Silver Falls State Park

ORECiON CiEOLOCiY formerly THE ORE BIN published by the Oregon Department of Geology and Minerallndustlfes Volume 41 , Number 1 January 1979 OREGON CiIOLOCiY To our readers: Volume 41, Number 1 J ~ nu t!lry 1979 This issue of OREGON GEOLOGY i ntroduces Publhhed IIIIOnthly by the State of Oregon Department of Geology and Hineral Industries (Volumes 1 through the fourth format for the Depart ment of Geol 40 were e ntitled The Or e 1111'1) . ogy and Mineral Industries' monthly publica tion. Changes began 40 years ago, when THE GOVERNING BOARD ORE BIN replaced the PRSSS· BULLETIN. Then, Leesnne C. MacCO!1, Chairperson, Po rtland in 1962, the mimeographed ORE BIN became the Robert W. Ooty • . talent popular printed version . John L. Schwabe Porthnd The magazine's readership has grown . STATE GEOLOGIST Well mo re than one-third of the copies mail ed Donald A. Hull in the U.S .A. go outside Oregon . In addition. DEPUTY STATE GEOLOGIST some of t oday's subscr iber s live in Canada. John D. 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Any r eview should be Doted in book reviews. the acknowledgmenta. Th~ March issue will bring you the De Second cl ass postage paid at Portland, Oregon. partment ' s annual reports on Oregon' s min Postmaster: Send address changel to OREGON GEOLOGY , eral and metallurgical industry. mined land 1069 State Office Building, Port land, Oregon 9720 1. reclamation. geothermal energy. and oil and gas explorat ion. The writers will be able t o treat these t opics more comprehensively COVER PHOTO: and accurately by waiting unt i l all t he data South Falls, Silver Falls State Pa rk. has been compiled before beginning their own See article beginning on page 3. (Photo articles . courtesy Oregon S t ate Highway Division) Remember to keep us informed of your whereabouts . The U. S. Postal Ser vice will CONTENTS not forward second class mail, even when it Silver Falls Stat e Park --- - Page 3 is a first class magazine. Columbia River Basalt Stratigraphy in western eregon - ---- - - Page 11 A.B. 2 OREGON GE:OLl)Gl:', VOL. 41; NO.1; JANlJARY 1979 Silver Falls State Park by Michael Freed, Department of Resource Recreation Management, Oregon State university This article introduces the reader to the general geology of one of Oregon's most popular state parks. A more technical discussion of the stratigraphy of the Co lumbia River Basalt Group in western Oregon follows on page 11. INTRODUCTION THE UPPER MIOCENE Silver Falls State Park embraces one of About 15 million years ago, during the the greatest concentrations of waterfalls in Miocene, a series of basaltic lava flows the West. Within a short radius, 14 falls called the Columbia River Basalt Group tumble over the narrow, rocky courses of erupted from great fissures or cracks in the the North and South Forks of Silver Creek; ground and then covered over 50,000 sq mi of dense groves of old growth timber, dotted by portions of Oregon, Washington, and Idaho. sunny meadows, surround the misty canyons. Silver Falls State Park is located on the It took millions of years of rock and soil western edge of the plateau formed by these deposition and subsequent erosion to form flows, labeled "Tcr" on the geologic map. this unique landscape. A 7-mi loop trail Although the exact source of the specific makes it possible to observe traces of the flows found in Silver Falls State Park has long geologic history at close range and to not yet been determined, swarms of feeder enjoy a rich variety of plant and animal dikes near Monument, La Grande, and Grand life. Ronde, eastern Oregon; near Yakima, Washing ton; and along the Oregon and Washington Largest of all Oregon's state parks, coasts indicate former centers of igneous Silver Falls lies about 30 mi east of Salem activity. No other area with such volumi in the foothills of the Cascade Mountains. nous flood basalt flows is known to exist The park is easily accessible by State in the continental United States. Highway 214 and remains open all year. Although each individual Columbia River THE BEGINNINGS Basalt flow is chemically homogeneous, vari ous zones within a flow may differ in ap The oldest rocks in the park date back pearance. As lava cools it shrinks, and to the Oligocene, more than 26 million years tensional stresses within the flow produce ago, when a sea covered most of western Ore fractures called joints. Because various gon and the Coast Range was an archipelago. parts of the flow cool at different rates, Marine sediments, shown as "Tm" on the geo zones with distinct jointing patterns may logic map, were deposited in the Silver often develop within anyone flow. For ex Falls area. The ocean receded, and the ample, slow cooling just above the base of Oligocene sandstone was tilted and deeply a thick flow may result in very regular ver eroded. tical jointing, often producing a zone with six-sided columns that together form a col onnade. Elsewhere in the flow, very irreg ular and closely spaced joints may combine to form the entablature. Either the entab lature or the colonnade may appear more than once in a flow and may be thick, thin, or absent. Where the lava was highly charged with ) gas, gas bubbles remaining throughout the i i cooling process produced holes called ves i icles within the rock. Basalt with many I I holes is described as vesicular basalt. i ._._. __ ._._. ..i During the several-thousand-year in tervals between basalt flows, soil zones developed on the surfaces of previous Index map showing location of flows. These zones are now represented by Silver Falls State Park, Oregon. thick interbeds of soil, sand, silt, and OREGON GEOLOGY, VOL. 41, NO.1, JANUARY 1979 3 organic material. parts of which were geologic map: the massive pumice tuff, "baked" by the heat of a new lava flow basaltic andesite flows , and agglomerate entering the area. conglomerate beds are called the Fern Ridge tuffs (Thayer, 19)9) and are labeled "Tst" All these features can be found within on the geologic map; pyroxene andesite flow the park boundaries. At least three Colum rock, labeled "Tsa" , overlies the Fern Ridge bia River Basalt flows have been observed in tuff . Although these rocks were deeply this area, and Barlow (1955) describes five eroded during the Pleistocene and relatively separate flows at South Silver Creek Falls. little of the softer units remains exposed today. the Fern Ridge tuff and associated THl PUOClME AHD PLlISTOClNE rocks may be seen at the North Falls parking lot . During the Pleistocene, volcanoes to the east blanketed the Silver Falls area RECENT TIMES with lava flows , breccias, and tuffs of the Sardine Formation. Two of the five Sardine The 14 waterfalls along the North and Formation subunits have been mapped in Silver South Forks of Silver Creek are of more re Falls State Park and are indicated on the cent origin, and the story of their forma- GEOLOGIC MAP OF CANYON TRAILS AREA, SILVER rALLS STATE PARK. (AFTER HAMPTON, 1972) LEGEND Quaternary landslides TC~ Columbia Piver Basalt Sardine Formation .J:{1( : Marine sandstone, - .. , Tsa, andesite flow rock ~ siltstone, and tuff Tst, Fern Ridge tuff Hiking trail , OREGON GEOLOGY, VOL. 41, NO. L JANUARY 1979 tion is of particular interest to park and water percolating from above. Manga visitors . nese, Silica, and iron have been leached out and r edeposited on the ceiling . Gap A tributary stream generally cuts its ing holes , called erosional chimneys, are channel as rapidly as the main stream it formed by continual enlargement of c racks joins. But in Silver Falls State Park, and fissures under the attack of ice and ~here the s treams fl o~ over Columb ia River percolat ing ~aters.

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