Stratigraphy, sedimentation, and petrology of the Tertiary rocks in the Bear Creek-Wickiup Mountain-Big Creek area, Clatsop County, Oregon

Abstract

Six Tertiary rock units are exposed in the Bear Creek-Wickiup Mountain-Big Creek area. They are: the late Oligocene to early Miocene Oswald West mudstones; the Big Creek sandstone member, the Pipeline member (a new unit), and the Silver Point mudstone member of the early to middle Miocene Astoria Formation; and the middle Miocene Depoe Bay and Cape Foulweather petrologic-type basalts. Quaternary alluvium locally overlies these units. The Oswald West mudstones consist of over 2,000 feet of mudstones, tuffaceous siltstones, and subordinate very fine- to fine-grained sandstones which were deposited in a shallowing, open-marine, continental-shelf environment. The tuffaceous nature indicates considerable input from explosive dacitic and/or rhyolitic volcanism in the ancestral Cascade Range. Regression subaerially exposed the thesis area during the middle early Miocene. The Big Creek sandstone member unconformably overlies the Oswald West mudstones. It is a transgressive, finingupward, 1,500-foot sequence of littoral and shallow-marine sandstones which were deposited on a subsiding, inner-continental shelf. The overlying Pipeline member consists of 2,600 feet of very thickly bedded medium-grained arkosic sandstones complexly intertonguing with deep-marine mudstones. Sandstone intrusions into the mudstones are common. Fossils, lithologies, and geometry suggest the member is a submarine-canyon facies. The structureless sandstones were probably deposited by grain-flow and fluidized-flow mechanisms. The Silver Point mudstone member consists of up to 1,600 feet of structureless to well-laminated hemipelagic mudstones deposited in a middle-continental shelf to upper-slope setting. The unit is lateral to, and overlies, the Pipeline member. Aphanitic to fine-grained Depoe Bay Basalt intruded all the Tertiary sedimentary units. Local submarine volcanic centers formed over 400-foot sequences of pillow basalts, structureless to well-bedded hyaloclastites, and thick, structureless basaltic breccias, which are intruded by feeder dikes. Stratified, locally graded deposits are thought to be the result of turbulent, hot volcanic glass-water vapor mixtures transported by density currents. Minor subaerial flows and basalt conglomerates indicate that some volcanic centers built up above sea level. Locally derived pillow basalts and breccias appear to underlie Columbia Plateau-derived subaerial basalt flows of the Columbia River. Group to the east of the thesis area. The overlying, sparsely porphyritic Cape Foulweather Basalt is also of local submarine eruptive origin. Overall lithologies are similar to the Depoe Bay Basalt. A fan-like topographic expression and fossils suggest accumulation of stratified hyaloclastites on a submarine apron similar to those recognized surrounding some modern seamounts and basaltic islands. Heavy minerals and sandstone petrology suggest both andesitic and regionally metamorphosed source areas, which are most likely the ancestral Cascades and areas to the east and north. A major river system, similar to the modern Columbia River drainage, is postulated to have been the main transporting agent of Oswald West and Astoria detritus to the thesis area. Some Big Creek sandstones were deposited or reworked by longshore currents. The Pipeline sandstones are generally more feldspathic and have higher K-feldspar/plagioclase ratios than the Big Creek sandstones. The two sandstones can be differentiated mineralogically on a quartz-plagioclase-potassium feldspar ternary diagram, and texturally on a bivariant plot of deviation versus mean diameter. The Depoe Bay and Cape Foulweather Basalts are chemically similar to the correlative units which occur on the Oregon Coast. In the thesis area, the two units are most easily differentiated by the consistently higher Ti02 in the latter unit. Post-Astoria deformation superimposed high-angle normal faults and gentle north-south folds on a regional northward dip. Petroleum potential within the area is low, but projection of the Pipeline member offshore suggests the existence of ideal accumulation sites off the mouth of the modern Columbia River.