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A complex history of Pleistocene erosion, sedimentation and basaltic fissure flows is recorded along the ancestral and present Snake River drainage of the Snake River Plain in southeastern Idaho. The area studied is between Blackfoot and Eagle Rock, but special emphasis was given to an area between Blackfoot and American Falls Reservoir. Five Pleistocene sedimentary units are recognized in the Fort Hall area and consist of three units of gravel separated by two fine-grained alluvial or lacustrine units. The units record two cycles of basalt damming of, and basalt-dam breaching by the Snake River. The upper lacustrine unit is known as American Falls Lakebeds and the lower fine-grained unit expressed only in the subsurface is called unit B. In the Blackfoot-Pocatello area south of the Snake River, Pleistocene sediments lie unconformably on Pliocene basalt. North of the Snake River near Tilden Bridge, the age relations among the numerous basalt flows that intertongue with the sediments are locally ambiguous, but general correlations are made that seem reasonable based on lithologies reported in well logs. Between Blackfoot and American Falls Reservoir, two major channels apparently were incised into Pliocene basalts by the ancestral Snake River. One channel was developed west of the present Snake River and west of Ferry Butte (Pingree channel). This channel is older than about 310,000 years, the age of basalt flow west of the Snake River at Tilden Bridge, on the west side of Ferry Butte, that apparently caused rerouting of the Snake River eastward into the Fort Hall channel on the east side of Ferry Butte. The ancestral Snake River was rerouted many times by the repeated outpouring of basaltic lavas during late Pleistocene time. Virtually all of the basalts that modified the course of the ancestral Snake River in the Blackfoot-Eagle Rock area were derived from vents that lie west of the Snake River. Basalt-dam breaching by the Snake River led to deposition of extensive gravel deposits between Blackfoot and American Falls Reservoir that consist of two gravel units separated by American Falls Lakebeds. Locally, in the Fort Hall channel, the American Falls Lakebeds were eroded prior to the deposition of gravel of Pinedale age, and the upper (Pinedale) and middle gravels are indistinguishable. Near Springfield, and near the northernmost shore of American Falls Reservoir, American Falls Lakebeds lie unconformably on the basalt of Crystal Springs, which is about 210,000 years old. At this locality, the basalt is only about 6-9 feet thick. The lake beds are about 3-6 feet thick, and they are unconformably overlain by gravel of Pinedale age that is generally less than 10 feet thick; this gravel is locally overlain by at least 3 feet of loess. INTRODUCTION This study of the Pleistocene alluvial and lacustrine deposits and basalts of the Snake River drainage between Eagle Rock and Blackfoot was undertaken to better understand low-grade placer gold occurrences that were mined around the turn of the century. The stratigraphic sequence of the sediments in the ancient courses of the river is important to locating the upstream source of the gold, as well as silver tellurides (Desborough and others, 1986) in the gravels. The surficial geology of the area was mapped by Scott (1982), and Scott and others (1982) have revised the Quaternary stratigraphy and chronology in the area. Earlier studies of Quaternary geology in the area include those of Stearns and others (1938), Stearns and Isotoff (1956), Trimble and Carr (1961a, 1961b, 1976), Carr and Trimble (1963), and Trimble (1976). Our study concentrated on the area between Blackfoot and American Falls Reservoir. BLACKFOOT-FORT HALL-SPRINGFIELD AREA The area is chiefly east of the Snake River, north of Pocatello and southwest of Blackfoot (fig. 1). No sites of subsurface data are shown on figure 1, and only a few (those used in the cross sections) are on figure 3. Most of the subsurface data is from drillers logs of waterwells and therefore may be subject to some uncertainty in both lithology and depth. The Fort Hall Bottoms lies east of the Snake River, between Ferry Butte and the America Falls Reservoir; the bottoms are generally enclosed on the east side by the 4,400-foot topographic contour. The Fort Hall upland area is an alluvial bench east of the Fort Hall Bottoms and Snake River; this area lies above the 4,400-foot contour and below the 4,500-foot contour. In the Fort Hall uplands we sampled and examined natural and artificial exposures of gravel and sand and examined subsurface data of water-well drillhole logs. In the Fort Hall Bottoms we dug 17 backhoe pits to depths of about 15 feet and drilled 8 shallow holes to depths of 120 feet. In addition, 85 drillers logs were used for the Fort Hall upland area between Blackfoot and Fort Hall. Stratigraphy of the Blackfoot-Pocatello area In the area between Pocatello and Blackfoot, Michaud Gravel covers much of the benchlands and was deposited by the Bonneville Flood that entered the area via the Portneuf River drainage (fig. 1) (Trimble and Carr, 1961a, 1961b). Scott and others (1982) show that the Bonneville Flood occurred about 14,000-15,000 years ago. Michaud Gravel, as well as extensive but thin Holocene eolian sand, obscures older deposits and makes interpretation of the Pleistocene history difficult. Trimble and Carr (1961a, 1961b) noted that the basal erosional contact of the Michaud Gravel with the underlying American Falls Lakebeds "is uniformly about 4,400 feet in altitude" in the Michaud Flats area. Table 1 contrasts the distinctive characteristics of the Snake River gravel deposits versus the Michaud Gravel, and the data suggests that except for basalt, igneous rocks are absent from Michaud Gravel, which is composed mostly of clasts of sedimentary and metasedimentary rocks. The Fort Hall upland area, north of Fort Hall, is thinly mantled by eolian sand and Michaud sand and gravel deposits, that total less than about 15 feet in thickness. The Fort Hall Bottoms is thinly mantled by Holocene soil developed on very fine grained alluvium that averages about 8 feet thick, and that unconformably overlies alluvial Pleistocene gravel and sand. Figure 2 shows a generalized sequence of late Quaternary sediments in the Fort Hall area as determined by the U.S. Geological Survey, drilling and examination of 85 water-well drill logs. The American Falls Lakebeds were named by Stearns and others (1938, p. 69) for sediments deposited in a lake formed by the Cedar Butte Basalt flow that dammed the Snake River southwest of Eagle Rock Formation. Carr and Trimble (1963, p. G28) and Trimble and Carr (1976) restricted the formation to the upper part of the beds named by Stearns and others, with the base of the formation as a 0.7 foot thick gravel at an altitude of about 4,320 feet. The beds below the gravel are referred to as the Raft Formation. The American Falls Lakebeds in the Michaud flats area (fig. 1) are well exposed in the bluffs that rim most of American Falls BT B. Ba sa lt of Ti ld en Br id ge BC B. Ba sa lt of Cr ys ta l Sp ri ng s CB B. Ce da r Bu tt e Ba aa lt R IV E R y P o ca te ll o F i g u r e 1 . L o c a t i o n of st ud y a r e a an d sa li en t g e o g r a p h i c l o c a l i t i e s an d g e o m o r p h i c f e a t u r e s in th e B l a c k f o o t E a g l e R o c k ar ea of th e S n a k e Ri ve r Pl ai n, s o u t h e a s t e r n Id ah o. Table 1. Characteristics of Snake River gravel deposits and Michaud Gravel in the Fort Hall and Pocatello area, Idaho Snake River gravel deposits Michaud Gravel deposits SORTING AND BEDDING Well-sorted and distinctly bedded with large-scale and small-scale crossbedding. Not sorted, bedding features virtually absent. CLAST AND GRAIN CONSTITUENTS Conspicuous obsidian grains 0.5 to 1.0 mm, pink to maroon pebbles and cobbles of Proterozoic metaargillite and metasiltstone, grains of rhyolite glass in the size range of 0.5-2.0 mm with red, blue and milk tints, sparse cobbles of rhyolite and welded tuff, sparse Archean granite clasts, cobbles of quartzite. Large boulders of basalt, limestone, and quartzite. Cobbles of chert limestone, dolomite, and quartzite common. SIZE OF LARGER CLASTS Generally smaller than 10 cm. and many larger than 30 cm. Generally larger than 10 cm Reservoir and along the valley walls of the lower parts of Bannock Creek and the Portneuf River. "The upper limit of the American Falls Lakebeds is at an altitude of about 4,400 feet, and the total thickness is about 80 feet" (Trimble, 1976, p. 54). Ridenour (1969) demonstrated that the lower part of the American Falls Lakebeds (unit D, fig. 2) is alluvial rather than lacustrine, because exposures along the south side of American Falls Reservoir consist of crossbedded sand. However, because of the generalized nature of the available drill hole logs we have used, the most logical distinction to be made was between coarse (gravel-bearing) material versus fine (sand-, silt-, and clay-size) material. Unit D (fig. 2) is American Falls Lakebeds; it includes fine sand and silt associated with clay-size material, the coarser portions of which may be of fluviatile origin. The ancestral Snake River occupied a broad, prominent channel cut on an erosion surface on Pliocene(?) basalt bedrock, based on drill-hole information for the subsurface (sections A4-E and F-F', figs. 3, 4, and 5 (at end of report)). Prior to the Bonneville Flood (about 14,000-15,000 years ago) the Snake appears to have had two main channels, one northwest of Ferry Butte (the Pingree channel) and the other between Ferry Butte and Buckskin dome (the Fort hall channel) (see figs. 5, 6, 7 (at end of report)). These channels are newly recognized and named in this report. Ferry Butte, composed of Pliocene(?) basalt flows and cinders, at times may have been an island in a broad channel; at least once it was the eastern buttress of a basalt dam formed by the basalt of Ti

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Pleistocene sediments and basalts along the Snake River in the area between Blackfoot and Eagle Rock, southeastern Snake River Plain, southeastern Idaho