617
The Yahoo Clay, a Lacustrine Unit Impounded by the
McKinney Basalt in the Snake River Canyon Near Bliss, Idaho
Harold
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.
E. Malde’
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
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
AND
CLASSIFICATION
SEQUENCE
Melon
:
z
E
h
5
PRESENT
AND
Gravel
Melon
5 McK,nney
Basalt
C? Wendell
Grade Basalt
u
Basalt
8 Sand Springs
2
Crowsnest
Gravel
a,
s
&
m
2
$,
Sand
Thousand
Springs
Madson
Basalt
Basalt
$
4
0
Bruneau
Formation
Tuana
Gravel
Glenns
Ferry
Formaton
Banbury
Basalt
ldavada
Volcamcs
Figure
2. Listing
stratigraphy
Melon
Valley,
in
of former
the area
Idaho.
GEOLOGIC
Gravel
2
Crowsnest
g
Yahoo
;
McKlnney
$
Wendell
Gravel
Clay
Basalt
Grade
Sprmgs
Thousand
Madson
(0
CLASSIFICATION
SEQUENCE
Basalt
Basalt
Springs
Basall
Basalt
Tuana
Gravel
Glenns
Ferry
Banbury
Basalt
ldavada
“olcamcs
Formation
and present
classifications
extending
upstream
from
of the
Bliss
to
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
of Idaho
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 margins 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
CLAY AND
OF THE LACUSTRINE
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
EAST
r
3500
3500h
Original top of
QTgf
A
2500
2500
WEST
EAST
3500-
Y
-3500
%
z2
3100-
?
>Q 2900Y
w
B
2700
2700
0
I
1
I
2 MILES
J
Figure
3. Diagrammatic
cross
sections
of the Snake
River
canyon
near Bliss (A) and at Hagerman,
identified
as follows:
Qm. Melon
Gravel;
Qcg, Crowsnest
Gravel;
Qy, Yahoo
Clay; Qmk, McKinney
Qss, Sand Springs
Basalt;
Qtm, Malad
Member
of Thousand
Springs
Basalt;
Qma, Madson
Basalt;
age; QTgf, Glenns
Ferry
Formation
of Quaternary
and Tertiary
age; and Tb, Banbury
Basalt
the north abutment at the proposed site had penetrated 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 causedme to reinterpret previously ambiguous 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
Idaho
(E). The geologic
units
are
Basalt;
Qwg, Wendell
Grade Basalt;
Qt, Tuana
Gravel,
all of Quaternary
of Tertiary
age.
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 basewhere 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
Cenoroic
620
Geology
of Idaho
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 McKinney 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
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
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
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 McKinney 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
Clay Near
Bliss
621
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 McKinney 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
622
Cenozoic Geology of Idaho
Figure6. Outcropat the southendof the Shoestring
Roadbridge(locality2 on FigureI) showinglayerscomposed
dominantlyof glassy
fragments
typicalof the McKinneypillowlavaoverlainby laminated
YahooClay.The upwardchangefrom fragmental
basaltlo clay
takesplacein an intervalabout8 inches
thick, whichischaracterized
by alternatinglaminae
of thesecontrasting
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 cascadethat
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. Remnants 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 aquiferhence, 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. Identifying the clay as Yahoo is consistent with the postThousand Springs age of the canyon, but correlating
the clay with the Bruneau Formation-a
stratigraphic
unit much older than the Thousand Springs Basaltrequires 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
ANCESTRAL
TO THE
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
624
Cenozoic
Geology
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 similarly 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 muscorum (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.
of
Idaho
According to E. B. Leopold (written communication, 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), Cyperaceae (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
625
BIiss
Figure 8. Deltaic foreset beds of McKinney
pillow lava plunging to the
which formed the rim of the ancestral Snake River canyon (locality
abruptly,
at an altitude of 3,175 feet and is overlain by a subaerial lava
outcrop is a roadcut on U. S. Highway
30 (2,OM) feet N., 1,100 feet
left (southwest)
over Madson Basalt, seen here at the lower right,
P on Figure 1). The pillow lava terminates
upward,
more or less
flow of McKinney
Basalt, which forms the present canyon rim. The
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. Meanwhile, 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 lifetime, and that it leaked enough to prevent overflow of
the lava dam. The path of leakage could only have
626
Cenozoic
Geology
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 equilibrium 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
of
Idaho
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 Pleistocene 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 Company report, “A J Wiley Hydro Electric Development,” 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 primarily 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
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 Pleistocene 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 Thousand 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 comprehensible, 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
Clay
Near
Bliss
627
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. Schuster, 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
measurements in Idaho, 1894-1967: U. S. Geological Survey, Boise, Idaho, Basic-Data Release, 310 p.
628
Cenozoic
of
Geology
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 Quaternary 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 Cenozoic stratigraphy of the western Snake River Plain,
Idaho: Geological Society of America Bulletin, v.
73, p. 1197-1220.
Idaho
1972, Geologic map of the Glenns FerryHagerman 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.
and streamflow
Simons, W. D., 1953, Irrigation
depletion in Columbia River basin above The
Dalles, Oregon: U. S. Geological Survey WaterSupply 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.
-7