Lava Dam Research Articles

History of Quaternary volcanism and lava dams in western Grand Canyon based on lidar analysis, 40Ar/39Ar dating, and field studies: Implications for flow stratigraphy, timing of volcanic events, and lava dams

A synthesis of the geochronology on basalt flows from the southern Uinkaret volcanic field indicates that basalts erupted within and flowed into Grand Canyon during four major episodes: 725–475 ka, 400–275 ka, 225–150 ka, and 150–75 ka. To extend the usefulness of these dates for understanding volcanic stratigraphy and lava dams in western Grand Canyon, we analyzed light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt-flow remnants along the river corridor. When projected onto a longitudinal river profile, these data show the original extent of now-dissected intracanyon flows and aid in correlation of flow remnants. Systematic variations in the elevation of flow bottoms across the Uinkaret fault block can be used to infer the geometry of a hanging-wall anticline that formed adjacent to the listric Toroweap fault.

Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene Sinker Butte Volcano, Western Snake River Plain, Idaho

Sinker Butte is the erosional remnant of a very large basaltic tuff cone of middle Pleistocene age located at the southern edge of the western Snake River Plain. Phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the Snake River Canyon, creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions. The main objectives of the study were to determine the overall evolution of the Sinker Butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions. Toward this end, twenty-three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180° of the circumference of the volcano. Three main rock units are recognized in canyon walls at Sinker Butte: a lower sequence composed of numerous thin basaltic lava flows, an intermediate sequence of phreatomagmatic tephras, and a capping sequence of welded basaltic spatter and more lava flows. We subdivide the phreatomagmatic deposits into two main parts, a series of reworked, mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits. Most of the reworked deposits are gray in color and exhibit features such as channel scour and fill, planar-stratification, high and low angle cross-stratification, trough cross-stratification, and Bouma-turbidite sequences consistent with their being deposited in shallow standing water or in braided streams. The overlying subaerial deposits are commonly brown or orange in color due to palagonitization. They display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges, wet to dry pyroclastic fall events, and water saturated debris flows. Proximal sections through the subaerial tephras exhibit large regressive cross-strata, planar bedding, and bomb sags suggesting deposition by wet base surges and tephra fallout. Medial and distal deposits consist of a thick sequence of well-bedded tephras; however, the cross-stratified base-surge deposits are thinner and interbedded within the fallout deposits. The average wavelength and amplitude of the cross strata continue to decrease with distance from the vent. These bedded surge and fall deposits grade upward into dominantly fall deposits containing 75–95% juvenile vesiculated clasts and localized layers of welded spatter, indicating a greatly reduced water-melt ratio. Overlying these “dryer” deposits are massive tuff breccias that were probably deposited as water saturated debris flows (lahars). The first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section. The “wetter” nature of these deposits suggests a renewed source of external water. The massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows. Field observations and two new 40Ar– 39Ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the Snake River canyon may all have been erupted from a closely related complex of vents at Sinker Butte. We propose that initial eruptions of lava flows built a small shield edifice that dammed or disrupted the flow of the ancestral Snake River. The shift from effusive to explosive eruptions occurred when the surface water or rising ground water gained access to the vent. As the river cut a new channel around the lava dam, water levels dropped and the volcano returned to an effusive style of eruption.

TROPICAL MARSH AND SAVANNA OF THE LATE PLEISTOCENE IN NORTHEASTERN SONORA, MEXICO

Abstract We recovered Pleistocene fossils from a lava-dammed river deposit along the Rio de Moctezuma in northeastern Sonora, at 29°41′N, 109°39′W, and 605 m elevation. Today the region is semiarid, with a foothills thornscrub community. The impoundment that resulted from the lava dam produced a short-lived marsh with an adjacent savanna. The extraordinary fauna is both diverse and rich, and includes ostracods, mollusks, fish, amphibians, turtles, a crocodilian, snakes, lizards, birds, and mammals, many with tropical affinities today. Most of the animals are either extralimital to the setting today or extinct. The recovery of Bison dictates a Rancholabrean Land Mammal Age for the fauna; a preliminary 40Ar/39Ar age suggests that the deposit is between 570,000 and 310,000 years old. The occurrence of cf. Crocodylus acutus (a crocodilian; generic assignment uncertain) and Pampatherium, the giant armadillo, is unique in the northern interior Sonora setting. We speculate that a well-developed riparian corridor...

Differentiating Pleistocene tectonically driven and climate-related fluvial incision: the Sanggan River, Datong Basin, North China

The Sanggan River is an alluvial river flowing through a graben basin system of the northern Shanxi Rift Zone, North China. During Pleistocene times, the river reach in the Datong Basin was affected successively by various external variables, such as invasion by basaltic flow, along-valley faulting and climatic change. Therefore, it provides excellent constraints for differentiating tectonically driven and climate-related fluvial incision in the context of tectonic subsidence. Based on equilibrium profile analysis, K–Ar dating of basalts (0.74–0.41 Ma), studies of the river terrace and of stream action history, we present a conceptual model for differentiating fault-driven and climate-related fluvial incision by the river. The results show that fluvial incision induced by tectonic lowering of the base-level due to along-valley movement on the Sanggan River fault is equal to fault displacement. The amount of post-basalt fluvial incision of the reach upstream from the lava dam is 23 to 25 m, of which the fault-driven and climate-related incisions are 15 m and 8 to 10 m, respectively, the former predominating over the latter. The total amount of incision in the lava dam reach is 40 to 47 m, of which the fault-driven and climate-related incisions are 10 m and 30–37 m, respectively; here the latter is predominant over the former. Since 0.41 ± 0.10 Ma, the rate of fluvial incision of the lava dam reach of the river has reached 98–115 m/Ma, which is 1.5–2 times as great as those of the reaches upstream and downstream from the lava dam. The higher rate of fluvial incision can be attributed to high water levels supplied by the onset and maintenance of backwater conditions in the reach upstream from the lava dam, due to the long period of warm and humid climate in this region. Plucking, abrasion and knickpoint migration appear to be the primary erosional processes in the lava dam reach.

Multiple constraints on the age of a Pleistocene lava dam across the Little Colorado River at Grand Falls, Arizona

Research Article| March 01, 2006 Multiple constraints on the age of a Pleistocene lava dam across the Little Colorado River at Grand Falls, Arizona Wendell Duffield; Wendell Duffield 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for other works by this author on: GSW Google Scholar Nancy Riggs; Nancy Riggs 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for other works by this author on: GSW Google Scholar Darrell Kaufman; Darrell Kaufman 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for other works by this author on: GSW Google Scholar Duane Champion; Duane Champion 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Cassandra Fenton; Cassandra Fenton 3GeoForschungsZentrum Potsdam, Telegrafenberg, Haus B, D-14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Steven Forman; Steven Forman 4Department of Earth and Environmental Sciences, University of Illinois, Chicago, Illinois 60607, USA Search for other works by this author on: GSW Google Scholar William McIntosh; William McIntosh 5New Mexico Bureau of Mines and Mineral Resources, 801 Leroy Place, Socorro, New Mexico 87801, USA Search for other works by this author on: GSW Google Scholar Richard Hereford; Richard Hereford 6U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001, USA Search for other works by this author on: GSW Google Scholar Jeffery Plescia; Jeffery Plescia 7Johns Hopkins Applied Physics Lab, 11100 Johns Hopkins Drive, Laurel, Maryland 20723, USA Search for other works by this author on: GSW Google Scholar Michael Ort Michael Ort 8Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2006) 118 (3-4): 421–429. https://doi.org/10.1130/B25814.1 Article history received: 11 Feb 2005 rev-recd: 02 Jun 2005 accepted: 13 Jul 2005 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wendell Duffield, Nancy Riggs, Darrell Kaufman, Duane Champion, Cassandra Fenton, Steven Forman, William McIntosh, Richard Hereford, Jeffery Plescia, Michael Ort; Multiple constraints on the age of a Pleistocene lava dam across the Little Colorado River at Grand Falls, Arizona. GSA Bulletin 2006;; 118 (3-4): 421–429. doi: https://doi.org/10.1130/B25814.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Grand Falls basalt lava flow in northern Arizona was emplaced in late Pleistocene time. It flowed 10 km from its vent area to the Little Colorado River, where it cascaded into and filled a 65-m-deep canyon to form the Grand Falls lava dam. Lava continued ∼25 km downstream and ∼1 km onto the far rim beyond where the canyon was filled. Subsequent fluvial sedimentation filled the reservoir behind the dam, and eventually the river established a channel along the margin of the lava flow to the site where water falls back into the preeruption canyon.The ca. 150 ka age of the Grand Falls flow provided by whole-rock K-Ar analysis in the 1970s is inconsistent with the preservation of centimeter-scale flow-top features on the surface of the flow and the near absence of physical and chemical weathering on the flow downstream of the falls. The buried Little Colorado River channel and the present-day channel are at nearly the same elevation, indicating that very little, if any, regional downcutting has occurred since emplacement of the flow.Newly applied dating techniques better define the age of the lava dam. Infrared- stimulated luminescence dating of silty mudstone baked by the lava yielded an age of 19.6 ± 1.2 ka. Samples from three noneroded or slightly eroded outcrops at the top of the lava flow yielded 3He cosmogenic ages of 16 ± 1 ka, 17 ± 1 ka, and 20 ± 1 ka. A mean age of 8 ± 19 ka was obtained from averaging four samples using the 40Ar/39Ar step-heating method. Finally, paleomagnetic directions in lava samples from two sites at Grand Falls and one at the vent area are nearly identical and match the curve of magnetic secular variation at ca. 15 ka, 19 ka, 23 ka, and 28 ka. We conclude that the Grand Falls flow was emplaced at ca. 20 ka. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 10 9 m 3 of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 10 5 m 3 s −1 for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 10 4 m 3 s −1) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 10 4 m 3 s −1). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>10 5 m 3 s −1) known worldwide and in the top ten largest floods in North America.

Geochemical Discrimination of Five Pleistocene Lava‐Dam Outburst‐Flood Deposits, Western Grand Canyon, Arizona

Pleistocene basaltic lava dams and outburst‐flood deposits in the western Grand Canyon, Arizona, have been correlated by means of cosmogenic 3He (3Hec) ages and concentrations of SiO2, Na2O, K2O, and rare earth elements. These data indicate that basalt clasts and vitroclasts in a given outburst‐flood deposit came from a common source, a lava dam. With these data, it is possible to distinguish individual dam‐flood events and improve our understanding of the interrelations of volcanism and river processes. At least five lava dams on the Colorado River failed catastrophically between 100 and 525 ka; subsequent outburst floods emplaced basalt‐rich deposits preserved on benches as high as 200 m above the current river and up to 53 km downstream of dam sites. Chemical data also distinguishes individual lava flows that were collectively mapped in the past as large long‐lasting dam complexes. These chemical data, in combination with age constraints, increase our ability to correlate lava dams and outburst‐flood deposits and increase our understanding of the longevity of lava dams. Bases of correlated lava dams and flood deposits approximate the elevation of the ancestral river during each flood event. Water surface profiles are reconstructed and can be used in future hydraulic models to estimate the magnitude of these large‐scale floods.

Cyclic Aggradation and Downcutting, Fluvial Response to Volcanic Activity, and Calibration of Soil-Carbonate Stages in the Western Grand Canyon, Arizona

In the western Grand Canyon, fluvial terraces and pediment surfaces, both associated with a Pleistocene basalt flow, document Quaternary aggradation and downcutting by the Colorado River, illuminate the river's response to overload and the end of overload, and allow calibration of soil-carbonate stages and determination of downcutting rates. Four downcutting–aggradation cycles are present. Each begins with erosion of older deposits to form a new river channel in which a characteristic suite of deposits is laid down. The current cycle (I) started ∼700 yr B.P. The oldest (IV) includes the 603,000 ± 8000 to 524,000 ± 7000 yr Black Ledge basalt flow, emplaced when the river channel was ∼30 m higher than it is now. The flow is overlain by basalt–cobble gravel and basalt sand. Soils reach the stage V level of carbonate development. Calibrated ages for soil stages are Stage V, ∼525,000 yr; stage IV, <525,000 yr, ≥250,000 yr; stage III, <250,000 yr, ≥100,000 yr. The monolithologic basalt sand beds represent overloading by volcanic ash produced by an eruption 30–50 km upstream. The basalt–cobble beds signal breaching and rapid destruction of lava dams and erosion of flows. These deposits show that the Colorado River responds to overload by aggrading vigorously during the overload and then downcutting equally vigorously when the overload ends. The overall downcutting rate for the interval studied is 1.6 cm/1000 yr, much lower than rates upstream. The current downcutting rate, 11–14 m/1000 yr, likely is a response both to the end of late Pleistocene and early Holocene overload and to the reduction of sediment supply caused by Glen Canyon Dam.

K-Ar ages of Pleistocene lava dams in the Grand Canyon in Arizona.

At least 13 times during the Pleistocene Epoch lava flowed into the inner gorge of the Grand Canyon and formed lava dams, as high as 600 m, that temporarily blocked the flow of the Colorado River. K-Ar ages on these lava dams indicate that the seven youngest formed within a short period of time between about 0.6 and 0.4 mega-annum (Ma). The physiography of the lava dam remnants within the canyon shows that each dam was destroyed by erosion, the Colorado River rapidly reaching its pre-existing grade level, before the next dam was emplaced by new eruptions. The total time for emplacement and destruction for an individual lava dam was probably as little as 0. 01-0.02 million years. The K-Ar ages of the two oldest dams, the Lava Butte dam (0.577 +/- 0.054 Ma) and the Prospect dam (0.684 +/- 0.051 Ma) are somewhat younger than the physiography of their remnants suggest.

Book review

Book Review in This ArticleOrbital Forcing Timescales and Cyclostratigraphy. By M. R. House and A. S. GaleLate Cenozoic Lava Dams in the Western Grand Canyon. By Kenneth Hamblin

Shallow seismic stratigraphy and pockmarks of a hydrothermally influenced lake, Lake Rotoiti, New Zealand

ABSTRACTLake Rotoiti formed 11 850–20 000 years ago when lava flows dammed the drainage system through the Okataina caldera, one of the major rhyolitic centres of the intracontinental, back‐arc, Taupo Volcanic Zone of North Island, New Zealand. The lake morphology reflects this complex origin, with remnants of the old caldera rim, the lava dam and relict river system being preserved. High resolution seismic reflection profiles (7 kHz) show the drowned river system channelled inflowing water into the lake basin, scouring and depositing sediments and maintaining an active sublacustrine channel in the former river valley. Airfall tephras are preserved in lake sediments. The tephrachronology, established from the surrounding catchments, can be correlated with the siesmic stratigraphy for eight major eruptions over the last 11 850 years. Hydrothermal fields underlying the lake generate gaseous sediments which mask seismic penetration. This is indistinguishable from seismic masking caused by biogenic gas accumulation within the sediments. Venting of hydrothermal gases at the surface creates pockmarks. From the seismic stratigraphy beneath pockmarks seven stages of pockmark genesis have been identified which show pockmarks grow, decay, migrate and persist over several thousand years.

Hawaiites and related lavas in the Atenguillo graben, western Mexican Volcanic Belt

Hawaiite and mugearite, alkali-olivine basalt, and basaltic andesite have erupted from three shield volcanoes in the Atenguillo graben, Jalisco, Mexico. The youngest shield volcano, La Laja (0.66 Ma), is the focus of this study. Basal pyroclastic surge deposits of basaltic ash, lapilli, and blocks are overlain by seven to eight, 5- to 10-m-thick hawaiite flows. Pillow lavas along a 4-km stretch on the southeast flank of the volcano and lake-bed sediments define a shoreline and lake bottom southward to the town of Atenguillo. Eruption of La Laja apparently dammed the northward-flowing Atenguillo River and formed a lake of as much as 100-m depth. Subsequent fluvial erosion has destroyed the lava dam, exposing the shield succession. Discharge rates for the Atenguillo River and rainfall estimates for the Atenguillo basin both suggest that this lake formed within 20 to 40 yr. Eruption of nearly 10 km 3 of material within 10 to 100 yr indicates an output rate of 0.1 to 1 km 3 /yr, which is larger than any rate calculated for volcanoes in the Mexican Volcanic Belt (MVB), but similar in magnitude to that of Mauna Loa and Kilauea volcanoes (Hawaii). The older volcanoes, El Vigia (2.7 Ma) and Tin Cleto (3.6 Ma), are composed of basaltic-andesite and alkali-olivine basalt, respectively. The composition of these lavas is unusual for a continental-arc setting. Total FeO, TiO 2 , and alkali concentrations are higher than those of basic calc-alkaline lavas from the western MVB. Alkaline earth/light-rare-earth and light-rare-earth/high-field-strength element ratios in these lavas are low relative to those of calc-alkaline lavas but similar to those of oceanic-island basalt values. Such compositional traits coupled with a high magma-output rate are characteristic of oceanic-island volcanoes. Both hydrous, fluid-enriched, sub-arc mantle, and oceanic-island-type mantle components are present beneath western Mexico; contemporaneous subduction and extension of this region of the western MVB allow tapping of both sources and thus the eruption of compositionally diverse lavas.

Paleomagnetism of tertiary tuffs from the ‘lava dam’, Southern House Range, Utah

At ‘Lava Dam’, Utah, the Tunnel Spring Tuff yields a reversed‐polarity characteristic magnetization; two units within the tuff yield similar but distinct directions of magnetization (lower: D=156°,I=−73°,α95=2°; upper: D=137°,I=−73°,α95=2°; no bedding corrections applied). If this difference reflects paleosecular variation, it suggests that the two units reflect different eruptions perhaps a few tens of years apart. The Cottonwood Wash Tuff Member (D=348°,I=50°,α95=2°) and the Wah Wah Springs Tuff Member (D=344°,I=47°, α95=3°) of the Needles Range Formation yield normal‐polarity magnetizations. The polarity of the Cottonwood Wash is consistent with the results of other workers, but the Wah Wah Springs has been reported to have reversed polarity. Whether the normal polarity at Lava Dam is due to pervasive remagnetization of that section or to miscorrelation of the Wah Wah Springs cannot be determined from the present data, but miscorrelation seems more likely.

K-Ar Age of Lava Dam in Grand Canyon

The K-Ar age of the basal basalt flow at the bottom of the “Lower Canyon group” of lavas near Toroweap fault is 1.16 ± 0.18 standard deviation (sd) m.y. This represents a minimum age of Grand Canyon, for at the time the lava formed, the canyon was essentially as deep as it is today. Since that time the Colorado River has cut through the 550-ft lava dam at the mouth of Toroweap Valley, an additional 50 ft of Paleozoic strata below, and through one or more younger lava dams in the area; downstream it has cut through 100 feet of younger intracanyon lavas.

Late Quaternary History of the Snake River in the American Falls Region, Idaho

While mapping the American Falls region, we found evidence that contributes to the middle Pleistocene to Recent history of the Snake River, and indirectly to the history of overflow of Lake Bonneville. Middle Pleistocene to recent rocks in the valley are mainly lacustrine and fluvial silts and clays, with some sand, gravel, basalt, and a few thin tuff beds. The formation of terraces can be correlated with events both up- and downstream. The Snake River was at least once, and possibly twice, dammed and diverted by eruptions of basalt, resulting in the formation of lakes and deposition of lacustrine beds. A rather flat-lying, thin, but persistent gravel at the base of one lake bed formation may represent a glacial period, possibly Illinoian, during which the Snake River had a large volume. Overflow of water from Lake Bonneville into the Snake River system, by way of the Marsh Creek-Portneuf valley, laid down a deltaic-fluvial deposit here named the Michaud Gravel. At this time the Snake River, greatly augmented by Lake Bonneville overflow, began to cut channels through and around a lava dam. Terraces between Aberdeen, American Falls, and Pocatello were formed during the existence of the lake in which the Michaud Gravel was deposited and by fluvial processes after drainage of the lake. At one stage in the downcutting, bars of huge basalt boulders were built across the mouths of abandoned spillways. Radiocarbon dating and geologic evidence from the area between Preston and Soda Springs, Idaho, suggest that basalt flows diverted the Bear River into Lake Bonneville, perhaps causing it to overflow. This diversion probably occurred about 33,000 years ago. This dating accords with events in the American Falls region.

Nature and origin of central Idaho blacksands

Fifty or more heavy minerals occur in blacksand placers adjacent to the Cretaceous Idaho batholith. They contain Nb (columbium), Ta, Y, Zr, Hf, U, Th, Ti, and the rare earths. These minerals are of present or future potential value as sources of metals characterized by versatility, including strength and resistance to heat and chemical corrosion. Natural concentrations of blacksand involve mineral hardness, texture, chemical composition, and specific gravity; mineral size, shape, and degree of subroundness; and major alterations in stream regimens affecting competency, capacity, and interruptions of transportation. The latter include climatic changes, lava damming, crustal warping, faulting, and mass-wasting phenomena. Local deep disintegration of bedrock is an important adjunct to blacksand concentration. The original source of heavy mineral elements is believed to be Precambrian placer accumulations. These concentrations were later partially dispersed by metasomatic reconversion during diastrophic maxima, and indistinct swarms of accessory minerals were produced. New placers were formed again later through the operation of erosional and depositional agents. Recent blacksand placers derived their heavy minerals from quartz monzonite, granodiorite, diorite, and metamorphic remnants of original country rock commonly found in adjacent exposures of the Idaho batholith. The batholith may appear deceptively normal in these areas. When present, pegmatite and aplite dikes provide an additional source of heavy detrital minerals. The domination of one or more minerals over others in the more recent placers, appears to be related to the domination of similar elemental constituents in earlier original placer deposits. Thus recent data on both the petrography of the Idaho batholith and the nature and origin of blacksand placers lead to the following conclusions: erosional processes concentrated the first placer ore deposits in the Precambrian; and subsequent events both geomorphic and metasomatic probably played more important roles in the genesis of all types of ore deposits, including placers, than previously has been supposed.

GEOLOGY OF THE UINKARET PLATEAU, NORTHERN ARIZONA

The Uinkaret Plateau, a major block of the Colorado Plateau province in northern Arizona, is bounded on the west by the Hurricane Ledge, developed along the Hurricane fault, and on the east by scarps developed along the Toroweap and Sevier faults. Over the southern part there are extensive outpourings of lava which fall into four age groups. The oldest eruptions preceded earliest movement along the Hurricane fault and antedated cutting of the present Grand Cayon. These lavas poured out on a relatively even surface believed to have been a large pediment or series of pediments extending at least 16 miles north from the Colorado River. Extensive dissection of the oldest lavas preceded the next eruptions, which took place at considerably lower elevations. The Inner Gorge of Grand Canyon was cut to approximately its present depth and later filled to a height of at least 600 feet and perhaps 1,200 by lava floods from Toroweap Valley. The lava dams were entirely removed before later flows, pouring into the Inner Gorge from the rim of the Esplanade, again dammed the river, which has removed all but small portions of these dams. Repeated movements along the Toroweap fault have occurred in post-lowan-Wisconsin time. In the very recent past, renewed volcanic activity formed a single small cone and lava flow.

Origin of the Large Springs and their Alcoves along the Snake River in Southern Idaho

A group of large springs, eleven of which rank among the largest springs in the United States, discharge 5,000 cubic feet a second from the north side of the Snake River between Bliss and Twin Falls, Idaho. Most of the water issues from pillow lava at the base of basalt flows that fill ancestral canyons of the Snake River carved in the older lake beds of the region. Six different canyon fills of basalt, indicating six displacements of the Snake River in Pleistocene time, have been mapped in this area by the author. Each of the flows that spilled into the Snake River Canyon caused a temporary lake on the upstream side of the lava dam and an alluvial fan containing huge blocks torn from the dam on the downstream side. Subsequent partial removal of the fans by the river has left concentrates of huge boulders in the form of trains, helpful in locating such displacements of the Snake River. Springs issue from these canyon fills of permeable basalt wherever the river has cut into them below the water table. Som...

Preliminary Notes on Some Igneous Rocks of Japan. II

Locality.-Kasa-yama, near Hagi city, prov. Nagato. Occurrence.-The rock occurs as a lava flow erupted at the volcano Kasa-yama, which consists merely of an isolated cone of small size, 112.5 meters above the sea-level and about 1,300 meters in diameter across its base. In the summit, there is a perfectly preserved crater, 25 meters in diameter and 13 meters in depth. This small and regular cone stands in strong contrast to the topography of the environs, where the geology is mainly composed of granites and mesozoic sedimentaries, and especially to that of table-lands or flat islands formed by basalt flows which poured out here and there through the ground. Age.-The eruption of the rock appears to be Diluvium and the latest of the basalt in this region, which seem to have been erupted at the period from the close of Tertiary to Diluvium. Megascopic characters.-The specimen collected from the lava dam near the Shinto shrine at the eastern foot of Kasa-yama is noteworthy for containing abundant quartz as porphyritic grains in a hypocrystalline groundmass. It is black in color and vesicular with small and irregular cavities, but has a high specific gravity. The quartz, varying in size from i mm. to 5 mm., shows an irregular outline, but sometimes almost hexagonal. Though the percentage of quartz grains varies in different portions of the lava, generally they are distributed uniformly and are clearly distinguishable from the groundmass by their color, as seen in the photograph. Besides these there are only a few crystals of yellow