Date of Award

4-23-2008

Document Type

Thesis open access

Abstract

Reconstructions of the Mesozoic paleogeography of the North American Cordillera depend on data that can link displaced terranes to each other and/or to the North American craton. The Pythian Cave Conglomerate (PCC) sits unconformably on the eastern terranes of the Klamath Mountains south of Yreka, California and has the potential to yield valuable information about the tectonic history of the region. The PCC is approximately 60 m thick, with the lower 40 m dominated by cobble conglomerate with thin sandstone lenses, and the upper 20 m containing thicker and more laterally continuous interbedded sandstone layers. Originally interpreted as Tertiary conglomerate based on petrologic similarity to other Tertiary units in the region, the PCC was dated as Albian (112 – 99 Ma) by a later pollen study. The youngest detrital zircon age peak in the PCC of 131 Ma provides a maximum depositional age and is consistent with Albian deposition. Paleoflow readings from cobble imbrication and sandstone cross bedding suggest that the source of the sediment was to the east-northeast. Thin-section petrography of cobbles throughout the PCC indicates a source dominated by hydrothermally-altered intermediate volcanic rocks, consistent with sandstone point-counts that plot in the arc and dissected arc provenance fields. Detrital zircon age spectra and conglomerate clast counts are consistent throughout the entire PCC section, implying a uniform provenance. Two PCC sandstone samples both yield two distinct detrital zircon age peaks of Early/Middle Jurassic (165-190 Ma) and Early Cretaceous (130-150 Ma), with few Late Jurassic (150-165 Ma) detrital zircon grains. Four igneous clasts also yield an Early Jurassic age. Together, these results suggest a proximal magmatic arc source to the east-northeast of the PCC that was active in Early-Middle Jurassic and Early Cretaceous time and may now be buried under Tertiary volcanic rocks. Youngest detrital zircon ages of 131 Ma, a lack of zircon ages older than 200 Ma, along with a southwesterly paleoflow, disqualify the Klamath Mountains as a source. These results indicate a potential source for the PCC in the northern Sierra Nevada Mountains or a now-buried northern continuation of the Sierra Nevada Mountains.

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