Graduate Thesis Or Dissertation

Stratigraphy, depositional history, and petrology of the Upper Cretaceous(?) to middle Eocene Montgomery Creek Formation, northern California

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  • The Upper Cretaceous to Eocene Montgomery Creek Formation is exposed between the Klamath and Cascade Mountains in the vicinity of Big Bend and Montgomery Creek, northern California. The formation is divided into a northern and a southern section. The rocks consist of a basal conglomerate succeeded by feldspathic to lithic graywackes, conglomerates, shales, carbonaceous shales, and minor lignites. The northern section is 3,188 feet thick. Fining upward point bar deposits, abundant foreset- and cross-bedding, and thick flood-basin deposits indicate the northen section strata were deposited by meandering rivers. Carbonaceous shales, lignites and fossil leaves define the swamps that were present in the basins. The southern section is 4,596 feet thick. Thick and laterally extensive sandstone bodies, numerous conglomerate interbeds, and abundant planar cross-bedding suggest that these strata were deposited by braided rivers. A few outcrops low in the section resemble the northern section strata, and are interpreted to be meandering river deposits. A progressive change in composition of the sandstones from the base to the top of the formation occurs in both the northern and southern sections. This reflects the change from metamorphic and plutonic source rocks of the Klamath Mountains to volcanic source rocks of the early Cascade Range. The southern section maintains a more lithic-rich composition, reflecting higher discharge rates and steeper gradients responsible for braided-river deposition. Paleocurrent readings from foreset bedding and imbricated pebbles indicate a north-to-south transport direction in most of both sections. Some westerly transport occurred high in the southern section. Eocene strata four miles north of the northern section reflect a meandering river depositional environment. These strata have a higher shale-to-sandstone ratio, larger proportions of carbonaceous shales and coal, and a different sandstone mineralogy than Eocene rocks to the south. Analysis revealed that the Montgomery Creek Formation carbonaceous shales and coal are sufficiently rich in organic carbon to be hydrocarbon source rocks. Most of the samples are thermally immature, but could produce gas through bacterial-fungal breakdown of organic matter. The Eocene sandstones have adequate porosity and permeability to be petroleum reservoir rocks. The Klamath Mountains acted as a source area for Cretaceous marine deposits in basins north and south of the study area. Either the Cretaceous seas did not transgress into the study area, or such deposits were subsequently removed by erosion.
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