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  • 2010-2014  (3)
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  • 1
    Publication Date: 2012-05-01
    Description: Re-evaluation of acoustic, gamma ray and dip meter logs from the Cominco American Federal No. 2 well in the Sevier Desert basin of west-central Utah sheds new light on the interpretation of Neoproterozoic and Cambrian stratigraphy and Mesozoic structure in a region that has been influential in the development of ideas about crustal shortening and extension. The most prominent of several major thrust faults (the Canyon Range and Pavant thrusts) have been interpreted by DeCelles and Coogan (2006) [Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah: Geological Society of America Bulletin, v. 118, n. 7-8, p. 841-864, http://dx.doi.org/10.1130/B25759.1] as having been cut and in part re-activated between late Oligocene and Holocene time by as much as 47 km of displacement on the gently west-dipping Sevier Desert detachment. This interpretation, which is based upon a combination of outcrop, seismic reflection and well data, depends critically on the Canyon Range thrust intersecting the Cominco well at a depth of 2,551 to 2,557 m (8,370-8,389 ft.), and terminating downwards against a re-activated Pavant thrust.Our work suggests that the fault at 2,551 m (8,370 ft.) is a strand of the Pavant thrust, and that the Canyon Range thrust cuts the well at a depth of 1,222 m (4,010 ft.). This alternative interpretation depends in turn on identification of the section between the two faults as terminal Neoproterozoic to middle Cambrian Prospect Mountain Quartzite through Chisholm Formation rather than Neoproterozoic “Pocatello Formation,” “Blackrock Canyon Limestone” and lower Caddy Canyon Quartzite. To support this interpretation we present evidence for stratigraphic repetition and for deformation at the 1,222 m (4,010 ft.) level. Use of the lithostratigraphic terms “Pocatello” and “Blackrock Canyon” in west-central Utah is shown to be inappropriate, and among the reasons that the critical interval in the Cominco well has been misinterpreted by some authors. If the Canyon Range and Pavant thrusts are both found in the Cominco well, as we suggest, then they cannot be used as a piercing point for the estimation of displacement on the Sevier Desert detachment or as justification for the existence of the detachment. Published estimates of extension across the Sevier Desert basin therefore need to be reduced, potentially to as little as ~10 km.
    Print ISSN: 0002-9599
    Electronic ISSN: 1945-452X
    Topics: Geosciences
    Published by HighWire Press on behalf of The American Journal of Science.
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  • 2
    Publication Date: 2011-04-01
    Description: Neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates. This stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions. In contrast to this, sequence stratigraphic field studies of an Ediacaran (ca. 635 Ma) glacial to postglacial transition in the Amadeus Basin of central Australia reveal a complex deglacial stratigraphy, in which more than 175 m of conglomerate, sandstone, marl, and carbonate at the basin margin, and portions of four unconformity-bounded sequences, pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies. The unconformities, which are characterized by as much as several tens of meters of erosional relief (oblique sections of incised valleys), separate intervals of contrasting sediment provenance, and are confidently mapped on the basis of both criteria. Comparable unconformities are absent in the overlying Neoproterozoic succession, which is 〉2 km thick and encompasses many tens of millions of years. The Amadeus Basin cap carbonate was thus deposited during a protracted interval of multiphase (cyclical) transgression more similar to Phanerozoic cyclical sea-level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate. The superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions. Facies lithologically similar to cap carbonates may be less obvious in Phanerozoic successions because of a secular change in carbonate composition to reefal and deep-sea pelagic deposits.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 3
    Publication Date: 2012-04-01
    Description: Kula et al. (2012) reanalyze our sequence stratigraphic interpretation and carbon isotope data for the younger Neoproterozoic cap carbonate interval in the northeastern Amadeus Basin of central Australia (Kennedy and Christie-Blick, 2011), and conclude that they are incompatible. This conclusion is unwarranted.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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