Record Details
Geology of the Tillamook Head--Necanicum Junction area, Clatsop County, northwest Oregon
ScholarsArchive at Oregon State University
| Field | Value |
|---|---|
| Title | Geology of the Tillamook Head--Necanicum Junction area, Clatsop County, northwest Oregon |
| Names |
Neel, Robert Holmes
(creator) Niem, Alan R. (advisor) |
| Date Issued | 1975-07-10 (iso8601) |
| Note | Graduation date: 1976 |
| Abstract | Five distinct lithologic units compose the Tertiary rocks in the Tillamook Head - Necanicum Junction area of the northern Oregon Coast Range. They are: the late Eocene to early Miocene Oswald West mudstones, the middle Miocene Angora Peak sandstone and Silver Point mudstone members of the Astoria Formation, and the middle Miocene intrusive and extrusive Depoe Bay and Cape Foulweather basalts. These units are locally overlain by Pleistocene marine terraces and Holocene beach and dune sands, stream alluvium, and landslide deposits. The Oswald West mudstones, Angora Peak sandstones, and Silver Point mudstones are informal stratigraphic names used in this study. The Oswald West mudstones consist of over 2, 000 feet of well-bedded, intensely burrowed, silty mudstones and siltstones interstratified with minor glauconitic sandstones, tuff beds, and thick bedded tuffaceous siltstones Foraminifera, trace fossils, glauconite, and the general fine-grained character of these rocks suggest that deposition occurred in an open marine deep-water environment, possibly as part of a prodelta or on the outer continental shelf. The Angora Peak member is composed of several hundred feet of moderately sorted, medium- to coarse-grained quartzose-feldspathic and lithic sandstones. The sandstones unconformably overlie the Oswald West mudstones and are thick bedded, laminated, and less commonly cross-laminated. Sedimentary structures, sorting, and stratigraphic relationships suggest that the sandstones were deposited in a high energy, wave-dominated environment, possibly as delta sheet sands. The sands were, in part, redistributed by waves and longshore drift to form offshore coastal barrier bars and.linear clastic shoreline deposits. The overlying 650-foot thick Silver Point member consists dominantly of dark gray, micaceous, laminated mudstones and very thin siltstones. Rhythmically interbedded mudstones and fine-grained turbidite sandstones occur in the lower part of the unit. Deposition occurred in cool, low energy, open marine conditions, probably of sublittoral to upper bathyal depths. Oversteepening of Angora Peak sheet sands on the delta front may have resulted in periodic slumping of sands which were transported by turbidity currents into a shelf basin of the deeper water Silver Point delta-slope environment. An east to west paleocurrent pattern in the Salver Point turbidite sandstonesis consistent with this model. Sandstone petrography and heavy mineral suites of the Astoria Formation suggests a dominantly volcanic and sedimentary provenance for these strata, probably the pre-Miocene volcanic rocks of the western Cascades and local Coast Range Eocene basalts and sandstones. Mineralogy and rare rock fragments in the sandstones also suggest that some detritus was derived from the granitic, metamorphic, and Paleozoic sedimentary terrains of eastern Oregon and Washington, and from British Columbia and western Idaho, possibly transported via an "ancestral Columbia River drainage system. Numerous dikes, sills, and irregular-shaped plutons of Depoe Bay Basalt and Cape Foulweather Basalt intruded the Oswald West mudstones and Astoria Formation. The major intrusive body in the area is the 900-foot thick sill that forms Tillamook Head. At Ecola State Park, the forceful intrusion of this sill into the semi-consolidated Silver Point strata at very shallow depths produced local synsedimentary folds. Over 1, 000 feet of extrusive Depoe Bay Basalt palagonitized pillow lavas and breccias and 2.00 feet of very local Cape Foulweather pillow lavas lie with angular unconformity over the Silver Point member. Deformation which accompanied the general Coast Range uplift during the late Miocene to Pleistocene formed four northeast-trending folds in the thesis area and two sets of high angle faults which strike north-south and northwest- southeast through the area. In addition, active landslides are abundant throughout the area, particularly along the coast. Economic resources of the area include basalt quarry rock for rip-rap and road base, basalt stream gravels for road aggregate, and potential petroleum resources may occur in the adjacent offshore area. |
| Genre | Thesis |
| Topic | Geology -- Oregon -- Clatsop County |
| Identifier | http://hdl.handle.net/1957/8981 |
