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At least 793 records · Page 44Linked to original sources
USGS1991

Lower Paleozoic host rocks in the Getchell gold belt: Several distinct allochthons or a sequence of continuous sedimentation?

The lower Paleozoic rocks that host gold deposits along the Getchell gold belt in Humboldt County, Nevada, represent several allochthonous terranes rather than a sequence of continuous deposition. The term "terrane" is used only in a descriptive sense. Evidence for allochthonous terranes in this area includes fault boundaries and differences in age, lithology, and structural style among several rock sequences. The two most widespread and distinct terranes in the area are (1) the Osgood terrane, which consists of intensely deformed, regionally metamorphosed, marine rocks (Lower Cambrian Osgood Mountain Quartzite, Lower Cambrian to Lower Ordovician Preble Formation, and some rocks currently mapped as Comus Formation) and (2) the Getchell terrane, which consists of less deformed chert, clastic sedimentary rocks, and volcanic rocks (rocks mapped as Valmy and Vinini Formations, including Lower and Upper Ordovician and Lower Silurian strata in this region). Osgood-terrane bedding and foliation dip predominantly eastward, and folds verge westward. Getchell-terrane folds verge southeastward. The Comus Formation, which is Middle Ordovician at its type locality on Edna Mountain, represents a third terrane (Iron Point terrane) situated structurally between the Osgood and Getchell terranes. Use of the unit name Comus Formation outside the type locality has created confusion and needs reexamination. Some of the rocks currently mapped as Comus Formation might really be part of the other terranes.

Geology↗
USGS1992

SeaMARC II mapping of transform faults in the Cayman Trough, Caribbean Sea

SeaMARC II maps of the southern wall of the Cayman Trough between Honduras and Jamaica show zones of continuous, well-defined fault lineaments adjacent and parallel to the wall, both to the east and west of the Cayman spreading axis. These lineaments mark the present, active traces of transform faults which intersect the southern end of the spreading axis at a triple junction. The Swan Islands transform fault to the west is dominated by two major lineaments that overlap with right-stepping sense across a large push-up ridge beneath the Swan Islands. The fault zone to the east of the axis, named the Walton fault, is more complex, containing multiple fault strands and a large pull-apart structure. The Walton fault links the spreading axis to Jamaican and Hispaniolan strike-slip faults, and it defines the southern boundary of a microplate composed of the eastern Cayman Trough and western Hispaniola. The presence of this microplate raises questions about the veracity of Caribbean plate velocities based primarily on Cayman Trough opening rates.

Geology↗
USGS2007

Characterizing infiltration through a mine-waste dump using electrical geophysical and tracer-injection methods, Clear Creek County, Colorado

Infiltration of surface water through mine waste can be an important or even dominant source of contaminants in a watershed. The Waldorf mine site in Clear Creek County, Colorado, is typical of tens of thousands of small mines and prospects on public lands throughout the United States. In this study, electromagnetic (EM) conductivity and direct current (dc) resistivity surveys were conducted in tandem with a NaCl tracer study to delineate ground-water flow paths through a mine-waste dump and adjacent wetland area. The tracer was used to tag adit water infiltrating from braided channels flowing over the top of the dump to seeps at the base of the dump. Infiltration from the braided channels had a maximum flow rate of 92 m/day and a hydraulic conductivity of 1.6 × 10 4 cm 3 /s. After rerouting of adit flow around the waste dump, discharge at some of the largest seeps was reduced, although not all seepage was eliminated entirely. Integrating results of the tracer study with those of the EM and dc geophysical surveys revealed two main flow paths of ground water, one beneath the dump and one through the dump. The main source of water to the first flow path is deeper ground water emerging from the fault zone beneath the collapsed adit. This flow path travels beneath the waste dump and appears to have been unaffected by rerouting of the adit discharge around the waste dump. The source of the second flow path is infiltration of adit water from braided channels flowing over the top of the dump, which is intermediate in depth and flows through the center of the waste dump. Following rerouting of adit flow, discharge to seeps at the toe of the dump along this flow path was reduced by as much as two-thirds, although not eliminated entirely. Improved understanding of ground-water flow paths through this abandoned mine site is important in developing effective remediation strategies to target sources of metals emanating from the adit, waste dump, and contaminated wetland area. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.

Colorado↗
USGS2008

Deciphering the mid-Carboniferous eustatic event in the central Appalachian foreland basin, southern West Virginia, USA

A prominent unconformity, present across shallow shelf areas of the Euramerican paleoequatorial basins, is used to demark the boundary between the Mississippian and Pennsylvanian subsystems. This unconformity, the mid-Carboniferous eustatic event, is generally attributed to a major glacio-eustatic sea-level fall. Although a Mississippian-Pennsylvanian unconformity is recognized throughout most of the Appalachian region, the record of the mid-Carboniferous eustatic event in the structurally deepest part of the basin has been controversial. Based on early reports that suggested the most complete Pennsylvanian section was present in southern West Virginia, various conceptual depositional models postulated continuous sedimentation between the youngest Mississippian Bluestone Formation and the oldest Penn-sylvanian Pocahontas Formation. In contrast, tabular-erosion models envisioned axial drainage systems that evolved in response to changing basin dynamics. These models predicted a Mississippian-Pennsylvanian unconformity. All these models suffered from a lack of biostratigraphic control. The presence of a sub-Pocahontas paleovalley, herein named the Lashmeet paleovalley, has been confirmed in southern West Virginia. The Lashmeet paleovalley was incised over 35 m into Bluestone strata and filled by lithic sands derived from the Appalachian orogen to the northeast and east. The polygenetic Green Valley paleosol complex marks the Bluestone-Pocahontas contact on associated interfluves. Together, these features indicate a substantial period of subaerial exposure and argue strongly in favor of a Mississippian-Pennsylvanian unconformity. Paleontologic data from the Bluestone Formation, including marine invertebrates and conodonts from the marine Bramwell Member and paleofloral data, support a late, but not latest, Arnsbergian age assignment. Marine fossils are not known from the Pocahontas Formation, but macrofloral and palynomorph taxa support a Langsettian age for most of the Poca-hontas. The biostratigraphic, sedimentologic, and paleogeographic data support the presence of an early Pennsylvanian (middle to late Namurian) disconformity in the Appalachian Basin that corresponds to the mid-Carboniferous eustatic event. ?? 2008 Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2009

Deformation of the late Miocene to Pliocene Inyo Surface, eastern Sierra region, California

A middle and late Miocene erosion surface, the Inyo Surface, underlies late Miocene mafic flows in the White Mountains and late Miocene and (or) early Pliocene flows elsewhere in the eastern Sierra region. The Inyo Surface is correlated with an erosion surface that underlies late Miocene mafic flows in the central and northern Sierra Nevada. The mafic flows had outpourings similar to flood basalts, although of smaller volume, providing paleohorizontal and paleolowland indicators. The flows filed and locally topped the existing landscape forming broad plateau-like flats. Topographic relief in the region was characterized by weathered and rounded slopesp rior to late Miocene mafic magmatism. Relicts of the older landscape lie adjacent to late Miocene and early Pliocene basalt-covered lowlands that now occur within the crests of ranges that have 2500-3000 m relief and dramatically steep escarpments. Late Miocene mafic flows that lie on the crest of the Sierra Nevada adjacent to the White Mountains predate significant activity on the Sierra Nevada frontal fault zone. These deposits and accompanying erosion surfaces provide excellent strain markers for reconstructing part of the Walker Lane north of the Garlock fault and west of the Amargosa drainage, here referred to as the eastern Sierra region. The Inyo Surface is a compound erosional surface that records at least four major erosion events during the Cenozoic. These four surfaces were first recognized on the Kern Plateau and named from oldest to youngest, the Summit Upland, the Subsummit Plateau, the Chagoopa Plateau, and the Canyon. The three older surfaces have also been subsequently modifi ed by Pleistocene glaciation. The compound erosion surface, which is locally overlain by late Miocene mafic flows in the northern and central Sierra Nevada, is here referred to as the Lindgren Surface. Correlatives in the eastern Sierra region are found in the White Mountains, Inyo Mountains, Darwin Plateau, Coso Range, and nearby ranges. Copyright ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2015

The 2011 Mineral, Virginia, earthquake and its significance for seismic hazards in eastern North America: overview and synthesis

The 23 August 2011 M w (moment magnitude) 5.7 ± 0.1, Mineral, Virginia, earthquake was the largest and most damaging in the central and eastern United States since the 1886 M w 6.8–7.0, Charleston, South Carolina, earthquake. Seismic data indicate that the earthquake rupture occurred on a southeast-dipping reverse fault and consisted of three subevents that progressed northeastward and updip. U.S. Geological Survey (USGS) "Did You Feel It?" intensity reports from across the eastern United States and southeastern Canada, rockfalls triggered at distances to 245 km, and regional groundwater-level changes are all consistent with efficient propagation of high-frequency seismic waves (∼1 Hz and higher) in eastern North America due to low attenuation. Reported damage included cracked or shifted foundations and broken walls or chimneys, notably in unreinforced masonry, and indicated intensities up to VIII in the epicentral area based on USGS "Did You Feel It?" reports. The earthquake triggered the first automatic shutdown of a U.S. nuclear power plant, located ∼23 km northeast of the main shock epicenter. Although shaking exceeded the plant's design basis earthquake, the actual damage to safety-related structures, systems, and components was superficial. Damage to relatively tall masonry structures 130 km to the northeast in Washington, D.C., was consistent with source directivity, soft-soil ground-motion amplification, and anisotropic wave propagation with lower attenuation parallel to the northeast-trending Appalachian tectonic fabric. The earthquake and aftershocks occurred in crystalline rocks within Paleozoic thrust sheets of the Chopawamsic terrane. The main shock and majority of aftershocks delineated the newly named Quail fault zone in the subsurface, and shallow aftershocks defined outlying faults. The earthquake induced minor liquefaction sand boils, but notably there was no evidence of a surface fault rupture. Recurrence intervals, and evidence for larger earthquakes in the Quaternary in this area, remain important unknowns. This event, along with similar events during historical time, is a reminder that earthquakes of similar or larger magnitude pose a real hazard in eastern North America.

Virginia↗
USGS2020

Geology of the Trout Rock caves (Hamilton Cave, Trout Cave, New Trout Cave) in Pendleton County, West Virginia (USA), and implications regarding the origin of maze caves

The Trout Rock caves (Hamilton Cave, Trout Cave, New Trout Cave) are located in a hill named Cave Knob that overlooks the South Branch of the Potomac River in Pendleton County, West Virginia (U.S.A). The geologic structure of this hill is a northeasttrending anticline, and the caves are located at different elevations primarily along the contact between the Devonian New Creek Limestone (Helderberg Group) and the overlying Devonian Corriganville Limestone (Helderberg Group). The entrance to New Trout Cave (Stop 1) is located on the east flank of Cave Knob anticline at an elevation of 585 m (1,920 ft) relative to sea level, or 39 m (128 ft) above the modern river. Much of the cave consists of passages that extend to the northeast along strike, and many of these passages have developed along primary joints that trend N40E or secondary joints that trend N40W. Sediments in New Trout Cave include mud and sand (some of which was mined for nitrate during the American Civil War), as well as large boulders in the front part of the cave. Gypsum crusts are present in a maze section of the cave ~213 to 305 m (700 to 1,000 ft) from the cave entrance. Excavations in New Trout Cave have produced vertebrate fossils of Rancholabrean age, ~300,000 to 10,000 years Before Present (BP). The entrance to Trout Cave (Stop 2) is located on the east flank of Cave Knob anticline ~100 m (328 ft) northwest of the New Trout Cave entrance at an elevation of 622 m (2,040 ft) relative to sea level, or 76 m (249 ft) above the modern river. Much of the cave consists of passages that extend to the northeast along strike, although a small area of network maze passages is present in the western portion of Trout Cave that is closest to Hamilton Cave. Many of the passages of Trout Cave have developed along primary joints that trend N40E or secondary joints that trend N40W. Sediments in Trout Cave include mud (also mined for nitrate during the American Civil War), as well as large boulders in the front part of the cave. Excavations in the upper levels of Trout Cave have produced vertebrate fossils of Rancholabrean age (~300,000 to 10,000 years BP), whereas excavations in the lower levels of the cave have produced vertebrate fossils of Irvingtonian age (~1,810,000 to 300,000 years BP). The entrance to Hamilton Cave (Stop 3) is located along the axis of Cave Knob anticline ~165 m (540 ft) northwest of the Trout Cave entrance at an elevation of 640 m (2,100 ft) relative to sea level, or 94 m (308 ft) above the modern river. The front (upper) part of Hamilton Cave has a classic network maze pattern that is an angular grid of relatively horizontal passages, most of which follow vertical or near-vertical primary joints that trend N40W and N50W and secondary joints that trend N60W and N80E. This part of the cave lies along the axis of Cave Knob anticline. In contrast, the passages in the back (lower) part of Hamilton Cave lie along the west flank of Cave Knob anticline at ~58 to 85 m (190 to 279 ft) above the modern river. These passages do not display a classic maze pattern, and instead they may be divided into the following two categories: (1) longer northeast-trending passages that are relatively horizontal and follow the strike of the beds; and (2) shorter northwest-trending passages that descend steeply to the west and follow the dip of the beds. Sediments in Hamilton Cave include mud (which was apparently not mined for nitrate during the American Civil War), as well as large boulders in the front part of the cave. Gypsum crusts are present along passage walls of the New Creek Limestone from the Slab Room to the Airblower. Excavations in the front part of Hamilton Cave (maze section) have produced vertebrate fossils of Irvingtonian age (~1,810,000 to 300,000 years BP). The network maze portions of Hamilton Cave are interpreted as having developed at or near the water table where water did not have a free surface in contact with air and where the following conditions were present: (1) Location on or near the axis of an anticline (the location of the greatest amount of flexure); (2) Abundant vertical or near vertical joints, which are favored by location in the area of greatest flexure and by a lithologic unit (chert-rich limestone) that is more likely to experience brittle rather than ductile deformation; (3) Widening of joints to enhance ease of water infiltration, favored by location in area of greatest amount of flexure; and (4) Dissolution along nearly all major joints to produce cave passages of approximately the same size (which would most likely occur via water without a free surface in contact with air). The cave passages that are located along anticline axes and along strike at the New Creek-Corriganville contact are interpreted as having formed initially during times of base level stillstand at or near the water table where water did not have a free surface in contact with air and where the water flowed along the hydraulic gradient at gentle slopes. Under such conditions, dissolution occurred in all directions to produce cave passages with relatively linear wall morphologies. In the lower portions of some of the along-strike passages, the cave walls have a more sinuous (meandering) morphology, which is interpreted as having formed during subsequent initial base level fall as cave development continued under vadose conditions where the water had a free surface in contact with air, and where water flow was governed primarily by gravitational processes. Steeply inclined cave passages that are located along dip at the New Creek-Corriganville contact are interpreted as having formed during subsequent true vadose conditions (after base level fall). This chronology of base level stasis (with cave development in the phreatic zone a short distance below top of water table) followed by base level fall (with cave development in the vadose or epiphreatic zone) has repeated multiple times at Cave Knob during the past ~4 to 3 million years, resulting in multiple cave passages at different elevations, with different passage morphologies, and at different passage locations with respect to strike and dip.

West Virginia↗
USGS2003

Widespread effects of middle Mississippian deformation in the Great Basin of western North America

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland-created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada- shed sediments eastward across the Antler foreland, burying the unconformity. Postorogenic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America. We propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations.

Geological Society of America Bulletin↗
USGS2019

300,000 yr history of water-table fluctuations at Wind Cave, South Dakota, USA—Scale, timing, and groundwater mixing in the Madison Aquifer

Deposits of calcite coating the lower passages of Wind Cave in the southern Black Hills of South Dakota were precipitated under phreatic conditions. Data from samples associated with a new cave survey and hydrologic studies indicate that past water tables within Wind Cave reached a maximum height of 45 m above modern levels but were mostly confined to 25 m or less. Uranium-series ages for basal layers deposited on weathered wall rock indicate subaerial conditions in this part of the cave persisted between 1000 and 300 ka. Ages and elevations of wall coatings and cave rafts establish a 300,000 yr paleohydrograph indicating that water-table highstands occurred during interglacial or interstadial-to-early glacial periods and lowstands occurred during full-glacial and stadial episodes. Isotopes of Sr, U, C, and O from dated calcite samples were obtained to evaluate potential shifts in paleo-groundwater composition. For comparison, Sr and U isotopic compositions were determined for modern groundwater from 18 sites previously classified into five hydrogeologic domains. Isotope data for different domains tend to cluster in separate fields, although several fields overlap. Compositions of Calcite Lake (informal name) water reflect modern recharge to shallow aquifers. In contrast, speleothem data indicate that paleo-groundwater highstands were not supported by increased infiltration associated with local recharge, or by upwelling from deeper Proterozoic sources. Instead, cave water was similar to deeper, warmer groundwater from the Madison Aquifer discharging at modern artesian springs flanking the southern Black Hills. Highstands were likely influenced by large-scale hydraulic processes associated with recharge to the Madison Aquifer under the Laurentide ice sheet on the northeast side of the Williston Basin, causing increased hydrostatic pressures in confined aquifers on the south side of the basin.

South Dakota↗
USGS1987

Rocky Mountains

The Rocky Mountain region is one of the most topographically distinct and impressive parts of North America. The Rocky Mountains rise abruptly above the bordering regions, particularly on the east and northeast where they are flanked by plains, less so on the west and southwest where they are bounded by high plateaus. The Rocky Mountains comprise more than 100 individually named ranges that form a belt extending for slightly more than 5,000 km, from near Santa Fe, New Mexico, on the south to the Bering Sea on the north (Fig. 1). The belt varies in width from less than 100 km in the Canadian Rockies to nearly 600 km in the Middle Rockies of Wyoming and northeast Utah. The summits of the ranges rise 1,500 to 2,100 m above adjacent lowlands, to heights 1,800 to 4,400 m above sea level. The Southern Rockies of Colorado have the greatest amount of area, between 3,300 and 4,400 m, and the highest peak, Mount Elbert (4,400 m). The largest area of low mountains is in the Northern Rockies of Idaho and Montana, where summits are commonly only 2,100 to 2,400 m above sea level. A substantial part of the Rocky Mountain region consists of lowlands, in the form of basins and fault-bounded troughs and trenches that lie between ranges. The Rocky Mountain Trench is perhaps the most spectacular fault-bounded lowland, even if it is not the most representative. It extends north from Flathead Lake, Montana, more than 1,500 km, and forms

Book chapter↗
USGS1994

Continent-ocean transition in Alaska: The tectonic assembly of eastern Denalia

Alaska is the eastern, subaerial part of a large subcontinent of distinctive tectonic character that serves as an isthmus between nuclear North America, with its fringing belt of allochthonous terranes, and the accreted terranes and volcanic belts that constitute northeastern Russia. Physiographically, this subcontinent, which we name Denalia, is a bulge in the continental platform in the vicinity of Alaska, the Chukotsk Peninsula, and the broad continental shelf of the Bering Sea. The bulge is convex to the south and is bounded on the east and west by constrictions in the width of the continental platform and on the north and south by the edge of the continental shelf (Fig. 1). Tectonically, Denalia is characterized by geologic youthfulness and complexity, an abundance of convergent and transcurrent faults, and absence of autochthonous cratonic rocks. It contains a profusion of lithotectonic terranes of diverse origin and age that were emplaced in late Mesozoic and Cenozoic time. In addition, it includes the superimposed Cenozoic Aleutian arc and subduction zone and the Queen Charlotte-Fairweather transform fault system. Parts of Denalia were created by pre-middle Mesozoic tectonic events, but these took place elsewhere, before the affected rocks were tectonically transported and incorporated into the landmass of Denalia. Except for a small area in the Porcupine Plateau region along the Alaska-Yukon boundary, the only Precambrian rocks that have been recognized in the subcontinent are in tectonically emplaced fragments, the largest of which is the Arctic Alaska terrane in the Brooks Range, Arctic Foothills, and Arctic Foothills.

Alaska↗
USGS1988

The Blake Plateau Basin and Carolina Trough

Presently, the continental margin of the southeastern United States (Fig. 1) forms a zone of transition between the actively building, steep-fronted carbonate platform of the Bahamas and the typical eastern North American terrigenous clastic-dominated, drowned, shelf-slope-rise configuration. This region of the continental margin is underlain by two major sedimentary basins—the Blake Plateau Basin and the Carolina Trough (Fig. 2)—which are different in shape, basement structure, and history. Indeed, the two southern basins show some of the greatest contrasts of any basins of eastern North America, especially in their early response to rifting and in the change from rifting to drifting. The region has experienced abrupt major changes in geological conditions, most notably the onset of Gulf Stream flow in the early Tertiary. Morphologically, the area is dominated by the broad, flat Blake Plateau at about 800-1,000 m water depth (Fig. 1). The plateau is bounded to the east by the extremely steep Blake Escarpment, descending to 5,000 m water depths. To the west, a short continental slope rises to a continental shelf. This Blake Plateau morphology characterizes the margin east of Florida and north of the Bahamas. North of Florida the margin merges into the typical shelf-slope-rise morphology. Just north of the Blake Escarpment and its northern projection, the Blake Spur, the Blake Ridge extends away from the continental slope at water depths exceeding 2,000 m (Fig. 1). This broad ridge is a Cenozoic, sedimentary drift deposit controlled by bottom currents. (For the reader who is beginning to wonder why half of the features of this region seem to be named "Blake", the Blake was a Coast Survey steamer from which investigations off the southeastern U.S. were carried out in 1877 to 1880. Ferromanganese nodules were discovered on the Blake Plateau at that time [Murray, 1885].)

Blake Plateau basin; Carolina Trough↗
USGS2007

Three-dimensional flow in the Florida platform: Theoretical analysis of Kohout convection at its type locality

Kohout convection is the name given to the circulation of saline groundwater deep within carbonate platforms, first proposed by F.A. Kohout in the 1960s for south Florida. It is now seen as an Mg pump for dolomitization by seawater. As proposed by Kohout, cold seawater is drawn into the Florida platform from the deep Straits of Florida as part of a geothermally driven circulation in which the seawater then rises in the interior of the platform to mix and exit with the discharging meteoric water of the Floridan aquifer system. Simulation of the asymmetrically emergent Florida platform with the new three-dimensional (3-D), finite-element groundwater flow and transport model SUTRA-MS, which couples salinity- and temperature-dependent density variations, allows analysis of how much of the cyclic flow is due to geothermal heating (free convection) as opposed to mixing with meteoric water discharging to the shoreline (forced convection). Simulation of the system with and without geothermal heating reveals that the inflow of seawater from the Straits of Florida would be similar without the heat flow, but the distribution would differ significantly. The addition of heat flow reduces the asymmetry of the circulation: it decreases seawater inflows on the Atlantic side by 8% and on the Guff of Mexico side by half. The study illustrates the complex interplay of freshwater-saltwater mixing, geothermal heat flow, and projected dolomitization in complicated 3-D settings with asymmetric boundary conditions and realistic horizontal and vertical variations in hydraulic properties.

Florida↗
USGS2010

The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons

U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Late Cretaceous McCoy Mountains Formation of southwestern Arizona (United States) are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south-trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region flowed northeast ~700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green River-Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau. ?? 2010 Geological Society of America.

Geology↗
USGS2008

Magmatic and tectonic evolution of the Caetano caldera, north-central Nevada: A tilted, mid-Tertiary eruptive center and source of the Caetano Tuff

The Caetano Tuff is a late Eocene, rhyolite ash-flow tuff that crops out within an ∼90-km-long, east-west–trending belt in north-central Nevada, previously interpreted as an elongate graben or “volcano-tectonic trough.” New field, petrographic, geochemical, and geochronologic data show that: (1) the east half of the “trough” is actually the Caetano caldera, formed by eruption of the Caetano Tuff at 33.8 Ma and later structurally dismembered during Miocene extension; (2) the west half of the trough includes both the distinctly younger and unrelated Fish Creek Mountains caldera (ca. 24.7 Ma) and a west-trending paleovalley partly filled with outflow Caetano Tuff; and (3) the Caetano Tuff as previously defined actually consists of three distinct units, two units of the 33.8 Ma Caetano Tuff and an older (34.2 Ma) tuff, exposed north of the Caetano caldera, herein named the tuff of Cove Mine. Miocene extensional faulting and tilting has exposed the Caetano caldera over a paleodepth range of >5 km, from the caldera floor through post-caldera sedimentary rocks, providing exceptional constraints on an evolutionary model of the caldera that are rarely available for other calderas. The Caetano caldera filled with more than 4 km of intracaldera Caetano Tuff, while outflow tuff flowed west and south of the caldera, primarily down Eocene paleovalleys. Caldera fill consists of two units of Caetano Tuff. The lower compound cooling unit is as much as 3600 m thick and is separated by a complete cooling break from a 500–1000-m-thick upper unit that consists of multiple, thin, ash flows interbedded with sedimentary deposits. Multiple granite porphyries, including the 25-km 2 Carico Lake pluton, intruded and domed the center of the caldera within 0.1 Ma of caldera formation; one of these porphyries is associated with pervasive argillic and advanced argillic alteration of the western half of the caldera. All exposed caldera-related rocks are rhyolites or granites (71–77.5 wt% SiO 2 ). Caldera collapse was significantly greater than the thickness of caldera fill and created a topographic depression that served as a depocenter until at least 25 Ma, filling with nearly 1 km of sediments and distally derived, ash-flow tuffs. The caldera is presently exposed in a series of 40–50°, east-tilted blocks bounded by north-striking, west-dipping normal faults that formed after 16 Ma. Slip on these faults accommodated ∼100% E-W extension, making the restored Caetano caldera ∼20 km east-west by 10–18 km north-south. The estimated volume of intracaldera Caetano Tuff is, therefore, ∼840 km 3 , and the minimum estimated total eruptive volume is ∼1100 km 3 . Although the Caetano magmatic system was probably too young to supply heat for nearby Carlin-type gold deposits in the Cortez district, earlier nearby magmatic activity may have contributed to formation of these deposits. Reconstruction of the late Eocene, pre-Caetano caldera geologic setting, immediately prior to caldera formation, indicates that the Cortez Hills and Horse Canyon Carlin-type deposits formed at ≤1 km depths.

Nevada↗
USGS2019

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular (AWP) terrane to the south, and the Farewell and other peri-cratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard peri-cratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the AWP terrane and mid to Late Cretaceous arc related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at about 81 Ma and they were deformed together by about 80 Ma by southeast-verging fold-and-thrust style deformation interpreted to represent final accretion of the AWP terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the backarc flank of an active, south-facing island arc system (AWP terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench-Nankai Trough on the east side of central Japan.

Alaska↗
USGS2021

Fast Pliocene integration of the Central Anatolian Plateau drainage: Evidence, processes, and driving forces

Continental sedimentation was widespread across the Central Anatolian Plateau in Miocene–Pliocene time, during the early stages of plateau uplift. Today, however, most sediment produced on the plateau is dispersed by a well-integrated drainage and released into surrounding marine depocenters. Residual long-term (10 6 –10 7 yr) sediment storage on the plateau is now restricted to a few closed catchments. Lacustrine sedimentation was widespread in the Miocene–Pliocene depocenters. Today, it is also restricted to the residual closed catchments. The present-day association of closed catchments, long-term sediment storage, and lacustrine sedimentation suggests that the Miocene–Pliocene sedimentation also occurred in closed catchments. The termination of sedimentation across the plateau would therefore mark the opening of these closed catchments, their integration, and the formation of the present-day drainage. By combining newly dated volcanic markers with previously dated sedimentary sequences, we show that this drainage integration occurred remarkably rapidly, within 1.5 m.y., at the turn of the Pliocene. The evolution of stream incision documented by these markers and newly obtained 10 Be erosion rates allow us to discriminate the respective contributions of three potential processes to drainage integration, namely, the capture of closed catchments by rivers draining the outer slopes of the plateau, the overflow of closed lakes, and the avulsion of closed catchments. Along the southern plateau margin, rivers draining the southern slope of the Central Anatolian Plateau expanded into the plateau interior; however, only a small amount of drainage integration was achieved by this process. Instead, avulsion and/or overflow between closed catchments achieved most of the integration, and these top-down processes left a distinctive sedimentary signal in the form of terminal lacustrine limestone sequences. In the absence of substantial regional climate wetting during the early Pliocene, we propose that two major tectonic events triggered drainage integration, separately or in tandem: the uplift of the Central Anatolian Plateau and the tectonic completion of the Anatolian microplate. Higher surface uplift of the eastern Central Anatolian Plateau relative to the western Central Anatolian Plateau promoted more positive water balances in the eastern catchments, higher water discharge, and larger sediment fluxes. Overflow/avulsion in some of the eastern catchments triggered a chain of avulsions and/or overflows, sparking sweeping integration across the plateau. Around 5 Ma, the inception of the full escape of the Anatolian microplate led to the disruption of the plateau surface by normal and strike-slip faults. Fault scarps partitioned large catchments fed by widely averaged sediment and water influxes into smaller catchments with more contrasted water balances and sediment fluxes. The evolution of the Central Anatolian Plateau shows that top-down processes of integration can outcompete erosion of outer plateau slopes to reintegrate plateau interior drainages, and this is overlooked in current models, in which drainage evolution is dominated by bottom-up integration. Top-down integration has the advantage that it can be driven by more subtle changes in climatic and tectonic boundary conditions than bottom-up integration.

Central Anatolian Plateau↗
USGS2022

Geologic and geomorphic evidence for multi-phase history of strands of the San Andreas fault through the San Gorgonio Pass structural knot, southern California

The San Gorgonio Pass region of southern California is a locus of extensive Quaternary deformation within a multi-strand section of the San Andreas fault zone. The geomorphology of the San Gorgonio Pass region reflects the complicated history of geologic events in the formation of this structurally complex region. We define fault-bounded blocks in San Gorgonio Pass and focus on two that are characterized by extensive crystalline bedrock outcrops with similar bedrock lithologies. These two blocks are separated by the San Bernardino strand of the San Andreas fault. Morphometric variables, including local relief, slope, slope distribution, and surface roughness, consistently demonstrate distinctions between the bedrock upland regions of the two blocks. Geologic observations of the region highlight differences in Quaternary units within the two blocks, reflective of the differing surficial processes active in each block. Within the Kitching Peak block, the morphology highlights a lineament that we informally name the Lion Canyon lineament. This boundary more clearly differentiates the two regions, as compared to the mapped San Bernardino strand, and may represent the previously active strand or bounding structure in this section. The distinction in morphology and surficial processes leads to our interpretation that the Kitching Peak and Pisgah Peak blocks have experienced different uplift histories. This further leads to the conclusion that the San Bernardino strand, broadly defined, has been integrated, at some point in the past, with the Banning strand, allowing for through-going rupture along the fault system. This connectivity may have occurred along the Burro Flats section of the San Bernardino strand or the Lion Canyon lineament. The fault connection along the mapped trace of the San Bernardino strand is not currently evident at the surface, however, suggesting that the integration has been disrupted. We propose this is due to intervals of N-S compression in the region, manifest as slip along the San Gorgonio Pass fault zone and other regional faults. We present evidence for lateral displacement along the San Bernardino and Banning strands of the San Andreas fault, discuss the implications of these displacements, and propose a sequence of fault activity, including multiple phases of activity along the San Bernardino and Banning strand pathway to account for the structural complexity and lack of surficial fault continuity.

California↗