Kurt N. Constenius

Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt

The Cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle Eocene to early Miocene (ca. 49–20 Ma) episode of crustal extension. The sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern Canada to central Utah. Extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy, structural relief, and sole faults of preexisting thrust-fold structures. The sole faults dip 3°–6° west above an undeformed Precambrian crystalline basement and accommodated tectonic transport of a thick (up to 20+ km) eastward-tapering hanging wall during regimes of crustal shortening and extension. The chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces. Dated movement on two extensionally reactivated thrusts, the Lewis thrust of northwest Montana and southeast British Columbia and the Medicine Butte thrust of southwest Wyoming and northeast Utah, suggests that the hiatus between the end of crustal shortening in the early or early middle Eocene and the start of extension in the early middle Eocene was brief. Lateral spreading and extensional basin formation in the Cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism. Conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late Paleogene Cordilleran orogenic wedge is accomplished by applying the extensional-wedge Coulomb critical-taper model. The rapid drop in North America–Pacific plate convergence rate and/or steepening of the subducted oceanic slab at ca. 50 Ma resulted in a large reduction in east-west horizontal compressive stress in the Cordillera. As a result, the Cordilleran orogenic wedge was left unsupported, and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca. 20 Ma. Subsequently, crustal extension and magmatism during the Basin and Range event (ca. 17–0 Ma) overprinted much of this earlier phase of extension.

Tectonic control on coarse-grained foreland-basin sequences: An example from the Cordilleran foreland basin, Utah

Newly released reflection seismic and borehole data, combined with sedimentological, provenance, and biostratigraphic data from Upper Cretaceous–Paleocene strata in the proximal part of the Cordilleran foreland-basin system in Utah, establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin. During Campanian time, coarse-grained sand and gravel were derived from the internally shortening Charleston-Nebo salient of the Sevier thrust belt. A rapid, regional Campanian progradational event in the distal foreland basin (>200 km from the thrust belt in <8 m.y.) can be tied directly to active thrust-generated growth structures and an influx of quartzose detritus derived from the Charleston-Nebo salient. Eustatic sea-level variation exerted a minimal role in sequence progradation.

Tectonic evolution of the Jurassic–Cretaceous Great Valley forearc, California: Implications for the Franciscan thrust-wedge hypothesis

Interpretation of seismic reflection data and restoration of depositional geometries of Cretaceous forearc basin strata in the northwest Great Valley of California provide important controls on structural reconstructions of the western margin of the Sacramento Valley and northern Coast Ranges. Monoclinal eastward dips of Great Valley Group strata and fault systems striking northwest-southeast, which are features proposed as evidence for a west-dipping blind Great Valley–Franciscan sole thrust and related backthrusts, instead are expressions of bedding geometry that resulted from folding of the Paskenta and related synsedimentary normal faults, depositional onlap, and a major structural-stratigraphic discontinuity. The discontinuity separates east-dipping Aptian and younger Great Valley Group strata from beds of lower Great Valley Group and Coast Range ophiolite that were deformed and erosionally or structurally truncated by mid-Cretaceous time. Dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient Great Valley forearc, but is also observed in modern forearc basins. Advocates of the Franciscan thrust-wedge model have also proposed that west-dipping, shingled patterns of seismic events imaged beneath the Sacramento Valley are imbricate thrust slices of the Great Valley Group. This hypothesis, however, is incompatible with borehole, potential-field, and seismic-refraction data that characterize the Sacramento Valley basement as ophiolitic. Seaward-dipping reflections in the ophiolitic basement of the Sacramento Valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges. Thus, late-stage tectonic mechanisms are not required to interpret a forearc that owes much of its present-day bedding architecture to processes coeval with deposition. Thickening of the Great Valley Group stratigraphic section (Valanginian–Turonian) in the hanging walls of the Paskenta, Elder Creek, and Cold Fork fault zones, combined with attenuation or complete omission of preextensional units (including the Coast Range ophiolite) and geometric evidence based on seismic reconstructions, suggest that these faults are Jurassic–Cretaceous normal faults that developed in a submarine setting. Down-structure views of the Great Valley outcrop belt simplify otherwise complex map relations and portray the Paskenta and related faults as half-graben bounding faults that accommodated significant northwestward tectonic transport of hanging-wall rocks. It is significant that these faults sole into the Coast Range fault, an enigmatic forearc structure that juxtaposes rocks of the Franciscan Complex (blueschists) with rocks of the Coast Range ophiolite and Great Valley Group that have sustained only zeolite-grade metamorphism. Discovery of Jurassic–Cretaceous crustal-scale extension in the Great Valley forearc suggests that a significant part of Coast Range fault-related attenuation developed early in the history of the subduction complex.

Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary petrologic data illuminate the history of development of the North American Cordilleran foreland basin system and adjacent thrust belt from Middle Jurassic through Eocene time in northwestern Montana. The oldest deposits in the foreland basin system consist of relatively thin, regionally tabular deposits of the marine Ellis Group and fluvial-estuarine Morrison Formation, which accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and sandstone modal petrographic data indicate that by ca. 170 Ma, miogeoclinal strata were being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt. The Jurassic deposits probably accumulated in the distal, back-bulge depozone of an early foreland basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A regional unconformity separates the Jurassic strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted as a combined effect of forebulge migration, decreased dynamic subsidence, and eustatic sea-level fall. The late Barremian(?)-early Albian Kootenai Formation is the first unit that consistently thickens westward, as would be expected in a foredeep depozone. The subsidence curve at this time begins to show the convex-upward pattern characteristic of foredeeps. By Albian time, the fold-and-thrust belt had propagated to the east and incorporated Proterozoic rocks of the Belt Supergroup, as indicated by sandstone compositions, detrital zircon ages in the Blackleaf Formation, and by crosscutting relationships in thrust sheets involving Belt Supergroup rocks in the thrust belt. A major episode of marine inundation and black shale deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian, and was followed by a regressive succession represented by the Upper Santonian–mid-Campanian Telegraph Creek, Virgelle, and Two Medicine Formations. Provenance data do not resolve the timing of individual thrust displacements during Cenomanian–early Campanian time. The Upper Campanian Bearpaw Formation represents the last major marine inundation in the foreland basin. By latest Campanian time, a major episode of slip on the Lewis thrust system had commenced, as recorded in the foreland by the Willow Creek and St. Mary River Formations in the proximal foredeep depozone. The final stage in the evolution of the Cordilleran fold-and-thrust belt and foreland basin system is recorded by the Paleocene–early Eocene Fort Union and Wasatch Formations, which were preserved in the distal foreland region. Regional extensional faulting along the fold-and-thrust belt began during the middle Eocene. The results presented here enable the establishment of links between previous geological work in Canada and the better known parts of the Cordilleran foreland basin in the United States.

Fault and fault-rock characteristics associated with Cenozoic extension and core-complex evolution in the Catalina-Rincon region, southeastern Arizona

Cenozoic extensional deformation in southern Arizona included (1) a Neogene phase of Basin and Range deformation recorded by high-angle normal faults and (2) an earlier phase of detachment faulting and brittle-ductile crustal shearing associated with tectonic denudation of metamorphic core complexes. In the Catalina-Rincon region, exposed fault zones produced at different crustal depths during successive extensional episodes display differing fault geometries and types of fault rocks formed during progressive crustal extension. Detachment faults are associated with both mylonites produced by ductile shear and cataclasites produced by brittle shear. Younger faults formed at shallower depths are associated with less intense cataclastic deformation and with brittle fracturing that includes transtensile phenomena at the shallowest crustal levels represented. Qualitative measures of net displacement along individual fault zones are provided by (1) the nature of contrasts among successively overprinted fabrics and internal structures in the footwall and (2) the degree of contrast between fabrics and structures of footwall and hanging-wall rocks. Footwalls of the oldest structures display varied brittle overprints of ductile fabrics and are juxtaposed across gouge zones along detachment surfaces with stratal successions cut by multiple brittle shear surfaces. Footwalls of younger structures formed at shallower depths display multiple generations of cataclastic features, including brecciation of variable intensity and cataclasite dikes, but are juxtaposed against hanging-wall strata that are only moderately deformed by subsidiary faults. The shallowest fault zones lack either structural overprints in their footwalls or any significant contrasts between footwall and hanging-wall deformation. Exposures of mid-crustal rocks within the core complexes reflect successive exhumation and uplift of fault footwalls during sequential episodes of deformation. The present high elevation of mylonitic rocks in the Catalina-Rincon metamorphic core complex reflects dip slip and isostatic footwall flexure during Basin and Range deformation as well as tectonic denudation during detachment faulting. Net uplift of core rocks resulted from multiple phases of deformation.

Basement complexes in the Wasatch fault, Utah, provide new limits on crustal accretion

New and reinterpreted isotopic data for crystalline rocks ex- posed in the Wasatch Range require a reevaluation of Precambrian crustal boundaries in Utah. Crystalline rocks of the Santaquin Com- plex underwent metamorphism prior to ca. 1670 Ma, consistent with Sr and Nd isotope data. Mafic to intermediate rocks have major element, trace element, and isotope ratios indicative of derivation in an arc accreted to the Archean craton in Proterozoic time, requiring the crustal suture to be north of the Santaquin Complex. Farther north, the Farmington Canyon Complex has been considered Ar- chean based on published Nd model ages and discordant U/Pb zir- con ages. However, Nd model ages and zircons could be inherited from sedimentary protoliths. U/Pb and electron microprobe ages of monazite have a mode at 1650 to 1700 Ma, concordant with the Santaquin Complex, and lack inheritance. We propose that the Farmington Canyon Complex was first cratonized from Archean- derived sediments in the Proterozoic, requiring a crustal suture to be north of it as well. Accretion ages of arc terranes in southeastern Wyoming are ;60-100 m.y. older than in Utah. Thus, a serious reevaluation of basement architecture in Utah is needed and a pre- viously unrecognized temporal complexity of accretion is indicated.

Recognition of a Santonian Submarine Canyon, Great Valley Group, Sacramento Basin, California: Implications for Petroleum Exploration and Sequence Stratigraphy of Deep-Marine Strata

Seismic stratigraphic and outcrop interpretation of Santonian-Campanian portions of the Great Valley group in the north-central Sacramento basin, California, reveal the presence of a large, north-south-oriented submarine canyon. Named after an overlying town, Williams canyon appears to have been cut during the middle Santonian and filled during the late Santonian and early Campanian (approximately 85-80 Ma). This fossil canyon is more than 100 km in length, ranges from 12 to 22 km in width, and contains compacted sedimentary fill with a maximum thickness of 1.5 km over its mapped extent. Williams canyon is comparable in scale to the Paleogene gorges of the Sacramento basin. Stratigraphic relations indicate active folding locally during the Turonian-Santonian and probable structural control on the location of the canyon. Regional correlations showing uplift and regression along the northern basin margin synchronous with transgression of the eastern basin margin during canyon incision suggest that tectonic tilting of the basin initiated cutting of Williams canyon, whereas the role of eustatic sea level change appears to have been negligible. Delineation of Williams canyon clarifies geometries of outcropping and subsurface strata because some of the canyon fill previously has been correlated to older strata, and canyon boundaries locally have been misinterpreted as faults. Over most of its areal extent, the sequence boundary associated with canyon incision is characterized by facies associations that do not conform to widely cited sequence stratigraphic models of deep-marine deposits, suggesting that such models are oversimplified. Combination trapping geometries created by Williams canyon cut-and-fill represent an untested gas exploration play.

Paleomagnetic determination of vertical-axis rotations within the Charleston-Nebo salient, Utah

The Charleston-Nebo salient is highly convex toward the foreland of the Sevier fold-thrust belt in Utah. To determine the importance of vertical-axis rotations to the kinematic history of the salient, paleomagnetic samples were collected from red Triassic strata of the Woodside Shale and Ankareh Formation from 68 sites distributed around the curvature of the salient. The paleomagnetic data indicate counterclockwise rotation of 33.3° ± 4.8° in the northern part of the salient and clockwise rotation of 66.7° ± 5.1° in the southern part of the salient. These data support divergent-flow models for thrust-salient development, augmented by relatively minor contributions from original basin geometry and local strike-slip along salient margins.

Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA

The Cordilleran thrust belt of northwestern Montana (United States) has received much less attention than its counterparts in the western interior of USA and Canada. The structure of the thrust belt in this region is well preserved and has not been strongly overprinted by Cenozoic extension, providing an opportunity to reconstruct its geometry and to relate it to the foreland basin system. The thrust belt in this region consists of a frontal part of highly deformed Paleozoic, Mesozoic, and Paleocene sedimentary rocks, and a western region dominated by a >15-km-thick succession of Proterozoic Belt Supergroup strata underlain by faults of the Lewis thrust system. The frontal part can be subdivided into the foothills and the Sawtooth Range. At the surface, the foothills show deformed Mesozoic and Paleocene rocks; at depth, refl ection seismic data indicate numerous thrust faults carrying Paleozoic strata. The Sawtooth Range, south from the Lewis thrust salient, is defi ned by steeply dipping imbricate thrusts that detach at the basal Cambrian stratigraphic level. The Sawtooth Range plunges northward beneath the Lewis thrust salient and diverges into a pair of independent thrust systems that form the Flathead and Waterton duplexes in Canada. The relatively minor internal deformation in the western part of the thrust belt resulted from the great rheological strength of the Belt Supergroup rocks and initial high taper of the preorogenic stratigraphic wedge. A new ~145-km-long balanced cross section indicates ~135 km of shortening, a value similar to that in the southern part of the Canadian thrust belt. Previous work and new conventional and isotopic provenance data from the foreland basin and U-Pb ages from crosscutting intrusive rocks establish a preliminary kinematic model for this segment of the Cordilleran thrust belt. The emerging pattern is a relatively simple forelandward progression of thrusting events. Most shortening in the Lewis thrust system, Sawtooth Range, and foothills occurred roughly between mid-Campanian and Early Eocene time (ca. 75‐52 Ma), yielding a shortening rate of ~5.9 mm/yr. This pattern differs from the pattern of shortening in the better known Sevier thrust belt to the south, where regional far-traveled Proterozoic quartzitebearing thrust sheets were mainly active during Early Cretaceous time. From Middle Eocene to Early Miocene time, this sector of the Cordillera collapsed, generating a number of extensional depocenters.

Blickomylus (Artiodactyla, Camelidae, Stenomylinae) and the age of the Moroni Formation, central Utah

The first, and thus far only, fossil vertebrate to be discovered in the Moroni Formation of central Utah (Fig. 1) is a new species of the Miocene stenomyline camel Blickomylus Frick and Tay lor, 1968, represented by parts of both rami, each with p4-m3, of a single individual (Fig. 2). Among the stenomylines, all of which have transversely narrow, hypsodont molars, Blickomylus is the most highly derived in the anteroposterior elongation of the third molars (Frick and Taylor, 1968). The only previously de scribed species of Blickomylus, B. galushai, has as its type speci men a crushed skull, F:A.M. 50840; the type and originally re ferred skulls, jaws, and metapodials are from Blick Quarry in the Chamisa Mesa Member of the Zia Sand Formation in Sandoval County, New Mexico (Frick and Taylor, 1968). The age of Blick Quarry is considered to be early Hemingfordian, He-1, and the genus ranges through the entire Hemingfordian (Tedford et al., 2004). Blickomylus galushai has also been reported from the Hemingfordian of the Browns Park Formation in northeastern Colorado (Honey and Izett, 1988) and the Split Rock Formation of central Wyoming (Munthe, 1988). The only other stenomyline having somewhat similar derived characteristics, although rela tively less elongated third molars, is Rakomylus Frick, 1937. The type and single known species, R. raki, is based on a skull, F:A.M. 30990, complete except for the nuchal area, and is known also from several mandibles, and metapodials from the Skull Ridge Member of the Tesuque Formation, Santa Fe Group, Santa Fe County, New Mexico, in deposits referred to the Bar stovian (Frick and Taylor, 1968; Galusha and Blick, 1971). Because the specimen from the Moroni Formation consists of rami, only characters of the lower jaw and dentition can be com pared to Blickomylus and Rakomylus. Both Rakomylus and Blickomylus exhibit dental changes after wear that tend to blur some of their generic distinctions. In topotypic specimens of B. galushai from the Blick Quarry, pl-4 are small, ml is reduced in size and laterally displaced, and m3 is more hypsodont and rela tively more elongated than in the other stenomylines. In at least one referred specimen of B. galushai from Straight Cliff Fork, F:A.M.50803, pl-3 are absent and p4 has three cusps, presum ably paraconid, protoconid, and metaconid. Rakomnylus raki ex hibits greater premolar reduction than in topotypic specimens of B. galushai, having pl vestigial or probably absent in adults, p2 absent, p3 vestigial, and p4 smaller than in B. galushai. However, the ml of R. raki is relatively well developed, and m3 is less elongated relative to m2 than in B. galushai. The stenomyline from the Moroni Formation combines features such as the re duced premolars that occur in Rakomylus with the more reduced ml and greatly expanded m3 of Blickomylus. The Moroni cam elid is smaller than B. gallishai and appears to represent a new species of Blickomylus. Institutional Abbreviations-F:AM, Frick Collection, Ameri can Museum of Natural History; CM, Carnegie Museum of Natural History.