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Home , Bruneau River, Butte, Hot spring, Red Rock Pass (Idaho)

Geologic Map of the Grand View-Bruneau Area, Owyhee County,

Margaret D. Jenks Bill Bonnichsen Martha M. Godchaux

Idaho Geological Survey Technical Report 98- 1 University of Idaho December 1998 Moscow, Idaho 83844-3014 Contents

Introduction ...... 1 Location ...... 2 General Geologic Setting ...... 2 Structure ...... 5 Hotsprings ...... 6 Acknowledgments ...... 7 References ...... 7 DescriptionofUnits ...... 9 SedimentqUnits ...... 9 Younger Unconsolidated ...... 9 Qal Alluvium (Holocene) ...... 9 Qil Intermittent sediments (Holocene) ...... 9 Qfs Fresh, unvegetated sand (Holocene) ...... 9 Qaf deposits (Holocene and Pleistocene) ...... :...... 9 Qds Vegetated dune sand (Holocene and Pleistocene) ...... 9 Qls Landslide deposits (Holocene and Pleistocene) ...... 9 Qbf Bonneville deposits (Pleistocene) ...... 9 Gravel Surfaces and Lenses ...... 10 QTg Gravel surface, lithology undivided (Quaternary or Tertiary) ...... 10 QTgq Gravel surface, abundant quartzite (Quaternary or Tertiary) ...... 10 QTgqb Gravel surface, abundant quartzite, signiticant basalt (Quaternary or Tertiary) ...... 10 QTgr Gravel surface, abundant rhyolite (Quaternaty or Tertiary) ...... 10 QTgrb Gravel surface, abundant rhyolite, significant basalt (Quaternary or Tertiary) ...... 10 QTge Gravel surface, abundant Eocene volcanics (Quaternary or Tertiary) ...... 11 Tgq Gravel lens or surface, abundant quartzite (Tertiary) ...... 11 Tgqb Gravel lens, abundant quartzite, significant basalt (Tertiary) ...... 11 Tgr Gravel lens or surface, abundant rhyolite (Tertiary) ...... 11 Tge Gravel lens or surface, abundant Eocene volcanics (Tertiary) ...... 11 Tgeb Gravel lens or surface, abundant Eocene volcanics, significant basalt (Tertiary) ...... 11 IdahoGroupSediiments ...... 11 Ti - to sand-sized sediments (Miocene to Pliocene) ...... 12 Tsi Silicic tuff (Miocene to Pliocene) ...... 12 Tf Sand- to gravel-sized sediments, fermginous and phosphatic cemented or stained (Miocene to Pliocene) ..... 13 Tbs Basaltic tuff (Miocene to Pliocene) ...... 13 Tls , undifferentiated (Miocene to Pliocene) ...... 13 Thot Reefal limestone of Hot (Miocene to Pliocene) ...... , .... 13

Tsoo Oolite of Shoofly Creek (Miocene to Pliocene) ...... 13 VolcanicUnits ...... 13 Basalt Volcanoes and Lava Flows ...... 13 NorthAreaUnits ...... 14 Qe Basalt of Rattlesnake Springs (Pleistocene) ...... 14 Qstr Basalt of Strike (Pleistocene) ...... 14 QTsirn Basalt of Simco Road (Pliocene or Pleistocene) ...... 15 QTcrf Basalt of Crane Falls (Pliocene or Pleistocene) ...... 15 Tcg Basalt of Chalk (Pliocene) ...... 15 Tfly Basalt of Flying H Ranch (Pliocene) ...... 15 Tcan Basalt of Creek (Pliocene) ...... 15 Tgsm Basalt of Goldsmith Road (Pliocene) ...... 15 Tkl Basalt of Katie Lane (Pliocene) ...... 15 Tdix Basalt of Dixie Road (Pliocene) ...... 15 SouthAreaUnits ...... 16 Twc Basalt of Winter Camp Butte (Pliocene) ...... 16 Tphb Tuff of the Pot Hole Butte volcanic complex (Pliocene) ...... 16 Tbrc Basalt of Browns Creek (Pliocene) ...... 16 Tpeb Basalt of Pence Butte (Pliocene) ...... 16 Tlbbu Upper basalt of Lower Crossing (Pliocene?) ...... 16 Tlbbl Lower basalt of Lower Black Butte Crossing (Pliocene?) ...... 16 Twfb Pillow basalt of West Fork Browns Creek (Pliocene) ...... 16 Tsog Basalt of Seventy One Gulch (Pliocene) ...... 16 Tcuf Basalt of Curlew (Pliocene) ...... 16 Thc Basalt of (Pliocene) ...... 17 Tid Basalt of Indian Bathtub (Pliocene) ...... 17 Tbuc Basalt of Buckaroo Dam (Pliocene) ...... 17 Tharu Upper basalt of Harris Dam (Pliocene) ...... 17 Tharl Lower basalt of Hams Dam (Pliocene) ...... 17 Tcro Basalt of Crowbar Gulch (Pliocene) ...... 17 ...... Tbm Upper basalt of Bruneau (Pliocene) ...... 17 Tbrm Middle basalt of Bruneau River (Pliocene) ...... 17 Tbrl Lower basalt of Bruneau River (Pliocene) ...... 17 Tbr Basalt of Bruneau River (Pliocene) ...... 18 Tmw Basalt of Miller Water (Pliocene) ...... 18 Troc Basalt of Lake (Pliocene) ...... 18 Tbib Basalt of Big Bend (Pliocene) ...... 18 Ttwi Basalt of Twin Butte (Pliocene) ...... 18 Tsal Basalt of Salvador Lake (Pliocene) ...... 18 Tab Basalt of Austin Butte (Pliocene) ...... 18 Ttab Basalt of Butte (Pliocene) ...... 18 Tbwf Basalt of Broken Wagon Flat (Pliocene) ...... 18 Tbgr Basalt of Blackstone Grasmere Road (Pliocene) ...... 19 Tsot Basalt of South Top (Pliocene) ...... 19 Thtc Basalt of Horse Trap Corral (Pliocene) ...... 19 Tsug Basalt of Sugar Creek (Pliocene) ...... 19 Tww Basalt of Windy Well (Pliocene) ...... 19 Tmbr Basalt of Missile Base Road (Pliocene) ...... 19 Tbla Basalt of Black Rocks (Pliocene) ...... 19 Tdea Basalt of Deadman Gulch (Pliocene) ...... 19 Tals Basalt of Al Sadie Ranch (Pliocene) ...... 19 Thir Basalt of Hole In Rock (Pliocene) ...... 19 Twhc Basalt of Wickahoney Creek (Pliocene) ...... 20 Twt Basalt of Whiclcney Tree (Pliocene) ...... : ...... 20 To1 Basalt of 0 X Lake (Pliocene) ...... 20 Rhyolite Lava Flows ...... 20 TN Rhyolite, lmdifferentiated (Miocene) ...... 20 Thb Rhyolite of Horse Basin (Miocene) ...... 20 Tsfc Rhyolite of Shoofly Creek (Miocene) ...... 20 Tsc Rhyolite of Sheep Creek (Miocene) ...... 20 Tpc Rhyolite of Perjue Canyon (Miocene) ...... 21 Thg Rhyolite of Halfway Gulch (Miocene) ...... 21 Tbtc Rhyolite of Behveen The Creeks (Miocene) ...... 21 Tts Rhyolite of Tigert Spring (Miocene) ...... 21 Top Rhyolite of 0 X Prong (Miocene) ...... 21 Trc Rhyolite of Rattlesnake Creek (Miocene) ...... 21 Older Volcanic Units ...... 21 Tev Eocene volcanics ...... 21

Figure

Figure 1. Location map of the study area...... 2

Plates

The following 7.5-minute quadrangles in this report are purchased separately:.... .

Pence Butte, Pot Hole Butte, Bruneau ,Hot Spring, Crowbar Gulch, Austin Butte, Bruneau, Sugar , Broken Wagon Flat, Table Butte, C.J. Strike Dam, Little Valley, Hole In Rock, Grand View, Chalk , Big Horse Basin Gap, Vinson Wash, Perjue Canyon, and 0 X Lake Geologic Maps of the Grand View-Bruneau Area, Owyhee County, Idaho

Margaret D. Jenksl, Bill Bonnichsen', and Martha M. Godchauxl

INTRODUCTION chemical analyses on samples of the basaltic and rhyoli- tic units at the laboratory facilities of the University of Massachusetts, Arnherst, Massachusetts. Bonnichsen A grant from the Idaho Department of Water Re- and Godchaux drilled cores in the rhyolitic units for sources funded the mapping of nineteen 1:24,000-scale analysis at the paleomagnetic- laboratory facilities of quadrangles. The grant was part of a much larger re- Eastern University, Cheney, Washington. search project funded by the U.S. Fish and Wildlife Our understanding of lacustrine sedimentation was en- Service. Our part provided detailed geologic informa- hanced by Jenks' attendance at two professional meet- tion on surface lithologic and structural elements. This ings: the Geological Society of America's Penrose information was used by scientists with the Idaho De- Conference on Large and the Friends of the Pleis- partment of Water Resources and the U.S. Geological tocene field trip to the basin. Survey's Water Resources Division in their hydro- geologic study of the Bruneau-Grand View area geo- In our maps we use many new concepts about the thermal . geologic history of the . Since the 1920s, scientists have believed that the western Our field work on the project began in the spring 1990 in the late Tertiary was a large river and ended in the fall 1991. The field work was supple- valley, sometimes occupied by ephemeral lakes. Based mented with observations from 1:31,680-scale color on our mapping and previous work, we have returned stereo air photographs. In addition, Bonnichsen performed instead to the nineteenth century idea that the western

'1daho Gmlogical Survey, University of Idaho, Moscow, Idaho 83843 Snake River was occupied in the Miocene to Pleisto- 2 Geologic Maps -Grand View-Brrineau Area

cene by a large lake, Lake Idaho. This concept of a sta- The land is mainly used for cattle raising and farming. ble, long-lived lake is an important change in how sci- Most of the farming is in the valleys and low areas entists view the sedimentary and volcanic history of the south of the Snake River. The region is plain. By mapping marker beds and def~ngmost of the arid, so crops are imgated from wells or from the complex volcanic stratigraphy, we have laid the founda- flows of the Btuneau and Snake . Major tions for detailed sedimentological and stratigraphic crops include hay, sugar beets, grain, beans, mint, and work. In addition, out new structural information will potatoes. The lower country is also used for wintering contribute to an understanding of the basin's complex cattle, which are moved to pastures in the higher eleva- tectonic history. tions for the summer. The Simplot Cotpotation operates a large cattle feedlot just north of Grand View. LOCATION The region usually receives less than 10 inches (25.4 cm) of rain a year at the lower elevations. Tempera- tures vaty from -30°F in winter to 110°F in summer. The study area is composed of nineteen 7.5-minute The sparse population is concentrated in the agricultural quadrangles (Figure 1): Pence Butte, Pot Hoie Butte, areas at the lower elevations. Much of the country is un- Btuneau Dunes, Hot Spring, Crowbar Gulch, Austin inhabited. The unpopulated areas are mainly federal Butte, Btuneau, Sugar Valley, Broken Wagon Flat, Ta- land managed by the U.S. Bureau of Land Management. ble Butte, C.J. Strike Dam, Little Valley, Hole In Rock, Militaty aircraft from nearby Home Air Force Grand View, Chalk Hills, Big Horse Basin Gap, Vinson Base also use the area around Pence Butte in the Pence Wash, Perjue Canyon, and 0 X Lake. These quadran- Butte quadrangle as a bombing range target. gles occupy longitudes 115" to 116"15' and latitudes 42'30' to 43". The study area lies primarily south of the The natural vegetation, the high Snake River within Owyhee County and includes the flora and fauna, varies depending on elevation and pre- towns of Btuneau and Grand View. Elevations range cipitation. Russian olive, willow, and cottonwood trees from 6,062 feet (1848 m) in the 0 X Lake quadrangle to grow at the lower elevations along the rivers. At the 2,360 feet (719 m) along the Snake River in the Grand middle elevations, sagebrush is the predominant plant, View quadrangle. Major topographic features include with lesser amounts of rabbit brush and salt brush. In the Bruneau, Little, and Sugar valleys and the Btuneau years of normal spring rainfall, wildflowers are plentiful River, Little Jacks Creek, Big Jacks Creek, and Snake and varied. At high elevations sagebrush is still the River . Btuneau Dunes State Park is on the east dominant shrub, but juniper trees are also abundant. side of the area, occupying Eagle Cove within the Snake The best access to the field area (Figure 1) is through River canyon. Mountain Home either on Idaho Highway 51 to Bru- neau or on Idaho Highway 67 to Grand View. Idaho Highway 78 connects these two towns. Highway 51 continues south to the border from a crossroads just west of Btuneau. The other two principal paved are the Hot Spring Road on the east side of Bru- neau Valley and the Mud ~lai~oadjust east of Gtand View. Graveled roads are numerous, and accdss to the entire area is generally good.

GENERAL GEOLOGIC SETTING

Regionally, the area is at the intersection of the west- em Snake River Plain graben and the NE-SW trend of that caused the formation of the eastern Snake River Plain. The western Snake River Plain is a classic rift graben, in some ways similar to the rift ba- sins of East Africa. It formed tectonically, and large normal faults define its northeast and southwest sides. Scale 1:360000 u Bimodal volcanism dominates the stratigraphy in both Figure 1. Location map of the study area. the eastern and western . Volcanism began with Geologic Maps - Grand View-BrrtneauArea 3 rhyolitic ash and lava flows and culminated with the able, the lake probably began sometime in the late Mio- eruption of basaltic volcanoes. cene after the eruption of all rhyolitic units. With nu- The oldest rocks in the mapped area are on the west merous major and minor fluctuations, the lake probably side. They are rhyodacite ash-flow tuffs correlative with continued in some form into the Pleistocene. During the Challis Volcanics (40 Ma) of south- most of this time the lake occupied a large slowly sub- (Ekren and others, 1981). This cotrelation is based on siding and deepening basin. Based on the present-day the lithologic characteristics of the rocks and on a single location of near-shore sediments, Lake Idaho -at its K-At date. highest stand-was approximately the size of the ptes- ent-day Lake Ontario. It covered the area from Twin The southern part of the area falls within the northern Falls on the east to beyond the -Idaho border on boundary of the Btuneau-Jarbidge eruptive center (Bon- the west. nichsen, 1982). The eruption of large amounts of ash- flow tuffs, the Cougar Point Tuff units, created this A thick section of sediments intercalated with vol- structutal at approximately 11.3 Ma (K-At canic units accumulated within the Lake Idaho basin. whole rock; Bonnichsen and Citron, 1982). skginning For the most part, shorelines in the .lake basin had rela- about 10 Ma (K-Ar whole rock), rhyolite lava flows and tively low gradients. Thus, the major thicknesses of basalt shield volcanoes filled and covered the depres- sediments deposited in the basin during most of its his- tory are all in the small pebble- to clay-sized fractions. sion of the eruptive center. During its final recession, alluvial processes eroded Bimodal volcanism continued with the eruption of the large amounts of these fine-grained sediments. Addi- rhyolite lava flows (Kauffrnan and Bonnichsen, 1990; tional sediments eroded as faulting continued to form Bonnichsen and Kauffrnan, 1987). The flows were the western Snake River Plain during and after the final erupted in the south and southwest parts of the area. The recession of the lake. In some areas we see only frag- largest, the rhyolite of Sheep Creek, has a total extent of ments of sedimentary sections, where they were pro- approximately 400 square miles (1,000 km2). These tected from by the formation of small lava flows probably erupted from north- to northwest- downfaulted grabens. Throughout the basin, the vast trending fissures. However, only one area, in the rhyo- quantities of sediments removed by erosion are more lite of Horse Basin, has been identified as a vent. impressive than the major thicknesses of sediments that Following the rhyolite lava flows, the volcanism remain. changed to the eruption of basaltic shield and hydroclas- As in most modem large lake systems, deposits in the tic volcanoes. Many of the units show interactions with Lake Idaho system were chiefly influenced by the water, probably the watets of Lake Idaho. Only one of amounts and types .of sediments eroded by the almost fifty basalt units we have identified has been drainages. Within the area the most dominant and long- dated (whole rock K-At). The 8.4 + 0.7 Ma and 8.8 + 0.5 lived tributary drainage apparently was the ancestral Ma dates (Armstrong and others, 1975; 1980) are from Btuneau River. The river constructed a complex net- a single sample collected by W. P. Leeman from the work of deltas where it deposited at the edge bottom of the Hot Creek drainage near the Indian Bath- of Lake Idaho. We have found gravels composed en- tub Hot Springs (Armstrong and others, 1975). The tirely of Btuneau River drainage lithologies as-far west sample is from either the basalt of Hot Creek or the ba- as the Vinson Wash quadrangle. Therefore, during low salt of Indian Bathtub. These dates may, however, be in- stands of the lake, the Bruneau River probably was the correct by some factor, because near the sample major though-going drainage. Our emphasis on tribu- location both units contain large areas of water-affected tary sources as the cause for lateral differences in the basalt (see the introduction to the volcanic units for a sediment packages around the lake basin contrasts with description of water-affected basalt). The exchange of the hypothesis of Malde and Powers (1963). They and ions that occurs in the process of making water-affected other earlier researchers believed the sediments were basalt probably skews the amounts of potassium and ar- deposited by a single through-going drainage, possibly gon now present in the basalt. Further research that even the present Snake River. dates more of the volcanic units will resolve whether or Most large lakes with the longevity of Lake Idaho not the present dating is accurate. (like present-day Lake Turkana in Africa and Lake Bai- With our mapping, we are developing a better under- kal in Asia) undergo many fluctuations in water level. standing of the ancient Lake Idaho system. Although an While occupying the basin, Lake Idaho probably never exact beginning date for the lake system is still unavail- dried up completely. However, it did experience at least 4 Geologic Mnps - Grnrrd View-Brrmearc Area

more than one major recessionaty event. These were sition with actual datable strata. The presence within the probably caused by both tectonic and climatic changes lake sediments of numerous datable silicic ashes, as to the basin. Tectonic events may have altered the con- well as basaltic flows and tuffs, creates a framework figuration of the outlet and therefore the base level of that can be used to divide the sediments into facies the lake. Increased or decreased amounts of precipita- packages. The datable volcanic units will allow sedi- tion undoubtedly caused other water-level fluctuations. mentologists to chonologically correlate sediment Although some fluctuations were linked to seasonal packages of divetse lithologies. cycles, others occurred at intervals of 100,000 or Paleontologists in the nineteenth century (Cope, 1883a, 1,000,000 years and possibly were the result of major 18831>; Meek, 1870; Newberty, 1869) fust recognized the climatic changes. These fluctuations reworked and rede- existence of the lake from their examinations of lacustrine posited the sediments in the neat-shore environment. fossils. In recent years Smith (1975) and Smith and others Thus, the near-shore environment is the most complex (1982) have worked extensively with the abundant fish and difficult for geologists to reconstruct. However, the fauna and found a diverse population that included me- environment is also the most interesting for geologists ter-long minnows and salmon. The 'fish' fed on an abun- because it contains a variety of sedimentaty facies. The dant population of rnicrofossils, including many species near-shore environments include reefs, gravel and sand of both and ostracods. The shells of these mi- , and oolite sandbars. The lacustrine reefs, like crofossils are well-preserved in the fine-grained sedi- marine barrier reefs, formed parallel to the shoreline and ments. contained a variety of organisms. In the sand to gravel A diverse mammal and vertebrate fauna that included beaches are found major deposits of fish bones, mollusc swans, saber-toothed tigers, frogs, and camels (Jonena tests, and petrified wood. Some areas appear to have Hearst, oral and written comm.) also flourished on the been relatively starved of clastic sediment, allowing the shores of the lake. The famous Hagerman horse locality formation of major thicknesses of prograding oolite probably is related in some way to the lake, either as a sandbars. The most widespread shoreline lithology - sand - is also the least consolidated and resistant to shoreline feature or as sediments from a tributary . Thus, sand is the most difficult lithology to stream, possibly the ancestral . To- map. However, we can infer the locations and sizes of day, ~~~~~~~~~~g~~~~are pursuing research in several the sand layers from the presence of large Holocene areas, including work with pollen that will document dune sand fields in parts of the area. variations thtough time in continental climates. Previously, we and other researchers thought the Lake Idaho probably drained slowly as the ancestral deeper environments would contain only very fine- Snake River cut thtough Hells Canyon and breached the grained sediment packages. However, large lakes mimic lake basin (Wheeler and Cook, 1954). Evidence for this oceans by producing currents and waves. These form final recession can be found in the fluvial gravel sur- that look similar to those made faces. The tributary first eroded the underlying by the same processes in oceans and streams. Thus, ear- sediments and then coveted them with gravel. The lake lier reseatchers have mistakenly assigned some lacus- possibly receded from the mapped area about 1.5 to 1.0 trine deposits to fluvial origins. Major storms can create Ma. Evidence for this hypothesis comes from K-At currents in lakes that reach depths of 330 feet (100 m). dates (whole rock) of basalt units that are located out- When large waves caused by these storms beat against side the present mapped area. The change from lacus- the lake shorelines, they move the coarser-fraction sedi- trine to subaerial conditions is documented in these ment particles into the central and deeper parts of the basalt units. With the final recession, the present drain- lake. Neat the river and larger stream deltas, both storm- ages formed within the basin, and the Snake River was generated currents and annual spring runoff may at last established as the dominant though-going drain- cause the formation of turbidity currents. The turbidity age. Tributary drainages, like the Bruneau River, also currents ate another way in which the coarser near- began to cut their canyons at this time. shore sediments are moved into the centers of the lake Since the demise of Lake Idaho, several large struc- basins. tural valleys have formed within the area. The largest is We are just beginning to fully understand these lacus- the Bruneau Valley, which runs northwest-southeast trine sediments. We believe that the sediments remain- though the eastern part of the area. Several alluvial fans ing from Lake Idaho arguably offer a unique oppor- have formed on both sides of the valley along the mar- tunity to study these complex facies relations in juxtapo- ginal scarps. The presence of the fans and their Geologic Maps - Gratrd View-BnrneauArea 5 youthful appearance suggest that some of the movement jue Canyon quadrangles, appears to have moved more on the valley faults may have been quite recent. We recently than other similar faults. It has a youthful es- have noticed that the Btuneau River has only cut into carpment that has been cut by local streams to form the fans on the southwest side of the valley. Perhaps small alluvial fans of sediment on the downthrown fault movement has caused the river to move in that di- block. rection. In the relatively unconsolidated sediments that under- The present Snake River canyon, located in the north- lie most of the area, we were unable to locate actual em part of the area, may have been largely the result of fault planes or other indicators of fault motion. Based the most recent major geologic event, the Bonneville on the few faults traceable through offsets in marker Flood. Several pulses of floods, flowing as much as 1 beds, much of the faulting appears to be normal in mo- million cubic feet a second, poured down the Snake tion. This is consistent with the idea that the western River drainage after ancient Lake Bonneville breached Snake River Plain is a rift-graben. In the area most of the Red Rock Pass divide, south of Pocatello. The dat- the fault movement appears to have,occurred after the ing of shoreline features within the Lake Bonneville ba- Lake Idaho sediments were deposited.;In several loca- sin, which surrounds the present in tions fine-grained sediments butt up against escarp- northern Utah, suggests the flood occurred about 14,500 ments of rhyolite as much as 400 feet (122 m) high. years ago (Oviatt, 1991). The incredible deluge of water Even immediately adjacent to the large boundary faults, down the Snake Rivet canyon cut canyons, stripped such as the faults in Broken Wagon Flat quadrangle that sediments from basalt surfaces, and deposited large step down the rhyolite of Sheep Creek to the north, sand, gravel, and boulder bars in the bottom of the can- sediments beneath the capping gravels are not larger yon. Evidence within the area suggests the flood than small pebbles. The present juxtaposition of fine- reached as high as 2,900 feet in elevation. As evidence grained sediments against rhyolite or basalt escarpments for this elevation, we have mapped both large landslides is further evidence for the presence of a relatively low and stripped basalt surfaces near and within the present gradient Lake Idaho shoreline during most of the lake's Snake River canyon. The flood waters saturated and un- history. dercut the sediments below the basalt canyon rims, The most obvious structural grain in the area strikes causing them to fail. northwest-southeast. The grain is approximately parallel to the overall trend of the western Snake River Plain STRUCTURE graben. The northwest-trending faults appear to be the youngest in the area. In some locations they offset all Two sets of fault directions dominate the structural sedimentary and volcanic units, including the highest grain of the area. In the sediments most of the fault elevation, basin-fill gravels and the uppermost basalt planes ate covered with slope wash and talus and thus flows. Examples of this trend are the large marginal are very difficult to locate. However, reliable local indi- faults that form the major rhyolite escatpments and the cators of faulting within the sediments include the fol- major faults that bound the Btuneau Valley graben. An- lowing: (1) the offset of marker beds known to occupy other major set of northwest-trending fa& is either only a certain stratigraphic position, (2) the presence of continuously or sporadically exposed from the Btuneau tilted bedding (more than lo0), and (3) the locations of River canyon, on the southeast, to the northwest comer cemented or more resistant areas of sediment. The off- of the Vinson Wash quadrangle. sets in the basalt and rhyolite units are easier to docu- The north- to northeast-trending faults, the second set ment, because in many places they are juxtaposed of faults within the area, are less obvious in their topo- against sedimentary lithologies. graphic expression. However, they may actually be In the southern part of the area is a zone up to several more important because they determine the stream pat- miles wide, containing numerous horst and graben terns over much of the area. Evidence for these faults blocks. Some faults have offsets of as much as 250 feet can be found in the offset of major marker beds and in (76 m), so that the sum of all displacements is in the the locations of fairly long and nearly straight canyon thousands of feet. In addition to this zone, even larger segments of major streams. A good example of the fault boundary fault-sets drop down all rhyolitic units to form sets is the boundary faults on the northwest and south- the present topographic southern edge of the western east sides of the Little Valley graben. At the intersec- Snake River Plain basin. One of these major boundary tions of the two major fault sets, the northeast-trending faults, located in the Big Horse Basin Gap and the Per- faults affect and perhaps offset the northwest-trending 6 Geologic Maps -Grand View-BruneauArea

structures. This effect suggests that the northeast-trend- As we suggested in the section on structure, the drain- ing structures are older and probably have been active age probably didnot begin eroding until at least 1 Ma. If for a longer time. erosion is the controlling factor for the presence and lo- As suggested earlier, the juxtaposition in many places cation of the Indian Bathtub Hot Springs, then the of fine-grained sediments next to thick sequences of springs have probably been in existence less than 1 mil- rhyolite is evidence that much of the faulting probably lion years. occurred after the final recession of Lake Idaho. The Another control on the location of the spring may gravel surfaces deposited during and after the lake's re- be partially related to the lithologies through which it cession obviously did not all form simultaneously in dif- issues. In other nearby locations, features in the basalt ferent parts of the basin. However, many of them still and sediment stratigraphy suggest that a local lower- must be relatively young, ranging from 1.5 to 0.5 Ma, ing of the level of Lake Idaho occurred between the because they represent the change in the basin from be- emplacements of the basalt of Buckaroo Dam and the ing occupied by a stable large lake to an environment basalt of Indian Bathtub. The basaltof Buckaroo Dam containing both rivers and lakes. In addition, base-level is completely water affected throughout its exposure, changes from post-lake faulting may have in part been suggesting that it flowed beneath the waters of Lake responsible for the rapid of some streams, Idaho. The overlying sequence of sediments grades like the Bruneau River. Therefore, the Bruneau River from a lower silty clay to an upper clean sand. This canyon and its must also be fairly young. A change in facies is consistent with a recession of the probable age for this and other canyons might be 1 mil- lake shoreline. The bottom two flows of the basalt of lion years or less. Indian Bathtub appear to have been subaerially em- placed, but the flows above them are extremely water HOT SPRINGS affected. Most water-affected basalt flows are mas- sive with few vesicles and with interstitial spaces The Indian Bathtub Hot Springs, the focus of this filled with clay. The overlying and underlying water- study, is in the floor of the Hot Creek drainage. It is affected basalts may confine the aquifer within the approximately 0.5 mile (0.8 krn) southwest of the more porous sediments and scoriaceous subaerial ba- of Hot Creek and the Bruneau River. Lo- salt flows. cal ranchers report that in the past the hot spring is- Another way to consider the age and future longev- sued from the Hot Creek streambed, upstream of a ity of the Indian Bathtub Hot Springs is to look at hot small . The springs presently issue below the springs as general geological phenomena. Geologi- waterfall from the red sediment bed between the ba- cally, hot springs are fairly ephemeral features in any salt of Indian Bathtub and the underlying basalt of landscape. Lowell (1992, p. 552-553) says: Buckaroo Dam. Hydrothermal systems are fundarnentally transient While it is impossible to date the actual origin of phenomena. The temporal variability may have a the hot springs, it is possible to speculate on their range of time scales. Heat input may vary as probable control and origin. At the springs the units chambers solidifj, and cool, or are peri- are not offset by faults. Elsewhere, particularly in the odically replenished. . . . Permeability may change upper end of the Bruneau Valley, other hot springs do as a result of tectonic, thermal, or chemical effects. appear to be directly related to the fault sets that Fluid recharge patterns may change because of cli- formed the Bruneau Valley graben. In addition, both matic changes (precipitation patterns, glacial epi- upstream and downstream of the Indian Bathtub Hot sodes, etc.). Springs, faults do offset the units of the basalt and sediment package. It is possible, but difficult to Large continental hydrothermal areas such as Yellow- prove, that the older northeast-trending structures stone Park may last for several tens of thousands of control the location of the Hot Creek drainage at its years, although individual subsystems may have a confluence with the Bruneau River. shorter lifetime. If the springs are not fault-controlled, then they Two geologic factors that can possibly affect the could be the result of stream erosion having reached spans of springs in this area are (1) the amount of the level of a confined part of the aquifer. Thus, the dissolved carried by the spring water and (2) springs would only have begun to flow as the erosion the presence of local or regional earthquake activity. of the Hot Creek drainage reached its present depth. All hot springs tend to plug the faults and pore spaces Geologic Maps - Grand View-Bru~reauArea 7 that are their plumbing systems. Where they issue from Charlotte Fullerton and Jennifer Churchill provided as- the ground, they meet a zone of lower pressure and tem- sistance in the preparation of the final report. perature. Thus, the dissolved solids carried in the hot, pressurized water are likely to be precipitated at, as well REFERENCES as below, the springlsurface interface. The precipitation tends to plug the subsurface fractures and sedimentary Armstrong, R.L., J.E. Harakal, and W.M. Neill, 1980, spaces through which the water travels. Precipitation K-Ar dating of Snake River Plain (Idaho) volcanic of dissolved fluids is one of the primary reasons why rocks -new results: Isochron/West, no. 27, p. 5-10. hot springs do not exist for geologically lengthy peri- ods. Although we have not observed precipitated de- Armstrong, R.L., W.P. Leeman, and H.E. Malde, 1975, posits at Indian Bathtub Hot Springs, the high K-Ar dating, Quaternary and Neogene volcanic percentage of dissolved solids in this and other rocks of the Snake River Plain, Idaho: American springs in the area (Charles Berenbrock, oral cornrn.), Journal of Science, v. 275, p. 225-25 1. Bonnichsen, Bill, 1982, Rhyolite, lava.' flows in the as well as the presence of older cemented areas in the . . sediments, suggests that subsurface precipitated de- Bruneau-Jarbidge eruptive center, southwestern posits may exist in the Indian Bathtub Hot Springs. Idaho, in Bill Bonnichsen and R.M. Breckenridge, The Indian Bathtub Hot Springs is within the Snake eds., Cenozoic geology of Idaho: Idaho Bureau of River Plain, adjacent to seismically active areas. Ma- Mines and Geology Bulletin 26, p. 283-320. jor earthquakes occurred in 1959 in the Yellowstone Bonnichsen, Bill, and G.P. Citron, 1982, The Cougar area and in 1983 near Borah Peak in Point Tuff, and vicinity, in Bill central Idaho. These earthquakes caused changes in Bonnichsen and R.M. Breckenridge, eds., Cenozoic both cold and hot springs in the immediately affected geology of Idaho: Idaho Bureau of Mines and Geol- region (Roy Breckenridge, oral comrn.). Following ogy Bulletin 26, p. 255-281. the earthquakes, some springs flowed more and oth- Bonnichsen, Bill, and D.F. Kauffrnan, 1987, Physical ers less. Piper (1924) reports that the 1906 San Fran- features of rhyolite lava flows in the Snake River cisco earthquake was felt in the Bruneau basin and Plain volcanic province, southwestern Idaho, in J.H. affected both the locations and the flows of the area's Fink, ed., The emplacement of silicic domes and lava hot springs. Earthquake activity anywhere within or flows: Geological Society of America Special Paper adjacent to the western Snake River Plain basin will 212, p. 119-145. have some effect on the hot artesian wells and springs Buwalda, J.P., 1921, Oil and gas possibilities of eastern in the Indian Bathtub Hot Springs area. However, we Oregon: Oregon Bureau of Mines and Geology, v. 3, cannot predict the actual manifestation of that effect no. 2, p. 1-47. on any particular spring. Cope, E.D., 1883a, On the fishes of the recent and Plio- With all these factors in mind -the typically short cene lakes of the western part of the Great Basin and of hot springs, the springs' tendency to become of the Idaho Pliocene lake: Academy of-Natural Sci- plugged by the dissolved in the water, and ences, Philadelphia Proceedings, p. 134-166. their vulnerability to seismic events -we believe it is important to view the Indian Bathtub Hot Springs , 1883b, A new Pliocene formation in the Snake geologically as well as historically. Geologically, the River valley: American Naturalist, v. 17, no. 8, p. 867- springs, and by extension the plants and animals that 868. inhabit them, are a transitory rather than a permanent Ekren, E.B., D.H. McIntyre, E.H. Bennett, and H.E. feature of the landscape. Malde, 1981, Geologic map of Owyhee County, Idaho, west of longitude 116" W.: U.S. Geological ACKNOWLEDGMENTS Survey Miscellaneous Investigations Series Map I- 1256, scale 1:125,000. The authors are happy to thank the Idaho Depart- Jenks, M.D., and Bill Bonnichsen, 1987, Lake Idaho - ment of Water Resources for funding the grant that new perspectives through basalt stratigraphy (abs.): made possible the mapping of this important part of American Association of Petroleum Geologists Bul- the Snake River Plain. We also want to thank Kurt letin, v. 71, p. 1008. Othberg, Loudon Stanford, Paul Castelin, Janet , 1989, Subaqueous basalt eruptions into Plio- Crockett, and Ken Neely for their helpful reviews. cene Lake Idaho, Snake River Plain, Idaho, in V.E. 8 Geologic Maps - Grand View-Bruneau Area

Chamberlain, R.M. Breckenridge, and Bill Bonnich- Russell, I.C., 1902, Geology and water resources of the sen, eds., Guidebook to the geology of northern and Snake River Plains in Idaho: U.S. Geological Survey western Idaho and surrounding area: Idaho Geologi- Bulletin 199, 192 p. cal Survey Bulletin 28, p. 17-34. , 1903a, Notes on the geology of southwestern Kauffrnan, D.F., and Bill Bonnichsen, 1990, Geologic Idaho and southeastern Oregon: U.S. Geological Sur- map of the Little Jacks Creek, Big Jacks Creek, and vey Bulletin 217, 83 p. Duncan Creek Wilderness Study Areas, Owyhee , 1903b, Preliminary report on the artesian ba- County, Idaho: U.S. Geological Survey Miscellane- sins in southwestern Idaho and southeastern Oregon: ous Field Studies Map MF-2142, scale 1:50,000. U.S. Geological Survey Water Supply Paper 78, 13 p. Kirkham, V.R.D., 1931, Revision of the Payette and Smith, G.R., 1975, Fishes of the Pliocene Glenns Ferry Idaho Formations: Journal of Geology, v. 39, no. 3, Formation, southwest Idaho: Ann Arbor, Mich., Uni- p. 193-239. versity of Michigan Museum of Paleontology Papers

Lindgren, Waldemar, 1898, Description of the Boise on Paleontology, v. 14, p. 1-68. ... . : .' quadrangle: U.S. Geological Survey Geologic Atlas, Smith, G.R., Krystyna Swirydczuk, P.G. Kimrnel, and Folio 45. B.H. Wilkinson, 1982, Fish biostratigraphy of late Lindgren, Waldemar, and N.F. Drake, 1904, Description Miocene to Pleistocene sediments of the western of the Nampa quadrangle: U.S. Geological Survey Snake River Plain, Idaho, in Bill Bonnichsen and Geologic Atlas, Folio 103. R.M. Breckenridge, eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, Lowell, R.P., 1992, Hydrothermal systems in W.A. p. 519-541. Nierenberg, ed., Encyclopedia of System Science, v. 2: Academic Press, p. 547-558. Straccia, F.G.. B.H. Wilkinson, and G.R. Smith, 1990, Miocene lacustrine algal reefs -southwestern Snake Malde, H.E., and H.A. Powers, 1962, Upper Cenozoic River Plain, Idaho: Sedimentary Geology, v. 67, p. 7- stratigraphy of western Snake River Plain, Idaho: Geo- 23. logical Society of America Bulletin, v. 73, p. 1197- 1220. Swirydczuk, Krystyna, 1980, The Pliocene Glenns Ferry oolite -sedimentology of oolitic lacustrine ter- Malde, H.E., H.A. Powers, and C.E. Marshall, 1963, Re- race deposits: Journal of Sedimentary Petrology, v. 50, connaissance geologic map of west-central Snake River p. 1237-1248. Plain, Idaho: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-373, scale 1: 125,000. Swirydczuk, Krystyna, B.H. Wilkinson, and G.R. Smith, 1979, The Pliocene Glenns Ferry oolite - Meek, F.B., 1870, Descriptions of fossils collected by lake margin carbonate in the southwestern the U.S. geological survey under the charge of Snake River Plain: Journal of Sedimentary Petrol- Clarence King, esq.: Academy of Natural Sciences, ogy, v. 49, p. 995-1004. Philadelphia Proceedings, v. 22, p. 56-64. Wheeler, H.E., and E.F. Cook, 1954, Structural and stra- Neill, W.M., 1975, Geology of the southeastern Owy- tigraphic significance of the -Snake River capture, hee and environs, Owyhee County, Idaho: Idaho-Oregon: Joutnal of Geology, v. 62, no. 6, p. 525- Palo Alto, Calif., Stanford University unpublished 536. report on student research project, 59 p. Newberry, J.S., 1869, On the flora and fauna of the Miocene Tertiary beds of Oregon and Idaho: Ameri- can Naturalist, v. 3, no. 8, p. 446-447. Oviatt, C.G., ed., 1991, Guidebook to Lake Bonneville stratigraphy and Quaternary volcanism in the Sevier and Black Rock , Utah: Friends of the Pleisto- cene, Rocky Mountain Cell Field Trip, October 11- 13, 1991,29 p. Piper, A.M., 1924, Geology and water resources of the Bruneau River basin, Owyhee County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 11,56 p. Geologic Maps - Grand View-BruneauArea 9

DESCRIPTION OF UNITS derlying volcanic lithologies also lie upon the surface of the fine-grained sediments. SEDIMENTARY UNITS Qfs Fresh, unvegetated dune sand (Holocene) - Large active dune fields that crop out only in the eastern Approximately 80 percent of the mapped area is part of the mapped area. The largest of these active underlain by sedimentary beds. For convenience, we dunes is in Btuneau Dunes State Park, which occupies have grouped the sediments into three categories: the Eagle Cove on the south side of the Snake River. With younger unconsolidated sediments, the gravel sur- the tallest dune in the park measuring 470 feet (143 m), faces and lenses, and the Idaho Group sediments. they are the tallest single-structure dunes in North Some units among the three groups overlap in age. America. Some active dune fields, located elsewhere We have noted these equivalencies within individual within the study area, were too small scale to map. unit descriptions. Qaf Alluvial fan deposits (Holocene and Pleisto- cene) -Deposited on the downthrown .sides of major Younger Unconsolidated Sediments faults. The greatest concentration of fans is along the walls of the Btuneau Valley. The drainages that created The unconsolidated sediments are generally the the fans have relatively small watersheds, some less youngest units in the area. They were formed either than 1 mile (1.6 km) in length. Thus, the alluvial fan de- by recent stream and aeolian activity or by the Bon- posits are composed of a mixture of clast sizes, eroded neville Flood. They are Holocene to Pleistocene in from the local sediment deposits, that range in size from age. The alluvial fan deposits and alluvium are the re- clay to gravel. Recent floods on the fans during our sult of recent stream activity. We have divided the dune sand into two units, vegetated and unvegetated. field seasons show that sheet wash during major thun- The locations of these pervasive and large sand de- derstorms is one of the principal mechanisms of fan posits are, of course, due in part to prevailing wind growth. directions. They also seem to be related to nearby Qds Vegetated dune sand (Holocene and Pleisto- large uncemented sand layers within the Idaho Group cene) -Large and widespread fields of vegetated dune that are sources for the dune sand. Thus, the dunes sand are a major feature of the area's landscape. De- may be used to some degree as indicators for locating rived from nearby sand beds, each field is a unique the positions of the older sand deposits. The intermit- shape and size, depending upon the underlying topogra- tent lake sediments are also the result of aeolian ac- phy and the local prevailing winds. tion. These clay-sized windblown sediments are Qls Landslide deposits (Holocene and Pleisto- accumulating in the shallow depressions in the under- cene) -Probably produced in part by the passage of lying volcanic units. The landslide deposits and the deposits are both located exclu- the catastrophic Bonneville Flood as well as by long- sively within the present Snake River course. They term canyon slope processes. Most deposits are along were formed by the massive Bonneville Flood, the Snake River canyon. There, valley walls, cut in sedi- caused by the rapid drainage of Pleistocene Lake ments capped by basalt rims, were saturated and under- Bonneville in northern Utah. cut by the flood water. The saturation and erosion caused numerous massive slides. Some of the slide ar- Qal Alluvium (Holocene) -Unconsolidated stream eas stretch for several miles along the uppermost as well and valley deposits, including clay, silt, sand, and as the lower canyon rims. other areas debris falls, de- gravel. The unit is located throughout the area in the In bottoms of the present stream and river drainages, bris flows, and slump deposits are the result of the nor- where the deposits conform to the present topogra- mal long-term mass wasting of the canyon walls. The only major slide area outside the Snake River canyon is phy. within the Btuneau River canyon in Crowbar Gulch Qil Intermittent lake sediments (Holocene) - quadrangle. It was probably caused by movement on the Clay- to silt-sized sediments transported largely as fault that offsets the units exposed in the canyon walls windblown loess and deposited into lakes formed by near the slide zone. depressions in the surfaces of the underlying basalt and rhyolite flows. When dry, the sediments form deep poly- Qbf Bonneville Flood deposits (Pleistocene) - gonal mud cracks, which in some lakes are outlined by Sand- to boulder-sized sediments that form sand, gravel, small pebbles. Cobble- to boulder-sized rocks of the un- and boulder deposits. Most deposits are within the 10 Geologic Maps - Grarrd Kew-Bruneau Area

Snake River canyon, but a few are also in the lower end means that they could be either Quaternary or Tertiary of the Bruneau Valley near the Bruneau Narrows. in age and that their process of origin is unknown. QTg Gravel surface, lithology undivided (Quater- Gravel Surfaces and Lenses nary or Tertiary) -Located throughout the area, the surfaces are placed in this general unit but are probably Extensive gravel surfaces are one of the most promi- correlative with the nearby surfaces whose lithologies nent topographic features of the mapped area, as well as we have verified and described. However, where we did other areas of the western Snake River Plain. The grav- not visit the localities, and their locations make designa- els are intercalated within the fmer grained sediments tions questionable, we have lumped them into this undi- and also make surfaces overlying the sediments. vided unit. Clast size ranges from pebble to boulder. The local streams and rivers that drained into the QTgq Gravel surface, abundant quartzite (Quater- Lake Idaho basin deposited the sediments they carried nary or Tertiary) -Principally located in the area sur- as deltas or alluvial fans. Therefore, these gravel depo- rounding the present Bruneau Valley; The unit is also to sits vaty from area to area in the predominant clast li- the north along and within the snake Aver canyon as thologies. The variability is directly correlative with the far west as Vinson Wash quadrangle. The gravel surface lithologies of the source rocks underlying the basins was deposited by the same drainage, the ancestral Bru- drained by each ancient tributary stream or river system. neau River, that laid down the Tgq unit. It is similar in We did not perform point counts for each gravel loca- physical characteristics and constituent lithologies to the tion; instead, we noted the quantities of the principal li- Tgq unit. Thus, in some locations where the unit is con- thologies for any gravel surface we visited. For the tiguous to an outcrop of Tgq, the surface is probably surfaces visited, we have differentiated the gravels by correlative with the adjacent Tgq outcrop and has been their approximate predominant lithology or lithologies. exhumed. In other locations it could be either down- The gravels of unvisited sites are lumped into an undif- faulted or reworked from the overlying Tgq unit. ferentiated unit. QTgqb Gravel surface, abundant quartzite, signifi- The gravel surfaces also lie at different elevations. cant basalt (Quaternary or Tertiary) -Located Where the surfaces lie at elevations lower in the strati- throughout the upper reaches of the Bruneau Valley and graphic section, we were unable to determine the par- east of the West Fork of Browns Creek in the Pence ticular process, or perhaps the combination of processes, Butte quadrangle. The unit contains the same lithologies that formed them. Talus and slope wash from the over- as found in Tgq and QTgq but also has significant quan- lying rocks and sediments have covered the back edges tities of basalt clasts. In addition, in contrast to the of the surfaces, obscuring the exact stratigraphic rela- quartzite clasts in the Tgq and QTgq units, the outsides tions. Thus, any individual surface could be the result of of the clasts in this unit are unstained. Because it is the one or more of the following processes: (1) downfault- lowest elevation gravel surface in the lower Bruneau ing of a surface from a higher to a lower elevation; (2) River drainage, the unit may have two possible origins. exhumation of an intercalated gravel deposit by the ero- First, as stratigraphic position suggests, it may be the sion of the overlying finer grained sediments; or (3) ter- oldest of the gravel surfaces and simply has been ex- race formation during the development of the present humed by erosion. Second, it may be the most recent drainage valleys. These thtee processes directly affect terrace formed by the Bruneau River within its present how we view the relative ages of the surfaces. In proc- canyon. In this case, it would be the youngest of the ess 1, the two surfaces are the same age; in process 2 the gravel surfaces in the Bruneau River area. upper elevation surface is the youngest; and in process 3 the upper elevation surface is the oldest. To deal with QTgr Gravel surface, abundant rhyolite (Quater- nary or Tertiary) -Covers surfaces in the central and this problem, we have chosen a mapping method that is southern parts of the area. The unit contains similar lith- strictly descriptive. We have not attempted to differenti- ologies and in some locations is probably correlative ate the relative ages of the gravel surfaces based on any single model or process. We suggest that the uppermost with the contiguous or higher elevation exposures of the gravels, both in elevation and stratigraphy, and any in- Tgr unit. tercalated gravels are Tertiary in age. These lenses and QTgrb Gravel surface, abundant rhyolite, signifi- surfaces are the result of the gradual base level changes cant basalt (Quaternary or Tertiary) -Located in caused by the filling and draining of the Lake Idaho ba- the center of the area, the unit outcrops at two different sin. The QT classification of the lower elevation gravels elevations. At both elevations it contains significant Geologic Maps -Grand View-Bruneau Area 11 quantities of basalt clasts. The fust surface location is at elevations and probably are correlative with the nearby lower elevations, just above the present creek bottom in QTgqb surfaces. the lower part of the Sugar Creek drainage in the Sugar Tgr Gravel lens or surface, abundant rhyolite (Terti- Valley and Broken Wagon Flat quadrangles. These de- ary) -Covers much of the south part of the area, north posits differ somewhat from the mote widespread QTgr of the marginal western Snake River Plain faults. The deposits; the rhyolite clasts are large vitrophyre boul- deposit is dominated by clasts eroded from the nearby ders instead of smaller devittified clasts. The second rhyolite lava flows. The nearby source conttasts with QTgrb surface elevation is higher and crops out on both the other gravel surfaces, also composed partially of sides of Idaho Highway 5 1 in the Hole In Rock and Lit- rhyolite clasts but from more distant sources, like the tle Valley quadrangles. In these deposits the rhyolite Jarbidge and Silver City rhyolites. Probably the Tgr sur- clasts are smaller and composed of both vitrophyre and faces ate the remnants of large alluvial fans or deltas. devitrified rock Thus, the rhyolite clasts in these higher The fans built out from the escarpments made by the elevation surfaces ate similar to the typical rhyolite movement of large margin-basin faults. The fans clasts in the QTgr and Tgr units. formed along the edge of Lake Idaho during its final re- QTge Gravel surface, abundant Eocene volcanics cession. The clasts range in size from pebble to cobble, (Quaternary or Tertiary) -Covers many of the sur- and in some places appear to be more angular than com- faces in the west part of the atea. The unit contains the parable clasts in the Tgq unit. The deposits of the unit same lithologies as the Tge unit. Thus, it may be correla- are probably correlative with any QTgr surfaces at sirni- tive with any Tge lenses that outcrop within the fine- lar elevations. grained sediments near this unit. However, the unit Tge Gravel lens or surface, abundant Eocene vol- probably is not correlative with the small Tge surface to canic~(Tertiary) -Located on the west edge of the the southeast of the QTge surfaces in Perjue Canyon area. The unit is intercalated with Idaho Group sedi- quadrangle. ments and also covers a small area on the north side of Tgq Gravel lens or surface, abundant quartzite Poison Creek. It contains several different lithologies, (Tertiary) -Covering a large part of the eastern side and its multicolored appearance is an identifying char- of the area, the unit underlies both Horse and the acteristic. Clasts include granite, quartz vein pieces, Eo- highest elevation surface east of the Bruneau River cene volcanics, rhyolite lava flow rocks, and obsidian. drainage. The deposits appear to be fluvial in origin and The clasts are sand- to cobble-sized and subangular. may be the remnants of a series of Btuneau River del- Where intercalated, the unit may be correlative with the tas. Clasts range in size from sand to large cobble. Com- adjacent and similar elevation QTge surfaces. However, mon lithologies are Valmy Formation quartzite, Cougar the surfaces underlain by this unit near Poison Creek are Point Tuff, and Jarbidge rhyolite, all eroded from their probably not correlative, given theit differences in ele- exposures near the Nevada-Idaho border in the upper vation and distribution. Instead, these areas may be the parts of the Btuneau River drainage. A prominent fea- result of deposition from an ancestral Poison Creek ture is the large, up to 2.0-feet (0.6 m) in diameter, drainage. quartzite cobbles whose outer surfaces are stained or- Tgeb Gravel lens or surface, abundant Eocene vol- ange to gold to tan. The surfaces of some clasts are also canic~,significant basalt (Tertiary) - Covers much covered with percussion marks, caused by the impacts of the upper surface on the west side of the area. The between clasts as they were being transported in the unit contains all lithologies found in the Tge and QTge river system. Basalt clasts are very tare in the gravel. units but additionally has significant quantities of basalt This absence is interesting because at the time the clasts. Grain size ranges from sand to cobble. Probably gravel surfaces were deposited, the Btuneau River because of the high sand content and smaller grain size, drainage flowed north from the Nevada border over the unit does not make horizontal and relatively undis- more than 50 miles (80 km) of previously emplaced ba- sected surfaces similar to those underlain by Tgr and salt. Tgq. Instead, the Tgeb surfaces cap more deeply incised sand sections. Tgqb Gravel lens, abundant quartzite, significant basalt (Tertiary) -Located only in the Hot Spring and Pence Butte quadrangles. The unit contains the Idaho Group Sediments same lithologies as Tgq, but it also has significant amounts of basalt clasts. These gravel lenses, interca- The first geologists mapping in the area, Lindgren lated with the Idaho Group sediments, are at the same (1898), Lindgren and Drake, (1904), and Russell (1902, Geologic Maps -Grand View-Bruneau Area

1903a, 1903b), divided the sedimentary units into two graphic information as possible, we have mapped sev- formations, the Payette and the Idaho. These two forma- eral of the many marker beds intercalated with the Idaho tional divisions were based on relative stratigraphic po- Group fine-grained sediments. Each of these beds is in- sition and on ages assigned by paleontologists to fossils dividually distinct and traceable over mappable dis- collected from the area by earlier scientific explorations tances. The marker beds are generally near-shore facies (Cope, 1883a, 1883b; Meek, 1870; Newberry, 1869). that include fossiliferous cemented sands and gravels, Buwalda (1921) and later Kirkham (1931) devoted their oolites, and reefal . We have also mapped research efforts in the Weiser area to distinguishing the layers of basaltic and silicic tephra as marker beds two formations in more detail and to suggesting deposi- where they are significantly thick or numerous. tional environments. Both Kirkham and Buwalda ar- Ti Clay- to sand-sized sediments (Miocene to Plio- gued that most sediments in the western Snake River cene) -The most widespread unit, covering much of Plain are fluvial in origin and were deposited in a-large the central part of the area. Intercalated within these river valley that at various times and in different areas fine-grained sediments are gravels, iron- and phos- contained pluvial lakes. Later, Malde and. Powers phatic-cemented sand and gravel laye&, basalt flows, (1962) adopted this hypothesis. They also raised the silicic and basaltic ashes, and limestone beds. Most ex- Idaho Formation to group status and divided it into sev- posures of the unit are poor, and where intercalated with eral primarily fine-grained sedimentary formations con- or overlain by basalt flows, they are covered by basaltic taining basalt interbeds -the Bruneau, the Glenns talus. The percentage of sand beds to clay beds changes Ferry, and the Chalk Hills as well as a number of gravel from basin edge to basin center (sand to clay). Percent- units. They appear to have distinguished these forma- ages also change longitudinally along the lake's shore- tions on the basis of a few scattered paleontological lo- line away from the deltas formed as tributary drainages calities and by the relative elevation differences. We intersected with the lake (gravel to sand to clay). The found, however, that because of the multilithologic clay layers are thin bedded in some areas and more re- character, the formations are not mappable. Either we sistant than the sand layers. Some clay units contain were unable to trace them laterally, or we could not fairly large amounts of as both strataform layers clearly distinguish them on the basis of lithology. and cross-cutting veins. The cross-cutting relationships We began our work by mapping the individual vol- suggest that some of the gypsum may be from postsedi- canic units. Our intention was to trace each volcanic mentation ground-water deposition. The sand layers are unit to its source. Unfortunately, this created problems rarely exposed, are more abundant beneath the gravel with fitting out new volcanic stratigraphy into the pre- surfaces, and are resistant only where they have been vious, basically sedimentary framework. In some areas cemented by ground water. This cementation commonly previous workers (Malde and others, 1963) had mapped follows the internal changes in porosity produced by the the vent area of a volcanic unit as one formation and its original depositional regime, creating abundant concre- related flows as another. Because only a single whole tions that reflect the primary sedimentary structures rock K-At date exists for all of the volcanic units in the (ripple marks, crossbeds, etc.). A few sand layers, de- area, we were thus unable to successfully assign out posited within thick clay sequences and closer to the new volcanic units as members of the generalized sedi- center of the basin, were probably the result of-turbidity mentary formations. In addition, evidence from the vol- curtents from delta fronts and steeper shorelines. There- canic units (Jenks and Bonnichsen, 1987) strongly fore, they do not necessarily represent low stands of the supported the existence of a large lake environment for lake. the deposition of most sediments. The idea of a large Tsi Silicic tuff (Miocene to Pliocene) -The greatest lake, of come, was very different from the fluvial envi- thicknesses of silicic ash are interbedded with the fme- ronment hypothesized by Malde and Powers (1962). grained sediments in the west part of the area. Individ- Given the unit assignment dilemmas posed by out ual beds range in thickness from less than an inch to 15 new volcanic stratigraphy and our new large lake sedi- feet (4.6 m). The ash colors vary from dark gray to mentary model, we have abandoned the most recent for- white, and the individual ash layers vary from clean ash, mation names (Bruneau, Glenns Ferty, Chalk Hills, etc.) with well-preserved glass shards, to reworked ashy silt. for the mapping in this report. Instead, we have returned In many locations the ashes have been cemented with to a simple designation and have mapped the fine- and are therefore more resistant to erosion than grained sediments as Idaho Group undifferentiated. In the surrounding fine-grained sediments. We have not addition, to portray as much basic lithologic and strati- identified sources for these ash beds, but some of the Geologic Maps -Grand View-BnmeauArea 13 thickest and most coarse grained of the clean ashes may spring activity along a fault formed while the lake occu- possibly have nearby Snake River Plain sources. Other pied the basin. clean ash layers are very fine grained and thin, and they Thot Reefal limestone of Hot Spring (Miocene to ate probably the distal air-fall deposition from eruptions Pliocene) -Located in the Broken Wagon Flat, Sugar to the east in the Snake River Plain or to the west in the Valley, Hot Spring, and Crowbar Gulch quadrangles in Cascade Mountains. the southeast part of the area. The limestone, which was Tf Sand- to gravel-sized sediments, ferruginous and studied in detail by Straccia and others (1990), is a thick phosphatic cemented or stained (Miocene to Pliocene) resistor within the fine-grained sediments of the area. -Located throughout the south part of the area. The The unit is thickest around the confluence of Hot Creek unit probably represents near-shore sands and and the Btuneau River. It apparently has grown in ways gravels. Clasts range in size from sand to cobble. The similar to the coral barrier reefs and contains channels cementation and the red to brown to black staining both and armored colonies. The unit's overall shape suggests vary in amount. The cementation vanations may be the that it formed parallel to the ancient lake shoreline, with result of postdepositional changes in lake water chemis- the channels at right angles to this trend: The channels, try. Where the layers of the unit are strongly cemented, which are less resistant than the reef, commonly contain the bed forms a resistor within the finer grained sedi- major deposits of mollusc shells. Straccia and others ments. Thus, over local areas, the unit is an important (1990) attributed an algal origin to this limestone. Our matker bed, and offsets of the unit by faults have helped work suggests instead that the reef may have been us to understand the structure of parts of the area. Indi- formed by fresh-water sponges like those presently vidual bed thickness generally ranges from 3 inches (7.6 growing in Lake Baikal in Siberia (Farley Fleming, oral cm) to 1.5 feet (0.5 m). The sediment layers contain fish comm.). Parts of the unit contain as few as two and as and mollusc fossils and petrified wood, but the fossil many as eleven separate growth layers. These layers are abundance varies greatly from place to place. Although truncated at their tops, suggesting that they were grow- for this mapping we have lumped all occurrences of the ing within the erosive reach of the waves. In some cemented and stained beds together in a single li- places, the bottommost layer sits on scattered pebbles. thologic unit, they probably are not all chronologically In other areas the bottom layer appears to rest directly correlative. Instead, they may represent the locations of on biotutbated silts. the beachlwater interface at various times in the long Tsoo Oolite of Shoofly Creek (Miocene to Pliocene) history of the lake. -Located in the Vinson Wash and Perjue Canyon Tbs Basaltic tuff (Miocene to Pliocene) -Like the quadrangles on the west side of the area. Swirydczuk silicic tuff, the unit is located throughout the area but is (1980) and Swirydczuk and others (1979) described the unit in detail. They suggested that the oolites formed as most abundant in the lower Btuneau Valley near the - - Bruneau Narrows and around the upper end of the large prograding dunes or bars in a near-shore beach en- Bruneau Valley. The distributions of the tuff layers sug- vironment during a fairly long still-stand of lake levels. gest that they may be from nearby hydroclastic volca- The ooids may have been aided in their growth by the noes. Their coarse-grained outcrops near the Btuneau presence of algae. In some areas the ooids are strongly Nattows are adjacent to pillow deltas and may be re- cemented and iron stained. In others, where they are worked debris shed during the formation of the pillows uncemented like the other uncemented sand *deposits, rather than air-fall tephra from hydroclastic volcanoes. they have been reworked by into vegetated sand dunes. Tls Limestone, undifferentiated (Miocene to Plio- cene) -Several areas of limestone are not spatially connected with the reefal and oolitic limestone pre- VOLCANIC UNITS viously defined by other researchers (Straccia and oth- ers, 1990; Swirydczuk, 1980; Swirydczuk and others, Basalt Volcanoes and Lava Flows 1979). Four of the areas are in the Sugar Creek drainage in the Broken Wagon Flat quadrangle. These outcrops We have divided our discussion of the basalt flows in are reefal or oolitic limestone and are adjacent to or near the area geographically into two sequences of units. thick sections of water-affected basalt. A linear vein of One is in the north part and is exposed in the Snake massive to reefal-appearing limestone crops out on the River canyon. The other is in the south and southeast common boundary of the Hot Spring and Sugar Valley parts and exposed in and around the canyons of the quadrangles. This deposit possibly is the result of hot Btuneau River, Little Jacks Creek, and Big Jacks Creek 14 Geologic Maps -Grarid View-Brirneau Area

and their tributaries. No direct stratigraphic relation- weathered. In outcrop they form a slope of dark, ships exist between the two geographic sequences of gruslike crystal grains with what appears to be a pow- units, making the production of an integrated strati- dery golden matrix. We believe that this apparent ex- graphic column impossible at this point in our investiga- treme weathering is caused not only by exposure to the tions. Also, in the south part of the area, sequences of weather but also by the further hydration of the palago- basalt units exposed in some of the river and stream val- nite, already formed in the rock, to clay. Since this leys are isolated from one another by the intervening change is on the interstitial level, the flows lose their and covering sections of sediments. Although Bonnich- usual resistant character and essentially fall apart from sen has chemically analyzed numerous samples of the the inside out. All WAB units contain some areas that basalt to help with correlations, the analyses were not have been less affected by the palagonitization process. ready for this report. Thus, we have not attempted to Thus, the WAB parts of the flows vary from small, 6- to correlate the separate sequences of units. When the 12-inch (15-30 cm) round areas to larger areas between chemical data are complete, we will be able to correlate joint surfaces and in the centers of columns, and finally units in adjacent areas and drop one set of unit names. to flows that are essentially all water affected, with only isolated areas of somewhat more competent rock. The The descriptions of the individual units are arranged more water-affected areas of the flows appear to be in the following way. First, the flows in the north part of massive, with few vesicles, and without the diktytaxitic the area are discussed from youngest to oldest and then texture common to subaerial flows. Thus, these units from east to west. Next, the units of the south part are may have more capacity to act as barriers to water flow arranged as spatially separate groups from east to west. than the vesicular to scoriaceous subaerially emplaced Within each of the southern geographic groups, the flows. Under some WAB areas, the heat from the flows units are discussed in their relative stratigraphic order, has "baked" the underlying sediments to a bright red from youngest to oldest. For more precise stratigraphic that is easily and continuously traceable over many relations, the reader should refer to the list of units in miles of outcrop. each quadrangle legend. Where the flows appear to have cascaded over a Flows in the units of the north sequence erupted from beach face or escarpment, they form pillow deltas in- volcanoes near the town of Mountain Home. Flows in stead of WAB flows. We suggest that for the lavas to the units of the south sequence erupted from volcanoes form into pillows, they must first flow down an es- to the southeast of their area of exposure. Flows in some carpment where each flow is divided into individual units of each sequence ran for tens of miles away from streams. These thin streams then interact with the their volcanoes. lake waters to form log and pillow shapes. The pillow Seven vents are in the area. Five are shield deltas are important evidence in delineating the his- volcanoes, and two are hydroclastic volcanoes. The tory of Lake Idaho, because they mark the interface at vents of the hydroclastic volcanoes had some interaction the time of their eruption between the air and the lake with water or saturated sediments, probably related to waters. Thus, because the basalt flows can be dated, the presence of Lake Idaho. The flows from the basaltic the basalt deltas establish water levels in the lake for shields also interacted with the lake in one of two particular periods. The debris formed by the pillow deltas at their fronts is the likely source for. some of ways. Most flows interacted by forming what we the basaltic tuffs intercalated with the lake sediments. have called water-affected basalt (WAB). When these flows ran into the lake, they did not interact in the fa- miliar way to form pillow deltas or sequences. In- North Area Units stead, they appear to have flowed en masse without fragmenting along the bottom of the lake. In the proc- Qrs Basalt of Rattlesnake Springs (Pleistocene) - ess, they absorbed some quantity of water into their The highest elevation rim and surface on the north side still-liquid lava. This water then interacted with the of the Snake River canyon in the C.J.Strike Dam, Bru- basaltic glass on a microcrystalline, interstitial level. neau Dunes, and Bruneau quadrangles. The unit appears The interaction caused the glass to be hydrated, either to be entirely subaerial. Flows in hand sample contain as the lava cooled or immediately after, to what we abundant, medium to large plagioclase and olivine phe- believe is a form of palagonite. nocrysts, some in clumps. Upon erosion and exposure to the elements, the Qstr Basalt of Strike Dam Road (Pleistocene) - WAB flows appear to be extremely altered and Located on the northwest side of the Snake River can- Geologic Maps - Grand View-Bruneau Area I5

yon in the C.J. Strike Dam and Grand View quadran- pieces of the pillowed flows are incorporated into a gles. In hand sample the flows contain only abundant, nearby exposure of QTgqb. In addition, on its west edge medium to large olivine phenocrysts. A roadcut on the the deposit is beneath an area of QTgq. In hand sample Strike Dam Road near the canyon rim has exposed an the flows contain some to abundant, medium to small area of pillowing at the south end of the unit. In another plagioclase phenocrysts. exposure along the west canyon rim, the unit is under- Tcan Basalt of Canyon Creek (Pliocene) -The lain by a thin-bedded cinder layer. The flows were per- lowest unit in elevation in the sequence of units exten- haps erupted from Little Joe Butte in the Little Joe Butte sively exposed along the rims of the Snake River can- quadrangle to the northwest outside the mapped area. yon in the Bruneau, C.J. Strike Dam, and Grand View QTsim Basalt of Simco Road (Pliocene or Pleisto- quadrangles. The unit is pillowed in only one place on cene) -Located on both the north and south rims of the west edge in the Grand View quadrangle. In other the Snake River canyon in the C.J. Strike Dam and areas it is water affected to some extent. Its south edge Bruneau quadrangles. In sevetal places at its edge, the is irregular, suggesting that the present exposure may unit formed thick and extensive pillow deltas. 'Because closely approximate the original flow 'margin. In all the flows are generally horizontal and the areas of pil- hand samples, the flows are essentially aphyric with lowing are not continuous, the flows in this unit prob- some samples containing scarce to some tiny to small ably were interacting with bodies of water, either Lake plagioclase phenocrysts. Idaho or perhaps smaller post-lake Idaho lakes. Tgsm Basalt of Goldsmith Road (Pliocene) -Lo- QTcrf Basalt of Crane Falls (Pliocene or Pleisto- cated on both sides of the Snake River canyon east cene) -Only exposed over a small area on the north and west of Idaho Highway 51 northeast of the town wall of the Snake River canyon in the C.J. Strike Dam of Btuneau, as well as north and south of the Bruneau and Bruneau quadrangles. We do not have a sample of Nartows west of the town of Bruneau. The unit out- this basalt for comparison, so its hand sample lithology crops in the C.J. Strike Dam, Bruneau, and Btuneau is unknown. Thus, the unit may be cotrelative with an- Dunes quadrangles. Throughout its exposure the unit other basalt unit exposed elsewhere in the canyon. is extensively water affected. It also contains several Tcg Basalt of Chalk Gulch (Pliocene) -Located on areas of pillowing, one of which is near a basaltic tuff both sides of the Snake River in the Bruneau Dunes in the area of Wilkins Gulch on the north side of the quadrangle. The unit also continues outside the area to Bruneau Ann of C.J. Strike Reservoir. West of Idaho the east in the Indian Cove quadrangle. In its eastern ex- Highway 51 in the lower canyon of Rattlesnake posure in the mapped area, the unit contains numerous Creek in the Bruneau Dunes quadrangle, each flow basalt flows and is up to 200 feet (61 m) thick It is within the unit has a pillowed base. The water-af- water affected to some degree over most of the expo- fected character of the flows meant that we could not sute and in several places also has pillowed basalt at its always identify its phenocryst content in hand sam- base. These features suggest that the unit interacted with ple. However, in some samples the unit has some to a standing . In some places along the rim abundant, small to large, felty plagioclase crystals. of the Snake River, where the river makes a steep- The size of the plagioclase- phenoctysts seems to walled canyon, the Bonneville Flood waters have range from larger in the east to smaller in thawest. In stripped the overlying sediments from the unit's surface. most samples the olivine crystals have been weath- In other places, the flood eroded into and created chan- ered out. Where visible, they are only small to me- nels in the uppetmost flows. Thus, the flood both dium in size. changed the thickness of the unit and in some areas Tkl Basalt of Katie Lane (Pliocene) -Only ex- made it appear to be mote than a single unit. In hand posed in a small area on both sides of the Bruneau Nar- sample the flows contain abundant to very abundant, rows and east along the Bruneau Arm in the C.J. Strike medium to large plagioclase phenocrysts as well as Dam quadrangle. In two places, the unit makes an ex- varying amounts (none to some) of olivine crystals. tensive pillow delta, which has shed detritus that looks Tfly Basalt of Flying H Ranch (Pliocene) -Lo- very much like a basaltic tuff. Both the tuff and one area cated north of the Snake River canyon in Bruneau of the pillows contain significant quantities of Tgq cob- Dunes quadrangle. The unit overlies the basalt of Chalk bles. Gulch. At its westernmost edge the flows of the unit Tdix Basalt of Dixie Road (Pliocene) -Makes the make a fairly large pillow delta. Just south of the delta, bench on the north side of the Snake River, west of C.J. 16 Geologic Maps -Grand View-BrrineauArea

Strike Dam in the C.J. Strike Dam and Grand View the Idaho Group sediments. The volcano that erupted quadrangles. The base is pillowed in at least two places. the unit is Pence Butte on the south side of the Pence In hand sample the flows contain varying amounts of Butte quadtangle. The U.S. Air Force presently uses the olivine ctystals including rare, vety large crystals. butte as its principal target on the Saylor Creek Bomb- ing Range. In all of its exposures, the flows in hand sample contain large, rosette-type clumps of large pla- South Area Units gioclase and olivine phenoctysts. Twc Basalt of Winter Camp Butte (Pliocene) - Tlbbu Upper basalt of Lower Black Butte Crossing Covers much of the Pence Butte and Pot Hole Butte (Pliocene?) -Located in a small area on the east side quadrangles and smaller parts of the Austin Butte and of Pot Hole Butte quadrangle, the unit also extends into Crowbar Gulch quadrangles. The flows in this wide- the adjacent Black Butte West quadrangle. The volcano spread unit erupted from the Winter Camp Butte vol- from which these flows erupted appears to be hill 3608 cano in the southeast comer of the Pot Hole Butte in the notth part of Black Butte West quadrangle. In quadrangle. The unit is the youngest on the south'side of hand sample the flows range in their phenoctyst content the area, and therefore it covets much of the underlying from abundant large plagioclase and olivine to sparse stratigraphy. Over most of its exposure the edges of the plagioclase. flows are pillowed and define the slope break between Tlbbl Lower basalt of Lower Black Butte Crossing the basalt that continues south to Nevada and (Pliocene?) -Exposed in only a small area in the bot- the Lake Idaho sediments to the north. The unit lies both tom of Pot Hole Canyon on the east side of Pence Butte above and below Tgq. The point where its bottom flow quadrangle. The unit extends to the notth down the forms a pillow delta probably marks one of the high- drainage in the Pot Hole Canyon quadrangle. The flows stand shorelines of Lake Idaho. have an inclined, scoriaceous and possibly pillowed as- Tphb Tuff of the Pot Hole Butte volcanic complex pect and in hand sample are aphyric. (Pliocene) -Erupted from at least three vents collec- Twfb Pillow basalt of West Fork Browns Creek tively called Pot Hole Butte. The deposits of this hydro- clastic unit ate in the Pence Butte and Pot Hole Butte (Pliocene) -Located in the bottom of the West Fork of quadrangles. Because of posteruption erosion, loess Browns Creek drainage at the intersection of the Hot cover, and the fine-grained character of the tuff, we Spring, Pence Butte, and Pot Hole Butte quadrangles. were unable to locate the exact boundaries of this unit. The unit is pillowed throughout its exposure and is one Features of the principal vent area are described in of the best three-dimensional exposures of a pillow delta Jenks and Bonnichsen (1989). The distal, layered parts that we have found in the Snake River Plain. In hand of the tuff are exposed to the south and east of the main sample the basalt flows contain numerous large plagio- vent area, in draws cut below the edge of the basalt of clase phenoctysts, in some samples as clumps or ro- Winter Camp Butte. Some of the subaerial-appearing settes. spatter deposits, which are the uppetmost rim in the Tsog Basalt of Seventy One Gulch (Pliocene) - main vent area, contain in hand sample very abundant This volcano and its related dike deposits are in the large plagioclase phenocrysts. south part of the Hot Spring quadrangle. The volcano is Tbrc Basalt of Browns Creek (Pliocene) - Covers hydroclastic, and some of the flows from the'dike are the surface notth and east of Pence Butte in the Pence pillowed. Adjacent to the volcano the dike's exposure Butte and Pot Hole Butte quadrangles. This relatively shows an intrusion of the lava into wet sediments. Most thin unit nonetheless is important because, with the un- of the deposits in the volcano itself are cinders. derlying Twc, its edge forms the slope break between Tcuf Basalt of Curlew Flat (Pliocene) -Located in the sediments to the notth and the basalt plateau to the only a small area on the east side of the Btuneau River south. In hand sample the lithology of its flows changes canyon in Crowbar Gulch quadrangle. The unit is di- from containing a few scattered, small to medium pla- rectly below the surrounding pillow delta of the basalt gioclase phenocrysts in the west to having a general of Winter Camp. The unit is probably correlative with aphyric appearance in the east. another unit within this part of the area, but its limited Tpeb Basalt of Pence Butte (Pliocene) -Located exposure prevents direct correlation. In hand sample the on the east side of the area in the Pence Butte and Pot unit is diktytaxitic to massive with sparse to some large Hole Butte quadrangles. The flows of most exposures clumps and ctystals of olivine and sparse large plagio- of the unit formed large pillow deltas intercalated with clase phenoctysts. Geologic Maps -Grand View-BruneauArea 17

Thc Basalt of Hot Creek (Pliocene) -Located over in the Hot Spring and Crowbar Gulch quadrangles. The a large area in the lower parts of the Btuneau River can- unit probably is correlative with one of the subunits yon and in Hot Creek. The unit is exposed in the Broken within the Tbrm composite unit. It is not generally as Wagon Flat, Sugar Valley, Hot Spring, and Crowbar extremely water affected as the overlying units. In hand Gulch quadrangles. It is correlative with one of the up- sample the flows contain abundant, medium to large per units in the Tbru composite unit. Thtoughout much plagioclase phenocrysts and some small olivine crystals. of its exposure, but particularly in its northern patt, the Tharl Lower basalt of Harris Dam (Pliocene) - basalt of Hot Creek is extremely water affected. Thus, The lowest unit in the downstream end of the Btuneau in many places, it either looks like a brown sediment River canyon in the Hot Spring and Crowbar Gulch slope or has been almost completely eroded, leaving quadrangles. The unit probably is part of the sequence only isolated exposutes of less affected basalt. It is diffi- of units in the Tbrm composite unit exposed to the cult to see the phenocryst assemblage of the flows in south. In hand sample it is water affected and felty, con- hand sample. It is mostly aphyric, but some samples taining numerous small plagioclase phenocrysts and also contain plagioclase phenocrysts, and others'olivine some small olivine crvstals. . . crystals. Tcro Basalt of Crowbar Gulch (Pliocene) -The Tind Basalt of Indian Bathtub (Pliocene) -Lo- uppermost unit in the area of Crowbar Gulch in the cated in the Hot Creek drainage and the lower Btuneau Crowbar Gulch quadrangle. Except for a subaerial ex- River canyon in the Broken Wagon Flat, Sugar Valley, posure in the upper part of the gulch drainage, the unit Hot Spring, and Crowbar Gulch quadrangles. Through- is very water affected throughout its exposure. In ap- out its exposure, the unit is above the thickest of any of pearance, stratigraphic position, and elevation, there- the sediment layers intercalated with the basalt units ex- fore, it is similar to the basalt of Hot Creek. However, it posed in the lower Btuneau Rivet canyon. This sedi- differs in phenocryst content. The flows are not aphyric ment layer appears to vary in thickness over short like Thc but are slightly diktytaxitic with varied sizes distances. The unit also changes in thickness within its and amounts of plagioclase phenocrysts, some in area of exposure. It ranges from only a single flow, clumps, and with abundant to scattered olivine ctystals. where it becomes correlative with Tbru south of the confluence of Crowbar Gulch with the Btuneau River, Tbru Upper basalt of Bruneau River (Pliocene) - to several flows that make a thick unit in the area sut- Located in the Btuneau River canyon in the Crowbar rounding the confluence of Hot Creek with the Btuneau Gulch quadrangle, north of the Miller Water draw and River. The unit may have been emplaced during or south of Crowbar Gulch. The unit is a composite, con- shortly after a fluctuation in lake levels (see previous sisting of a sequence of three to four subunits exposed Hot Springs discussion). The phenocryst content varies in the upper part of canyon. It is separated from Tbnn in hand sample throughout the area of exposure. This by a slope break that, where exposed, contains a signifi- variability can be explained in one of two ways. First, cant thickness of sediment. In the part of the canyon up- unlike the other basalt units, the flows of this unit may stream from the Btuneau Scenic Overlook, some of the simply vary in their phenocryst content. Second, what upper subunits within this composite unit are extremely we have assumed is straightforward stratigraphy is really water affected. more complex, and thus, some of the samples may be- Tbrm Middle basalt of Bruneau River (Pliocene) long to other units. -Located within the same area as the upper basalt of Tbuc Basalt of Buckaroo Dam (Pliocene) -Lo- Bruneau River in the Crowbar Gulch quadrangle. This cated in the lower parts of Hot Creek and the Btuneau is the middle composite unit exposed in the canyon and River in the Hot Spring and Crowbar Gulch quadran- also contains three to four subunits of basalt flows, each gles. The unit is patt of the sequence of basalt units that separated by a thin sediment. It is above a covered slope form the walls of the lower Btuneau River canyon. It is break that probably marks the position of a section of probably correlative with one of the units in the Tbr17i sediments. composite unit. It is water affected throughout much of Tbrl Lower basalt of Bruneau River (Pliocene) - its exposure and is beneath a silty clay sediment. The The lowest sequence of units within the Bruneau River flows in hand sample contain medium to large olivine canyon in the Crowbar Gulch quadrangle, north of ctystals as well as very sparse large plagioclase. Miller Water draw and south of Crowbar Gulch. The Tharu Upper basalt of Harris Dam (Pliocene) - composite unit forms the shear in the bottom of the Located in the lower patt of the Btuneau River canyon canyon. The unit contains four or five individual 18 Geologic Maps -Grand View-BruneauArea

subunits, depending on the depth of the canyon. Its the flows contain only abundant, medium to large oli- subunits appear to be less rubbly than any of the vine phenocrysts and no visible plagioclase crystals. subunits in the other two overlying composite units. The ~~~l ~~~~l~ of salvador ~~k~ pliocene) -me competent appearance suggests that these subunits most extensively exposed unit in the northwest part of may have been emplaced subaeriall~.The subunits are Austin Butte quadrangle. In hand .ample the unit con- separated from one another by very thin, red sediment him large- and numerous clum~of olivine crvstals with layers. some large plagioclase phenocrysts. One of the subunits Tbr Basalt of Bruneau River (Pliocene) -Located in the T'rcomposite unit probably is a continuation of in the Austin Butte quadrangle between the Miller the flows ofthis unit. Water draw and the south edge of the area. The compos- Tab Basalt of Austin Butte (Pliocene) -One of the ite unit is above the basalt of Austin Butte and below best exposed volcanoes in the area. The unit is in the the basalt of Miller Water and the basalt of Winter Austin Butte quadrangle. The volcano is Austin Butte, Camp. Over most of its exposure, the'unit contains only one or two subunits. However, in the area of the Miller located at the confluence of Clover..creek and the Bru- Water draw and across from the Austin Butte volcano, it neau River. The flows are exposed in three directions, to contains as many as five subunits. The contact of this the north and south in the Bruneau River canyon and to unit with the sediments exposed northwest of Austin the east in Clover Creek. The unit's thickness ranges Butte is uncertain and therefore queried. from 40 feet to 400 feet (12-122 m), because its flows were emplaced directly on the irregular top of the un- Tmw Basalt of Miller Water (Pliocene) - Exposed derlying rhyolite lava flow. At the volcano the character on both sides of the Bruneau River canyon in the area of the flows varies up-section. The lower section is atound Miller Water draw in the Austin Butte and thicker and characterized by a few, fairly massive flows. Crowbar Gulch quadrangles. In hand sample the unit The upper section contains as many as fifteen thin and has a diktytaxitic texture with moderate amounts of large plagioclase crystals and some olivine crystals. very scoriaceous flows. In hand sample the flows con- Both chemically and petrographically, the unit appears tain clumps of plagioclase and olivine in a rnicrocrys- to be similar to Twc. However, stratigraphic relations re- talline matrix. quire that the units be separate. Ttab Basalt of Table Butte (Pliocene) -This small Troc Basalt of Rock Lake (Pliocene) -Located in exposure in Table Butte quadrangle is one of the most the south part of the area west of the Bruneau River in enigmatic basalt units in the area. A subaerially emplaced Austin Butte quadrangle. The unit is exposed in only unit, its exposure is neatly circular in plan view. On its two small areas. In hand sample the flows are felty with north side it rests directly on the rhyolite of Sheep Creek abundant medium to large plagioclase and sparse small On its south side it rests on at least 20 feet (6.1 m) of fine- olivine crystals. grained sediments. Flow-layering is a prominent feature within individual flows. The exposure does not appear to Tbib Basalt of Big Bend (Pliocene) -.Located in the southern part of the Austin Butte quadrangle on the west be a volcanic vent. Instead its flows have characteristics side of the Bruneau River. The volcano that erupted the that we have in other units associated with the more distal flows is to the south of the area on the north side of parts of flows. Proximity suggests that the exposure at Ta- Sheep Creek. Within the mapped area hand samples are ble Butte may be related to two other isolated basalt expo- nearly aphyric with sparse small plagioclase phe- sures to the southwest. In hand sample the flows contain nocrysts. Elsewhere, samples from outside the area vary abundant, small plagioclase phenocrysts. in content. Some samples contain large olivine crystals Tbwf Basalt of Broken Wagon Flat (Pliocene) - and small plagioclase laths while others have sparse, Located in the Hot Creek drainage in the Broken Wagon very large, typically equant plagioclase phenocrysts. Flat quadrangle. Even though the exposures suggest a Ttwi Basalt of Twin Butte (Pliocene) -Thee un- single unit, its phenocryst content is varied. In the Hot connected exposures: the prominent Twin Butte loca- Creek drainage, the basalt flows in hand sample are dik- tion, a large area in the Austin Butte quadrangle, and a tytaxitic or felty with abundant, medium to large plagio- single small exposure in the northeast comer of the Table clase phenocrysts. In exposures to the west the lithology Butte quadrangle. All exposures have been displaced by is water affected and, in hand sample, appears to be postemplacement faults. At its thickest exposure, the nearly aphyric with some medium-sized plagioclase unit contains only two subaerial flows. In hand sample phenocrysts. Thus, the unit may actually need to be fur- Geologic Maps - Grand mew-Bruneau Area 19 ther subdivided into several units from more than one fected. It is above red "baked" sediment, which in one source. area includes a small lens of pebble gravel. The basalt Tbgr Basalt of Blackstone Grasmere Road (Pliocene) of Missile Base Road may be correlative with other ba- -Located in the bottom of the Hot Creek drainage in salt units in the Sugar Creek drainage and is probably the Broken Wagon Flat quadrangle. In some areas it is cotrelative with units exposed in the Hot Creek drainage water affected; in others, it appears to have been and Btuneau River canyon. subaerially emplaced. Its lithology in hand sample is Tbla Basalt of Black Rocks (Pliocene) -Located to abundant large olivine crystals with varied amounts and the west of Idaho Highway 51 in the southern part of sizes of plagioclase phenocrysts. the Little Valley quadrangle. The unit is extremely Tsot Basalt of South Top (Pliocene) -Two isolated water affected throughout its exposure. Thus, in many outcrops in the Sugar Valley and Broken Wagon Flat areas only the bottom flow and its underlying red sedi- quadrangles. The unit is entirely water affected and en- ment remain of what was originally a fairly thick unit. closed within the Idaho Group sediments. It is probably In areas of less affected basalt, the unit in hand sample correlative with one of the units in the Hot Creek drain- is olivine phyric with some large single crystals as well age to the east. as clumps. Since no nearby source exists for this unit, we assume that it is the westward extension of one of Thtc Basalt of Horse Trap Corral (Pliocene) - the basalt units exposed in the Sugar Creek, Hot Creek, Makes the flat and rim along and east of Idaho Highway and Bruneau River drainages. 51 in the Broken Wagon Flat and Hole In Rock quad- rangles. The outline and extent of the unit suggest that it Tdea Basalt of Deadman Gulch (Pliocene) -Ex- flowed north from the area of a small hill surrounded by posed over a fairly large area in the drainages just west the 3600-foot contour in SWV4SWV4 sec. 32, T. 8 S., R. of Idaho Highway 51 in the Hole In Rock quadrangle. 5 E. However, our traverse in that area did not prove Except in its farthest southem exposures, the unit is ex- conclusively whether this hill is a vent. At its edges the tremely water affected. In one place the top of the unit's flows are extensively water affected. In hand sample it upper flow contains a breccia mixed with the overlying is a dense and aphyric lithology with a few olivine phe- silts. The bottom flow of the unit rests on a red sediment noctysts, some in clumps, and very sparse plagioclase layer. In at least two areas, the unit has been completely phenocrysts. eroded away, with nothing remaining but the underlying Tsug Basalt of Sugar Creek (Pliocene) -Located red sediment layer. Less affected hand samples contain on the east side of the Sugar Creek drainage in the Bro- some small plagioclase phenoctysts. The phenocryst ken Wagon Flat quadrangle. Throughout its exposure content of the unit, the widespread nature of its flows, the flows are extremely water affected and above a red and its proximity to the basalt of Windy Well to the east fine-grained sediment layer. An oolitic limestone con- in the Sugar Creek drainage strongly suggest that these tinues for a short distance at the same elevation at the two units may be cotrelative. Presumably, it is also cor- unit's north edge. At its south edge, a small area of ree- relative with one of the basalt units in the Hot Creek fa1 limestone is above as well as at the same elevation. drainage and the Btuneau River canyon. The unit is probably cotrelative with one of the basalt Tals Basalt of Al Sadie Ranch (Pliocene) -Ex- units exposed in the Hot Creek and Bmeau River posed in the bottoms of the small grabens at the adja- drainages. cent comets of the Hole In Rock, Little Valley, and Tww Basalt of Windy Well (Pliocene) - Floots the Chalk Hill quadrangles. The unit is in the area between, bottoms of the draws east of the Sugar Creek drainage and east and west of, the Big and Little Jacks Creek in the Broken .Wagon Flat and Hole In Rock quadran- drainages. Throughout its exposure the unit is extremely gles. The u& is water affected in some areas. It is prob- water affected. In some places very little of the unit is ably cotrelative with one of the units exposed in the Hot still intact above the red sediment layer. Everywhere the Creek and Bruneau River drainages. Throughout its ex- unit is enclosed within a section of fine-grained Idaho posure, hand samples of the flows contain numerous Gtoup sediments. In hand sample the flows have a phe- small plagioclase phenocrysts. nocryst content that ranges from a few individual, small Tmbr Basalt of Missile Base Road (Pliocene) -Lo- plagioclase phenocrysts to clumps of olivine and small cated in the bottom of the Sugar Creek drainage in the plagioclase crystals. northwest part of the Broken Wagon Flat quadrangle. Thir Basalt of Hole In Rock (Pliocene) -The vol- Throughout its exposure, the unit is extremely water af- cano for this unit is in the southwest comer of the 20 Geologic Maps -Grard View-Brrrtlea~cArea

Hole In Rock quadrangle. Its flows extend to the north into a middle zone of massive, devitrified rhyolite (red, of the volcano within a small area next to Wickahoney crystalline). In canyon exposures these devitrified mid- Creek in the Hole In Rock and Big Horse Basin Gap dle zones contain vertical shrinkage joints that propa- quadrangles. The flows evidently followed a paleodrain- gated from the upper and lower zones of the flow. The age, filling it and overlying a Tgq layer. All hand Sam- areas of these joints are commonly easy to locate be- ples from the flows contain plagioclase phenocrysts cause they form prominent cliffs. Where the two joint that range in abundance from sparse to plentiful and surfaces meet, however, the flow weathers more easily in size from small to large. This range may indicate a to a steep rubble-covered slope. This gives the flows compositional variability in the unit itself or the pres- their characteristic "two-tiered" appearance. The upper ence of more than a single unit. zones are flow layered and extensively folded, espe- Twhc Basalt of Wickahoney Creek (Pliocene) - cially toward their uppermost surface. This zone con- Only outcrops in a very small area in the adjacent south- tains both devitrified, brecciated areas and vitrophyre em comers of the Hole In Rock hd Big Horse. Basin sections. Some of the rhyolite flows are underlain by Gap quadrangles. Since the unit is beneath the .basalt of thin air-fall ash deposits. The ends of the.flows are thick Hole In Rock, it also has presumably followed a paleo- and bulbous with large flow lobes. topographic low and flowed north from sources to the Tru Rhyolite, undifferentiated (Miocene) -Located south. Outside the mapped area, the unit is composite as horsts separated from the main parts of the named rhy- and contains at least two subunits separated by a sedi- olite units by sediment-filled grabens. The exposures are ment layer. Within the area the unit contains a single se- in two separate areas: east and west of Idaho Highway quence of flows with no intercalated sediment. 51 in the Broken Wagon Flat, Little Valley, and Hole In Twt Basalt of Whickney Tree Reservoir (Pliocene) Rock quadrangles; and east of the Mud Flat Road in the -Erupted from a volcano (hill, elevation 6062) on the Perjue Canyon quadrangle. west edge of the 0 X Lake quadrangle. In some areas Thb Rhyolite of Horse Basin (Miocene) -Exposed its flows appear to have followed paleoslopes. The between and east of the Big and Little Jacks Creek can- flows have also been cut and displaced by numerous yons in the Hole In Rock, Little Valley, Chalk Hills, and postemplacement faults. Big Horse Basin Gap quadrangles. In its more northern To1 Basalt of 0 X Lake (Pliocene) - Located in the exposures the unit contains hundreds of feet of rhyolite area surrounding the 0 X Prong of Little Jacks Creek in breccia intercalated with dikes and sills of unbrecciated the southeastern part of the 0 X Lake quadrangle. The and jointed devitrified rhyolite. At its southern edge the unit probably erupted from a volcano to the south of the unit apparently flowed against a northeast-facing pa- area. Our examination of a single hand sample suggests leoescarpment that has been completely eroded away. In only that the flows have abundant phenocrysts. fact, the flow may have been erupted from a buried northwest-trending fissure near its southern edge. In Rhyolite Lava Flows some places the unit is underlain by 0.3 to 3.0 feet (0.1- 1.0 m) of air-fall ash. Plagioclase and pigeonite phe- A series of rhyolite lava flows are in the southern part nocrysts are more abundant in this, lava flow than in of the area. A single large unit, the rhyolite of Sheep others in the region. The unit has normal palemagnetic Creek, dominates the landscape in the southeastern part. polarity. Other flows are exposed in the upper canyons of the Big Jacks, Little Jacks, Shoofly, and Poison creeks, and their Tsfc Rhyolite of Shoofly Creek (Miocene) -Lo- tributaries. cated in the southwestern part of the mapped area in the 0 X Lake and Perjue Canyon quadrangles. No informa- Bonnichsen (1982) and Bonnichsen and Kauffrnan tion about its internal or petrographic characteristics is (1987) described the rhyolite lava flows in detail; some available at this point in our research. of the characteristics of individual units are taken from these reports. The general rhyolite flow characteristics Tsc Rhyolite of Sheep Creek (Miocene) -The larg- are as follows. Each lava flow has three distinct internal est single unit within the mapped area, it is in the Crow- zones: a lower, a middle, and an upper. The lower zone bar Gulch, Broken Wagon Flat, Hole In Rock, Table is made up of red and black flow breccia and is up to 30 Butte, Austin Butte, and Big Horse Basin Gap quadran- feet (9 m) thick. It may also have from 1 to 3 feet (0.3- gles. This enormous lava flow is exposed over a broad 1.0 m) of flow-layered vitrophyre (black, glassy, non- area between the Bruneau River and the Big Jacks crystalline rhyolite). The lower zone grades vertically Creek canyons. It also extends for many miles to the Geologic Maps -Grand View-Brrmeau Area 21

south outside the area and has an overall extent of ap- Top Rhyolite of 0 X Prong (Miocene) -Exposed proximately 400 square miles (1,000 km2). The unit only in the walls of the Big and Little Jacks Creek can- mainly contains phenocrysts of plagioclase, augite, yons in the southwestern part of the area in the 0 X pigeonite, and opaque oxides, but it also has very Lake and Big Horse Basin Gap quadrangles. In Little sparse, tiny quartz crystals. Hart and Aronson (1983) Jacks Creek the unit forms a persistent thin upper cliff dated a sample of this unit at 9.88 ? 0.46 m.y. using the that distinguishes it from the other rhyolite units in the whole rock K-Ar method. The unit has normal paleo- area. The unit contains sanidine, plagioclase, and pige- magnetic polarity. onite phenocrysts. It shows erratic paleomagnetic polar- ity. Tpc Rhyolite of Perjue Canyon (Miocene) -Lo- cated in the Petjue Canyon, 0 X Lake, and Big Horse Trc Rhyolite of Rattlesnake Creek (Miocene) - Basin Gap quadrangles and underlies a large area be- Located in the bottom of the Little Jacks Creek canyon in the 0 X Lake and Big Horse Basin Gap quadrangles. tween Big Jacks Creek canyon and the western edge of The base is not exposed. In addition its characteristics the area. Phenocrysts are principally plagioclase and are ambiguous as to whether the unit is.a'lava flow or a pigeonite. The unit has reverse paleomagnetic polarity. welded tuff. Petrographically, it is similar to the rhyolite Thg Rhyolite of Halfway Gulch (Miocene) - Ex- of 0 X Prong above it. The unit has normal paleomag- posed in and between the large in the Perjue netic polarity. Canyon, 0 X Lake, and Big Horse Basin Gap quadran- gles just south of the major Snake River Plain margin Older Volcanic Units faults. Parts or all of this unit may be correlative with the rhyolite of Tigert Spring, exposed to the east in the Tev Eocene volcanics -Located on the southwest Little Jacks Creek canyon. side of the area on both sides of the Mud Flat Road in Tbtc Rhyolite of Between The Creeks (Miocene) - the Perjue Canyon quadrangle. Ekren and others (1981) Located only in a small part of the western edge of correlated the units with the Challis Volcanics that are the area in the bottom of Perjue Canyon in the 0 X Lake extensively and widely exposed over south-central quadrangle. The unit also is exposed to the west of the Idaho. Neil1 (1975) reported a whole rock K-Ar date of area. Although the base is not exposed in the area, to 43.6 + 0.8 m.y. for the exposures on the west side of the west it rests on a tilted surface of Eocene volcanics. Poison Creek at the extreme western edge of the area. The unit is a red, pink, tan, brown, white, or green rhyo- Tts Rhyolite of Tigert Spring (Miocene) -Located dacite. It is somewhat more phenocryst-rich in hand in and between the canyons of the Big and Little Jacks sample than the overlying rhyolitic rocks, and according Creeks in the 0 X Lake and Big Horse Basin Gap quad- to Ekren and others (1981) contains quartz, alkali feld- rangles. Although the unit is sporadically exposed on spar, plagioclase, biotite, hornblende, and opaque ox- the west side of the Little Jacks Creek canyon, the creek ides. Another distinguishing feature of the unit is its follows its approximate western and southwestern mar- fairly extensive inclusions of jasper. Clasts eroded from gins. The phenocryst assemblage is sahdine, plagio- this unit are a characteristic lithology in- the gravels clase, pigeonite, and augite. The unit has reverse paleo- above and within the Idaho Group sediments to the magnetic polarity. north.