the yahoo clay, a lacustrine unit impounded by the...

617

The Yahoo Clay, a Lacustrine Unit Impounded by the McKinney Basalt in the Snake River Canyon Near Bliss, Idaho

Harold E. Malde’

ABSTRACT

Damming of the ancestral Snake River by the late Pleistocene McKinney Basalt formed a temporary, but long lasting, lake as much as 600 feet deep, which is here called McKinney Lake. The lake extended upstream from the town of Bliss along a canyon much like the present canyon through Hagerman Valley and the Thousand Springs area to Melon Valley. Pillow lava of the McKinney Basalt partly filled the lower reach of McKinney Lake, and lacustrine clay then filled the remainder of the lake. The clay-here named the Yahoo Clay of the Snake River Group for deposits found near the mouth of Yahoo Creek southwest of Hagerman-is misidentified on published geologic maps as being part of the Bruneau Formation, an older unit which is now considered to be limited to the area west of Bliss. Accordingly, evidence for relatively cool and moist conditions, which is provided by fossil mollusks and pollen from lake clay upstream from Bliss, pertains to the Yahoo Clay, not to the Bruneau Formation. The relations of the Yahoo Clay to McKinney pillow lava and to older deposits are described here from features found at numerous outcrops, and certain puzzling relations of the lake clay that can be found on the published maps are explained in light of this new stratigraphic knowledge. Curiously, McKinney Lake did not overflow until the Yahoo Clay had accumulated nearly to the height of the McKinney lava dam. This finding suggests that the buildup of thick lacustrine deposits behind other lava dams in the region may have been aided by a controlled process of leakage, which inhibited overflow and downcutting. Finally, the age of the Crowsnest Gravel at Hagerman is now considered to be younger than the McKinney Basalt because of its position on the Yahoo Clay. Gravel downstream from Bliss that appears to have been deeply incised before eruption of the McKinney Basalt probably has been wrongly correlated with the Crowsnest Gravel at Hagerman.

iU. S. Geological Survey, Denver, Colorado 80225.

INTRODUCTION

The interplay of late Pleistocene lava flows and the ancestral Snake River in southern Idaho has produced many complex, sometimes bewildering, stratigraphic and physiographic features. No features found thus far are as puzzling as those associated with the McKinney Basalt, which erupted from McKinney Butte 9 miles northeast of the town of Bliss, Idaho (Figure 1).

The role of the McKinney Basalt in damming a former course of the Snake River was explained in an earlier paper (Malde, 1971), thereby accounting for the accumulation of thick deposits of McKinney pillow lava in the resulting temporary lake. The present paper is concerned with evidence that lacus-

I j

Figure I. Index map showing numbered localities of Yahoo Clay described in the text. The letter P marks an outcrop of pillow lava shown in Figure 8.

618 Cenozoic Geology of Idaho

trine clay in the canyon upstream from the lava dam, which was previously assigned to an older lacustrine unit (the Bruneau Formation), was also deposited in the same lake. The significance of the lake clay for understanding the physiographic history of the Snake River canyon is then briefly mentioned. Finally, the age of river gravel that overlies the lake clay (the Crowsnest Gravel) is also redefined. The former and present classifications of the local stratigraphy are shown in Figure 2.

\GE FORMER CLASSIFICATION PRESENT CLASSIFICATION

AND SEQUENCE AND SEQUENCE

Melon Gravel Melon Gravel

: 5 McK,nney Basalt 2 Crowsnest Gravel

C? Wendell Grade Basalt z u

g Yahoo Clay

E 8 Sand Springs Basalt ; McKlnney Basalt

h 2 Crowsnest Gravel $ Wendell Grade Basalt

a, Thousand Springs Basalt 5 s

m Sand Sprmgs Basalt

& Madson Basalt 2 $, Thousand Springs Basall

Madson Basalt

Bruneau Formation Tuana Gravel (0

$ Tuana Gravel Glenns Ferry Formation

4

Glenns Ferry Formaton Banbury Basalt

Banbury Basalt ldavada “olcamcs 0

ldavada Volcamcs

Figure 2. Listing of former and present classifications of the stratigraphy in the area extending upstream from Bliss to Melon Valley, Idaho.

GEOLOGIC SETTING

The McKinney Basalt is the youngest and most westerly volcanic unit in an area distinguished by the overlap of late Pleistocene basaltic lava flows on thick, older formations of basalt and poorly consolidated stream and lake deposits (Malde, 1965). The boundary between these two major geologic divisions is steep and is broadly traced by the canyon of the Snake River, with the older deposits forming a dissected canyon wall 700 feet high to the left of the river, and the younger lava flows forming cliffs that reach more than 500 feet above the river on theright. (Left and right are to be understood in the sense of looking downstream.) The canyon floor is variously composed of both the older and younger geologic units, depending on vagaries of past deposition and erosion (Figure 3).

When the geologic units are subdivided and mapped in greater detail, the Snake River is seen to have become entrenched in its present canyon at Bliss by downcutting that was impeded from time to time by

the eruption and spread of the late Pleistocene lava flows. As the river cut downward, it was forced progressively farther south and west along the mar- gins of successive canyon-filling lava flows (Malde, 1971, Figure 11). With respect to the present canyon, the most significant geologic event was the eruption of the canyon-filling McKinney Basalt. The basalt dammed the ancestral Snake River at a place still concealed from 2 to 3 miles west of Bliss, thus forming a temporary lake more than 600 feet deep that extended many miles upstream along the canyon of that time. In this paper, the body of water impounded by the lava dam of McKinney Basalt is informally named McKinney Lake. The McKinney Basalt was deposited in two facies: upland lava flows at the level of the canyon rim and pillow lava that formed within the ancestral canyon near Bliss where basalt spilled into the canyon (Figure 4).

From evidence described later in this paper, lacustrine clay began to accumulate in McKinney Lake immediately after deposition of the pillow lava and ultimately reached a height nearly equal to the level of the McKinney lava dam. Eventually, the lake overflowed, and the lava dam was breached along its southern margin by downcutting in poorly consolidated silt and clay of the Glenns Ferry Formation. The Snake River thus began to cut its present canyon west of Bliss, and McKinney Lake was correspondingly lowered and drained. When downcutting had reached a level about 300 feet above the present Snake River in Hagerman Valley, about 8 miles upstream from Bliss, terrace deposits called the Crowsnest Gravel were formed, mostly on the lacustrine clay of McKinney Lake. After the Snake River canyon had nearly achieved its present width and depth, flood debris from catastrophic overflow of Pleistocene Lake Bonneville was deposited in large bars along the canyon floor, in places reaching heights more than 200 feet above the modern river (Malde, 1968). Erosion during entrenchment of the canyon and the havoc produced by the Bonneville Flood have limited the exposures of the McKinney pillow lava to discontinuous outcrops, and the lacustrine clay is correspondingly preserved in widely scattered remnants.

RELATION OF THE LACUSTRINE CLAY AND THE McKINNEY BASALT

The knowledge that McKinney pillow lava is overlain by lacustrine clay came initially from stratigraphic information in a proposal by the Idaho Power Company, dated August 26, 1980, to build a dam on the Snake River near Bliss. Core drilling in

Malde- Yahoo Clay Near Bliss 619

WEST

3500h

EAST

r 3500

QTgf

Original top of

A 2500 2500

WEST EAST 3500- Y -3500

%

z -

2 3100-

?

>Q

Y 2900-

w

B

2700 2700

0 1 2 MILES I I J

Figure 3. Diagrammatic cross sections of the Snake River canyon near Bliss (A) and at Hagerman, Idaho (E). The geologic units are identified as follows: Qm. Melon Gravel; Qcg, Crowsnest Gravel; Qy, Yahoo Clay; Qmk, McKinney Basalt; Qwg, Wendell Grade Basalt; Qss, Sand Springs Basalt; Qtm, Malad Member of Thousand Springs Basalt; Qma, Madson Basalt; Qt, Tuana Gravel, all of Quaternary age; QTgf, Glenns Ferry Formation of Quaternary and Tertiary age; and Tb, Banbury Basalt of Tertiary age.

the north abutment at the proposed site had pene- trated as much as 90 feet of thinly laminated clay resting on a southward-dipping surface of McKinney pillow lava (Figure 5).

This finding prompted me in September and October, 1980, to review the geology extending upstream from the proposed dam site. Several new exposures were found, showing lacustrine clay on McKinney pillow lava, and the new stratigraphic knowledge caused me to reinterpret previously ambig- uous field relations. My understanding in the 1960s because I had not then seen the lacustrine clay on McKinney Basalt, was that the clay is correlative with clay of the Bruneau Formation and, hence, is older than the McKinney Basalt. To my chagrin, but to my satisfaction, the investigations by the Idaho Power

Company, together with my own recent observations, now show that the Bruneau Formation does not exist at the proposed dam site or in the present canyon area above Bliss. Rather, the Bruneau Formation is found only farther downstream, where it occupies a broad older canyon. Its base where intercepted by the Snake River canyon some 11 miles west of Bliss is at an altitude of 2,675 feet-an elevation considerably too high to account for lacustrine clay at the same altitude near river level at the dam site. In short, all the clay previously mapped as Bruneau lake beds upstream from the proposed dam (that is, unit Qbs on Sheet I of the map by Malde and Powers, 1972) is incorrectly identified. The lacustrine clay is a unique geologic unit that accumulated behind the lava dam formed by the McKinney Basalt, and the clay occupies

620 Cenoroic Geology of Idaho

L Figure 4. Sketch map showing the distribution of McKinney Basalt

near Bliss, Idaho. This geologic unit consists of pillow lava that occupies a former canyon of the Snake River and lava flows that rest on the pillow lava as well as on the surrounding upland of older rocks. Edges of the lava flows are exposed as rimrock by erosion along the right side of the present canyon.

an ancestral canyon of the Snake River that is much younger than the canyon occupied by the Bruneau Formation.

The contact of the clay on McKinney pillow lava is not widely exposed, but certain outcrops that are described below show that the clay was deposited directly on the pillow lava. In one particularly instructive outcrop, layers of glassy basaltic fragments that were produced during formation of the pillow lava grade upward into laminated clay (locality 2 described below). This relationship suggests that the

clay began to accumulate immediately after the pillow lava was deposited, at least in the initial deposits of clay at low altitudes in the canyon of that time. Other outcrops show that the lacustrine clay accumulated from an altitude of 2,650 feet at present river level near Bliss to an altitude of 3,180 feet, thus deeply filling the canyon upstream from the Mc- Kinney lava dam. The distribution of the clay suggests that the ancestral canyon was physiographically much like the present canyon in its width, depth, and location.

STRATIGRAPHY AND DISTRIBUTION OF THE LACUSTRINE CLAY

In retrospect, H. A. Powers recognized many aspects of the stratigraphy of the lacustrine clay during his field work in 1954 and 1955, although he obviously did not know its relation to the McKinney Basalt. Powers found that the most continuous and best exposed deposits of the lacustrine clay are in a dissected area near the mouth of Yahoo Creek, which joins the Snake River about 4 miles south-southwest of Hagerman.

NAME AND TYPE LOCALITY

Accordingly, although the physical relations of the clay to McKinney Basalt can be determined only at other outcrops, the lacustrine clay associated with the McKinney Basalt is here named the Yahoo Clay from remnants of the clay in the area of the lower reach of Yahoo Creek. The clay at its type locality rests on an inclined surface cut on the Glenns Ferry Formation (part of the left wall of a former canyon of the Snake River) in sections 3,4, and 5, T. 8 S., R. 13 E., and in

Figure 5. Cross section between the left and right abutments of a proposed dam on the Snake River about a mile downstream from Bliss, Idaho (at mile 565 near the center of sec. 12, T. 6 S., R. 12 E., Bliss quadrangle). The unit labeled “Older alluvium”is a local body of mixed debris at the base of the pillow lava facies of the McKinney Basalt. Beds l-4 are local sedimentary layers interbedded in the Banbury Basalt. (Adapted from a drawing submitted by the Idaho Power Company to the Federal Energy Regulatory Commission, August 26, 1980.)

Ma/de- Yahoo Clay Near Bliss 621

the adjacent parts of sections 8, 9, and 10 to the south, as shown on the map by Malde and Powers (1972).The abrupt contact on the Glenns Ferry is well exposed where crossed by the Crowsnest Road (3,400 feet N., 2,000 feet E., SW car., sec. 10, T. 8 S., R. 13 E., Yahoo Creek quadrangle).

The Yahoo Clay in this area rises continuously from its base at an altitude of 2,900 feet near the mouth of Yahoo Creek to an altitude of 3,180 feet in sections 4, 5, 8, and 9. (Because of mistakes made when compiling the published map, the clay is erroneously shown as locally reaching altitudes higher than 3,180 feet in this area.) Throughout this area, the Yahoo Clay is monotonously uniform in its lithology, consisting of laminated clay and sparse amounts of silty clay, commonly in laminae about 1/ inch thick. The color, when freshly exposed, ranges from pinkish white (7SYR 8/2) to light yellowish brown (IOYR 6/4), based on terminology used on the Munsell Soil Color Charts (1954 edition). The clay is compact and hard when dry, breaking with a conchoidal fracture, but readily slakes in water. The eroded surface weathers to form a loose, “popcorn” surficial layer a foot or more thick, possibly because of the presence of clays that swell when wet.

At the top of the Yahoo Clay in this area, as well as at other places described later in the cliffs to the west of Hagerman, the clay is weathered to form a soil profile about 6 feet thick. On the soil is as much as 15 feet of surficial sand and silt that have been mapped as some of the deposits of an erosion surface related to the Crowsnest Gravel (Malde and Powers, 1972). In 1955, I noted that the upper part of the soil on the Yahoo Clay has weathered to dark brown (IOYR 4/3) and that the lower part of the soil is mottled with nodules of secondary calcium carbonate.

REFERENCE LOCALITIES

In addition to the type locality of the Yahoo Clay at Yahoo Creek, several other localities provide further information about the stratigraphy and distribution of the clay. These places, which are indicated by numbers on Figure 1, are described below as reference localities.

Locality 1. The farthest downstream remnant of Yahoo Clay is preserved in the right abutment of the proposed dam at altitudes between 2,675 and 2,800 feet, resting on a southward-dipping surface of Mc- Kinney pillow lava (Figure 5). Contiguous outcrops of the clay extend half a mile upstream and rise to an altitude of 2,900 feet.

Locality 2. At the south end of the Shoestring Road Bridge (2,150 feet N., 800 feet E., SW car., sec. 7, T. 6 S., R. 13 E., Bliss quadrangle) and at an

altitude of 2,675 feet, a recent excavation for the road shows that Yahoo Clay overlies rubble composed of unsorted angular blocks of Madson Basalt mingled with McKinney pillow lava. The rubble grades upward to as much as 5 feet of bedded fragments of glassy basalt and then to lacustrine clay (Figure 6). The transition to Yahoo Clay is within an interval 8 inches thick marked by alternating laminae of clay, silt, and basaltic fragmental material. The relations suggest an uninterrupted transition from deposition of the McKinney pillow lava to deposition of the Yahoo Clay, In short, the pillow lava and the clay appear to be nearly contemporaneous.

Locality 3. Above and below old U. S. Highway 30, 1 mile south of Bliss along an east-west nose in the canyon wall, the Yahoo Clay rests on a steep, westward-plunging slope of McKinney pillow lava, which drops from an altitude of about 3,050 feet to 2,925 feet (the contact on the highway is exposed at 1,750 feet N., 500 feet W., SE car., sec. 7, T. 6 S., R. 13 E., Bliss quadrangle).

Locality 4. Yahoo Clay makes up much of the exposed material of a large landslide immediately south of Bliss. The highest exposure of clay (1,600 feet S., 200 feet E., NW car., sec. 8, T. 6 S., R. 13 E., Bliss quadrangle) is at an altitude between 3,150 and 3,175 feet, adjacent to a cliff of Madson Basalt at the head of the landslide, The exposure is contiguous to an outcrop of the Glenns Ferry Formation at the same altitude.

Locality 5. A recent excavation made for the King Hill Canal (1,250 feet N., 2,500 feet E., SW car., sec. 20, T. 6 S., R. 13 E., Bliss quadrangle) exposes Yahoo Clay on as much as 6 feet of steeply dipping colluvium, which in turn rests on the eroded Glenns Ferry Formation. The colluvium contains abundant angular blocks from the Madson Basalt (which is preserved as a remnant 1/ mile southeast and 125 feet higher) as well as rounded pieces of older basalt and pebbles similar to those found in Tuana Gravel higher in the canyon wall. Good exposures in this area make the Yahoo Clay continuously recognizable from river level to an altitude of 3,175 feet in the canyon wall to the west.

Locality 6. Landsliding and gully erosion in Yahoo Clay at the west end of the former siphon for the King Hill Canal (750 feet N., 150 feet E., SW car., sec. 27, T. 6 S., R. 13 E., Hagerman quadrangle) has exposed the base of the clay at an altitude of about 2,800 feet. The clay rests on rubble that has some of the characteristics of rubble at the Shoestring Road Bridge (locality 2 described above). The rubble contains substantial amounts of fragmental glassy Mc- Kinney lava as well as scattered McKinney pillows. The rubble may have originated as colluvium (such as a former talus deposit), but it locally contains sorted


Figure 6. Outcrop at the south end of the Shoestring Road bridge (locality 2 on Figure I) showing layers composed dominantly of glassy fragments typical of the McKinney pillow lava overlain by laminated Yahoo Clay. The upward change from fragmental basalt lo clay takes place in an interval about 8 inches thick, which is characterized by alternating laminae of these contrasting materials.

layers in the upper part. Irregularities on the upper surface of the rubble, which dips steeply toward the river, are filled by laminae of the overlying Yahoo Clay (Figure 7). Like locality 2 at the Shoestring Road Bridge, these features suggest the lack of a hiatus between deposition of the McKinney pillow lava and the Yahoo Clay.

Locality 7. Along a road known as the Tupper Grade, on the east side of Billingsley Creek, which here forms the eastern edge of the Snake River canyon, a new road cut shows that Yahoo Clay overlies Wendell Grade Basalt at an altitude of 3,090 feet (800 feet S., 1,400 feet E., NW car., sec. 19, T. 7 S., R. 14 E., Tuttle quadrangle). The clay is continuously exposed at least as high as 3,120 feet. The Wendell Grade Basalt here forms a small cascade that spilled from the canyon rim onto talus and landslide debris along the ancestral canyon wall. Much of this old debris is still in place and is now exposed, having been exhumed by erosion of the Yahoo Clay. Rem- nants of Yahoo Clay are also found in an alcove in the eastern wall of Billingsley Creek 3A mile to the

south (SWl/,SWl/, sec. 19). The alcove is evidently inherited from an embayment in the canyon wall that existed when the clay was being deposited.

Locality 8. Excavation of a new ditch to collect spring water from the base of Sand Springs Basalt half a mile downstream from Lower Salmon Falls and about 30 feet above the Snake River (550 feet S., 2,050 feet E., NW car., sec. 2, T. 7 S., R. 13 E., Hagerman quadrangle) has exposed Yahoo Clay lying against a cliff formed by the basalt and on talus derived from the basalt. The basalt has previously been mapped as part of the Glenns Ferry Formation (Malde and Powers, 1972), but the new exposure shows that all the basalt here is the Sand Springs.

Locality 9. At an altitude of 3,050 feet along the Brailsford Ditch southeast of Hagerman, the terminus of the Thousand Springs Basalt is overlain by Yahoo Clay (the outcrop extends half a mile southeast from 700 feet S., 1,500 feet E., NW car., sec. 6, T. 8 S., R. 14 E., Tuttle quadrangle). The clay is capped at an altitude of 3,100 feet by a terrace deposit of Crowsnest Gravel. The Thousand Springs Basalt at this locality

Ma/de- Yahoo Clay Near Bliss 623

Figure 7. Outcrop at the west end of the former siphon for the King Hill Canal (locality 6 on Figure 1) showing laminae of Yahoo Clay that fill irregularities at the upper boundary of blocky rubble. The rubble contains substantial amounts of glassy fragments typical of the McKinney pillow lava. The glassy fragments are visible in the photograph as darker areas of granules and small pebbles, which occur as interstitial material and as larger masses surrounded by the blocks.

has been erroneously mapped as a detached remnant, resting against the lacustrine clay (Malde and Powers, 1972). Instead, the basalt appears to be connected with the main body of the Snake Plain aquifer- hence, with the main body of the Thousand Springs Basalt-as shown by newly exposed springs that discharge from pillow lava at its base. The Thousand Springs Basalt in this outcrop probably rests on the Banbury Basalt, as it does elsewhere.

Locality IO. The canyon stretching upstream from Thousand Springs to Melon Valley, which is cut in Banbury Basalt along the margin of the Thousand Springs Basalt, has many scattered remnants of Yahoo Clay. The highest outcrops of the clay reach an altitude of 3,180 feet, where the clay provides the base level for an upland erosion surface related to the Crowsnest Gravel, now considerably dissected. In places, some of the clay is close to river level. Identi- fying the clay as Yahoo is consistent with the post- Thousand Springs age of the canyon, but correlating the clay with the Bruneau Formation-a stratigraphic

unit much older than the Thousand Springs Basalt- requires highly improbable geologic circumstances.

Locality II. The canyon wall west and southwest of Hagerman, besides the extensive deposits of Yahoo Clay near Yahoo Creek, has many scattered remnants of the clay, which rest on the dissected Glenns Ferry Formation. The highest remnants reach an altitude of 3,180 feet and are overlain by surficial sand and silt that are considered to be part of the Crowsnest erosion surface.

RELATION TO THE ANCESTRAL CANYON

The reference localities described above show that the Yahoo Clay was deposited in the ancestral canyon of the Snake River as it existed at the time of the eruption of the McKinney Basalt. Near Hagerman, the canyon was limited on the east by the canyon wall of Billingsley Creek, covered in at least one place by a


cascade of Wendell Grade Basalt. On the west the canyon was defined by a wall eroded in the Glenns Ferry Formation. These limits still exist today, having been exhumed by erosion of the clay. The depth was also much like the present, as shown by Yahoo Clay on eroded Sand Springs Basalt near river level at Lower Salmon Falls. Upstream from Hagerman, where the canyon is outlined on the east by a rim of Thousand Springs Basalt, remnants of the clay simi- larly suggest that the present canyon is simply the exhumed equivalent of the former canyon. As explained elsewhere (Malde, 1971), this reach of the canyon was not cut until after eruption of the Sand Springs Basalt.

FOSSILS

In 1955, D. W. Taylor collected molluscan fossils from the Yahoo Clay west of Hagerman, as well as sediment samples that have yielded identifiable pollen.

From sandy colluvium at the base of the Yahoo (USGS Washington Cenozoic catalog number 19225; center SW% sec. 16, T. 7 S., R. 13 E., altitude 2,875 to 2,940 feet, Hagerman quadrangle), Taylor reported the following mollusks (manuscript report, May 23, 1967): Fossaria da/Ii (F. C. Baker), Lymnaea caperata Say, Gyraulus circumstriatus (Tryon), Pupilla mus- corum (Linnaeus), Vertigo ovata Say, Vallonia gracilicosta Reinhardt, Succinea?, and Discus cronkhitei (Newcomb). In addition, Taylor reported Lymnaeu caperata from three nearby localities of the brown soil at the top of the Yahoo Clay-all at an altitude of 3,180 feet, and all buried by surficial deposits related to the Crowsnest Gravel-as follows: USGS 19219 (500 feet N., 250 feet E., to 700 feet N., 350 feet E., SW COT., sec. 16, T. 7 S., R. 13 E., Hagerman quadrangle); USGS 19222 (1,525 feet N., 150 feet W., to 1,625 feet N., 50 feet W., SE car., sec. 17, T. 7 S., R. 13 E.); and USGS 20410 (1,750 feet S., 175 feet W., NE car., sec. 17, to 1,950 feet S., 350 feet E., NW car., sec. 16, T. 7 S., R. 13 E.). Although the fauna is modern, Taylor pointed out that some of the mollusks no longer live in the immediate vicinity. Gyruulus circumstriatus is characteristic of seasonal ponds, a habitat lacking in southwest Idaho, and its nearest occurrences are at higher elevations in southeast Idaho and northern Nevada. Vertigo ovata and Vallonia gracilicosta are land snails that might be expected in mountains of southwest Idaho, but their nearest known occurrences are also in the southeast part of the state. Thus, he explains, the fossil mollusks provide some evidence that the summers were less hot and dry than at present, at least during the onset of Yahoo time.

According to E. B. Leopold (written communica- tion, January 24, 1957), a pollen sample collected by Taylor from the brown soil at the site of USGS 20410, which is identified as Pollen Locality D1120, yielded the following counts of pollen grains: Picea (16 grains), Pinus (2), Populus (I), Abies (I), Cyper- aceae (2), Gramineae (I), Compositae (5), pollen type ProR-I (I), pollen type CjR-I (I), unidentified dicots (1 I), and spores (3). Taylor’s sample of Yahoo Clay 3.5 feet below the soil (Pollen Locality DI 121) yielded single grains identified as cf. Lycopodium and as Gramineae. Colluvium at the base of the Yahoo where Taylor collected USGS 19225 yielded 2 grains of Pinus and one of Chenopodiaceae (Pollen Locality D1117). Leopold comments that the abundant spruce pollen in the brown soil (D1120), together with the pollen of pine, poplar, and fir, indicates the presence of trees now found several thousand feet higher in central Idaho. Hence, she suggests that the buried soil formed during a time that was wetter and cooler than the present.

RELATION OF YAHOO CLAY TO PILLOW LAVA OF McKINNEY BASALT

In addition to the outcrops already mentioned, the distribution of Yahoo Clay with respect to the McKinney pillow lava upstream from Bliss supports the conclusion that the clay began to accumulate in McKinney Lake immediately after the pillow lava was deposited. In particular, the Yahoo Clay in the area of the McKinney Basalt is largely limited to the left side of the canyon, in places reaching the canyon floor. McKinney pillow lava, on the other hand, is virtually confined to the right side of the canyon. This distribution of the clay and the pillow lava takes on added significance when it is realized that the existing canyon in this reach closely matches the course of the ancestral canyon when the pillow lava was deposited. it appears that the contact between the pillow lava and the clay was steep and that the pillow lava was deposited first on the right side of the ancestral canyon, leaving the remainder of the canyon to be filled with Yahoo Clay.

Some signs of the steepness of the contact are preserved in the right abutment of the proposed dam and in contiguous outcrops that climb 100 feet higher a short distance upstream (locality 1 described above). A steep contact of clay on pillow lava is also expressed by the field relations a mile south of Bliss (locality 3 described above).

The existence of discrete depositional areas for the pillow lava and the clay, and the evidence of a steep

Ma/de- Yahoo Clay Near BIiss 625

Figure 8. Deltaic foreset beds of McKinney pillow lava plunging to the left (southwest) over Madson Basalt, seen here at the lower right, which formed the rim of the ancestral Snake River canyon (locality P on Figure 1). The pillow lava terminates upward, more or less abruptly, at an altitude of 3,175 feet and is overlain by a subaerial lava flow of McKinney Basalt, which forms the present canyon rim. The outcrop is a roadcut on U. S. Highway 30 (2,OM) feet N., 1,100 feet W., SE COT., sec. 21, T. 6 S., R. 13 E., Bliss quadrangle).

contact between them, are understandable from the origin and mode of deposition of the McKinney Basalt. McKinney lava spilled southward and westward into McKinney Lake, building steeply inclined beds of subaqueous pillow lava that advanced into the canyon until the supply of lava ceased. Mean- while, lava flows at the level of the upland advanced over the subaqueous pillow lava much like the advance of topset beds over the foreset beds of a delta. The result was a rim of lava flows along the right side of the canyon from which a face of pillow lava plunged steeply to the bottom of the lake (Figure 8). Because the lava flows of the canyon rim are subaerial, their contact on the pillow lava marks the level of McKinney Lake when these lavas were deposited. On this basis, lake level was at times as high as about 3,175 feet, virtually the same as the altitude of the highest deposits of Yahoo Clay. The floor of the lake was in places at least as low as an altitude of 2,550 feet, or 100 feet below present river level, as shown by the base of the McKinney pillow

lava under the right abutment of the proposed dam (Idaho Power Company drill hole DH-69, 2,550 feet S., 1,950 feet E., NW car., sec. 12, T. 6 S., R. 12 E., Bliss quadrangle).

DRAINAGE OF McKINNEY LAKE

The geologic record of McKinney Lake suggests that its lava dam was more or less tight but also that the dam was surprisingly leaky. On the one hand, the lava dam was capable of holding water deep enough to account for deposits of pillow lava that reach the canyon rim, and to account for deposits of lacustrine clay that reach virtually the same height. On the other hand, the geologic record shows that McKinney Lake must have leaked voluminously throughout its life- time, and that it leaked enough to prevent overflow of the lava dam. The path of leakage could only have


been via the canyon fill of McKinney pillow lava and thence through the lava dam of McKinney Basalt farther downstream. The evidence of a lava dam that was surprisingly leaky, yet effective in obstructing the ancestral Snake River, is the subject of the remarks that follow. Besides the significance of this discussion for understanding the local late Pleistocene history, the example of Lake McKinney may be of interest when considering the geomorphic consequences of other lava dams on the Snake River.

The argument that McKinney Lake leaked voluminously is based on the inference that the lake could not have overflowed for any substantial period of time while the Yahoo Clay was being deposited. Overflow, if it had occurred, would have brought running water in contact with clays and silts of the Glenns Ferry Formation at the southern margin of the lava dam, and rapid erosion of these poorly consolidated sediments would have lowered the lake level, thus preventing buildup of the clay to its observed height. The clay, in fact, as explained above, accumulated throughout the area of McKinney Lake to an altitude of 3,180 feet, a height only slightly below the highest McKinney lava flow in the area of the lava dam. Clearly, the lake surface was virtually at the level of the lava dam when the last of the clay was being deposited. Eventually, of course, McKinney Lake did overflow, downcutting in the Glenns Ferry Formation was thereby initiated, and the lake was progressively lowered. The process of downcutting ultimately formed the present canyon of the Snake River west of Bliss.

The conclusion is inescapable that water was discharged from McKinney Lake not by overflow but by leakage through the canyon-filling McKinney Basalt. Evaporation from McKinney Lake probably was not greater than the present annual evaporation of 37 inches (Meyers, 1962, plate 3) and obviously would have been insufficient to prevent overflow, given the large discharge of the Snake River. (The discharge is discussed below.) Further, the geologic evidence suggests that the lake leaked at a more or less controlled rate, thus maintaining a high stand of lake water, not only when the last of the Yahoo Clay was accumulating, but also when the McKinney pillow lava was being deposited. A condition of equili- brium is implied. The water level was probably achieved and maintained by a relationship between the hydrostatic pressure, the rate of leakage, and the cross-sectional area of the canyon-filling McKinney Basalt at a given depth of lake water. As mentioned below, the accumulation of Yahoo Clay must also have influenced the rate of leakage.

The volume of water lost from McKinney Lake by leakage through the McKinney lava dam must have

been substantial, and this loss must have been maintained continuously while the Yahoo Clay was being deposited. The present discharge of the Snake River at this place averages 10,720 cubic feet per second and amounts to about 8 million acre-feet annually (U. S. Geological Survey, 1974, streamflow data for Snake River at King Hill). To this amount must be added the water depleted for irrigation, which is typically 2.5 million acre-feet annually (Simons, 1953). Thus the present annual discharge is about 10.5 million acre-feet. Probably the late Pleis- tocene discharge of the Snake River was even greater.

The conclusion that McKinney Lake lost water by seepage must be considered in light of the evidence for present-day leakage through the McKinney Basalt. The concealed lava dam formed by the basalt, given the low hydrostatic head produced by the present Snake River, now leaks only a small amount, although the river flows on permeable canyon-filling deposits of McKinney pillow lava that are obviously connected with the lava dam. The leakage is thought to be represented by a discharge of somewhat more than 200 cubic feet per second that rises in the bed of the Snake River at Bancroft Springs, 9 miles downstream, where the canyon-filling McKinney Basalt is intercepted by the present canyon (data credited to Carl Tappan, in an unpublished Idaho Power Com- pany report, “A J Wiley Hydro Electric Develop- ment,” June 5, 1953). The above-river flow from Bancroft Springs is only 17 cubic feet per second (Decker and others, 1970). Probably, the explanation for the small amount of leakage under existing conditions lies in the characteristics of the concealed canyon-filling deposits between Bliss and Bancroft Springs, which are still to be investigated by drilling. The hydrologic properties of McKinney Basalt above present river level, within the vertical range of the Yahoo Clay, are also uncertain,

A final matter worthy of consideration is the means by which the water in McKinney Lake was channeled to the leaky lava dam. Conclusions about the path of leakage are necessarily speculative, but geologic relations suggest that the lake leaked pri- marily by underflow through the McKinney pillow lava. Most significantly, like the clay used to seal man-made reservoirs, the deposition of the Yahoo Clay almost surely impeded direct leakage from McKinney Lake. Also, leakage from the upper few feet of the lake when it became nearly filled with clay cannot be assumed to have been sufficient to discharge the full flow of the Snake River-although some leakage at this level presumably took place until it was reduced by further buildup of the clay. In short, substantial flow through pillow lava under the Yahoo Clay is indicated.

Malde- Yahoo Clay Near Bliss 627

REVISED AGE OF CROWSNEST GRAVEL

The fluvial deposits in Hagerman Valley that are known as the Crowsnest Gravel were previously thought to represent an episode of aggradation during canyon cutting associated with the late Pleis- tocene lava flows of the area. In particular, the gravel was thought to be older than the Sand Springs Basalt, which terminates 300 feet lower on the canyon floor, and hence was considered to be older than the McKinney Basalt (Malde, 1971). It is now clear, however, that the Crowsnest Gravel rests on a surface that was formed during dissection of the Yahoo Clay, and that the gravel is younger than the McKinney Basalt. Accordingly, the Crowsnest Gravel of the Hagerman area is here assigned to a stratigraphic position above the Yahoo Clay.

Downstream from Hagerman, deposits identified as Crowsnest Gravel are not found until the area around King Hill. These deposits form a terrace that appears to have been deeply incised by canyon cutting before eruption of the McKinney Basalt. Thus, although the field relations have not yet been examined from the perspective of the findings discussed here, it is probable that the deposits called Crowsnest Gravel downstream from Hagerman are older than the McKinney Basalt and, hence, have been wrongly correlated with the typical deposits at Hagerman.

DISCUSSION

The stratigraphic relations described here, besides resolving several inconsistencies in my previous understanding of the local geology, provide a new perspective on the Pleistocene history of the western Snake River Plain. On the one hand, the discovery that the McKinney Basalt is associated with a thick body of lake deposits-the Yahoo Clay-explains puzzling and contradictory relations of lake clay to the Thou- sand Springs Basalt, the Sand Springs Basalt, and the McKinney Basalt itself, as represented on published geologic maps. Furthermore, placing the Yahoo Clay in its proper stratigraphic position makes its presence in a canyon of late Pleistocene age fully comprehen- sible, whereas the prior assignment of the lake clay to the Bruneau Formation was physiographically improbable. The Bruneau Formation, on the other hand, is now found to be limited to the area west of Bliss, at least in its exposed deposits. Consequently, the evidence that mollusks and pollen from lake clay upstream from Bliss indicate relatively cool and moist

conditions applies to the Yahoo, not to the Bruneau. The conclusion that the McKinney lava dam

maintained the height of its impounded water through a controlled process of leakage suggests a mechanism that may explain comparatively thick sections of lacustrine deposits behind other lava dams in the region, such as those of the Bruneau Formation farther west (Malde, 1965). Overflow of such lava dams, unless entirely across basaltic rocks, probably would have caused rapid downcutting and would have prevented the buildup of lake deposits. In other words, the presence of a substantial thickness of lake deposits behind a lava dam appears to be related to the leakiness of the dam and to the resulting delay in initiating a new canyon by overflow.

The revised age of the Crowsnest Gravel is of interest for comprehending the late Pleistocene history, to the degree that the gravel may reflect climatic conditions that influenced the regimen of the ancestral Snake River. It has been suggested that deposition of the Crowsnest may have coincided with mountain glaciation (Malde and Powers, 1962, p. 1215). If so, the glacial episode was younger than the McKinney Basalt. By the same token, the older gravel downstream from Bliss, which I have mistakenly correlated with Crowsnest Gravel in previous reports, may represent glaciation older than the McKinney Basalt.

ACKNOWLEDGMENTS

1 thank Peter Leitzke of the Federal Energy Regulatory Commission for arranging a field conference on October 20-21, 1980, to review the geology of the McKinney Basalt and the Yahoo Clay. Douglas E. Sprenger and Donald E. Westcott, representing the Idaho Power Company, participated in the field conference and freely provided stratigraphic and other geological information from their investigations of the proposed dam site at Bliss, Some of the geologic features described here were also reviewed in the field with Harry R. Covington, Robert L. Schus- ter, and Richard L. Whitehead, all of the U. S. Geological Survey, and 1 am grateful to them for their advice and counsel.

REFERENCES

Decker, S. O., R. E. Hammond, L. C. Kjelstrom, and others, 1970, Miscellaneous streamflow measure- ments in Idaho, 1894-1967: U. S. Geological Sur- vey, Boise, Idaho, Basic-Data Release, 310 p.


Malde, H. E., 1965, Snake River Plain, in H. E. Wright, Jr., and D. G. Frey, editors, The Quaternary of the United States, A Review Volume for the VII Congress of the International Association for Qua- ternary Research: Princeton, New Jersey, Princeton University Press, p. 225-263.

-, 1968, The catastrophic late Pleistocene Bon- neville Flood in the Snake River Plain, Idaho: U. S. Geological Survey Professional Paper 596, 52 p.

-, 1971, History of Snake River canyon indicated by revised stratigraphy of Snake River Group near Hagerman and King Hill, Idaho, with a section on paleomagnetism by Allan Cox: U. S. Geological Survey Professional Paper 644-F, 21 p.

Malde, H. E. and H. A. Powers, 1962, Upper Ceno- zoic stratigraphy of the western Snake River Plain, Idaho: Geological Society of America Bulletin, v. 73, p. 1197-1220.

-7 1972, Geologic map of the Glenns Ferry- Hagerman area, west-central Snake River Plain, Idaho: U. S. Geological Survey Miscellaneous Geologic Investigations Map I-696.

Meyers, J. S., 1962, Evaporation from the 17 western States: U. S. Geological Survey Professional Paper 272-D, p. 71-100.

Simons, W. D., 1953, Irrigation and streamflow depletion in Columbia River basin above The Dalles, Oregon: U. S. Geological Survey Water- Supply Paper 1220, 126 p.

U. S. Geological Survey, 1974, Surface water supply of the United States, 1966-70, Part 13, Snake River basin: U. S. Geological Survey Water-Supply Paper 2134, 821 p.

the yahoo clay, a lacustrine unit impounded by the...

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