Studies in Oregon. 1976 : Bowen and Others

APRIL 1977 VOLUME 39, No. 4 STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES The Ore Bin Published Month ly By STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTR I ES Head Office: 1069 State Office Bldg., Portland, Oregon 9nOl Telephone, [5031 229-5580 FIE LD OFFICES 2033 First Street 521 N.E. "E" Street Boker 97814 Grants Pass 97526 Subscr~tion Rates 1 year, $3. ; 3 years, $8.00 Available back issues, S.25 at counter, $ .35 moiled Second closs postoge paid at Portland, Oregon GOVERNING BOARD R. W. deWeese, Portland, Chairman Leeonne Mac:Coll, Portland Robert W. Dofy, Talent STATE GEOLOGIST Rolph S. Moson GEOLOGISTS IN CHARGE OF FIELD OFFICES Howard C. Brooks, Baker Len Ramp I Grants Pass Permission is granted to reprint information contained herein. Credit given the Stale of Oregon Department of Geology and Mineral Industries for compiling this informationwitl be appreciated. State of Oregon The ORE BIN Department of Geology and 1'1ineral Industries Volume 39, No.4 1069 State Office Bldg. April 1977 Port 1 and, Oregon 97201 FIELD GUIDE TO THE GEOLOGY OF CORVALLIS AND VICINITY, OREGON R.D. Lawrence, N.D. Livingston, S.D. Vickers, and L.B. Conyers Geology Department, Oregon State University, Corvallis Introduction and Geologic Background This field guide had its origin in a class in environmental geol ogy taught in the spring of 1971. The original road log has been modified so that those with an introduction to elementary geology will find it self-guiding. It emphasizes the practical aspects of local geology. It is divided into two parts, each of which provides a pleasant bicycle trip for an afternoon. The first half of the trip discusses the area west and north from Corvallis and deals largely with the bedrock geology of the area and the structure responsible for major landforms. The second half covers the area east and south of town and deals largely with surface sediments and the behavior of the Willamette River. Western Oregon is divided into four geo f.10rphi c provi nces: the Cascade Range, the \oJ il lamette Valley, the Coast Range, and the Kla- ma th Mounta ins. Corva 11 is, 1oca ted at the wes t ern edge of the Wi 11 amette Va 11 ey adjacent to the Coas t Range (Fi gure 1), 1 i es on the wes t bank of the Willamette River, mostly north of the mouth of Marys River. The Coast Range reaches its maximum elevation about 15 miles west of town at Marys Peak. From Marys Peak, a long ridge called Vineyard Mountain (MacDonald State For est) trends northeast past town. The Corvallis i§ fault, located along the southeastern edge of < u this ridge, separates erosion-resistant volcanic Vl< U rocks on the northwest from soft sedimentary rocks whi ch are mostly covered by va 1 iey all uvi um on the southeast (Figure 2). The Corvallis fault forms the eastern boundary of the Coast Range near Corvall is. The four bedrock units under the Corvall is Figure 1. Geomor area are, from oldest toyoungest: the Siletz phic provinces River Vol canics, the Tyee Formation*, the Spencer of western Ore Formation, and diabase dikes and sills (Vokes and gon. others, 1954). The fi rst three units were * Mapped as Flournoy Formation by Baldwin (1974). 53 deposited during Eocene time (40-60 million years ago), and the dikes and sills were intruded into them during the mid-Oligocene (25-30 million years ago). No younger materials were preserved until the accumulation of unconsolidated terrace, flood, and floodplain deposits of Pleistocene to Recent age (less than 100,000 years old). The bedrock units are more widely exposed in the Coast Range, whereas the unconsolidated materials are typical of the Willamette Valley. The Siletz River Volcanics are basaltic rocks many thousands of feet thick that erupted onto the early to early-middle Eocene sea floor from submarine vents (Macleod and Snavely, 1973). They are the oldest rocks in the area and include basaltic pillow lavas, breccias, tuffs, and associated basaltic sedimentary rocks. The lower part of the unit is similar to oceanic crustal basalts and is thought to be oceanic crust which accreted onto the continen tal margin. In the upper part of the unit, rocks of more varied composition accumulated locally in sufficient thickness to form island similar to modern seamounts (Snavely and others, 1968). The Siletz River Volcanics crop out only northwest of the Cor vallis fault in the vicinity of Corvallis. The unit is widely exposed in the Coast Range. The Tyee Formation, which overlies the Siletz River Volcanics with little or no unconformity, is a thick sequenc;eofrhythmically bedded sandstones and siltstones of middle Eocene age (Snavely and others, 1964). Many of the individual beds ( 6 inches to 3 feet thick) are composed of sediments which are graded in size from coarse at the bottom to fine at the top and are interpreted as marine turbidite deposits derived from the Klamath Mountains, to the south. The sediments, which become finer to the north, were deposited in a relatively deep trough which they gradually filled. The Tyee Formation is widespread in the Coast Range and is over 6,000 feet thick in places, such as west of Marys Peak (Baldwin, 1955). The few deep exploratory wells near Corvallis (Newton, 1969) suggest that it thins rapidly to the east. The Tyee For mation is exposed in several hills along the southeast side of the Corvallis fault. The Tyee Formation is overlain unconformably by the Spencer Formation, a 4,500-feet thick series of basaltic, arkosic, and micaceous marine sandstones. Most outcrops of the Spencer For mation are thick bedded without the grading typical of the Tyee Formation. The unconformity, which is generally sl ight, is quite pronounced near the Corvallis fault northeast of lewisburg, where a 50° discordance is reported (Vokes and others, 1954). The Cor vallis fault apparently developed, at least in part, during the time between the deposition of the Tyee and Spencer Formations. Because the latter was deposited during late Eocene times, the fault must have been active during late-middle to early-late Eo cene and probably continued to be active through late Eocene, as locally it cuts beds of the Spencer Formation. Indeed, the upper Spencer Formation was probably derived in large part from erosion of the Tyee Formation, which formed the block rising northwest of the Corvallis fault. 54 F'igW"e 2 . AeJ"iaL oblique view oj COY'V llis showing tY'aee oj Cor vallis JauU along the base oj Vineyapd Mountain . Note old millY'ace south oj Ma Y'ys River. (Photo aoupiesy oj" Western Waus ~ Inc. F'igW"e 3. Origin and Y'oute of the Spokane Flood UXlter s that de posited t he Wil lamette sitt. 111 Route of flood Waters C. lee Lobes o " -miles 55 Marine Oligocene rocks. present to the south near Eugene (Baldwin. 1964) and to the east under the alluvium of the Willa mette Valley (Newton. 1969). are not present near Corvallis. If they ever were present. they have been removed by erosion. During the middle Oligocene. dikes and sills of diabase (shallow intru sive rocks of basaltic composition) were injected into the Eocene deposits. Near Corvallis. sills seem to'be common along the Tyee/ Spencer contact. These dikes and sills are relatively erosion resistent and so support many of the ridges and hills. such as r~arys Peak (4.097 feet). in the area. Neither Miocene nor Plio cene materials are present. and it is unlikely that they ever were. Thus the geologic record has a gap of about 25 million years. during which erosion probably occurred. before the most recent materials were deposited. The most recent episode of uplift of tne Coast Range occurred near the end of this period of erosion and may be continuing at the present. The Willamette Valley,a structural depression be tween the Coast Range and the Cascades. collects water draining from both the east and west to form a major north-flowing river. the Willamette. Thus. this is one case where a river did not carve its own valley. The bottom of the structural depression is interrupted by several sets of hills. and as it has only a gentle slope to the north. the river has often been unable to carry away all of its sediment load. Most of this sediment comes from the Cascades. as the largest part of the Coast Range drainage goes directly into the sea. During the Pleistocene. great fans of sediments from the Cascades repeatedly accumulated in the valley and forced the Willamette River over to the western edge of the valley (Beaulieu. Hughes. and r'1athiot. 1974). Remnants of these sediments. sometimes deeply weathered. are preserved as terraces in the valley. In general. the higher the terrace is above the valley floor. the older it is (Balster and Parson. 1968). Toward the end of the most recent of these periods of accu mulation. the Wi11amette Valley was a minor participant in one of the most dramatic events in the geologic history of the Pacific Northwest - the Spokane Flood (McKee. 1972). In northern Idaho a lobe of the great continental ice sheet dammed the Clark Fork River. creating Lake Missoula. an enormous lake which extended into Montana. The ice dam failed abruptly. and the lake emptied in a period of a few days. Grand Coulee and the channeled scab lands of southeastern Washington are a result of this great (es timated at 50 cubic miles). but brief. flood of water. All of this water reached the ocean via the Columbia River; and at the sharp bend in the river at Portland.

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