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At least 1,351 records · Page 75Linked to original sources
USGS1969

Cretaceous, Tertiary, and early Pleistocene rocks from the continental margin in the Bering Sea

Rocks dredged from the continental margin in eastern Bering Sea in and near the Pribilof Canyon indicate that the acoustic basement represents the upper surface of thoroughly lithified turbidite beds of graywacke and siltstone of Late Cretaceous age. The stratified sequence covering the acoustic basement is gently deformed and includes marine clastic and diatomaceous sediments ranging in age from middle or late Miocene through early Pleistocene. Dense argillite, siltstone, and calcareous sandstone of early Tertiary age in and near the Zhemchug Canyon probably represent an older part of this sequence. The main layered sequence accumulated above the acoustic basement in shallow water, and, because the older beds now lie as much as 1000 m below sea level, the continental margin must have undergone considerable subsidence during late Tertiary and Quaternary time. A rich pollen flora indicates that the shoreline lay only a few tens of kilometers away from the site of the Pribilof Canyon during late Miocene time.

Alaska↗
USGS1969

Active metamorphism of upper Cenozoic sediments in the Salton Sea geothermal field and the Salton Trough, southeastern California

The Salton Sea geothermal system is entirely within Pliocene and Quaternary sediments of the Colorado River delta at the north end of the Gulf of California. At the time of deposition, these sediments consisted of sands, silts, and clays of uniform original mineralogic composition, but under the elevated temperatures and pressures of the geothermal system they are being transformed to low-grade metamorphic rocks of the greenschist facies. We have studied these transformations by X-ray, petrographic, and chemical analyses of cuttings and core from deep wells that penetrate the sedimentary section. Temperatures within the explored geothermal system range up to 360° C at 7100 feet. The wells produce a brine containing over 250,000 ppm dissolved solids, primarily Cl, Na, Ca, K, and Fe, plus a host of minor constituents.

California↗
USGS1970

Sea-floor spreading at the junction between Gorda Rise and Mendocino Ridge

A Quaternary deformation pattern revealed by new sub-bottom acoustic profiles supplements previous knowledge derived chiefly from magnetic and seismic evidence on the contemporary tectonics off northern California. An inferred age for the sedimentary cover along the axis of the southern part of Gorda Rise suggests that no volcanism has occurred along that part of the rise for more than 100,000 years. Sea-floor spreading at the rise crest has been accommodated by sinking of a keystone block that forms the floor of Escanaba Trough, the median valley of the rise. Differential movement between Pacific and American tectonic plates could have caused the deformation pattern, and during this movement, the displacement offshore beyond the northwest end of the San Andreas fault is inferred to have divided at Cape Mendocino between a fault along Mendocino Ridge and a fault segment that connects with the Blanco fault farther northwest.

California↗
USGS1971

Sedimentary and gravity-slide emplacement of serpentinite

Large deposits of serpentinite in alpine-type orogenic areas have been formed by sedimentary processes ranging from the detrital accumulation of bedded serpentinite sandstone and shale to the emplacement of chaotic breccias (olistostromes) and gigantic slide blocks. Known occurrences of sedimentary serpentinite are listed, and eight deposits from the circum-Pacific, Caribbean, and Mediterranean areas are described in detail. Sedimentary serpentinites range in age from early Paleozoic to Quaternary, although most are Cretaceous or Tertiary. Most were deposited in eugeosynclinal environments, early in the geosynclinal cycle. Individual deposits range in thickness from a few centimeters to nearly 3 km, and several extend laterally for tens of kilometers. Graded bedding is common, and many deposits contain marine fossils. Serpentinite is the dominant rock constituent, and clasts foreign to the alpine ultramafic assemblage are rare. Chemical analyses often detrital serpentinites show that these rocks contain slightly more silica and alumina than do nondetrital serpentinites, due to contamination by aluminosilicate minerals and quartz during deposition. This and nine other criteria are potentially useful in the recognition of sedimentary serpentinites. Several features suggest that most sedimentary serpentinites were deposited very rapidly by submarine landslides, mudflows, or turbidity currents. The sources of this serpentinite debris are postulated to be upward-migrating serpentinite protrusions which penetrate the seafloor or Earth's surface upslope from eventual depositional sites. Sedimentary serpentinites are much more abundant in alpine-type orogenic areas than is commonly thought, and many ultramafic masses presently regarded as igneous intrusions or tectonic protrusions may in fact be coeval with, instead of younger than, their enclosing sedimentary or metasedimentary rocks. In eugeosynclinal sequences such as the Franciscan Formation, some elongate bodies now regarded as serpentinite sills may be beds of ultramafic detritus whose sedimentary features have been masked by post-depositional shearing; isolated masses may be exotic slide blocks. A sedimentary origin can explain some of the most persistent and perplexing characteristics of many alpine serpentinites: their conformity with enclosing sedimentary rocks, their grossly planar shapes, and the absence of metamorphism along their contacts.

California↗
USGS1972

Hawaiian-emperor chain and its relation to cenozoic circumpacific tectonics

The Hawaiian Ridge and Emperor Seamounts appear to form a single chain of tholeiitic shield volcanoes that erupted sequentially on the sea floor of the central Pacific Ocean during Tertiary and Quaternary time. The chain cuts obliquely across the older Cretaceous structural patterns of that sea floor. While the pattern of the chain as a whole is linear, the individual volcanoes lie on short, sigmoidal, en echelon loci that are subparallel with respect to each other and that may represent extensional features in the crust and upper mantle. In general, the order of eurption progressed from northwest to southeaśt along the chain, but the rate of progression of volcanism along individual loci is nonlinear where best studied in the southeastern part of the chain. Furthermore, simultaneous eruptions appear to have occurred within a distance along the chain of about 200 to 400 km. The available data are consistent with a genesis related to the motion of the Pacific crust over a melting spot in the mantle. This melting spot, which may be due to either excess heat or pressure release, appears to have a diameter of about 300 km and is presently centered slightly north of the island of Hawaii. We concur with the idea that the bend in the Hawaiian-Emperor chain probably reflects a significant change in the motion of the Pacific plate. Our best estimate of the age of the Hawaiian-Emperor bend, based on the existing radiometric data, is 24.6 ± 2.5 m.y., which correlates with a time of increased tectonic activity in the western Pacific island arcs and along the northern and eastern boundaries of the Pacific plate. The vector change in the motion of the Pacific plate (with respect to the melting spot) that is required to produce the bend is about 12 cm/yr in a west-southwest-ward direction. © 1972, The Geological Society of America, Inc.

Hawaii↗
USGS1973

Reconnaissance study of the strontium isotopic composition of Cenozoic volcanic rocks in the northwestern Great Basin

Sixteen mafic and intermediate lava flows of Eocene to Pleistocene age from the northwestern Great Basin have initial Sr 87 /Sr 86 ratios of from 0.7029 to 0.7047. Seven upper Miocene mafic and intermediate lava flows have initial ratios of from 0.7037 to 0.7041, suggesting a common source for the Steens Basalt and contemporaneous rocks of the northwestern Great Basin. Values of 0.7047 and 0.7033 obtained on olivine basalts of early Miocene and Quaternary ages, respectively, suggest that these lavas were derived from different source materials than were the late Miocene rocks. Unusually low values of 0.7029 obtained on two specimens of andesite from the Eocene Cedarville Series of Russell (1928) suggest that these lava flows were derived from still another source material. Ten silicic volcanic rocks, most from widespread and voluminous ash-flow sheets and lava complexes, have initial ratios of approximately 0.7023 to 0.7057. Some of the more radiogenic values, obtained on very highly differentiated and strontium-poor rock units, provide only an upper limit for the initial Sr 87 /Sr 86 ratios of the parent magmas. The close similiarity of the strontium isotopic composition of the silicic rocks to those of spatially and temporally associated intermediate lavas supports the concept that the widespread Miocene silicic volcanic rocks of the northwestern Great Basin were produced by the fractional crystallization of enormous volumes of mantle-derived mafic magma. The data also provide indirect support for the interpretation that the relatively radiogenic character of many salic volcanic rocks from other parts of the Great Basin largely reflects the presence of an unusually radiogenic mantle rather than the involvement of crustal material.

California, Idaho, Nevada, Oregon↗
USGS1973

Structure and development of the continental margin of British Honduras

The continental margin of British Honduras is characterized by elongate ridges that are sub-parallel with the coast and that diverge slightly northward. Each ridge apparently is formed by aligned fault blocks composed of continental basement rock that rotated and subsided during rifting of the margin. The western part of the Cayman Ridge may be underlain by similar rock that extends southwestward into the margin. Little sediment has been trapped in the basins between the ridges, presumably as a result of long-term, low rates of sedimentation. A canyon system that is controlled by both fault-block ridges and cross faults transects the margin and opens into the Yucatan Basin. Deep-sea sediments are characterized by a succession of pelagic deposits above acoustic basement rocks; these deposits are covered by turbidites that have formed deep-sea fan deposits in some places. The change from pelagic to turbidite deposition may have occurred at the end of Cretaceous time when regional uplift provided a source for clastic materials. The principal channel for transport of sediment to the deep sea is believed to be a graben that extends northeastward from the southwest corner of the Gulf of Honduras. Development of the margin may have begun in Late Jurassic time when sphenochasmic rifting created the ancestral Gulf of Honduras. Near the end of Cretaceous time, a new set of motions began, with the Caribbean plate moving relatively eastward and slightly away from the North American plate. This new motion resulted in continued movement of basement blocks through Tertiary and Quaternary time. Although this margin is not young, it has a geomorphically youthful aspect in that the sedimentary sequences do not obscure the rift structures.

British Honduras, Caribbean Sea, Gulf of Honduras↗
USGS1980

Leg 67: the Deep Sea Drilling Project Mid-America Trench transect off Guatemala.

Drilling on the Cocos plate recovered a basal chalk sequence deposited during early and mid-Miocene time, a short interval of abyssal red clay, and an upper sequence of late Miocene and younger sediment deposited within an area influenced by a terrigenous source. In the trench, a mud and sand fill less than 400,000 yr old overlies the oceanic sequence. The entire section shows no evidence of compressive deformation. In contrast, the section cored on the trench's landward slope 3 km from the trench axis is affected by tectonism. The section contains a Cretaceous to Pliocene claystone sequence capped by Pliocene to Quaternary hemipelagic slope deposits.- from Authors

Geological Society of America Bulletin↗
USGS1981

Sedimentary framework of the Potomac River estuary, Maryland

Analyses of seismic-reflection profiles, sediment cores, grab samples, and side-scan sonar records, along with previously collected borehole data, reveal the characteristics, distribution, and geologic history of the shallow strata beneath the Potomac River estuary. The lowermost strata are sediments of the Chesapeake Group (lower Miocene to lower Pleistocene) that crop out on land near the shore but are buried as much as 40 m below the floor of the estuary. The top of these sediments is an erosional unconformity that outlines the Wisconsinan valley of the Potomac River. This valley has a sinuous trend, a flat bottom, a relief of 15 to 34 m, and axial depths of 34 to 54 m below present sea level. During the Holocene transgression of sea level, the ancestral valley was filled with as much as 40 m of sandy and silty, fluvial-to-shallow estuarine sediments. The fill became the substrate for oyster bars in the upper reach and now forms most marginal slopes of the estuary. Since sea level approached its present position (2,000 to 3,000 yr ago), the main channel has become the locus of deposition for watery, gray to black clay or silty clay, and waves and currents have eroded the heterogeneous Quaternary sediments along the margins, leaving winnowed brown sand on shallow shoreline flats. Pb-210 analyses indicate that modern mud is accumulating at rates ranging from 0.16 to 1.80 cm/yr, being lowest near the mouth and increasing toward the head of the estuary. This trend reflects an increased accumulation of fine-grained fluvial sediments near the turbidity maximum, similar to that found in nearby Chesapeake Bay. The present annual accumulation of mud is about 1.54 million metric tons; the cumulative mass is 406 million metric tons.

Maryl↗
USGS1984

The Loma Seca tuff and the Calabozos caldera: a major ash-flow and caldera complex in the southern Andes of central Chile

A 26 × 14-km composite ring-structure caldera of late Pleistocene age has been discovered and mapped near the Andean crest in central Chile (35°30′S). Rhyodacitic to dacitic zoned ash-flow sheets, each representing 150 to 300 km 3 of magma, were emplaced 0.8, 0.3, and 0.15 m.y. ago; the youngest of the associated collapses was closely followed by resurgent doming of the caldera floor and development of a longitudinal graben. Postcaldera eruptions of dacite and andesite have persisted into Holocene time, and active hot springs are abundant along caldera-marginal and resurgent fault systems, suggesting a significant geothermal-energy resource. The Pleistocene eruption rate of this district and the abundance of older Quaternary to Miocene ash-flow remnants in the 33°S to 36°S segment of the glaciated southern Andes indicate that ash-flow magmatism has been no less important here than in the arid central Andes (16°S–28°S), where ash-flow sheets are far better preserved.

Geological Society of America Bulletin↗
USGS1986

A fan dam for Tulare Lake, California, and implications for the Wisconsin glacial history of the Sierra Nevada

Historic fluctuations and late Quaternary deposits of Tulare Lake, in the southern San Joaquin Valley, indicate that maximum lake size has depended chiefly on the height of a frequently overtopped spillway. This dependence gives Tulare Lake a double record of paleoclimate. Climate in the Tulare Lake region has influenced the degree to which the lake fills its basin during dry seasons and dry years: during the past 100,000–130,000 yr, incidence of desiccation of Tulare Lake (inferred from stiffness, mud cracks, and other hand-specimen properties) has been broadly consistent with the lake's salinity and depth (inferred from diatoms and ostracodes) and with regional vegetation (inferred from pollen). Climate, however, also appears to control basin capacity itself: Tulare Lake becomes large as a consequence of glacial-outwash aggradation of its alluvial-fan dam. Late Wisconsin enlargement of Tulare Lake probably resulted from the last major glaciation of the Sierra Nevada. The lake's spillway coincides with the axis of the glacial-outwash fan of a major Sierra Nevada stream; moreover, sediment deposited in the transgressive lake resembles glacial rock flour from the Sierra Nevada. Differential tectonic subsidence and deposition by a Coast Range creek facilitated the building of Tulare Lake's fan dam during the late Wisconsin but were less important than deposition of Sierra Nevada outwash. Four stratigraphically consistent 14 C dates on peat and wood give an age of 26,000 yr B.P. for the start of Tulare Lake's late Wisconsin transgression. The last major Sierra Nevada glaciation (Tioga glaciation) thus may have begun about 26,000 yr B.P., provided that vigorous glacial-outwash deposition began early in the glaciation. Onset of the Tioga glaciation about 26,000 yr B.P. is consistent with new stratigraphic and radiocarbon data from the northeastern San Joaquin Valley. These data suggest that the principal episode of glacial-outwash deposition of Wisconsin age began in the San Joaquin Valley after 32,000 yr B.P., rather than at least 40,000 yr B.P., as previously believed. An earlier enlargement of Tulare Lake probably resulted from a fan dam produced by the penultimate major (Tahoe) glaciation of the Sierra Nevada. Average sedimentation rates inferred from depths to a 600,000-yr-old clay and from radiocarbon dates indicate that this earlier lake originated no later than 100,000 yr B.P. The Tahoe glaciation therefore is probably pre-Wisconsin.

Geological Society of America Bulletin↗
USGS1995

Tectonic setting of the Portland-Vancouver area, Oregon and Washington: Constraints from low-altitude aeromagnetic data

Seismic activity in the Portland-Vancouver metropolitan area may be associated with various mapped faults that locally offset volcanic basement of Eocene age and younger. This volcanic basement is concealed in most places by young deposits, vegetation, and urban development. The U.S. Geological Survey conducted an aeromagnetic survey in September 1992 to investigate the extent of these mapped faults and possibly to help identify other seismic and volcanic hazards in the area. The survey was flown approximately 240 m above terrain, along flight lines spaced 460 m apart, and over an area about 50 × 50 km. These magnetic data indicate a pronounced northwest-striking magnetic lineation east of the Willamette River in downtown Portland associated with a fault concealed beneath Quaternary sedimentary deposits and previously inferred from shallow well data. The magnetic lineation confirms the existence of the fault and suggests that it has had a prolonged history: (1) Although well data indicate <200 m of vertical offset of underlying volcanic basement, models based on the aeromagnetic data from downtown Portland suggest reverse faulting with up to 1 km of offset deeper in the section. (2) The magnetic lineation associated with this fault extends southeast to the Clackamas River drainage, a distance of 50 km and considerably beyond the mapped extent of the fault. A northwest-striking magnetic anomaly located southwest of the Tualatin Mountains corresponds closely with another mapped fault and with mixed reverse and strike-slip faulting during a seismic swarm (M ≤ 3) in 1991. We believe these and other anomalies in the aeromagnetic data reflect the Portland Hills fault zone, believed to be the southwestern boundary of a structural basin now occupied by Portland and Vancouver. The postulated northeastern boundary of the basin, the Frontal fault zone, is also evident, although less well represented in the aeromagnetic data. Aeromagnetic anomalies, geologic mapping, and earthquake focal-plane solutions demonstrate a complex deformational history in the Portland-Vancouver area since middle Miocene time that includes elements of compression, extension, and dextral slip. These complexities reflect Portland-Vancouver's unique position within a north-south transition in tectonic styles along the Cascadia margin, from compressional in the north to extensional in the south.

Oregon, Washington↗
USGS1997

Volcanism and erosion during the past 930 k.y. at the Tatara-San Pedro complex, Chilean Andes

Geologic mapping, together with 73 new K-Ar and 40 Ar/ 39 Ar age determinations of 45 samples from 17 different volcanic units, plus paleomagnetic orientations, geochemical compositions, and terrestrial photogrammetry are used to define the chronostratigraphy of the Tatara–San Pedro complex, an eruptive center at 36°S on the volcanic front of the Andean southern volcanic zone. The Tatara–San Pedro complex preserves ≈55 km 3 of lavas that erupted from at least three central vent regions. Remnant, unconformity-bound sequences of lavas are separated by lacunae that include significant periods of erosion. Quaternary volcanism commenced ca. 930 ka with eruption of voluminous dacitic magma, followed 100 k.y. later by the only major rhyolitic eruption. From 780 ka onward, more than 80% of the preserved volume is basaltic andesite (52%–57% SiO 2 ), but petrographically and geochemically diverse dacitic magmas (63%–69% SiO 2 ) erupted sporadically throughout this younger, dominantly mafic phase of activity. A few basaltic lavas (49%–52% SiO 2 ) are present, mainly in portions of the complex older than 230 ka. The number of vents, the petrologic and geochemical diversity, and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the Tatara–San Pedro complex over the past million years.

Geological Society of America Bulletin↗
USGS1998

Tectonic controls on magmatism in the Geysers-Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

Geological Society of America Bulletin↗
USGS1999

Northern San Andreas fault near Shelter Cove, California

The location of the San Andreas fault in the Shelter Cove area of northern California has been the subject of long-standing debate within the geological community. Although surface ruptures were reported near Shelter Cove in 1906, several subsequent workers questioned whether these ruptures represented true fault slip or shaking-related, gravity-driven deformation. This study, involving geologic and geomorphic mapping, historical research, and excavation across the 1906 rupture zone, concludes that the surface ruptures reported in 1906 were the result of strike-slip faulting, and that a significant Quaternary fault is located onshore near Shelter Cove. Geomorphic arguments suggest that the Holocene slip rate of this fault is greater than about 14 mm/yr, indicating that it plays an important role within the modern plate-boundary system. The onshore trace of the fault zone is well expressed as far north as Telegraph Hill; north of Telegraph Hill, its location is less well-constrained, but we propose that a splay of the fault may continue onshore northward for at least 9 km to the vicinity of Saddle Mountain.

California↗
USGS2000

New aeromagnetic data reveal large strike-slip (?) faults in the Northern Willamette Valley, Oregon

High-resolution aeromagnetic data from the northern Willamette Valley, Oregon, reveal large, northwest-striking faults buried beneath Quaternary basin sediments. Several faults known from geologic mapping are well defined by the data and appear to extend far beyond their mapped surface traces. The Mount Angel fault, the likely source of the Richter magnitude (M1) 5.6 earthquake in 1993, is at least 55 km long and may be connected in the subsurface with the Gales Creek fault 25 km farther northwest. Northeast of the Mount Angel fault, a 60-km-long, northwest-striking anomaly may represent a previously unrecognized dextral-slip fault beneath the towns of Canby and Molalla. Vertical offsets along the Mount Angel fault increase with depth, indicating a long history of movement for the fault. Dominantly northwest- trending, relatively straight faults, consistent stepover geometries, offset magnetic anomalies and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip. The 1993 earthquake may have occurred on a left-stepping restraining bend along the Mount Angel-Gales Creek fault zone.

Oregon↗
USGS2000

Potential seismic hazards and tectonics of the upper Cook Inlet basin, Alaska, based on analysis of Pliocene and younger deformation

The Cook Inlet basin is a northeast-trending forearc basin above the Aleutian subduction zone in southern Alaska. Folds in Cook Inlet are complex, discontinuous structures with variable shape and vergence that probably developed by right-transpressional deformation on oblique-slip faults extending downward into Mesozoic basement beneath the Tertiary basin. The most recent episode of deformation may have began as early as late Miocene time, but most of the deformation occurred after deposition of much of the Pliocene Sterling Formation. Deformation continued into Quaternary time, and many structures are probably still active. One structure, the Castle Mountain fault, has Holocene fault scarps, an adjacent anticline with flower structure, and historical seismicity. If other structures in Cook Inlet are active, blind faults coring fault-propagation folds may generate M w 6–7+ earthquakes. Dextral transpression of Cook Inlet appears to have been driven by coupling between the North American and Pacific plates along the Alaska-Aleutian subduction zone, and by lateral escape of the forearc to the southwest, due to collision and indentation of the Yakutat terrane 300 km to the east of the basin.

Alaska↗
USGS2002

Drainage reversals in Mono Basin during the late pliocene and Pleistocene

Mono Basin, on the eastern flank of the central Sierra Nevada, is the highest of the large hydrographically closed basins in the Basin and Range province. We use geomorphic features, shoreline deposits, and basalt-filled paleochannels to reconstruct an early to middle Pleistocene record of shorelines and changing spillways of Lake Russell in Mono Basin. During this period of time, Lake Russell repeatedly attained altitudes between 2205 and 2280 m-levels far above the present surface of Mono Lake (~1950 m) and above its last overflow level (2188 m). The spill point of Lake Russell shifted through time owing to late Tertiary and Quaternary faulting and volcanism. During the early Pleistocene, the lake periodically discharged through the Mount Hicks spillway on the northeastern rim of Mono Basin and flowed northward into the Walker Lake drainage basin via the East Walker River. Paleochannels recording such discharge were incised prior to 1.6 Ma, possibly between 1.6 and 1.3 Ma, and again after 1.3 Ma (ages of basaltic flows that plugged the paleochannels). Faulting in the Adobe Hills on the southeastern margin of the basin eventually lowered the rim in this area to below the altitude of the Mount Hicks spillway. Twice after 0.76 Ma, and possibly as late as after 0.1 Ma, Lake Russell discharged southward through the Adobe Hills spillway into the Owens-Death Valley system of lakes. This study supports a pre-Pleistocene aquatic connection through Mono Basin between the hydrologically distinct Lahontan and Owens-Death Valley systems, as long postulated by biologists, and also confirms a probable link during the Pleistocene for species adapted to travel upstream in fast-flowing water.

Geological Society of America Bulletin↗