Extensional tectonics of the Cordilleran foreland fold and thrust belt and the Jurassic-Cretaceous Great Valley forearc basin

Following cessation of contractional deformation, the Sevier orogenic belt collapsed and spread west during a middle Eocene to middle Miocene (∼48-20 Ma) episode of crustal extension coeval with formation of metamorphic core complexes and regional magmatism. The sedimentary and structural record of this event is a network of half-grabens that extends from southern Canada to at least central Utah. Extensional structures superposed on this fold-thrust belt are rooted in the physical stratigraphy, structural relief and sole faults of preexisting thrust-fold structures. Commonly, the same detachment surfaces were used to accommodate both contractional and extensional deformation. Foreland and hinterland extensional elements of the Cordillera that are normally widely separated are uniquely collocated in central Utah where the thrust belt straddles the Archean-Proterozoic Cheyenne belt crustal suture. Here, the Charleston-Nebo allochthon, an immense leading-edge structural element of the Sevier belt collapsed during late Eocene-middle Miocene time when the sole thrust was extensionally reactivated by faults of the Deer Creek detachment fault system and the allochthon was transported at least 5-7 km back to the west. Concurrently, the north margin of the allochthon was warped by flexural-isostatic rise of a Cheyenne belt crustal welt and its footwall was intruded by crustal melts of the Wasatch igneous belt. Collectively, these elements comprise the Cottonwood metamorphic core complex. Extensional processes were also important in the formation of the Jurassic-Cretaceous Great Valley forearc basin. Advocates of a thrust-wedge hypothesis argued that this forearc experienced prolonged Jurassic-Cretaceous contraction and proposed that northwest-southeast-striking fault systems were evidence of a west-dipping blind Great Valley-Franciscan sole thrust and related backthrusts. Based on interpretation of seismic reflection, borehole, map and stratographic data, I propose that these faults and associated bedding geometries are folded synsedimentary normal faults and half-grabens. Thus, late-stage diastrophic mechanisms are not required to interpret a forearc that owes much of its present-day bedding architecture to extensional processes coeval with deposition

Thermochronological and Geochronological Constraints on Late Cretaceous Unroofing and Proximal Sedimentation in the Sevier Orogenic Belt, Utah

A source-to-sink analysis incorporating geochronometric and thermochronometric data from the Sevier fold-thrust belt (SFTB) and proximal synorogenic strata of the Canyon Range Conglomerate (CRC) and Indianola Group (IG) provides new insights into orogenic exhumation, erosional unroofing, and the interplay between thrusting and coarse clastic deposition in the Cretaceous Cordilleran foreland basin of western North America. Zircon (U-Th)/He ages from the Pavant and Nebo thrust sheets record significant Cenomanian cooling indicative of synchronous exhumation and thrusting along a large segment of the SFTB in central and northern Utah. Detrital zircon (U-Th)/He (DZHe) ages indistinguishable from depositional ages from the Cenomanian Dakota Formation and lower CRC also record rapid unroofing of the SFTB and synchronous deposition. DZHe data from wedge-top deposits of the CRC record two significant unroofing episodes: Albo-Cenomanian exhumation of the Pavant thrust and progressive unroofing of the Canyon Range culmination. For the IG, the presence of Paleozoic DZHe ages along with Paleozoic-Mesozoic DZ U-Pb ages in the Cenomanian Sanpete Formation suggests derivation from Paleozoic to Jurassic strata exhumed in the frontal Pavant and Nebo thrust sheets. After the Cenomanian episode of rapid exhumation, proximal foredeep strata recorded a widespread DZ provenance shift in the Turonian. Short DZHe lag time values from Campanian CRC and IG deposits reveal rapid exhumation of the SFTB during the Campanian. The synchroneity of major shortening and Campanian and Cenomanian changes in foreland basin architecture and provenance supports models proposing that active shortening in the fold-thrust belt coincides with coarse clastic influx in foreland basins.6 month embargo; first published: 23 May 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Identification of seasonal varves in the lower Pliocene Bouse Formation, lower Colorado River Valley, and implications for Colorado Plateau uplift

The cause of Cenozoic uplift of the Colorado Plateau is one of the largest remaining problems of Cordilleran tectonics. Difficulty in discriminating between two major classes of uplift mechanisms, one related to lithosphere modification by low-angle subduction and the other related to active mantle processes following termination of subduction, is hampered by lack of evidence for the timing of uplift. The carbonate member of the Pliocene Bouse Formation in the lower Colorado River Valley southwest of the Colorado Plateau has been interpreted as estuarine, in which case its modern elevation of up to 330 m above sea level would be important evidence for late Cenozoic uplift. The carbonate member includes laminated marl and claystone interpreted previously in at least one locality as tidal, which is therefore of marine origin. We analyzed lamination mineralogy, oxygen and carbon isotopes, and thickness variations to discriminate between a tidal versus seasonal origin. Oxygen and carbon isotopic analysis of two laminated carbonate samples shows an alternating pattern of lower δ18O and δ13C associated with micrite and slightly higher δ18O and δ13C associated with siltstone, which is consistent with seasonal variation. Covariation of alternating δ18O and δ13C also indicates that post-depositional chemical alteration did not affect these samples. Furthermore, we did not identify any periodic thickness variations suggestive of tidal influence. We conclude that lamination characteristics indicate seasonal genesis in a lake rather than tidal genesis in an estuary and that the laminated Bouse Formation strata provide no constraints on the timing of Colorado Plateau uplift.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Microbial-caddisfly bioherm association from the Lower Cretaceous Shinekhudag Formation, Mongolia: Earliest record of plant armoring in fossil caddisfly cases

Caddisfly larvae construct underwater protective cases using surrounding materials, thus providing information on environmental conditions in both modern and ancient systems. Microbial bioherms associated with caddisfly cases are found in the Berriassian-Hauterivian (similar to 140-130 Ma) Shinekhudag Formation of Mongolia, and yield new insights into aspects of lacustrine paleoecosystems and paleoenvironments. This formation contains the earliest record of plant-armored caddisfly cases and a rare occurrence of microbial-caddisfly association from the Mesozoic. The bioherms are investigated within the context of stratigraphic correlations, depositional environment interpretations, and basin-evolution models of the sedimentary fill. The bioherms form 0.5-2.0 m diameter mound-shaped bodies and are concentrated within a single, oil shale-bound stratigraphic interval. Each bioherm is composed of up to 40% caddisfly cases along with stromatolites of millimeter-scale, micritic laminations. Petrographic analyses reveal these bioherms are composed of non-systematic associations of columnar and oncoidal microbialites, constructed around colonies of caddisfly cases. The cases are straight to curved, slightly tapered, and tube-shaped, with a progressively increasing length and width trend (7-21 mm by 1.5-2.5 mm). Despite these variations, the case architectures reveal similar construction materials; the particles used for cases are dominated by plant fragments, ostracod valves, carbonate rocks, and rare mica and feldspar grains. Allochems within the bioherms include ooids, ostracods, plant fragments, rare gastropods, feldspar grains bound in micritic matrices, and are consolidated by carbonate dominated cements. The combination of microbial-caddisfly association, plant fragment case particles, and ooids/oncoids are indicative of a shallow, littoral lake setting. Stratigraphic juxtaposition of nearshore bioherms and the bounding distal oil-shale facies suggests that the bioherms developed in an underfilled lake basin, resulting from an abrupt and short-lived lake desiccation event. Lake chemistry is believed to have been relatively alkaline, saline to hypersaline, and rich in Ca, Mg, and HCO3 ions. Through analyzing bioherm characteristics, caddisfly case architecture, carbonate microfacies, and stratigraphic variability, we infer larger-scale processes that controlled basin development during their formation.Fulbright AssociationOpen access journal.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Taphonomy of the fossil insects of the middle Eocene Kishenehn Formation

The lacustrine oil shales of the Coal Creek Member of the Kishenehn Formation in northwestern Montana comprise a relatively unstudied middle Eocene fossil insect locality. Herein, we detail the stratigraphic position of the fossiliferous unit, describe the insect fauna of the Coal Creek locality and document its bias towards very small but remarkably pre-served insects. In addition, the depositional environment is examined and the mineral constituents of the laminations that comprise the varves of the Kishenehn oil shale are defined. Fifteen orders of insects have been recorded with the majority of all insects identified as aquatic with the families Chironomidae (Diptera) and Corixidae (Hemiptera) dominant. The presence of small aquatic insects, many of which are immature, the intact nature of >90% of the fossil insects and the presence of Daphnia ephippia, all indicate that the depositional environment was the shallow margin of a large freshwater lake. The fossil insects occur within fossilized microbial mat layers that comprise the bedding planes of the oil shale. Unlike the fossiliferous shales of the Florissant and Okanagan Highlands, the mats are not a product of diatomaceous algae nor are diatom frustules a component of the sediments or the varve structure. Instead, the varves are composed of very fine eolian siliciclastic silt grains overlaid with non-diatomaceous, possibly cyanobacteria-derived microbial mats which contain distinct traces of polyaromatic hydrocarbons. A distinct third layer composed of essentially pure calcite is present in the shale of some exposures and is presumably derived from the seasonal warming-induced precipitation of carbonate from the lake’s waters. The Coal Creek locality presents a unique opportunity to study both very small middle Eocene insects not often preserved as compression fossils in most Konservat-Lagerstätte and the processes that led to their preservation

Resolving mid- to upper-crustal exhumation through apatite petrochronology and thermochronology

Double-dating using the apatite U-Pb and fission-track systems is becoming an increasingly popular method for resolving mid- to upper- crustal cooling. However, these thermochronometers constrain dates that are often difficult to link through geological time due to the large difference in temperature window between the two systems (typically >250 °C). In this study, we apply apatite U-Pb, fission-track, and apatite and whole rock geochemistry to fourteen samples from four tectonic domains common in Cordilleran orogenic systems: (1) basement-cored uplifts, (2) plutons intruded through a thick crustal column, (3) metamorphic core complexes and associated detachment faults, and (4) rapid, extrusive volcanic cooling, in order to provide a link between in situ geochemical signatures and cooling mechanisms. Comparisons of trace element partitioning between apatite and whole rock provide insights into initial apatite-forming processes and/or subsequent modification. Apatite trace element geochemistry and the Th/U and La/LuN ratios provide tools to determine if an apatite is primary and representative of its parent melt or if it has undergone geochemical perturbation(s) after crystallization. Further, we demonstrate that by using a combined apatite U-Pb, FT, trace element, and whole rock geochemistry approach it is possible to determine if a rock has undergone monotonic cooling since crystallization, protracted residence in the middle crust, and provide unique structural information such as the history of detachment faulting. Insights provided herein offer new applications for apatite thermochronology.Fonds De La Recherche Scientifique - FNRS24 month embargo; first published online 21 January 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]