Citation: Herrmann, A.D.; Haynes, J.T.; Robinet, R.; Clift, P.D.; Goggin, K.E. Testing Tectonostratigraphic Hypotheses of the Blountian Phase of the Taconic Orogeny in the Southern Appalachians through an Integrated Geochronological and Sedimentological Study of Ordovician K-Bentonites and Quartz Arenites. Minerals 2023, 13, 807. https://doi.org/10.3390/ min13060807 Academic Editor: Paul Alexandre Received: 1 April 2023 Revised: 19 May 2023 Accepted: 1 June 2023 Published: 13 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article Testing Tectonostratigraphic Hypotheses of the Blountian Phase of the Taconic Orogeny in the Southern Appalachians through an Integrated Geochronological and Sedimentological Study of Ordovician K-Bentonites and Quartz Arenites Achim D. Herrmann 1,2, *, John T. Haynes 3 , Richard Robinet 1,2 , Peter D. Clift 1,2 and Keith E. Goggin 4 1 Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA 2 Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA 3 Department of Geology and Environmental Science, James Madison University, MSC 6903, Harrisonburg, VA 22807, USA; haynesjx@jmu.edu 4 810 Bent Knoll Ct., Sugar Land, TX 77479, USA * Correspondence: aherrmann@lsu.edu Abstract: The tectonic setting of the southernmost part of the eastern margin of Laurentia during the Blountian tectophase (~472–452 Ma) of the Ordovician Taconic Orogeny remains unresolved. Tephras produced by explosive volcanism during this early phase of the orogeny are now K-bentonites, and in many locations, they are interbedded with mature to supermature quartz arenites. We conducted U-Pb analyses of detrital zircons from the sandstones, and of zoned magmatic zircons from the K-bentonites, to constrain the tectonostratigraphic setting with more precision. We also used geochemical fingerprinting of apatite phenocrysts to correlate the K-bentonites in these sandstones along the tectonic front, and we then integrated these results with a depositional systems study of the quartz arenites to further constrain and test competing models of the regional tectonomagmatic setting during that time. The general dearth of detrital zircons that have ages contemporaneous with the volcanic activity, coupled with the predominantly Precambrian ages of the zircons in these Lower Paleozoic quartz arenites that otherwise lack volcaniclastic components—such as detrital VRFs or a muddy matrix derived from an eroding volcanic arc—suggests that magmatic zircons from the tephra layers entered the depositional system only occasionally, and that the volcanic centers were separated geographically from where these quartzose sands were being deposited. Our findings support a tectonostratigraphic and tectonomagmatic model analogous to a combination of select modern settings in the western Pacific and Indonesia, specifically (1) New Guinea, where mature quartz arenites occur in the Cenozoic foreland succession, and (2) Sumatra, where the enormous Toba caldera formed in association with subduction beneath the Cretaceous-aged continental crust of Sumatra. Keywords: Sandbian; Katian; zircon geochronology; apatite geochemistry; tephrochronology; tectonostratigraphy; inherited cores; tephra; Deicke; Millbrig 1. Introduction and Background Many details of the tectonostratigraphic evolution of the southern Appalachians during the initial part of the Taconic Orogeny (Ordovician, Sandbian to Katian), known gen- erally as the Blount or Blountian phase of the orogeny [1], are still poorly understood. The tectonic setting and tectonic evolution of the northern and north-central parts of the Taconic orogeny have, by comparison, been largely resolved. To explain the Taconic Orogeny in that area (southern Pennsylvania and Maryland to Newfoundland), it is postulated that a volcanic island arc formed above a subduction zone along the eastern margin of the Laurentian continent. This volcanic island arc system then collided with the Laurentian Minerals 2023, 13, 807. https://doi.org/10.3390/min13060807 https://www.mdpi.com/journal/minerals