensional fault systems but re- within the Kayenta Formation and rare in the mid- the mean apparent slip is 98 ± 24 m. veals that they are only half of the extensional dle Wingate Sandstone. Measurements from pho- What was the effect of these faults on tectonic picture. As summarized in Figure 8, an equal tographs of oblique sections through exposed transport? Shoemaker and Herkenhoff’s (1984) number of extensional faults dip outward. As a faults offsetting the base of the Kayenta Forma- schematic section portrays inward-dipping listric group, their abundance, lengths, and stratigraphic tion provide rough separations for the largest fault faults linking with outward-dipping thrust sys- separations are similar to the inward-dipping in each panorama. For the outward-verging faults, tems in the core of the dome (Fig. 5A). That link- faults. The outward-dipping faults cannot be the mean stratigraphic separation is 35 ± 15 m, age would transport rock inward to the core of linked with the inner thrust faults in the manner and the mean apparent slip is 120 ± 55 m (stand- the dome. advocated by Shoemaker and Herkenhoff (1983, ard deviation). For the inward-verging faults, the Our mapping confirms the existence of the in- 1984). Most normal faults in the northern half of

1558 Geological Society of America Bulletin, December 1998 B

Figure 11. (B) Photograph of a similar view to that in A; on the far right is the eastern por- tal of the alcove.

Figure 12. Northward view of radial growth folds in the lower Wingate Sandstone in the northeast cliffs of the inner depression. The lowermost Wingate strata are isopachous and are interpreted to be prekinematic. Above here, strata thin over the antiform and thicken in the synform, indicating growth of the fold early during deposition of the Wingate Sand- stone. The growth-wedge taper is extreme (~15°) on the right limb of the antiform but much less on the left limb; we interpret this as an initially asymmetric fold verging to the right. After a period of stability, indicated by overlying isopachous units, the fold was tight- ened further into a symmetric fold late in Wingate Sandstone deposition.

Figure 13. Photograph southwest across Upheaval Canyon showing truncation surfaces of variable dip within the Kayenta Formation. The view superimposes one spur (panorama 14) on another (panorama 12) immediately to the southwest. Compare with Figures 14 and 17. Navajo Sandstone on Syncline Butte forms the foremost skyline, behind which are flat-lying strata outside the rim monocline and, just visible, the snow-capped peaks of the Henry Mountains laccoliths.

Geological Society of America Bulletin, December 1998 1559 Fluvial incision surface Outward verging extension Navajo (Jn) Cliff bend (not fold) Kayenta (Jk) Eroded Wingate (Jw) in mid Jk Pan 14 View to SW Outer thrust Chinle (TRC ) Extension = early Jk imbricates 0 100 m

Upper Jk 0 400 ft Erosional base, EB2

Pan 13 EB0 View to NE (mirrored) Dune scour surface Extension = early Jk Jk EB2

EB1 Back-erg complex

Central-erg complex

0 100 m

Syncline Butte 0 400 ft

Upper Kayenta channel-fill complex C5 EB2 C3 C4 C2 Lower and middle Kayenta channel-fill complex

Pan 12 C1 View to SSW Extension = early Jk 0 100 m

0 400 ft

Jn Pan 2 View to SE (mirrored) Extension = syn or post Jk Jk

0 100 m

0 400 ft

Figure 14. Panoramas (Pan) traced from photographs of outward-verging extensional fault systems encircling Upheaval Dome. Panorama locations and view directions are shown in Figure 6. The view directions of panoramas 2 and 13 have been reversed (“mirrored”) from the actual view so that the outward direction from the dome center is consistent throughout the figure. EB—erosional base of fluvial channel complex; PH—paleohigh draped by Kayenta shale that grades laterally into channel sandstones filling an erosional scour in the top of the Wingate Sand- stone. C1 through C5 are channels. Scales are approximate because of perspective.

1560 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

2000 Azimuth of TRC Jn A section = 301° of map Boundary Bend in section Elevation (m)

1000 0 500 m 0 2000 ft

B Azimuth of section = 106°

of map 6000 Boundary Bend in section Elevation (ft)

3000

Figure 15. Linear cross section across Upheaval Dome and surroundings as inferred from surface exposures and projection from the Buck Mesa well. Trace of section is shown in Figures 6 and 7 (ends of the section project beyond the maps). Geology inboard (east) of northwest Wingate Sandstone cliff (gray in section) was projected from the cliff and slope exposures just north of the section line. Contacts and faults are dashed beyond the zone of direct projection from the surface. Dotted lines show bedding traces within units. Figure 26 shows the inferred geometry of deep units, including the Paradox salt. the dome transported rock outward, whereas Kayenta Fold-and-Thrust Belt section typifies deformation elsewhere in the con- most normal faults in the southern half trans- tracted Kayenta Formation. ported rock inward. This is true whether the faults As noted by Shoemaker and Herkenhoff In Upheaval Canyon, ramp-flat thrust faults in are exposed inside or outside the trace of the pre- (1984), stratigraphic repetition due to thrusting ac- the Kayenta Formation form a thrust duplex. The sent rim syncline. counts for the anomalously thick outcrop belt of floor thrust of the duplex detached along the the Kayenta Formation (Figs. 6, 15, and 19). The lower Kayenta Formation contact; the roof thrust INNER CONTRACTIONAL SYSTEMS Kayenta Formation appears to be folded and (poorly exposed and conjectural) is near the top faulted everywhere in its outcrop belt between the of the formation (Figs. 15 and 19). Fault spacing Contractional structures dominate the center inner Navajo Sandstone and Wingate Sandstone decreases and folding associated with the faulting of Upheaval Dome (Fig. 7). Shoemaker and cliffs. As in the outer extensional systems, litho- increases inward. Some thrust faults show nor- Herkenhoff (1984) inferred “convergent dis- logic variations in the fluvially deposited Kayenta mal separation over parts of their trajectories, placement” of rocks. We agree and recognize Formation caused faults to form ramps and flats. particularly those that continue downward into both radial and circumferential shortening, which The typically lenticular depositional units resem- the Wingate Sandstone near the rim syncline was volumetrically balanced by vertical exten- ble the anastomosing fault-bounded bodies, mak- (Fig. 19B, center), suggesting that the lower parts sion. This bulk constriction is expressed by a dif- ing faults difficult to recognize. However, the de- of the faults had multiple displacement histories. ferent structural style in each formation, details of these structures are spectacularly exposed The thrust duplex in the Kayenta Formation scribed in the following in order of increasing where Upheaval Canyon cuts a radial section (Fig. appears to accommodate much more shortening proximity to the dome’s center. 19). From photogeology we infer that this exposed than in the underlying Wingate and overlying

Geological Society of America Bulletin, December 1998 1561 JACKSON ET AL.

A East South West

1600 Elevation (m)

0 100 m 0 400 ft No vertical exaggeration 1300

B West North East

5000 Elevation (ft)

4500

Figure 16. Approximately circular cross section in the inner depression. The right and left ends of the section connect on the east side to form a continuous ring. Trace of section is shown in Figure 7B. The section plane is viewed outward from the map center.

Navajo Sandstones. That anomaly is explicable if Wingate Structures (Figs. 8 and 15). The most complete dog tongue is the duplex formed mainly by shear due to flex- in the northeast corner of the Wingate Sandstone ural slip between the adjoining massive sand- Both radially and circumferentially trending cliffs. Synformal Wingate strata there dip inward stones as they folded. Tanner (1992) described folds have been mapped in the Wingate Sand- to levels far below the contact with the Chinle smaller versions of such duplexes, which formed stone cliffs facing the central depression. The ra- Formation on either side of the lobe. The radial on the limbs of larger chevron folds (Fig. 19B, in- dially trending antiforms distort the basal Wingate axis of the dog tongue forms a trough bordered set); his duplexes have smooth roofs, no hanging- Sandstone contact (Figs. 12 and 20) and indicate by radial antiforms. Farther inward, the same wall anticlines above each link thrust, and rela- circumferential shortening. Most intervening ra- Wingate strata reverse steeply upward along a tively small fault offsets. The roof and floor dial synforms have been eroded back to the same circumferential fold axis, like the upward-curled thrusts may both continue beyond the flexural- encircling cliff as the radial antiforms and are tip of a tongue (Fig. 20C). slip duplex. This type of duplex evolves where partly hidden by talus (Figs. 8 and 20B). A few ra- Wingate strata in the tip of the best-exposed dog thrust propagation was resisted by facies or thick- dial synforms are less eroded and retain a com- tongue appear to onlap the basal Wingate Sand- ness changes, or by transfer zones between faults plex three-dimensional curvature. For brevity, we stone contact (Fig. 20C). The basal Wingate Sand- propagating on different movement horizons informally refer to these doubly curved synformal stone contact in the dog tongues is also discordant (Cruikshank et al., 1991). Thus, the Kayenta radial lobes as “dog tongues” (name suggested by to Chinle Formation bedding (Fig. 20C) and to a thrust duplex may be caused as much by upward Rudy Kopf, 1992, personal commun.). bedding-parallel cleavage in folded Chinle Forma- flexure of the layered succession as by inward The dog tongues preserve the lowest and most tion siltstone and shale. The contact is typically contraction. inward exposures of the Wingate Sandstone marked by rubble. We interpret this doubly discor-

1562 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

A Inward verging extension Pan 11 View to NE (mirrored)

Extension = late syn 0 100 m or post Jk 0 400 ft Extension = syn or post Jk

Pan 10 ? View to SSE

0 50 m

0 200 ft Pan 6

Holeman Alcove View to NE Extension = syn or post Jk

0 50 m

0 200 ft

Pan 5 View to WNW (mirrored) Pan 4 Extension = syn Jk Alcove Spring View to NW Shale diapir Extension = syn to post Jk

0 50 m 0 50 m 0 200 ft 0 200 ft

Navajo (Jn) Kayenta (Jk) Wingate (Jw) Chinle (TRC )

Figure 17. Panoramas (Pan) traced from photographs of inward-verging extensional fault systems encircling Upheaval Dome. Panorama locations and view directions are shown in Figure 6. The view direction of panoramas marked “mirrored” have been reversed from the actual view so that the inward direction is consistent throughout the figure. Scales are approximate because of perspective. dant contact as a salt weld (see reconstruction sec- posed upper Wingate strata are deformed into up- of Wingate Sandstone exposures (Figs. 20B and tion following). Just below the folds, the Chinle right, nearly isoclinal folds that trend circumfer- 21). These folds do not appear to affect the spo- Formation appears to be relatively undeformed, entially (Fig. 20C) and are cut by steep reverse radically exposed basal Wingate Sandstone con- but exposures are poor in quality. faults. Such folds are well exposed on the eastern tact. Dips in much of the lower Wingate Sand- At the upper, outer end of the dog tongues, ex- side of the central depression near the inner edge stone and upper Chinle Formation are typically

Geological Society of America Bulletin, December 1998 1563 JACKSON ET AL. much less than the subvertical orientations ob- B served in the upper Wingate Sandstone. As with the radial antiforms and synforms described here, Pan 3 View to WNW it is difficult to envision a well-lithified Wingate Sandstone folding internally in this extreme, Extension = syn or end Jk strongly discordant manner. Compared with the intensely faulted Kayenta Formation, few visible faults cut the underlying Wingate Sandstone. The Wingate faults strike radially and dip steeply (Fig. 8). They tend to cut through the radial antiforms and along the boundaries of the dog tongues. The radial faults ? ? can be traced outward into the Kayenta Forma- 0 100 m ? tion, where they commonly curve into oblique or circumferential orientations. 0 400 ft Deformation along the inner Wingate Sand- Figure 17. (Continued). stone cliffs thus indicates both radial and circumferential shortening induced by radially inward movement. Farther inward, the circumferential has the most regular structure of the interior de- white sandstone, presumably derived from the un- shortening in the older exposed units dominates pression. The hanging walls moved in a counter- derlying White Rim Sandstone of the Cutler the strain, as described in the following. clockwise direction and imbricated the clockwise- Group. Other authors have described the White dipping Moenkopi strata (Figs. 7 and 16). We liken Rim outcrops as coherently emplaced blocks Chinle Deformation in Central Depression that movement to the closing of the leaves of a (McKnight, 1940; Mattox, 1968). However, the camera diaphragm, where circumferential short- main lenticular dikes anastomose, branch into Most of the Chinle Formation in the inner de- ening increases inward. No marker lines are avail- veins, and are discordant with country rock. All pression crops out on inward-facing slopes below able to determine the strike component of slip on these features indicate that the dikes are intrusive, the Wingate Sandstone cliffs (Fig. 6). The slopes these thrusts. The left-lateral separation may have as inferred by Fiero (1958), Shoemaker and are largely covered with talus exposing only small been derived all or in part from dip slip. Two of Herkenhoff (1984), and Kriens et al. (1997). In windows of the Chinle Formation. The resistant these thrusts continue outward as steep faults cut- places, the sandstone is laminated but is generally Moss Back Member (basal Chinle) and Black ting upward through the Wingate Sandstone. To- massive. The dike thickness is generally less than Ledge (mid-Chinle) sandy conglomerates crop ward the center, the thrusts merge, die out, lose de- 15 m but as much as 30 m. Lengths are typically out as ledges typically segmented by thrusts and finition where parallel lower Moenkopi strata are less than 70 m, but the long dike complex in the reverse faults. Because most of the Chinle Forma- on both sides, or continue as clastic dikes. center extends more than 400 m. In addition, tion shales are weathered or covered, their inter- The regular radial thrusts in the southwest more-isolated clastic quartzose dikes of unknown nal structure is poorly known. However, structural grade into highly variable structures in the north- source are exposed as far out as the outer part of variation increases toward the center of the inte- eastern half of the interior depression. Structures the thrust duplex in the southern wall of Upheaval rior depression from the relatively uniform out- show more local variation, including small folds, Canyon. Some clastic dikes are sourced by the ward dips below the Wingate Sandstone cliffs short faults, and abrupt attitude changes that re- Moss Back Member conglomerate (D. Bice, (Fig. 7). The innermost outcrops of the Chinle sult in complex, extremely detailed outcrop pat- 1997, personal commun.). Formation are folded and faulted similarly to the terns rather than the radial stripes exposed to the We infer that some of these steep dikes were underlying Moenkopi Formation. southwest (Fig. 22). Thrusts vary in strike and emplaced along faults. In some places, faults vis- vergence. Folds are common and rarely cylindri- ibly connect with the ends of the dikes. Else- Central Moenkopi Formation Constriction cal, varying from conical to irregular, and gener- where, the same rock type is on either side of the ally persist less than 100 m. dike such that an aligned fault would be obscure The Moenkopi Formation occupies most of the Although the structural pattern in the northeast (Fig. 7). Most dikes intrude the lower Moenkopi dome’s core. Structures are expressed well in defies simple characterization, the overall strain Formation, but extend upward into the middle bare, rugged terrane by variations in color and remains constrictional, as shown by subhorizon- and upper Moenkopi Formation in their northern lithology (Fig. 7). The Moenkopi Formation in tal radial and circumferential shortening and sub- outcrops. Quartz cementation makes the dikes the central uplift was radially and circumferen- vertical extension indicated by the presence of much more resistant than the surrounding rocks, tially shortened. A linear cross section (Fig. 15) the central uplift. Crowding from inward move- and they form towering spires and irregular walls captures structures with circumferential strikes, ment of the rocks caused the constriction. The in the center of Upheaval Dome. whereas a subcircular section (Fig. 16) crosses complexity and intensity of the central deforma- The microstructure of the clastic dikes is de- subradial strikes. Attitudes vary greatly over short tion, which has obscured large-scale synsedi- scribed and discussed in Part 3 (Data Repository distances, and structures tend to be irregular and mentary structures, preclude detailed reconstruc- item 9891)1 but can be summarized as follows. discontinuous. The Moenkopi Formation out- tion of the inner deformation history. Our nine thin sections contained abundant mi- crops can be divided into two sectors having different structural styles: converging radial thrusts Dome Center and Clastic Dikes 1GSA Data Repository item 9891, supplemental in the southwest half, and more circumferentially data, is available on request from Documents Secretary, oriented folds and thrusts in the northeast half. Clastic dikes are abundant in the center of Up- GSA, P.O. Box 9140, Boulder, CO 80301. E-mail: The southwestern zone of steep radial thrusts heaval Dome. The dikes are composed of clean [email protected].

1564 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

TABLE 1. EVIDENCE FOR PINCH-OFF AND IMPACT HYPOTHESES last few million years and a smaller modified Observation Pinch-off favored Impact favored crater. Kriens et al. (1997, p. 20 and 26) used a incompatible Compatible Compatible Incompatible crater diameter of 5 km to calculate the rise of the with impact with pinch-off with impact with pinch-off central uplift, and advocated a modified transient Positive evidence crater diameter “close to . . . . the walls of the pres- Circularity X X Central uplift X X ent topographic crater,” which is only 2 km wide. Clastic dikes X X The current central depression of Upheaval Dome Crushed quartz grains X X Inner constrictional zone X X was thought to be entirely erosional in origin by Outer extensional zone X X Shoemaker and Herkenhoff (1984). Radial flaps (dog tongues) X X Table 1 summarizes the evidence for or against Presence of underlying salt X X Gravity and magnetic anomalies X X meteoritic impact and diapiric pinch-off at Up- Contiguous anticline X X heaval Dome. A full discussion of each criterion Nearby salt structures X X to explain our evaluation of the evidence can be Rim syncline X X Rim monocline X X found in Part 3 (see footnote 1). Despite the fact Growth folds X X that Upheaval Dome is far better exposed than Growth faults X X Shifting rim synclines X X any terrestrial impact crater, we consider that the Truncations and channeling X X evidence for impact there is sparse and equivocal. Onlap X X Part 3 contests the evidence for shatter cones, pla- Multiple fracturing and cementation X X Steep zones X X nar deformation structures in clastic dikes, and Outward-verging extension X X ejecta breccia. Kriens et al. (1997, p. 28) con- Volume imbalance X X cluded that the most diagnostic evidence for im- Shatter cones XX Planar microstructures in quartz X X pact (planar deformation features and shatter Ejecta breccia XXcones) were “not well developed.” The scarcity Salt below rim syncline X X of features diagnostic of impact in Upheaval Negative evidence Dome might be ascribed to too much or too little Lack of salt at the surface X X erosion. However, the many apparently missing Lack of nearby piercement diapirs X X Lack of meteoritic material X X features would cover the complete range of shock Lack of melt X X zonation and depths. Thus, it seems implausible Lack of in-situ breccia X X that only a few contestable features have been Lack of shock-metamorphic minerals X X Lack of outer fault terracing X X found in a superbly exposed, high-relief structure Lack of overturned peripheral flap X X containing abundant thick sandstones. In contrast, a wide range of structures (Table 1) attests to the gradual growth of Upheaval Dome crofractures, typically masked by recementation where. That possibility cannot be excluded be- as a salt structure. The strongest argument for di- of authigenic quartz between and in optical con- cause of the similar appearance of the Organ apirism is the stratigraphic evidence indicating tinuity with the shards. Some microfractures are Rock and lower Moenkopi shales. However, if protracted growth of Upheaval Dome over at transgranular and radiating; they must have the Organ Rock Member shale were present, we least 20 m.y., which is incompatible with hyper- formed by cataclasis after or during a late stage of would expect to see much of the overlying velocity impact. dike injection. Electron beam-induced cathodo- White Rim Sandstone—an exceptionally promi- luminescence of our Upheaval Dome sample re- nent and resistant marker—in place rather than VOLUME IMBALANCE veals that the quartz undulatory extinction is as discordant dikes. INDICATES SALT LOSS caused by slightly misoriented grain fragments AND PRIMARY THICKNESS CHANGES separated by microfractures sealed with cement. SUMMARY OF EVIDENCE Cross-cutting quartz veinlets provide evidence FOR IMPACT VERSUS PINCH-OFF The pinch-off and impact hypotheses can be for at least two events of alternating fracturing tested in two ways by comparing selected rock and diagenesis. Multiple deformation is consist- Shoemaker and Herkenhoff (1984) proposed volumes in Upheaval Dome. This section sum- ent with episodic faulting during diapiric pinch- that a meteorite impact near the end of Cretaceous marizes the data presented, together with the off but not with the instantaneous deformation in- or in Paleogene time excavated a transient crater methodology and assumptions, in Part 2 (see duced by an impact. 1.3 to 1.4 km deep. That became instantly modi- footnote 1). The intruded lower Moenkopi Formation in fied by gravity to a final diameter of 8 to 9 km, First, we examined the variation in elevation of the center dips steeply or is overturned. Stratal forming Upheaval Dome. Evidence they cited to the base Wingate Sandstone horizon relative to its strikes are grossly similar to those of nearby support that hypothesis included the circular dim- regional elevation. That variation includes the de- dikes, but discordances are common. Attitudes pled shape, inward-verging listric normal faults pressed rim syncline and the elevated central in the Moenkopi Formation can vary abruptly near the periphery, inward-verging thrust faults mound (the center of which is eroded). The vol- and nonsystematically near the larger dikes. near the center, folds plunging radially outward ume of rock depressed below the base Wingate Mattox (1968) and Huntoon and Shoemaker from the center, crushed quartz grains in clastic Sandstone regional datum is 0.74 km3, compared (1995) identified some of the dark brownish-red dikes, and geophysical anomalies. They thought with only 0.29 km3 to 0.33 km3 above the regional shales in the dome’s core as the Organ Rock that erosion removed 1–2 km thickness of the up- datum. This volumetric mismatch of 220% to Member shale, which underlies the White Rim per part of the crater. Kriens et al. (1997) revised 260% can only be plausibly explained by the loss Sandstone in exposures of the Cutler Group else- these estimates by proposing impact within the of 0.45 km3 to 0.41 km3 salt from Upheaval

Geological Society of America Bulletin, December 1998 1565 JACKSON ET AL.

Dome since the Chinle Formation was deposited. Impact would allow lateral flow of Paradox salt but would entail no loss of salt volume because the salt was not exposed to dissolution. Con- versely, the pinch-off hypothesis requires a con- siderable loss of salt as the emergent diapir and glaciers dissolved. The gross volumetric imbalance therefore supports diapirism and is incompatible with impact.

SALT-TECTONIC HYPOTHESIS: A RECONSTRUCTION

We now attempt to reconstruct the evolution of Upheaval Dome by diapiric pinch-off. Figure 18 shows a seismic example with some similarities to the diapiric structure that we envisage. The pinched-off stem is marked by a vertical sec- ondary salt weld. An erosional section through the necked stem would expose strata steepening inward to form a central uplift, above which the subhorizontal tertiary salt weld represents a salt glacier largely evacuated of salt by the overburden load. The former salt glacier here is strongly asymmetric because the Gulf of Mexico has an extensive slope. In contrast, we envisage a symmetric salt diapir and glacier above Upheaval Dome, now eroded off.

Initiation of Diapirism

Published subsurface data are lacking, except for one well, so the deep structure and early evolution are highly speculative. The sparse and am- biguous evidence for what initiated the inferred diapirism can be accommodated in two hypotheses for the structural evolution: (1) salt tectonics alone, or (2) initiation by impact followed by salt Figure 18. A pinched-off diapir shown by reflection seismic image in the Gulf of Mexico (from tectonics. Hall and Thies, 1995, Fig. 4.3). Our hypothesis is illustrated in Figure 23. Geo- physical anomalies indicate basement uplift or dense rocks below the Paradox salt (see footnote the circular dome did not undergo plane strain: in- stock surrounded by a gentle rim syncline. In 1). One possibility is that emplacement of an ig- ward-moving rocks increased the cross-sectional general, crests of passive stocks are periodically neous intrusion into the salt initiated salt up- area, whereas outward-moving rocks decreased buried but continually break through their veneer welling by arching the overburden and softening the area. Thus, the restoration had to be schematic of overburden to remain close to the depositional the salt by adding heat and water. Alternatively, and was carried out as follows. (1) To emphasize surface. an irregular configuration of basement fault the strains, we ignored compaction. (2) Individual scarps generated by Pennsylvanian to Permian faults were omitted, but progressive thickening Stage 2: End of Chinle Formation Deposition extension might have obstructed southwestward and thinning by strain is schematically portrayed. flow of salt, squeezed ahead of prograding sedi- (3) Strata were unfolded by vertical shear. (4) The At this stage, we envision that the stock was still ments. Any such obstruction would favor local cross section was pinned at both ends, so that the emergent. The gradually thickening overburden upwelling of salt (Ge et al., 1997). Upwelling and overall length remained constant. (5) The diapir would have increased the pressure on the salt local stretching of overburden draped above the diameter approximates the value estimated for the source layer, causing the salt to flow at increasing tilted corner of a basement fault block might ini- time of Chinle Formation deposition in Part 2 (see rates up the emergent stock. The climate was rela- tiate a diapir that evolved into a passive stock dur- footnote 1). tively wet, so much of the salt probably dissolved ing continued sedimentation. as fast as it emerged. However, enough salt sur- The following history (Fig. 23) combines the Stage 1: End of Pennsylvanian Time vived that the stock began to extrude as a salt glac- most internally consistent interpretation of avail- ier, forming a flange that overhung surrounding able data with mechanically reasonable processes. By the end of Honaker Trail Formation depo- strata (for detailed discussions of this type of salt The restoration cannot be area balanced because sition (stage 1), we envisage an emergent, passive tectonics, see Fletcher et al., 1995; Talbot, 1995;

1566 Geological Society of America Bulletin, December 1998 A Figure 19. (A) Photograph and (B) broader interpreted panorama 15 (Pan—traced from photographs) of the Kayenta Formation thrust duplex and the abrupt transition into the adjoining extensional system farther west (left). Both views are northward across Up- heaval Canyon; the dome center is to the right. Note the steepened deformation front of the duplex (extreme right). See Figures 14 or 15 for stratigraphic explanation; one (or more) of the Kayenta Formation EB units is stippled. Scale varies greatly because of perspective, but the Wingate Sandstone (black) is about 80–100 m thick. Inset shows a similar but smaller duplex produced by folding in southwest England (after Tanner, 1992).

Figure 20. Views of dog tongues. Wingate strata in the lobes are ~60 m below the adjoining elevation of their basal contact. (A) Photo- graph looking radially outward at the northeast cliffs of the inner depression.

Figure 21. Northward view of several circumferential, upright, tight to isoclinal folds in upper Wingate and lower Kayenta strata. Note the abrupt transition between the Wingate- Kayenta steep zone in the twin spires (center) and open-folded Kayenta Formation on the right. At the extreme left are the northern cliffs of the inner depression.

Geological Society of America Bulletin, December 1998 1567 West East Dome center

Pan 15

e dg Le ck Bla

Roof thrust

Floor thrust ~1 m B

Figure 19. (Continued).

B CC Jk

Dome center

Jw ?

Upturned TRc tip Inferred Dome salt weld center

Upturned tip of tongue

Upright isoclinal folds, E DD Variably verging thrusts E Flap = future dog tongue

Jw Dome Jk Dome center center Salt flange Jw

Basal TRc flap

Weld at base of TRc dog tongue

Figure 20. (Continued). (B) to (E) idealized diagrams showing (B) radial fold traces on a Wingate horizon, where the central trace defines the dog tongue flanked by two higher antiforms (cf. Fig. 12); (C) radial cross section, and (D–E) schematic restoration of the dog tongue, showing its inferred origin as a flap overlying a flange of glacial salt that was subsequently removed to form a salt weld.

1568 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

Figure 22. Annotated photograph looking north-northeast from the dome center at folds (defined by dotted bedding), imbricated thrust faults (teeth on hanging wall), and clastic dikes. Turret Rock (located in Fig. 7A) is shown by arrows in the center. The larger trees are 2–3 m tall.

Talbot and Alavi, 1996). Chinle Formation strata dipping and outward-dipping Kayenta growth zone suggests that the outer edge of the extruding currently truncated against the basal Wingate faults may have initiated near the hinge lines salt extended well outside the present limits of Sandstone contact in the dog tongues may have of these localized depotroughs. The dog the central depression during Navajo Sandstone originally terminated against the overhanging salt tongues are thought to represent upper Wingate deposition (Figs. 6 and 23). face (Figs. 20E and 23). strata onlapping emergent salt, the presence of Diapir Pinch-Off. Two lines of evidence sug- If primary, the angular truncations sporadi- which prevented lower Wingate strata from ac- gest—but do not prove—that the most plausible cally exposed in the upper Chinle Formation cumulating locally (Figs. 20 and 23). This time of pinch-off was during the extrusion of salt suggest that local tilting took place (e.g., alcoves emergent salt would have been an early aborted that occurred during Navajo Sandstone deposi- B, C, and D). The apparently radial tilts of these phase of glacial extrusion before the main ex- tion. (1) Most of the constrictional deformation in truncations suggest that they were related to trusion during Navajo Sandstone deposition. the Wingate Sandstone appears to have occurred growth of a rim syncline around a dome. The de- Growth folds in the lower Wingate Sandstone before it was lithified, indicating that pinch-off gree of faulting during deposition of the Chinle suggest that the central part of the dome had could not have occurred too late in the geologic Formation is unknown; some of the faults ex- begun to fold radially, indicating that the history. The radial growth folds indicate that wall posed today in the core of the dome may have flanks of the dome were beginning to slide in- rocks may have begun to move radially inward formed during the growth phase of the inferred ward. This process accelerated during final during early deposition of the Wingate Sand- diapir and were reactivated during pinch-off. pinch-off later. stone. (2) Physical models (Jackson et al., 1997; B. C. Vendeville, 1996, personal commun.) indi- Stages 3 and 4: End of Wingate Sandstone to Stage 5: End of Navajo Sandstone Deposition cate that where salt is forced rapidly up and out of End of Kayenta Formation Deposition a feeder stock closing by lateral compression, salt Salt Extrusion. We infer that glacial salt extrudes vigorously during stem pinch-off. Onlap, truncation, and channeling in Wingate spread fastest during deposition of the Navajo The outward migration of steep zones (Fig. 8) and Kayenta strata indicate shifting axes of Sandstone. The inferred glacier reached a width suggests that the dome originally widened up- subsidence and deposition. We deduce shifts in of at least 3 km. Collision between the outward- ward. Pinch-off of the feeder produced different the sites of active salt withdrawal, an erratic spreading glacier and inward-gliding strata is results at different structural levels. For example, pattern typical of growing salt domes (e.g., recorded by the zone of abrupt steepening in the in the narrowest part of the diapir (Moenkopi Seni and Jackson, 1983). Many of the inward- Navajo Sandstone (Figs. 8 and 23). The steep Formation and Cutler Group), pinch-off continued

Geological Society of America Bulletin, December 1998 1569 Stage 6: Post-Navajo Rim Buck Rim Elevation Elevation syncline Central uplift Mesa monocline ft m 6000

5000 1500 Weld 4000

1000 3000

2000 500 Paradox salt 1000 msl 0 0 ms Base salt geometry unknown

Glacial spreading Steepening Stage 5: End Navajo Jn Jk Jw Rc R

Base salt omitted

Outward- Regional verging extension Contraction Inward-verging extension Stage 4: End Kayenta tilt Jk Dog tongue Jw Rc R Weld Pc

Stage 3: End Wingate Salt flangeFuture dog tongue Eroded bulge Jw Rc R

Stage 2: End Chinle Salt flange

Rc R

Stage 1: End Pennsylvanian Passive diapir Rim syncline

Figure 23. Schematic reconstruction of the salt-tectonic evolution of Upheaval Dome.

1570 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME until rocks from opposite sides met at the center, strata below the pinched-off diapir, peripheral inates the periphery of the dome core, but cir- completely closing the feeder during deposition of normal faults, and numerous unconformities. cumferential shortening predominates farther in- the Navajo Sandstone. That resulted in the highly However, because gravity gliding was not per- ward. In the western sector of the dome’s core, evolved constrictional features preserved in the mitted in Lemon’s (1985) physical models, no the hanging walls of radial thrusts moved coun- Moenkopi Formation in the central depression. central zone of contraction formed in them. terclockwise and imbricated the persistently The White Rim Sandstone clastic dikes, which How do inward-moving strata overcome the clockwise-dipping Moenkopi Formation, like the could have been emplaced along Moenkopi outward pressure exerted by the salt stem? The closing of a camera’s leaf diaphragm. Elsewhere Formation faults during the earlier growth phase most likely mechanism for pinch-off is gravity in the dome core, strain is highly variable, but is of the dome, would have been rotated, distorted, gliding of the country rocks down an inward- constrictional overall. and possibly reactivated during pinch-off. dipping top salt surface into the weaker rock 4. In the core of the dome, many steeply dip- The soft-sediment response of the Wingate composing the salt diapir (Fig. 5B). The process ping clastic dikes link with the radial fault sys- Sandstone in the dog tongues also suggests that would be enhanced by salt dissolution. The diapir tems. The lenticular sandstone dikes form a the weight of Wingate strata overburden expelled pinches off as a result of being the partially but- continuous network more than 400 m long, pre- salt from the early, abortive Chinle Formation tressing toe of a radially inward-dipping spread- sumably derived from the underlying White salt flanges at that time. Expulsion produced a ing system. Gravity spreading is driven by the Rim Sandstone. Their intrusive origin is indi- salt weld, juxtaposing suprasalt Wingate strata loss of gravity potential, which results in an ex- cated by branching, anastomosing veins and against subsalt Chinle strata (Figs. 20 and 23). cess volume of rock depressed below regional el- discordance with country rock. The unlithified dog tongues were then strongly evation compared with the volume of rock raised 5. Arguments have been advanced for the folded during constrictional pinch-off. above regional elevation. Part 2 (see footnote 1) formation of Upheaval Dome by either meteoritic Closing of the diapir stem at Cutler Group examines the implication of volume imbalances impact or salt tectonic processes. We think that and Moenkopi Formation levels would have for understanding the process of pinch-off, for evidence favoring meteoritic impact is sparse, be- ended diapiric pinch-off. Higher stratigraphic explaining primary thickening, and for estimat- ing restricted to a lag deposit of cobbles, part of units originally adjoined wider parts of the over- ing the diameter of the vanished diapir. two weakly developed shatter cones, and undocu- hanging diapir, so they did not completely close mented planar structures in quartz. The paucity of off before inward gliding stopped (Fig. 23). Stage 6: Present Day typical impact features in Upheaval Dome might Thus, higher stratigraphic units never developed be ascribed to too much or too little erosion. How- structures such as in the Moenkopi Formation Pinch-off would have ended salt rise, which al- ever, these missing features cover the full range of because they never moved close enough to the lowed Cretaceous strata to bury the allochthonous shock zonation and depths. It seems implausible center to produce the requisite amount of strain. salt sheet. During Tertiary time, Upheaval Dome that so few have been found in a structure that is Pinch-off ended late during, or after, deposition was uplifted and eroded. Because soluble salt was better exposed in three dimensions than any ter- of the Navajo Sandstone. The base of the more easily removed than the surrounding rocks, restrial impact structure. Navajo Sandstone is strongly folded by the rim the present topographic surface may partly reflect 6. We think that the weight of the evidence syncline, which is unlikely to have continued the original shape of the overhanging lower con- strongly favors salt tectonics. Evidence favoring deepening after pinch-off ended. The upper tact of the allochthonous salt. However, outside diapiric pinch-off includes a lack of the following contact of the Navajo Sandstone is no longer the mesa surrounding Upheaval Dome, erosion features associated with hypervelocity impact: preserved. was greater and all strata were deeply dissected. meteoritic material, melt, in situ breccia, shock- Examples of allochthonous salt sheets overly- metamorphic minerals, outer fault terracing, and ing pinched-off stems are common in the Gulf of CONCLUSIONS overturned peripheral flap. Conversely, the pres- Mexico (Wyatt et al., 1993; Diegel et al., 1995; ence of the following features at Upheaval Dome Fletcher et al., 1995; Rowan, 1995) and Red Sea 1. We propose that pinch-off of a salt diapir all favor diapirism: rim syncline, rim monocline, (Heaton et al., 1995) and have also been observed best explains the sedimentary and deformational steep zones in inner limb of rim syncline, out- by us on proprietary seismic data from the North structures at Upheaval Dome. Evidence summa- ward-verging extension, radial synformal flaps Sea, northwest Germany, and west Africa. How- rized here suggests that inward constriction of (dog tongues), underlying mother salt and nearby ever, the mechanism for diapiric pinch-off is un- overburden strata accompanied necking of the salt structures, multiple episodes of microfractur- known. Pinch-off is the core of our hypothesis for stem of a broadly overhanging salt extrusion, ing and sealing, and postemplacement microfrac- Upheaval Dome, yet we are unsure how it occurs subsequently removed by erosion. turing in the clastic dikes. Volume imbalance be- (Fig. 5B). 2. In the rim syncline, the Wingate Sandstone tween uplifted and depressed regions of the dome We do not regard this gap in knowledge as ev- and overlying Kayenta Formation are greatly indicates massive salt loss of at least 0.4 km3 af- idence against the pinch-off hypothesis for the thinned by circumferential extensional faults. ter deposition of the Chinle Formation. following reasons. (1) The lack of detailed me- These low-angle normal faults are either simple The most compelling evidence for diapirism in chanical knowledge reflects a gap in salt tecton- listric faults or series of ramps and flats. Most ex- Upheaval Dome is the wide range of synsedi- ics theory generally rather than an anomaly spe- posed faults slipped during or after deposition of mentary structures spatially restricted to this cir- cific to Upheaval Dome. (2) Three-dimensional the Kayenta Formation. Normal fault vergence cular structure. They include angular truncations, seismic data indicate that the process occurs shows a strong dichotomy: in the north, most onlap surfaces, channeling, growth faults, growth whether or not we understand it. (3) The limited faults transported rock outward from the dome, folds, shale diapirs, and shifting rim synclines. physical modeling of dome pinch-off in brittle whereas in the south, most faults transported rock These signs of synsedimentary deformation lo- country rock (Lemon, 1985) reproduces several inward. calized around the dome indicate gradual growth of the main features observed at Upheaval Dome: 3. In the core of the dome, the bulk strain is over at least 20 m.y. and exclude the possibility a rim syncline that deepened markedly when constrictional, and includes both radial and cir- of geologically instantaneous deformation re- pinch-off began, a central uplift with subvertical cumferential shortening. Radial shortening dom- quired by hypervelocity impact.

Geological Society of America Bulletin, December 1998 1571 JACKSON ET AL.

in western Colorado and eastern Utah: American Associ- 7. Many details of the structural evolution are leum, Petroleo Brasileiro S. A., Phillips Petro- ation of Petroleum Geologists Bulletin, v. 11, p. 111–133. speculative because subsurface data are sparse. leum Company, Saga Petroleum, Shell Oil, Sta- Heaton, R. C., Jackson, M. P. A., Bamahmoud, M., and Nani, We envisage that a passive stock less than 1 km in toil, Texaco Inc., and Total Minatome Corpora- A. S. O., 1995, Superposed Neogene extension, contraction, and salt canopy emplacement in the Yemeni Red diameter and surrounded by a gentle rim syncline tion. Permission to publish was granted by the Sea, in Jackson, M. P. A., Roberts, D. G., and Snelson, S., emerged in Pennsylvanian time. The salt re- Director, Bureau of Economic Geology, and by eds., Salt tectonics: A global perspective: American Asso- mained at or near the surface throughout Permian, the management of Exxon Production Research. ciation of Petroleum Geologists Memoir 65, p. 333–351. Hite, R. J., 1960, Stratigraphy of the saline facies of the Para- Triassic, and Early Jurassic time. Increasing sedi- dox Member of the Hermosa Formation of southeastern mentary load on the Paradox source layer below REFERENCES CITED Utah and southwestern Colorado: Farmington, New Mex- increased the flow rate of salt up the diapir, even- ico, Four Corners Geological Society, Third Field Con- Baars, D. L., 1966, Pre-Pennsylvanian paleotectonics—Key to ference Guidebook, p. 86–89. tually resulting in extrusion of salt glaciers near basin evolution and petroleum occurrences in Paradox Hudec, M. 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1572 Geological Society of America Bulletin, December 1998 STRUCTURE AND EVOLUTION OF UPHEAVAL DOME

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