Chapter 2 Quaternary Geology of the Channeled Scabland and Adjacent Areas VICTOR R. BAKER Department of Geological Sciences The University of Texas at Austin Austin, Texas 78712 PRECEDING PAGE BLANK NOT FILME* ABSTRACT ville affected the southern margin of the Chan- neled Scabland about 30,000 years B.P. The Quaternary history of the Channeled The last major episode of flooding occurred Scabland is characterized by discrete episodes of between about 18,000 and 13,000 years ago. It catastrophic flooding and prolonged periods of probably consisted of two outbursts from Glacial loess accumulation and soil formation. The loess Lake Missoula. The earlier outburst predates the sequence is correlated with Richmond's Rocky Vashon maximum (= Withrow Moraine of the Mountain glacial chronology. Two pre-Bull Lake Okanogan ice lobe). This flood aflccted Moses (pre-lllinoian) loess units are characterized by Coulee, the Grand Coulee (prior to its present siliceous petrocalcic horizons. The Bull Lake configuration), and the eastern Channeled Scab- loess (Palouse Formation) apparently accumu- land (Telford-Crab Creek and Cheney-Palouse lated episodically be downwind accretion, fol- scabland tracts). A second flood, probably in- lowed by periods of relative stability and soil volving less volume than the first, coincides with formation. The Palouse paleosols have platy the deglacial phase of the Okanogan ice lobe. It calcic horizons but do not show petrocalcic hori- mainly affected the Columbia River northwest of zons. Pinedale (= Fraser Glaciation) loess is the Channeled Scabland downstream from a hy- paler in color than the older units; its palcosol draulic constriction of the canyon at the site of calcic horizons lack the platy structure of the Coulee Dam. The last phase of that flood prob- older loess paleosols. ably also involved catastrophic flow down the At least five major catastrophic flood events oc- Grand Coulee. Slackwatcr fucics of this flood curred in the general vicinity of the Channeled contain the Mount St. Helens set "S" ash erupted Scabland. The earliest episode occurred prior to about 13,000 years B.P. according to D. R. Mul- the extensive deposition of the Palouse Forma- lineaux and co-workers. tion. Its surviving records are but fragmentary. Probable Missoula flood deposits in the Cheney- Palouse scabland tract are overlain by the younger pre-Palouse loess and a petrocalcic soil INTRODUCTION profile. Flood deposits in the Quincy Basin came from an unknown western source across Babcock The Channeled Scabland of eastern Wash- Ridge. The Quincy Basin flood deposits are over- ington (Fig. 2.1) consists of a spectacular com- lain by other flood deposits of probable Bull Lake plex of anastomosing channels, cataracts, loess age (Illinoian) also derived from a western "islands," and immense gravel bars created by the source. Catastrophic flooding from Lake Bonne- catastrophic fluvial erosion of the loess and basalt 17 Preceding page blank of the Columbia Plateau (Bretz and others, in the Quaternary history of eastern Washington. 1956). The erosion and deposition that produced This paper will summarize the general Quaternary the scabland topography resulted from the failure geology of the Channeled Scabland and present of the ice dam impounding glacial Lake Mis- some new data on the number of catastrophic soula. At its maximum outflow, near the end of floods. the last major Pleistocene glaciation, the lake discharged as much as 21.3 x 10° mVsec into the vicinity of Spokane, Washington (Baker, PHYSIOGRAPHY, CLIMATE, AND SOILS 1973a). Recently the origin and history of the Channeled Scabland has assumed new significance Freeman and others (1945) have formalized because of morphologic similarities to outflow the physiographic divisions of the regions covered channels of probable flood origin on Mars (Baker in this report. The entire study area lies in the and Milton, 1974; Sharp and Malin, 1975; Columbia Basin Subprovince of the Colum- Masursky and others, 1977). Despite detailed bia Intermontane Physiographic Province. The study since the 1920's (Bretz, 1923a, 1928c, Columbia Basin is a regional lowland surrounded 1932b, 1959, 1969; Richmond and others 1965), by the Blue Mountains to the south, Cascade the exact number and timing of major floods Mountains to the west, the Okanogan Highlands which resulted from the outpourings of Lake to the north, and the mountains of northern Idaho Missoula remains a major unresolved problem in the east. The region is also informally called 120° 119° 118° 47° 120° 118° Figure 2.1. Map of the Channeled Scabland in eastern general extent of loess (Palouse Formation) that was not Washington showing the distribution of channels and the stripped away by the last major episode of flooding. 18 OFifilNM. PAGE W OF FOUR QUALITY the "Columbia Plateau," but intense folding in The precipitation pattern is paralleled by a the western part produces a series of basins with change in Great Soil Groups to the northeast. intervening ridges. The basalt bedrock of the Sierozem soils form in the dry, southwestern basin has the regional aspect of a structural basin. portions of the plateau. Toward the northeast, The western part of the Columbia Basin, Brown, Chestnut, and Chernozem soils appear in termed the "Yakima Folds," consists of a series successively wetter portions of the plateau. These of anticlinal ridges extending eastward from the soils occur only on the loess and other fine- Cascade Mountains. Several of these ridges are grained sediments. The eroded scabland channels transected by the Columbia River. From north are generally devoid of these parent materials for to south these are the Frenchman Hills, Saddle soil formation. Mountains, and Horse Heaven Hills. The northwest portion of the Columbia Basin, the Waterville Plateau, was not appreciably dis- turbed by the Neogene folding. A major canyon, ANTEDILUVIAN EVENTS ON THE Moses Coulee, is deeply excavated into the Water- COLUMBIA PLATEAU ville Plateau and extends southwestward from its center to the Columbia River Valley. The Yakima Basalt comprises the bedrock in all but a few parts of the Channeled Scabland. The far eastern part of the Columbia Basin is This basalt unit is part of the extensive Neogene characterized by relatively undeformed basalt eruptions of plateau basalts that cover over 250,- overlain by as much as 75 m of Pleistocene loess. 000 km2 in parts of Washington, Oregon, and The loess has been dissected to form a rolling Idaho. Most of the lava was erupted during the topography known as the Palouse Hills. Eleva- Miocene. The lava flows are exceptionally thick, tion of the Palouse Hills declines from about 750 and several can be traced over 150 km. Consid- m on the northeastern margin to 100-120 m in erable structural and lithologic variation can be the southwest. This gradient reflects the regional found in the basalt sequence, including joint- dip of the basalt toward the center of the basin. ing patterns, pillow-palagonite complexes, sedi- Between the Palouse Hills and the Yakima mentary interbeds (from lakes on the Miocene Folds, extensive stripping occurred of the loess land surface) and geochemical variation. On the mantle by the catastrophic flooding of glacial north and east margins of the plateau, the basalt Lake Missoula. The maturely dissected loess is interbedded with extensive deposits of siltstone presents an abrupt contrast with the black cliffs and shale of the Latah Formation, deposited as and ragged appearance of the flood-eroded scab- drainages were blocked by the basalt outpour- land tracts. Steep scarps on the eroded channel ings. margins resemble wave-cut headlands rising above Deformation of the basalt sequence was most a desiccated sea. Bretz (1923b) named this re- extensive during the Pliocene. The entire Colum- gion the Channeled Scabland. bia Plateau was regionally tilted from an eleva- The Columbia Plateau lies in the rain shadow tion of about 760 m in the northeast to about of the Cascade Mountains. Precipitation in the 120 m in the southwest near Pasco, Washington. western basins (elevation 180-300 m) is less than Superimposed on the regional structure are the 20 cm per year, but increases with elevation east-west fold ridges described earlier. The up- northeastward, reaching nearly 50 cm per year raised northern rim of the plateau is especially on the updip margins of the plateau (elevation significant for the flood history of the Channeled 850 m). The low precipitation results in a lack Scabland. Only a truly phenomenal quantity of of perennial drainage through the huge ancient water could fill the great canyon of the Columbia channels, called "coulees." The Columbia and River between Spokane and Coulee Dam. That Spokane Rivers, which are deeply entrenched filling would be necessary to have water spill over along the northern and western margins of the the northern rim of the plateau and flow south- plateau, intercept most of the drainage from the westward, carving the great scabland channels. areas of higher precipitation in the bordering During the Pliocene, the great structural basins mountains. of the western scablands accumulated a sequence 19 of partly consolidated silt, gravel, and clay known ology of Pierce and others (1976), Bull Lake as the Ringold Formation (Flint, 1938a; New- time correlates to lllinoian, about 125,000 to comb, 1958). The age of this formation was 200,000 years ago. Two pre-Bull Lake loess units poorly interpreted until Eric Gustafson studied can be recognized in the field by the well-in- the extensive upper Ringold vertebrate fauna from durated, siliceous petrocalcic horizons that formed the White Bluffs area. Gustafson's White Bluffs on them. The older of the two petrocalcic hori- fauna is early Blancan (Pliocene) in age. The zons is colored pinkish by an associated oxi- faunal assemblage correlates best with the Hag- dized tuff. These petrocalcic horizons consist of erninn fauna of Idaho, dated at about 3.5 x 10° as much as 0.6 m of roughly horizontal carbonate years B.P.