Geology of the Saddle and Humbug Mountain area, Clatsop County, northwestern Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5712m968n

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  • Six early to middle Tertiary geologic units crop out in the Saddle and Humbug Mountain area. They include the late Eocene to early Miocene Oswald West muds tones, the lower Silver Point and the upper Silver Point tongues of the middle Miocene Astoria Formation, and the middle Miocene Depoe Bay and Cape Foulweather Basalts. One new lithologically distinct unit, the Falls Creek member of the Oswald West muds tones, is mapped, described, and named informally herein. These Tertiary units are locally overlain by Quaternary stream alluvium and landslide deposits. The Oswald West mudstones consist of more than 500 meters of thickly bedded grayish to yellowish orange bioturbated tuffaceous siltstones and mudstones interstratified with minor glauconitic sandstones and tuff beds. An open-marine, deep-water, slightly reducing depositional environment (outer shelf-continental slope) is indicated by Foraminifera, trace fossils, glauconite, and the predominantly fine-grained character of the unit. Deposition of the Falls Creek silts tone member which contains a very shallow water molluscan fossil assemblage probably occurred during gradual shallowing near the end of the late Oligocene, possibly due to adjacent deltaic progradation. An angular unconformity separates the Oswald West muds tones from the overlying lower Silver Point tongue and suggests that a broad uplift occurred during an early Miocene hiatus. Thin feldspathic sandstone lenses occur locally above the unconformity and mark the beginning of a middle Miocene marine transgression during lower Silver Point time. The approximately 300-meter thick Silver Point member is composed of two intertonguing lithosomes. The lower tongue consists of rhythmically bedded, light gray, laminated, micaceous and carbonaceous sandstones and dark gray siltstones. Graded bedding and partial Bouma sequences suggest that these sandstones were deposited by turbidity currents, possibly on a delta slope or outer delta platform. Paleocurrent measurements and facies patterns indicate that these turbidity flows originated at a south to southeasterly source, probably by slumping off the nearby Angora Peak delta front. Thick-bedded, structureless, arkosic sandstones which locally interfinger with the basal part of the lower Silver Point tongue probably also reflect redeposition of "clean" delta front sheet sands into deeper water delta slope environments. The lower Silver Point tongue grades upward into the 200-meter thick upper Silver Point tongue which consists of dark gray structureless to finely laminated mudstones and rare thin sandstone beds. Foraminifera and the overall fine-grained lithology indicate that this unit was deposited under deep marine (upper bathyal), low energy conditions and reflects continuing marine transgression over the region. Postulated underthrusting caused by convergence of the Juan de Fuca oceanic plate and the North American continental plate along the middle Miocene Oregon continental margin (Ku lm and Fowler, 1974a, 1974b), may have caused the uplift and high-angle faulting and development of an unconformity which followed upper Silver Point marine deposition. Rapid subsidence and marine transgression ended this short-lived erosion period as evidenced by the local eruption of over 600 meters of palagonitized Depoe Bay submarine basaltic breccias and pillow lavas and more than 200 meters of sparsely porphyritic Cape Foulweather submarine pillow lavas. The Depoe Bay Basalt lies with angular unconformity over the faulted Silver Point member and Oswald West mudstones. A local basaltic conglomerate interbed within the Depoe Bay breccias (near the base) suggests that some early Depoe Bay volcanic buildups developed above wave base. Chemical analyses show that these basalts are tholeiites and are comparable to the type Cape Foulweather and Depoe Bay petrologic-types along the central Oregon Coast. All Tertiary sedimentary units are intruded by numerous dikes, sills, and irregular-shaped plutons. Regional dike swarms and local feeder dikes to Depoe Bay Basalt eruptive centers such as Saddle Mountain, commonly, either parallel or coincide with northeasttrending or northwest-trending lineaments and high-angle faults, suggesting a structural control for the emplacement of the intrusives and location of the volcanic centers. The largest fault in the study area (Humbug Mountain fault) is co-linear with the northwest-trending Gales Creek Fault and another unnamed fault southeast of the study area. In total, these three faults form an almost continuous line of faults across the northern Oregon Coast Range for a distance of 100 km.
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  • File scanned at 300 ppi (Monochrome, 24-bit Color) using Capture Perfect 3.0 on a Canon DR-9050C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR. Plates: Master map file scanned at 600 dpi, 24-bit color on a Paradigm ImagePRO GxT 42 HD (OEM version of ColortracSmartLF Bx 42). Image manipulated by SmartLF1.3.05.
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