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

Thrusting of the Claron Formation, the Bryce Canyon region, Utah ERIK R. LUNDIN Conoco Norway, Inc., P.O. Box 488, 4001 Stavanger, Norway ABSTRACT -80 km east of the east limit of known thrusting parowits and Wahweap Formations (Fig. 2). At in the Sevier orogenic belt, but it is located outcrop scale, however, the unconformable rela- A thrust system at least 40 km long and within the western edge of the region containing tionship is commonly subtle. In the northeastern with as much as 2,000 m of shortening has compression-related structures produced during part of the study area, the Claron Formation lies been mapped in the southern High Plateaus the Late Cretaceous-early Tertiary Laramide conformably on the Upper Cretaceous to Paleo- of Utah, partly within Bryce Canyon National orogeny (Fig. 1). The middle Tertiary Marys- cene(?) Canaan Peak Formation, which in turn Park. Seismic data clearly reveal that the vale volcanic field lies on the north side of the lies unconformably above the Kaiparowits thrusts are of thin-skinned, ramp-flat style, area. Formation. Well data from the northeastern part soling out in evaporite-rich layers of the Ju- The thrusts are unusual in two aspects. First, of the area disclose that the Canaan Peak For- rassic Carmel Formation. Mapping discloses the constrictional sense is approximately north- mation and another formation of local extent, that large portions of the thrusts terminate south, which is anomalous for both the Sevier the Paleocene(?) Pine Hollow Formation, lie upward in fault-propagated folds, marked by and Laramide orogenies. Second, the thrusts in- with angular unconformity above the Upper vertical to overturned beds. Spaced cleavage volve the Claron Formation, which generally Cretaceous Dakota Formation. reveals that some shortening is accommo- rests with angular unconformity on rocks that With the exception of a small outcrop of un- dated by pressure dissolution. Shallow drill- are involved in compressional deformation in named conglomeratic sandstone in Little ing for coal provides data suggestive of more the region (Gregory and Moore, 1931; Gregory Henderson Canyon (Figs. 2, 3), tentatively as- than one episode of thrusting. An older event and Williams, 1947; Gregory, 1950, 1951; signed to the middle Tertiary, there are no strata predates deposition of the Upper Cretaceous Threet, 1952,1963; Mackin, 1954,1960; Cook, that depositionally overlie the Claron Formation and Paleocene(?) Canaan Peak Formation. 1957; Blank, 1959; Bowers, 1972; Hintze, within the map boundaries. Near the center of This deformation is presumably related, in the 1983). The purpose of this study is to provide the map area, brecciated volcanic rocks and broadest sense of the term, to the Sevier documentation of these anomalous aspects by middle Tertiary volcanic rocks of the Mount orogeny. The younger thrusting is south- investigating the geometry, kinematics, extent, Dutton Formation are in contact with the vergent and involves rocks as young as the and age of the thrusts and related structures and, Claron Formation. The nature of the contact is lower to middle Eocene, white limestone if possible, to interpret their tectonic signifi- difficult to determine, but Rowley and others member of the Claron Formation, a forma- cance. To accomplish this, the area embracing (1987) interpreted it to be the eroded lower portion that is considered post-orogenic by most the thrusts was mapped at 1:24,000 scale, well tion of a listric normal fault or possibly a geologists. The upper age of the younger and seismic reflection data (provided by Chev- denudation-type normal fault. The writer has event is not constrained in time, and its tec- ron USA) were analyzed, stratigraphic markers adopted their interpretation but has not been tonic significance remains poorly understood. needed to evaluate the amount of shortening able to independently confirm it. Also in fault were identified and mapped, and the thrusts and contact against the Claron Formation are INTRODUCTION related structures were studied in detail. Quaternary basalts along the Sevier fault zone (Fig. 2). Thrust structures of compressional origin STRATIGRAPHY were recently discovered in the Bryce Canyon Claron Formation area of the southern High Plateaus of Utah General Relations (Davis and Krantz, 1986; G. H. Davis and oth- Bowers (1972) subdivided the Wasatch For- ers, unpub. data). The Bryce Canyon area is Rocks within the map area consist of sedi- mation (Claron Formation in this paper) in the located in the late Cenozoic structural and phys- mentary formations of Late Cretaceous age, sed- Table Cliffs Plateau into three members, from iographic transition zone (High Plateaus), be- imentary strata of the lower to middle Tertiary oldest to youngest: the pink limestone member, tween the Basin and Range on the west and the Claron Formation, volcanic rocks and minor in- the white limestone member, and the variegated main part of the Colorado Plateau on the east. terbedded sedimentary rocks of middle Tertiary sandstone member. These will be referred to Major north- to northeast-striking normal faults age, and sedimentary and volcanic rocks of here as the pink, white, and variegated Claron, form the boundaries of the broad and gently Quaternary age (Fig. 2). respectively. tilted High Plateaus. The thrusting in the Bryce In the map area, the Claron Formation gener- The pink Claron occurs extensively Canyon area appears to be confined between the ally lies disconformably over (from youngest to throughout the study area. It is the colorful unit Paunsaugunt fault zone on the east and the Sev- oldest) the Kaiparowits, Wahweap, and Straight responsible for most of the famous erosional fea- ier fault zone on the west—two of the major late Cliffs Formations. Regionally, the unconformity tures of Bryce Canyon National Park and the Cenozoic normal fault zones. The area is located is obvious, in places cutting out the entire Kai- distinctive Pink Cliffs of the region. Lithologi- Geological Society of America Bulletin, v. 101, p. 1038-1050, 12 figs., August 1989. 1038 THRUSTING OF THE CLARON FORMATION, UTAH 1039 crops of the variegated Claron do not reveal whether this member was affected by the thrusting, or whether it was deposited on already de- formed pink and white Claron. Age of the Claron Formation The age of the Claron Formation is poorly known, and thus the age of the thrust-related deformation to be described cannot be firmly established. The pink Claron was considered to be Eocene by Gregory (1951). Subsequent to Gregory's work, however, other workers have suggested that the member may be Paleocene to lower Eocene(?) (Bowers, 1972), or perhaps even as old as latest Cretaceous (Anderson and Rowley, 1975; Rowley and others, 1978b; Rowley and others, 1979). The age of the white Claron would appear to be more tightly constrained. Gregory (1950) reported upper Eocene fossils from the uppermost part of the Claron Formation at Cedar Breaks National Monument. Fresh-water mol- lusks collected by Bowers from the white Claron in the Table Cliffs Plateau yielded an age of "early to middle Eocene but not earliest Eo- cene" (Bowers, 1972, p. 26). The variegated Claron contains no diagnostic age indicators, but Bowers (1972) reported that some of the sandstone beds of the member appear to be tuffaceous. This member is overlain, just north of the study area, by ash-flow tuff of the Wah Wah Springs Formation (Rowley, 1968; Rowley and others, 1987), dated by Best and Grant (1987) at 30 m.y. (middle Oligo- cene). Presently, the lower to middle Eocene white Claron is the youngest known formation involved in the Bryce Canyon thrusting. STRUCTURAL GEOLOGY Overview Figure 1. Index map of the southern High Plateaus of Utah. Shaded portion indicates area of study. Insert map displays generalized regional geologic relationships. Abbreviations: The major thrust faults in the area are the BP, Bryce Point; RI, Rubys Inn; STB, eastern leading edge of Sevier thrust belt, as recognized Rubys Inn and Johns Valley thrusts (Fig. 2). by previous work. The east-striking Rubys Inn thrust extends from the Sevier to the Paunsaugunt fault zone, a dis- tance of ~20 km. For convenience of descrip- tion, the Rubys Inn thrust is subdivided into cally, the member consists mostly of thick- cliff-forming rim cap in Bryce Canyon. It is three segments: the Highway 12 segment, Ahl- bedded to massive, clastic limestone, inter- composed mostly of massive limestone with strom Hollow segment, and Hillsdale Canyon bedded with marl, calcareous siltstone and minor interbeds of siltstone and mudstone segment (Fig. 2). On the basis of seismic pro- sandstone, and some conglomerate lenses. The (Bowers, 1972). The uppermost variegated files, the northeast-striking Johns Valley thrust base of the formation is marked in many places Claron is an inconspicuous, slope-forming extends at least 10 km beyond the northeastern by a distinctive 5- to 10-m-thick pebble to cob- member, exposed only in Johns Valley of the map boundary, giving it a minimum total length ble conglomerate (Gregory, 1951; Bowers, map area (Fig. 2). This member is made up of of 17 km. 1972). Recent investigations indicate that the mostly varicolored, fine-grained, friable sand- Other faults in the area of possible compres- pink Claron is largely a sequence of stacked stone, siltstone, and mudstone (Bowers, 1972). sional origin are the east-striking Pine Hills fault paleo-soil horizons (Mullett and others, 1988). Of the three members, only the lower two can (known only from one seismic profile) and the The overlying white Claron is the characteristic unequivocally be shown to be thrusted. The out- inferred northeast-striking Elbow fault (Fig. 2). Figure 2. Generalized geologic map of the Bryce Canyon region. Rubys Inn thrust is subdivided into the Hillsdale Canyon (HC), the Ahlstrom Hollow (AH), and the Highway 12 (H-12) segments.

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