The Geology of a Portion of the Quincy 15' Quadrangle, Plumas County, California

\n ABSTRACT\n \n The rocks studied for this thesis are exposed in a portion of the northern Sierra Nevada that is within the Quincy 15’ Quadrangle, Plumas County, California. The rocks belong to two great stratigraphic groups that are separated by a “profound unconformity.” The older group is the Ordovician-Silurian Shoo Fly Complex and its rocks are designated “Subjacent Units.” The younger group includes a series of Miocene to possibly Pliocene volcanic, sedimentary, and volcaniclastic deposits and are designated “Superjacent Units.”\n The Shoo Fly Complex is composed predominantly of metamorphosed and tectonized sandstone, siltstone, shale, and chert. Subordinate rock types include limestone, felsic dikes, serpentinite, a quartz porphyry granite stock, and an exposure of meta-diabase. Bedding and facing evidence preserved in the clastic meta-sediments is rare, but is generally to the east. The limited bedding evidence includes partial Bouma-sequence layering. All Shoo Fly rocks display lower-greenschist facies regional metamorphism and varying degrees of deformation attributed to the Jurassic Nevadan Orogeny and an earlier orogeny. The clastic meta-sediments and cherts are the most deformed. They are folded and sheared and display a dominant slaty cleavage that strikes northwest and is vertical to sub-vertical; dipping steeply northeast or southwest. Four episodes of deformation have affected the Shoo Fly rocks. The first episode led to the development of the angular unconformity that separates the Shoo Fly from the overlying Devonian Sierra Buttes Formation. The second and third episodes involved, respectively, isoclinal and kink folding that are displayed at outcrop scale. The fourth episode involved high-angle Cenozoic faulting that is evidenced by the exposures of the Tertiary units. The up to approximately 15-kilometer exposed thickness of the Shoo Fly is anomalous when compared to other ancient and modern sedimentary accumulations and may be best attributed to imbricate thrusting that attended obduction of the assembled Complex onto the continental margin of western North America. However, evidence of the Sierra City melange, the youngest of the allochthons that form the Complex and which was mapped previously as extending through the study area, was not found during this study.\n The metasandstones in the Shoo Fly studied for this thesis are quartz rich and contain significant amounts of quartz-phyllosilicate “pseudomatrix.” The source of the pseudomatrix and mode of its formation are important questions that remain to be resolved. The metasandstones involve material derived from continental crust, based on petrologic examinations and new whole-rock geochemical analyses. A “Recycled Orogen” provenance for the metasandstones is inferred, based on averaged QFL statistics. The depositional setting of the Shoo Fly sediments was most likely along a passive continental margin. However, continental rift and/or arc depositional settings are also possible. The northern Laurentian margin is a possible source of the clastic sediments in the Shoo Fly Complex. A possible alternative source is the Peace River Arch region of Canada. Both possibilities require transport of the assembled Complex over significant distances before arriving at its present geographic position relative to western North America.\n The Tertiary units in the study area include the Lovejoy basalt, Bonta formation and its associated basal gravels, Warner basalt and presumably associated intrusives, and an areally-limited intrusive dike complex. The Lovejoy, Bonta, and Warner exposures form a belt across the lower half of the study area that marks the location of a paleovalley. The paleovalley was one of a series through which these materials moved to the west and into the ancestral Sacramento Valley from sources located in northeastern California. The Lovejoy also helps to indicate that its main paleochannel was bifurcated into two routes across the study area. The Lovejoy has also become the focus of increased interest and importance because of its distinctiveness as a stratigraphic marker, providing a strain gauge to help identify and quantify late-Cenozoic tectonic deformation of the northern Sierra Nevada region, and because its age has been redefined as Middle Miocene (i.e., ~16 Ma) through improved radiometric dating techniques. The ages for the younger Tertiary units are reported, based on radiometric dating, to range from Middle Miocene to possibly Pliocene as follows: the Ingalls and Bonta formations are Middle Miocene; the Penman formation, “intrusive andesite,” and Warner basalt are Late Miocene; and the Warner intrusives are possibly Late Miocene to Pliocene. However, the mapping for this thesis demonstrates that the reported overlapping isotopic age ranges for these units carry significant uncertainties. These Tertiary units also help to define the Cenozoic faulting that has affected the northern Sierra Nevada region. The faulting includes the Mohawk Valley fault system which led to the formation of the Plumas Trench; a regionally-important structural feature. The faulting and available seismic data add to the evidence that basin-and-range deformation continues to migrate westward into the northern Sierra Nevada.

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