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1,129 records · Page 63Linked to original sources
USGS1974

Paleozoic tectonics in the Edna Mountain quadrangle, Nevada

Geologic mapping at scale 1:24,000 of the Edna Mountain 15-minute quadrangle, Humboldt County, Nev., revealed two episodes of pre-Mesozoic deformation that are difficult to reconcile with either the Antler or the Sonoma orogeny. We believe that the older episode predated the Antler orogeny and may be as old as Late Cambrian. The younger episode may have been more localized, predated the Sonoma orogeny, and was probably Late Pennsylvanian to Permian in age. Deformation related to Antler and Sonoma orogenies also occurred. These four episodes suggest that cycles of uplift, folding, faulting, and erosion began early in the development of the southern Cordillera and continued intermittently throughout Paleozoic time. West of the continental shelf was a broad subsiding basin marked by narrow troughs and elongate structural highs which emerged, matured, and diminished at different times. Waxing and waning deformation in various parts of the geosyncline acting upon local "highs" and troughs can explain the seemingly erratic distribution in north-central Nevada of different fades of time-correlative stratigraphic units and structural blocks of Paleozoic age. Telescoping of facies by thrust faulting certainly took place, but large displacements within brief periods are not essential to the validity of the explanation.

Nevada↗
USGS1974

Stratigraphic relationships within the Baraga Group of Precambrian age, central Upper Peninsula, Michigan

Details of the stratigraphic section in parts of northern Michigan have been known for many years, but correlation of units between geographically separated areas has been partly speculative. Mapping in the Witch Lake quadrangle has filled the gap between well-studied areas of the Marquette trough and parts of Iron and Dickinson Counties and has helped to correlate units in the Baraga Group (Precambrian X). Iron-formations are important marker beds; because of their magnetic properties they can be traced with certainty through poorly exposed areas. The magnetic Fence River Formation is now known to be stratigraphically beneath the Bijiki Iron-formation Member of the Michigamme Formation but may be equivalent to the Greenwood Iron-formation Member which crops out in parts of the Marquette trough to the east The Hemlock Formation (volcanic) underlies the Fence River and has been traced from near its type locality in Iron County to the south flank of the Marquette trough. These relationships provide the key to understanding the lateral facies changes responsible for the stratigraphic complexities of the Baraga Group.

Michigan↗
USGS1976

Geology of the Brysch uranium mine, Karnes County, Texas

Approximately 13,700 tons (12,400 tonnes) of oxidized uranium ore, averaging about 0.1 percent U 2 O 8 , was mined during 1966 and 1967 from the lower unit of the Deweesville Sandstone Member of the upper Eocene Whitsett Formation, from depths of 75 to 90 feet (23-27 m). The mine is in the Karnes County uranium area, 3 miles (5 km) east of Falls City, Tex. Meta-autunite, Ca(UO 2 ) 2 (PO 4 ) 2 2-6H 2 O, and meta-tyuyamunite, Ca(UO 2 ) 2 (PO 4 ) 2 3-5H 2 O, were identified in samples of the ore. The host rock is light-colored, medium-grained, well-sorted feldspathic sandstone that contains fossil wood and Ophiomorpha . It also contains clinoptilolite, cristobalite, and montmorillonite that probably formed as alteration products of volcanic grains in the original sediment. The host rock was deposited in a beach environment near or on a delta formed at the mouth of a stream channel oriented roughly normal to the beach. The Deweesville can be traced for many miles on the surface and represents in general the point of maximum transgression in a transgressive-regressive cycle. The upper unit of the Deweesville, which overlies the host rock, is now exposed in the mine and contains, in ascending order, storm-beach, shoreface, beach, and tidal-flat facies. The shape of the ore body suggests that it originated as an unoxidized ore roll. The ore-bearing fluids may have entered through porous fluvial rock extending updip. Plant material, abundant in the original host sediment, probably supplied reductant necessary for the roll formation.

Texas↗
USGS1975

Stratigraphy, conodont dating, and paleotectonic interpretation of the type Milligen Formation (Devonian), Wood River area, Idaho

The Milligen Formation at and near its type locality in the Wood River area is considerably older than and unrelated to rocks of Early Mississippian age called Milligen Formation in the Lost River Range and other ranges of east-central Idaho. Conodont faunas were found in limestones of a thin upper member of the sparsely fossiliferous marine Milligen Formation in its principal reference section at Milligen Gulch, at Fisher Canyon, and near Bellevue, Idaho. The faunas include indigenous conodonts here assigned to the early Late Devonian (early Frasnian) Lower and Middle Polygnathus asymmetricus Zones, and reworked conodonts derived from several Middle and Early Devonian conodont zones. An underlying much thicker argillite member of the Milligen contains fewer limestones, but a thin encrinite interbed near the middle of the member yielded early Middle Devonian (Eifelian) conodonts. This lower member probably represents most of Middle and Early Devonian time. Although its base is nowhere exposed in the Wood River area, the Milligen is inferred to have been deposited on the Silurian Trail Creek Formation, which crops out just to the east in the Pioneer Mountains. The age of the Milligen is therefore wholly Devonian and the highest fossiliferous beds are no younger than early Late Devonian. Reworked Middle and Early Devonian conodonts in limestone turbidites of the upper member of the Milligen Formation are identical to conodonts found in shelf (miogeosynclinal) carbonate rocks farther east. A postulated eastern source for the turbidites is supported by new data on the distribution, thickness, and tectonic facies of Devonian rocks that suggest the presence of a Late Devonian ridge on the continental shelf east of the Milligen depositional area. The Milligen Formation was intensely folded and was emergent during most of the Mississippian time when it formed part of the Antler Highlands, which shed flysch sediments eastward into the Copper basin. The Wood River Formation of Pennsylvanian and Permian age was then deposited over a subdued topography on the Milligen Formation. The Hailey Conglomerate Member at the base of the Wood River filled many irregularities in the surface. This depositional contact later was largely destroyed and the contact between the Milligen and Wood River is now a regional thrust fault at most places.

Idaho↗
USGS1976

Revision of Mississippian stratigraphy, eastern Idaho and northeastern Utah

New paleontologic evidence requires a revision of previous interpretations of the stratigraphy of Mississippian sequences in the Cordilleran miogeosyncline of eastern Idaho and northeastern Utah. A postulated unconformity between rocks of early Osagean age and rocks of middle Meramecian age is no longer tenable in the light of new data. The new evidence supports continuous deposition from the Lodgepole Limestone (Kinderhookian and early Osagean age) into the Little Flat Formation (early Osagean to middle Meramecian age) and its equivalents, and the interpretation of a phosphatic siltstone and shale interval in the lower part of the Little Flat Formation as a starved-basin facies.

Idaho, Utah↗
USGS1977

Geology of the gabbroic complex along the northern border of the Josephine Peridotite, Vulcan Peak area, southwestern Oregon

The terrane bordering the alpine-type Josephine Peridotite on the north in the Vulcan Peak area, southwestern Oregon, is composed of intrusive hornblende gabbro (Middle Jurassic) and scattered remnants of clinopyroxene-bearing ultramafic rocks and amphibolite. The amphibolite, which preliminary analyses suggest is of andesitic composition, has undergone regional metamorphism to the amphibolite facies and three episodes of plastic folding. The ultramafic rocks overlie the amphibolite with a possible magmatic sedimentary contact, although the contact is not entirely clear and a fault cannot be ruled out. The ultramafic rocks are partially recrystallized owing to the gabbro intrusion and later partially serpentinized; they have undergone two episodes of plastic folding, the second of which correlates with the third in the amphibolite. The intrusion of the gabbro began as early as the second folding episode in the amphibolite and probably continued at least intermittently throughout the third episode of folding and possibly later. After the high-temperature deformation, the Josephine Peridotite was thrust northward over the gabbroic complex along an east-striking south-dipping thrust fault, after which both of these terranes were thrust westward over the Upper Jurassic Dothan Formation along a major north-striking east-dipping thrust fault. The nature of the ultramaflc rocks, their association with an extensive gabbro terrane, and the proximity of a large alpine-type peridotite suggest that the gabbroic and ultramafic complexes are part of an ophiolite sequence. However, if the ultramaflc rocks are cumulates and were deposited on the amphibolite terrane, then the gabbroic complex is somewhat different from the ideal ophiolite model.

Oregon↗
USGS2022

Remote sensing and mapping Miocene paleovalleys of the Marble, Bristol, and Old Dad Mountains in the Trilobite and Bristol Mountain Wildernesses, California

Wilderness areas in the Mojave Desert, California, are remote and rugged terrain, but they contain important geology for understanding faults of the eastern California shear zone (ECSZ), and remote sensing offers techniques that can optimize mapping. The Bristol–Granite Mountain fault zone (BGMFZ) is the easternmost fault of the ECSZ with the Marble, Bristol, and Old Dad mountains on either side of the fault, as are the Trilobite and Bristol Mountain Wildernesses. In the northern Marble Mountains, a west-trending Miocene paleovalley has been proposed to have a correlative in the Old Dad Mountains and provides a constraint for right-lateral separation across the BGMFZ; however, this correlation is based on the premise that there was a unique paleovalley with a well defined geometry. In the northern Marble Mountains, a paleovalley was mapped by the distribution of (1) thickness and facies within the Lost Marble gravel (LMg) and 18.8 Ma Peach Spring Tuff (PST), and (2) adjacent highlands where the PST was deposited on basalt and dacite lava flows. Whether this paleovalley is unique, or there are other paleovalleys farther south in the Marble Mountains, requires mapping of the entire 5 by 28 km area of Miocene volcanic rocks. In the south Bristol and Old Dad mountains, there is a similar 12 by 22 km area of Miocene basalt and dacite with deposits of PST and local sedimentary rocks, including the proposed offset Lost Marble paleovalley, but the entire range needs to be mapped to establish a unique correlate. The mountains are in the Mojave Trails National Monument, and the Trilobite and Bristol Mountains wilderness areas, so access is limited. Remote sensing data, including aerial photography and hyperspectral images, are important for identification and characterization of rocks. Airborne hyperspectral Mako data can distinguish the distinctive spectral characteristics of the PST as well as several more map units identified by detailed field mapping in the Bristol Mountains. Reconnaissance maps derived from high spatial resolution Mako data can guide the detailed mapping needed to identify paleovalley or paleohighland deposits and can be used to optimize field time.

California↗
USGS2022

Geologic map of the Mount Blue Sky (formerly Mount Evans) quadrangle, Clear Creek and Park Counties, Colorado

The Mount Blue Sky (formerly Mount Evans) 7.5’ quadrangle lies in Park and Clear Creek counties, Colorado, about 60 km west of Denver. The highest elevation in the quadrangle is 14,265 ft (4,348 m) at the top of Mount Blue Sky. The lowest is at about 9,200 ft (2,804 m) on Guanella Pass Road at the southern edge of the quadrangle. Bedrock directly underlies most of the map area, with surficial deposits primarily in the valleys. The geology of the quadrangle was previously mapped at 1:100,000 scale as part of a regional compilation by Kellogg and others (2008). The oldest rocks in the Mount Blue Sky 7.5-minute quadrangle are Paleoproterozoic metasedimentary rocks, and mafic to felsic metaigneous rocks (all units starting with ‘X’ on Plate 1). These rocks were metamorphosed under upper amphibolite facies conditions and intruded by Mesoproterozoic felsic igneous rocks of the ~1442 Ma Mount Blue Sky (YgR, Yt, Ygdm, Ymgm and ~1424 Ma Silver Plume (Yg) batholiths (Spurr and others, 1908; Tweto, 1897; Aleinikoff and others, 1993; du Bray and others, 2018) and, in the southern part of the quadrangle, by rocks that may also be part of the Mount Blue Sky batholith, but may alternatively interpreted as part of the ~1115 Ma to ~1066 Ma Pikes Peak batholith (Unruh and others, 1995; Guitreau and others, 2016). Four generations of folds affected the area. The oldest, F1 folds are isoclinal of various orientations, but primarily northerly-plunging in the southern part of the quadrangle (Mahatma, 2019; Mahatma and others, 2022). In the northern part of the quadrangle (Powell, 2020), open to close F2 chevron folds exist with various orientations. F3 folds in the northern part of the quadrangle are open to close with upright axial planes and plunges to the north and south, and in the southern part of the quadrangle they are open centimeter- to meter- scale northerly-plunging folds, possibly overprinted by another generation of northerly-plunging folds based on orientations of axial planes (F2 and F3 of Mahatma and others, 2022). F4 folds throughout the quadrangle are open to gentle with upright axial planes and shallow plunges to the east and west. The Mount Blue Sky batholith displays a pervasive moderately NW-dipping biotite-hornblende foliation (Fig. 1) in addition to a flow foliation near the margins, indicating NW-directed shortening after ~1442 Ma (Powell, 2020). The relationship between this foliation and the folds is not clear. Various joint sets are present in the area. The most pervasive joint set strikes 355°-020° and is subvertical. It is best developed in the western to southwestern part of the map area, and may be related to late Cenozoic extension associated with the Rio Grande Rift. Joint orientations are generally consistent with the trends of topographical lineaments. Surficial deposits include two series of glacial till deposits (Qtb and Qtp), and outwash (Qgp) deposits. They correlate with the Bull Lake (170-120 ka) and Pinedale (30-12 ka) glacial periods (Dahms, 2004) based on original depositional morphology, geomorphic and topographic position, deposit weathering and pedogenic properties. Possible older glacial deposits (Qti) have been observed along topographically higher surfaces.

Colorado↗
USGS2023

UAS mapping of surface roughness and digital grain size to assess pre-dam removal baseline conditions along the mainstem Klamath River corridor below Iron Gate Dam, California

Surface roughness and grain size in river corridors are fundamental indicators of river hydraulics. In hydraulic models for coarse-grained rivers, the roughness parameter is often assumed to be related to a representative grain diameter. This paper documents a workflow for using aerial imagery and Structure-from-Motion (SfM) photogrammetry to map surface roughness and digital grain size (DGS) on gravel bars. In June 2022, an Uncrewed Aerial System (UAS) was used to collect aerial imagery at 7 study sites distributed longitudinally downstream from Iron Gate Dam along the mainstem Klamath River. The study sites were selected for repeat monitoring as part of a larger effort to monitor post-dam removal response of the Klamath River to a temporary increase in fine-sediment flux. Advantages of SfM, in comparison to other methods used to acquire high-resolution topography data, is the ability to generate topographic data and orthophotos for DGS analysis from imagery easily collected at multiple spatial scales. UAS imagery was collected at three altitudes (62-m, 15-m, and 4-m), and a workflow for creating georeferenced surface roughness and grain size maps at the patch (100 m2) to site-scale (100,000 m2) was developed. The multi-scale UAS imagery was post-processed, and Digital Surface Models (DSMs) and orthophotos were exported, using AgiSoft Metashape Professional and standardized SfM methods. The DSMs were detrended using an automated tools implemented in ArcGIS to remove bedform gradients and the reach slope. Surface roughness maps were created using the standard deviation of detrended elevations. Plots for DGS analysis were randomly selected from the orthophotos, and DGS was estimated using a wavelet-based method, which minimizes surface disturbance and does not require calibration. For the 62-m survey, the grid-cell resolutions for the DSMs and surface roughness maps was 4 cm, and the orthophotos have a resolution of 2 cm. For the 15-m survey, the resolution of the DSMs and surface roughness maps was 8 mm, and the orthophotos have a resolution of 4 mm. For the 4-m survey, the resolution of the DSMs and surface roughness maps was 3 mm, and the orthophotos have a resolution of 2 mm. The surface roughness maps for the 62-m, 15-m, 4-m surveys provided surface roughness maps suitable for assessing variations in roughness heights across the study sites. The 15-m survey also provided orthophotos suitable for heads-up digitizing of coarse- and fine-grained facies. The 4-m survey provided orthophotos with the grains clearly resolved and suitable for DGS. The wavelet-based DGS analysis used in this study is most appropriate for assessing relative differences in the grain size distributions across a single site or among multiple sites, instead of absolute estimates. This workflow produced a series of digital products to support multi-scale investigations at resolutions appropriate for detecting textural changes in grain size on gravel bars and for producing roughness maps to define roughness parameters for hydraulic modeling.

California↗
USGS2023

Reconnaissance mineral and cathodoluminescence studies of gold occurrences in the Pogo-Black Mountain area, eastern interior Alaska, USA

The Pogo Au deposit is the largest of a number of gold occurrences in eastern interior Alaska, that occur along a broad trend from west of Pogo to Black Mountain. Some of these occurrences are hosted in amphibolite facies gneisses and others in mid-Cretaceous igneous rocks that intruded the older metamorphic rocks. All occurrences contain arsenopyrite and pyrite. Whole rock geochemical trends distinguish most metamorphic rock-hosted vein prospects (strong Bi-Te-Au correlations) and intrusion-hosted occurrences (weak As-Au correlations). Brecciated quartz veins in metamorphic rocks have paragentically late Bi-Te (±S) + Au that post-dates Fe-As sulphide deposition. High grade vein samples from the Tibbs Creek intrusion-hosted deposits contain pyrite and arsenopyrite, generally lack Bi-Te minerals, but can contain paragentically younger euhedral quartz, stibnite and carbonate. Cathodoluminescence studies of gold-rich samples indicate that quartz dissolution occurred during the syn- to post-tectonic Bi-Te-Au deposition, and the later stibnite event. In the case of metamorphic rock-hosted deposits (e.g., Pogo, Gray Lead), Bi-Te and gold deposition commonly occurs in microfractures within quartz veins; the limited quartz in these fractures have distinctive CL response. We propose that gold deposition is related to changes in P-T conditions rather than fluid-rock chemical reactions. Similar quartz dissolution textures affect the void-filling euhedral quartz before or during stibnite and carbonate mineralization in the high-grade Au samples from Blue Lead.

Alaska↗
USGS2025

Deformation mechanisms in quartz veins and shear zones elucidate the origin of gold mineralization at Pogo, Alaska

Pogo is a quartz vein hosted, ca. 8 Moz gold deposit. Although it has similarities to orogenic and magmatic-hydrothermal deposits, its origin remains enigmatic. Observations from surface exposures, underground workings, and drill core provide new constraints on quartz vein origins with implications for mineralization. Abundant, largely barren metamorphic segregation quartz veins are found throughout interior Alaska and share characteristics with mineralized quartz veins at Pogo. Pogo quartz veins show crystal plastic deformation fabrics consistent with greenschist to amphibolite facies metamorphism and a lack of internal shear. In contrast, gold in Pogo quartz veins occurs in brittle microfaults and fractures with sulphides such as arsenopyrite and pyrrhotite. Major and minor cataclastic shear zones also exist at Pogo. Cataclastic shear zones commonly cut the mineralized quartz veins and porphyroclasts are dominated by quartz with the same plastic deformation fabrics found in the major Pogo quartz veins. The porphyroclasts, and the carbonaceous clay-rich matrix they sit in, also contain gold indicating that the shear zones postdate quartz and earliest gold deposition. These observations suggest that competency contrasts between the quartz veins and their phyllosilicate-rich host rocks controlled transient permeability formed by late, preferential brittle deformation events localizing sulphide ± gold deposition.

Alaska↗
USGS2025

Paleoproterozoic vein graphite mineralization caused by decarbonation in the Ruby Range, Montana, USA

Hydrothermal graphite veins are a possible source for modern battery materials and require better understanding of their carbon source(s) and absolute timing to develop mapable criteria for exploration models. We present new observations of graphite vein and alteration paragenesis and U-Pb LA-ICP-MS titanite age data from the Ruby prospect, Montana, USA, that constrain mineralization timing and source. The graphite veins cut high-temperature metamorphic rocks of the lower Christensen Range suite and are associated with intense diopside (Di0.69Hd0.27Jo0.04) alteration of marble. The oldest titanite ages in fresh marble and partially altered calc-silicate gneiss are ca. 2,500 – 2,450 Ma, show elevated REE values, and Eu/Eu* <1.5, consistent with growth during early regional metamorphism. Titanite in diopside-graphite alteration, interpreted as part of the hydrothermal vein-forming episode, cluster around 1,750 Ma, are characterized by lower REE values, and show Eu/Eu* >2; suggesting a low fO 2 fluid generated from metamorphism during the Big Sky orogeny. Our paragenetic observations and titanite ages indicate graphite vein formation via skarnoid decarbonation reactions in marble late in regional orogenesis. Granulite-facies, carbonate-bearing, supracrustal rock terranes are thus favorable for hydrothermal graphite vein deposits.

Montana↗
USGS2018

Middle Pleistocene infill of Hinkley Valley by Mojave River sediment and associated lake sediment: Depositional architecture and deformation by strike-slip faults

Hinkley Valley in the Mojave Desert, near Barstow about 140 km northeast of Los Angeles and midway between Victorville Valley and the Lake Manix basin, contains a thick sedimentary sequence delivered by the Mojave River. Our study of sediment cores drilled in the valley indicates that Hinkley Valley was probably a closed playa basin with stream inflow from four directions prior to Mojave River inflow. The Mojave River deposited thick and laterally extensive clastic wedges originating from the southern valley that rapidly filled much of Hinkley Valley. Sedimentary facies representing braided stream, wetland, delta, and lacustrine depositional environments all are found in the basin fill; in some places, the sequence is greater than 74 m (245 ft) thick. The sediment is dated in part by the presence of the ~631 ka Lava Creek B ash bed low in the section, and thus represents sediment deposition after Victorville basin was overtopped by sediment and before the Manix basin began to be filled. Evidently, upstream Victorville basin filled with sediment by about 650 ka, causing the ancestral Mojave River to spill to the Harper and Hinkley basins, and later to Manix basin. Initial river sediment overran wetland deposits in many places in southern Hinkley Valley, indicating a rapidly encroaching river system. These sediments were succeeded by a widespread lake (“blue” clay) that includes the Lava Creek B ash bed. Above the lake sediment lies a thick section of interlayered stream sediment, delta and nearshore lake sediment, mudflat and/or playa sediment, and minor lake sediment. This stratigraphic architecture is found throughout the valley, and positions of lake sediment layers indicate a successive northward progression in the closed basin. A thin overlapping sequence at the north end of the valley contains evidence for a younger late Pleistocene lake episode. This late lake episode, and bracketing braided stream deposits of the Mojave River, indicate that the river avulsed through the valley, rather than continuing toward Lake Manix, during the late Pleistocene. Two dextral strike-slip fault zones, the Lockhart and the Mt. General, fold and displace the distinctive stratigraphic units, as well as surficial late Pleistocene and Holocene deposits. The sedimentary architecture and the two fault zones provide a framework for evaluating groundwater flow in Hinkley Valley.

Hinkley Valley↗