Introduction Evaluation Prospects










Mesozoic(?) Folding

Large-scale, broad and open to locally isoclinal folds of Mesozoic or younger age can be found within the Pinon, Adobe, Sulphur Spring, northern Antelope, Monitor and Pancake Ranges, Buck Mountain-Bald Mountain, northern Cherry Creek, Medicine, and Maverick Springs Ranges, Diamond, Fish Creek, Butte, southern Pequop, Dolly Varden, and Leach Mountains, and in the Currie Hills area. Precise dating of these folds is made difficult by the lack of Mesozoic sediments over large areas, and by Tertiary volcanic and sedimentary cover. It is possible that more than one Mesozoic folding event is represented across the area, perhaps related to both the Golconda and Sevier deformational events. Much of the folding in the eastern portion of the area is in the upper plate of low-angle normal faults which appear to be mainly Middle Tertiary in age. The upper or youngest age limit for this folding is essentially unconstrained and may at least in part be related to Tertiary extensional processes. The following paragraphs discuss some of these folds.

The Diamond Mountains form a north-south trending, eastward verging, asymmetric syncline which deforms the Upper Permian Garden Valley and Cretaceous Newark Canyon Formations, indicating post-Early Cretaceous folding. To the south near Eureka in the northern Fish Creek Range, Mississippian through Cretaceous sediments are folded around north-northwest-trending axes, again indicating post-Cretaceous folding (Nolan and others, 1974). In the northern Simpson Park Range the Cretaceous Newark Canyon Formation is gently to tightly folded indicating post-Cretaceous deformation (Stewart and McKee, 1977). In the Portuguese Mountain area of the Pancake Range, Mississippian through Cretaceous rocks are folded into a northeast-trending overturned syncline suggesting post-Cretaceous folding (Kleinhampl and Ziony, 1985).

In the Carlin-Pinon area large and small-scale, north-northeast-trending folds in Mississippian rocks are overturned to the west and northwest. These folds are older than Late Cretaceous and younger than Late Permian as bracketed by deformed Permian sediments and undeformed Cretaceous Newark Canyon Formation (Smith and Ketner, 1977). A major north-northeast-trending, locally overturned syncline in the Adobe Range and to the south in the Cortez Mountains deforms Triassic sediments and may deform Upper Jurassic Pony Trail Group rocks (Ketner and Smith, 1974). These east verging folds indicate post-Early Triassic and perhaps post-Late Jurassic deformation.

In the Buck Mountain-Bald Mountain area to the east of the Diamond Mountains (Rigby, 1960) rocks as young as the Pennsylvanian and Permian Ely and Riepe Spring Limestones are in north-northwest-trending asymmetric folds, indicating post-Early Permian deformation. In the central Egan Range near Ely, northwest-trending anticlines and synclines that are locally overturned and west verging deform the Lower Permian Loray and Pequop Formations indicating post Early-Permian deformation. In the Butte Mountains (Douglass, 1960), Permian through Triassic sediments are exposed in a broad north-northwest-trending syncline with adjacent anticlines indicating post-Early Triassic deformation.

In the Pequop (Thorman, 1970) and Leach Mountains (LeCompte, 1978; Martindale, 1981) and in the Currie Hills area (Nelson, 1956; Snelson, 1955), folds are oriented northeast-east rather than northwest. In the southern Pequop and Leach Mountains these large-scale, locally overturned folds deform Permian through Early Triassic rocks indicating west directed post-Early Triassic deformation. In the Currie Hills northeast-trending folds are overturned to the west and northwest and affect rocks as young as the Jurassic Nugget Sandstone, indicating post-Early Jurassic deformation.

Any or all of these folds may be the result of Tertiary deformation associated with low-angle normal faulting, or Mesozoic Sevier and Golconda thrusting, or any combination of these events in a given area, since the upper limit of deformation is essentially unconstrained. The change in fold vergence from east in the western portion of the evaluation area, to west in the eastern portion suggests at least two separate deformational events. The east vergence in the Adobe and Diamond Mountains area is consistent with east-directed Golconda compression while west vergence in the Currie Hills and Pequop Mountains is consistent with west-directed transport during Sevier compression.

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Last modified: 09/12/06