Semester of Graduation

Fall 2019

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

A provenance study of quartz arenites that stratigraphically are closely associated with major Ordovician K-bentonites has been conducted in order to further our understanding of the tectonic setting of eastern Laurentia during the Late Ordovician. Using laser ablation ICP-MS, detrital zircons separated from Ordovician sandstone samples in the southeastern Appalachian Mountains (Virginia to Alabama) were dated using U-Pb geochronology. Analytical results show three dominate age ranges for the zircons from these sandstones: ~440-490 Ma, ~900-1300 Ma, and ~1300-1500 Ma. In addition, some zircon ages grouped into older ranges of ~1600-1800 Ma, ~1800-1900 Ma, and ~2600-2800 Ma. Zircon ages from ~900-2800 Ma have all been previously identified in passive margin Cambrian sandstones, supporting a multi-cycle source for the Ordovician quartz arenites. The younger age signal of ~440-490 Ma, which becomes stronger to the south, is attributed to the K-bentonites because the sandstones were being deposited right after or, in some locations, during the eruptions. The tectonic setting for the Laurentian margin at this time was likely analogous to the current subduction of the Philippine Sea Plate underneath the east Asian continental margin (Ryukyu Arc area). The Laurentian continental crust was warped during the orogeny, and the Taconic Highlands and associated foredeep were created, in which detritus from older sedimentary rock and volcanics (tephras) were deposited. The eruptions that created these tephras have been estimated to have been larger than the historic Tambora and Toba eruptions, leading to inconsistencies with an island arc volcanic system due to no clear silica-rich source for these massive eruptions. However, the aforementioned Ryukyu Arc system helps explain a process by which volcanics can be erupted through continental (silica-rich) margin sediments via the dehydration and melting of a subducted slab. The proximity between a possible immature island arc system and quartz-rich sandstones is also troublesome. The U-Pb ages determined from the detrital zircons, as well as the size and nature of the K-bentonites found, led to the conclusion that these quartz-rich sandstones were produced in a humid and tropical climate with a high degree of chemical weathering and transport through an extended flood/coastal plain.

Committee Chair

Herrmann, Achim

DOI

10.31390/gradschool_theses.5022

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