VOLUME 38 NUMBER 2

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Paul K. Sims and Holly J. Stein

The Colorado province is a major component of a >1000-km-wide belt of Paleoproterozoic ocean-arc rocks that occupies the southwestern United States. Known as the Transcontinental Proterozoic provinces, this belt of largely juvenile rocks was added to the southern margin of the North American craton during the interval 1.8–1.70 Ga by convergent tectonism along the Cheyenne belt. A growing body of data suggests that these rocks were deposited, at least locally, on older rocks of earliest Proterozoic and Archean ages, probably correlative with the Trans-Hudson and Penokean orogens.

The volcano-plutonic and associated sedimentary rocks of the Colorado province record two major, regional orogenies: (1) an older, protracted thermotectonic episode (1.78–1.70 Ga), named the Colorado orogeny, which involved mainly amphibolite-facies metamorphism during piecemeal assembly of various ocean-arc terranes; and (2) a younger, Mesoproterozoic intra-continental orogeny, named the Berthoud orogeny, which involved associated regional heating and A-type plutonism chiefly during the interval 1.45–1.40 Ga. The term "Colorado orogeny" is proposed for the regional Paleoproterozoic dynamothermal deformation. To distinguish differences in the geodynamics and ages of deformation and facilitate comparisons from place to place, type localities are proposed for separate phases or events of the Colorado orogeny. The central Front Range is suggested as a type area for the Berthoud orogeny, because the character and orientation of structures there are readily distinguished from those of the older Colorado orogeny.

Key Words: Proterozoic • Colorado province • Southern Rocky Mountains • Trans-Hudson orogeny • Cheyenne belt • A-type granite • shear zones • aeromagnetic anomalies • strike-slip faulting • reactivation • Colorado mineral belt

Donald S. Stone

The northwest-striking, northeast-directed Piney Creek thrust on the northeastern flank of the Bighorn Mountains in north-central Wyoming is traditionally interpreted as abruptly terminated on the northwest by the transverse Granite Ridge fault zone (new name). The structure of the mountain front north of this fault generally has been described as an unfaulted monocline. However, seismic-reflection, gravity, and borehole data, and the requirement of reasonable shortening balance along the mountain front indicate subsurface continuity of the (buried) Piney Creek thrust northward from the Granite Ridge fault zone—perhaps to the Tongue River lineament near the Montana border.

Based on these subsurface data, the Granite Ridge fault is described here as a true tear fault confined to the hanging wall of an uninterrupted, 30°-dipping Piney Creek thrust, segmenting hanging-wall thrust displacement between the Piney Creek block on the south and Walker Mountain block on the north. Slip on the Granite Ridge tear fault is expressed at the surface along the eastern margin of the Bighorn Mountains, west of Story, Wyoming, as a resequent fault-line scarp with 2000 ft (600 m) of relief, and with 3.3 mi (5.3 km) of left separation of steeply dipping, Paleozoic rocks. Based on measurement between piercing points produced by the lines of intersection of the steeply, east-dipping Cambrian-Precambrian unconformity with the Piney Creek thrust surface along either side of the Granite Ridge tear fault, a (reverse) left-oblique slip of 15,000 ft (4500 m) is measured on this tear fault zone. Thus, 30,000 ft (9100 m) of net slip on the Piney Creek thrust in the Piney Creek block decreases to 15,000 ft (4500 m) of net slip in the Walker Mountain block, and slip on the thrust further declines northward. The described Granite Ridge tear fault–Piney Creek thrust geometry may provide a useful model in the investigation of similar fault-offset mountain fronts within the Laramide foreland province of the Middle Rocky Mountains.

Key Words: Bighorn Mountains • basement-involved uplift • fault-related folding • tear fault • piercing points • left-oblique slip • Piney Creek thrust • Beartooth thrust • seismic-reflection profiles • shortening disparity • Laramide contraction

Edward C. Beutner and Steven P. DiBenedetto

The Blacktail anticline in northwestern Wyoming was interpreted by Pierce and Nelson (1973) to be a pop-up anticline which rose into a pull-apart chasm created during an early stage of movement of the Heart Mountain detachment fault (HMD). The Crandall Conglomerate was thought to be the sedimentary fill of that chasm. Alternatively, Hauge (1990) suggested that the channel formed by normal erosional processes and that the Blacktail structure is a late Laramide thrust-fold localized along this channel. Based upon new field observations, we interpret the Blacktail structure to be a northeast-verging, early Laramide thrust-fold, similar to nearby structures. All Crandall Conglomerate exposures in the HMD footwall are always just northeast of the crest of the thrust-fold, a geometry which suggests that a canyon incised in a plateau surface was initially localized along fractured rock on the crest of the thrust-fold. As the canyon eroded vertically, it departed with depth from the southwest-inclined thrust-fold. Crandall Conglomerate deposited in this canyon and the adjacent thrust-fold were then decapitated by movement of the HMD. This history negates the need for an early period of HMD movement.

Key Words: Heart Mountain detachment fault • Crandall Conglomerate • Blacktail thrust-fold • Beartooth Mountains • Little Bear Creek fault • Laramide orogeny

Pennilyn Higgins

A succession of fossil localities from the Hanna Basin, south-central Wyoming, brackets the boundary between Torrejonian and Tiffanian North American Land Mammal "Ages" (NALMAs). Unusually high rates of deposition in mid-Paleocene time in the Hanna Basin led to a greatly expanded section relative to classic mid-Paleocene sedimentary accumulations. Outcrops of the Hanna Formation, in an area of badlands in the northeast corner of the Hanna Basin called "The Breaks," yield abundant vertebrate fossils. Mammalian fossils from a 550 m-thick interval in The Breaks document the latest Torrejonian (To3) though middle Tiffanian (Ti3) NALMAs. A 55 m-thick overlap zone between faunas typical elsewhere of To3 or earliest Tiffanian (Ti1) lies centrally within this interval.

The entire overlap zone in The Breaks represents the earliest parts of Ti1 as based upon presence of Plesiadapis praecursor and Nannodectes intermedius, index taxa for the Tiffanian. This does not affect the traditional definition of the To–Ti boundary. It does, however, extend ranges of several mammals typically considered exclusively Torrejonian into early Tiffanian time.

The mammalian fauna from The Breaks is one of the most diverse earliest Tiffanian faunas yet described, with 72 species of mammals recognized. The high diversity facilitates correlation with less diverse faunas of western North America. This is especially valuable for faunas of late Torrejonian or early Tiffanian age that lack critical index taxa (i.e., members of the Plesiadapidae) necessary for assigning a definitive age to the fauna. Furthermore, recognition of an overlap zone, the fauna of which is formally defined as The Breaks local fauna, within earliest parts of Ti1 provides greater age resolution for faunas near the Torrejonian–Tiffanian boundary. The greater detail about first and last appearances of mammalian species near the To–Ti boundary has resulted in complications to biostratigraphic zonation that have been undetectable elsewhere in thinner sections.

Key Words: Torrejonian • Tiffanian • North American Land Mammal "Ages," • Hanna Basin • Wyoming • vertebrate paleontology

Arthur W. Snoke

Department of Geology and Geophysics, University of Wyoming,Laramie, WY 82071-3006, U.S.A.
email: snoke@uwyo.edu