A refined northeast Pacific plate‐motion model provides a framework for analysis of the Tertiary volcanic and tectonic history of western Oregon and Washington. We examine three possible models for the origin of the allochthonous Paleocene and Eocene oceanic basalt basement of the Coast Range: (1) accretion to the continent of hot spot generated linear seamount chains; (2) accretion of thick oceanic crust and seamounts generated during Farallon‐Kula spreading reorganizations between 61 and 48 Ma; and (3) eruption of basalt during oblique rifting of the continental margin as it overrode an active Yellowstone hot spot on the Kula‐Farallon ridge. The plate model suggests that microplate rotation and accretion of hot spot generated linear aseismic ridges cannot be easily reconciled with rapid northeast motion of the KuIa and Farallon plates and the well‐established paleomagnetic rotations. Following emplacement of the Coast Range basement, changes in the character of forearc, back arc and Cascade arc volcanism correlate with a marked decrease in the rate of Farallon‐North America convergence between 43 and and 28 Ma. This slowdown may be responsible for (1) westward stepping of the volcanic arc front from the Challis axis to a Cascade axis at about 42 Ma; (2) a subsequent episode of increased ash flow tuff volcanism and extension in the Cascade arc between 37 and 18 Ma that correlates with the “ignimbrite flare‐up” in the Basin and Range; and (3) a period of extensional basaltic and alkalic volcanism and intrusion in the Coast Range between 44 and 28 Ma. Reduction of the convergence rate and westward stepping of the flexure in the subducted slab may have reduced the horizontal compressive stress on the continent, allowing increased injection of magma into the crust, development of large, shallow magma chambers, and the outbreak of extensional volcanism over a large area behind the Farallon‐North America subduction zone.
The Makah Formation of the Twin River Group crops out in a northwest-trending linear belt in the northwesternmost part of the Olympic Peninsula, Wash. This marine sequence consists of 2800 meters of predominantly thin-bedded siltstone and sandstone that encloses six distinctive newly named members four thick-bedded amalgamated turbidite sandstone members, an olistostromal shallow-water marine sandstone and conglomerate member, and a thin-bedded water-laid tuff member. A local unconformity of submarine origin occurs within the lower part of the Makah Formation except in the central part of the study area, where it forms the contact between the older Hoko River Formation and the Makah. Foraminiferal faunas indicate that the Makah Formation ranges in age from late Eocene (late Narizian) to late Oligocene (Zemorrian) and was deposited in a predominantly lower to middle bathyal environment. The Makah Formation is part of a deep-marginalbasin facies that crops out in the western part of the Olympic Peninsula, in southwesternmost Washington and coastal embayments in northwestern Oregon, and along the central part of the coast of western Vancouver Island. On the basis of limited subsurface data from exploratory wells, correlative deep-marginal-basin deposits underlie the inner continental shelf of Oregon and the continental shelf (Tofino basin) along the southwestern side of Vancouver Island. Directional structures in the Makah Formation indicate that the predominantly lithic arkosic sandstone that forms the turbidite packets was derived from the northwest. A possible source of the clastic material is the dioritic, granitic, and volcanic terranes in the vicinity of the Hesquiat Peninsula and Barkley Sound on the west coast of Vancouver Island. Vertical and lateral variations of turbidite facies suggest that the four packets of sandstone were formed as depositional lobes on an outer submarine fan. The thin-bedded strata between the turbidite packets have characteristics of basin-plain and outer-fan fringe deposits.
A gravity and magnetic survey of the Strait of Juan de Fuca and adjacent Pacific continental shelf was conducted to define the tectonic framework in this 20 to 35 km wide seaway and its relation to that of Vancouver Island and the Olympic Peninsula. The offshore extensions of large onshore faults are delineated by linear magnetic and gravity anomalies. One of these, the Leech River fault of southern Vancouver Island, marks the northern limit of oceanic-type basaltic basement present in western Washington and Oregon. This fault probably continues southeast-ward from Vancouver Island across the strait to near the northeastern coast of the Olympic Peninsula, and westward across the strait to the continental shelf off Cape Flattery. The Calawah fault, which extends northwestward from near Cape Flattery onto the Pacific shelf, terminates the Leech River fault. Northwest of the Leech River fault on the shelf, the Calawah fault probably is the contact between oceanic and continental crustal types. The gravity and magnetic data also indicate the location of folds, other faults, and areas of shallow basement rocks.
The geology was originally compiled at 1:48,000 scale on a composite base made from 24 seven and a half minute quadrangles. The geology was subsequently scanned into ARCINFO, a proprietary geographic information system, to produce the digital map. The geology was composited with a scanned topographic base derived from the old 15 minute base maps to expedite Open File publication.A major goal of our investigation was to establish the basic stratigraphy and structure necessary to guide energy, mineral, hydrologic, and geologic hazard investigations in northwest Oregon. The map area is centered on the Eocene Tillamook Volcanics and related rocks which form the core of the Coast Range uplift, and includes Tertiary marine strata ranging from Eocene to Miocene age on its eastern and western flanks (Figure 1). Most of the Eocene volcanic sequence consists of thick accumulations of submarine and subaerial basalt interbedded with deep water marine sedimentary rocks. At present, hydrocarbon exploration is concentrated in the shallow marine sequence which overlies the Tillamook Volcanics. About 30 km north of the map area is Mist, Oregon's only producing gas field. At Mist, the producing zones are in the shallow marine sandstones of the Eocene Cowlitz Formation above the Tillamook Volcanics. In the map area, repetitive basalt sequences dominate the stratigraphy, but arkosic sandstone that locally interfingers with Tillamook Volcanics to the northeast could be a potential reservoir at depth. Oil shales that are locally exposed in the Yamhill Formation beneath the Tillamook Volcanics may indicate petroleum source rock potential in the region.Also of interest in the Tillamook Highlands are local areas of mineralization that are concentrated in previously unrecognized silicic flows and intrusions of the Tillamook Volcanics (see map). Semi-quantitative emission spectroscopic analyses of small quartzsphalerite-pyrite veins in silicic flow breccias and fault zones indicate the presence of zinc, arsenic, silver, and gold (Table 1).Another important goal of this study was to determine the tectonic environment of the Eocene Tillamook Volcanics, which comprise a large part of the northern Oregon Coast Range. These basalt flows interfinger with marine strata and are chemically and GEOLOGIC SUMMARYThe Tillamook map area crosses a broad, northeast-plunging structural arch in Tertiary volcanic and sedimentary strata that form the northern Oregon Coast Range (Figure 1). The core of the uplift consists of Eocene basalt and interbedded marine strata which were previously correlated with the Siletz River Volcanics of the central Oregon Coast Range (Wells and Peck, 1961). In this study, the Eocene volcanics have been divided into five units, and a distinction is made between the lower Eocene Siletz River Volcanics and the overlying Tillamook Volcanics of late middle Eocene age. Marine mudstone and sandstone are interbedded with all of the volcanic units and comprise most of the late Eocene to Miocene stratigraphic section which forms the flank...
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