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

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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 over- hanging diapir of partly extrusive salt was pinched off from its stem and subsequently eroded. Many features support this inference, especially synsedimentary structures that in- dicate 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 exten- sional faults verging both inward (toward the center of the dome) and outward. Near the dome’s core, radial shortening produced con- strictional bulk strain, forming an inward- verging thrust duplex and tight to isoclinal, circumferentially trending folds. Farther in- ward, 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 varia- tion and weathering character define structures well. The middle Moenkopi Formation comprises reddish shale containing layers of fine-grained, yellow, flaggy, calcareous sandstone. This mem- ber 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 bot- tom to top, (1) basal Moss Back Member sand- stone 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 mem- ber 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.
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