Paleomagnetic and Rock Magnetic Analysis of Upper Cretaceous Synorogenic Conglomerates From the Cordilleran Foreland Basin, Utah; Middle Triassic Moenkopi Formation From the Petrified Forest National Park, Arizona; and Pseudotachylytes From the Albany-Fraser Orogenic Belt, Southwest Australia




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This dissertation consists of three induvial projects that focus on the application of paleomagnetism and rock-magnetism to decipher the magnetic mineralogy and magnetic remanence carrier, magnetization acquisition processes, magnetic petrofabric to understand the depositional and post-depositional deformation fabrics, and to construct a high-resolution magnetochronology of studied sedimentary rock sequences. Project 1 addresses the magnetic polarity stratigraphy of Late Cretaceous long run-out coarse-grained facies exposed in the Cordilleran Foreland basin in northeast Utah. For an improved understanding of kinematic history of the Sevier fold-thrust belt, depositional ages of syntectonic units and the integration of geologic and geochronologic constraints are important. To construct an independent age constraint for the Upper Cretaceous synorogenic units from the Cordilleran foreland basin in northeast Utah, we have sampled the fine-grained, hematitic interbeds in the Upper Cretaceous Echo Canyon Conglomerate and Weber Canyon Conglomerate. Results show that hematitic, fine-grained sandstone to siltstone intervals carries a geologically stable magnetization with directions and polarity consistent with the Late Cretaceous geomagnetic field. Magnetic polarity data coupled with the existing palynologic ages indicate that these coarse-grained strata in the northeast Utah were most likely deposited over the time span of the magnetic polarity Chron (C) 34n to C33r interval, which includes the Santonian-Campanian stage boundary (ca. 83.4 Ma/83.1 Ma) and the younger C33r to C33n interval. The new age constraints demonstrate a complete temporal overlap between proximal and distal coarse-grained deposits in this part of the Cordilleran foreland basin suggesting a coeval sedimentation in both the proximal and distal parts linked with the active thrust displacement and rapid hinterland exhumation. Project 2 involves establishing a robust age constraint for the Triassic Moenkopi Formation intersected in the Colorado Plateau Coring Project Phase 1. Moenkopi Formation strata in the Little Colorado River drainage basin preserves one of the richest non-marine macrofloral and vertebrate fossil assemblages from the whole of North America, and a robust age constraint of this sequence is important to estimate the ages of its constituent fossil assemblages, document key biotic events, and for lithologic and biostratigraphic correlation across continents. To construct a highresolution magnetic polarity timescale for the Moenkopi Formation, samples were collected from the entire interval of Moenkopi Formation rocks intersected in two continuous cores (CPCP-1A and CPCP-2B). Demagnetization data yield a well-defined, interpretable paleomagnetic results, and six normal and reverse polarity couplets were identified in both cores, which are denoted from youngest to oldest, MF1n to MF6r. CA-TIMS calibrated CPCP-1 magnetozones exhibited a well-defined correlation with the astronomically tuned polarity timescale for the Middle Triassic deep-marine Guandao (GD) section of South China, and ties the magnetozone MF1n with GD8 and MF6r with GD2r, and implies that the Moenkopi Formation spans, at most, the earliest Anisian (Aegean) to latest Anisian (Illyrian)/ earliest Ladinian stages (ca. 246.8 Ma to 241.5 Ma). This refined age estimate for the Moenkopi Formation suggests that the vertebrate fossil assemblages preserved in east-central Arizona are millions of years (minimally 3 - 4 Ma) younger and are all Anisian in age. Project 3 involves the magnetic petrofabric characterization of fault generated pseudotachylyte vein rocks and adjacent Mesoproterozoic host rock from the Cape Arid region of southwest Australia. We have examined the magnetic properties of faultgenerated pseudotachylyte (PST) veins, typically less than a cm in width, and surrounding granitic host rocks (HR) from the mid-Proterozoic Albany Fraser orogenic belt of the Yilgarn Craton. Analysis showed that the PST and HR samples contain contrasting rock-magnetic properties and magnetic fabrics. Microscopic observations of pseudotachylyte rocks show grainsize reduction, enhancement of specific mineral phases (e.g., magnetite and biotite) and grain elongation. Rock magnetic data suggest an overall enhancement of ferrimagnetic (s.l.) phases, largely magnetite, in the pseudotachylyte rocks, and the magnetic fabric is primarily controlled by the shape prepared orientation of magnetite grains. Paleomagnetic results yield highly dispersed magnetization vector directions, and unable to provide a geologically significant interpretation. PST magnetofabric shows a steep foliation suggesting a vertical slip, and a NESW oriented shear suggests that generation of these vein rocks are unrelated to the Albany-Fraser Orogeny and may plausibly represent the early-stage deformation related to the breakup of supercontinent Rodinia at ca. 750 Ma.



Geology, Geophysics