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Geological Sciences Seminar: Mark Pecha, University of Arizona

Tuesday, Feb 25, 2020 12:00 PM
GeoCorner 220 - 450 Serra Mall, Building 320
More Info:


Faculty/Staff, Students, Alumni/Friends
Department of Geological Science

** Please join us for coffee and cookies in the GeoCorner Undergraduate lounge (bldg. 320, rm 114) before the talk, at 11:30am! - Seminar will be in room 220~

Linking the Gulf of Mexico and Coast Mountains batholith during the Late Paleocene: Insights from Hf isotopes in detrital zircons

Paleocene Lower Wilcox Group sedimentation rates are three-times the Cenozoic average for the Gulf of Mexico region and are attributed to Laramide tectonism within the Laramide-Rocky Mountains region. These increased rates likely represent the erosion of the easily weathered sedimentary cover that once blanketed the Laramide-age basement cored uplifts.  Geologic observations and U-Pb geochronology are not sufficient to fully address this hypothesis alone, so we conducted 439 Lu-Hf isotopic analyses on detrital-zircons from eight samples from the San Juan basin and five samples from the Gulf of Mexico basin.  Focusing on the <300 Ma zircons allowed us to make direct comparisons to the eight principal components that comprise the North American Cordilleran magmatic arc: 1) Coast Mountains batholith; 2) North Cascades Range; 3) Idaho batholith; 4) Sierra Nevada batholith; 5) Laramide Porphyry Copper Province; 6) Transverse Ranges; 7) Peninsular Ranges; and 8) Sierra Madre Occidental.  The eHf(T) results range from +8.9 to -27.0 for the San Juan basin samples and +13.0 to -26.6 for the Gulf of Mexico samples.  Using the San Juan basin samples as a proxy for the eroded Mesozoic strata that was shed from the Laramide uplifts, we show that most of the sediment entering the Gulf of Mexico through the Houston and Mississippi embayments during the Late Paleocene was derived from this reworked cover.  However, the Gulf of Mexico samples also include a distinct juvenile suite (eHf(T) ranging from +13 to +5) of zircons ranging in age from ca. 220 to 55 Ma which we have traced the Coast Mountains batholith in British Columbia, Canada.  This trans-continent connection argues for an extension to the headwaters of the previously defined paleo-Mississippi drainage basin from ca. 58-55 Ma.  Therefore, we propose a through-going fluvial system (referred to here as the “Coast Mountains River”) that was routed from the Coast Mountains batholith all the way to the Gulf of Mexico.  This expands the previously defined paleo-Mississippi drainage basin area by an estimated 280,000 km2.  Our comprehensive Hf isotopic compilation of the North American Cordilleran magmatic arc also provides a benchmark eHf(T) fingerprint map, which can be used to determine provenance of detrital-zircons (85-50 Ma) at a scale of specific region(s) within the Cordillera based on their eHf(T).

Mark E. Pecha, Manager of the Arizona LaserChron Center, University of Arizona, 2009-Present. Received BS (2008), MS (2013), and Ph.D. (2019) in Geosciences from University of Arizona. Mark Pecha's bio: "My research focus is the geologic and tectonic history of the North American Cordillera.  I am primarily interested in thrust belts and their associated sedimentary basins where I apply a combination of geologic mapping, sedimentological analysis, detrital mineral geochronology (U-Pb geochronology and Hf isotope geochemistry), and provenance analysis to address broad- regional questions.  I’ve principally worked in southeastern Alaska, British Columbia, and the southwestern United States, and am currently focused on Laramide tectonism.  I am also interested in pushing analytical techniques to their limits via laser ablation ICP mass spectrometry and the development of geochronological and geochemical mineral standards."

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