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USGS2006

Regional surficial geochemistry of the northern Great Basin

The regional distribution of arsenic and 20 other elements in stream-sediment samples in northern Nevada and southeastern Oregon was studied in order to gain new insights about the geologic framework and patterns of hydrothermal mineralization in the area. Data were used from 10,261 samples that were originally collected during the National Uranium Resource Evaluation (NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) program in the 1970s. The data are available as U.S. Geological Survey Open-File Report 02-0227. The data were analyzed using traditional dot maps and interpolation between data points to construct high-resolution raster images, which were correlated with geographic and geologic information using a geographic information system (GIS). Wavelength filters were also used to deconvolute the geochemical images into various textural components, in order to study features with dimensions of a few kilometers to dimensions of hundreds of kilometers. The distribution of arsenic, antimony, gold, and silver is different from distributions of the other elements in that they show a distinctive high background in the southeast part of the area, generally in areas underlain by the pre-Mesozoic craton. Arsenic is an extremely mobile element and can be used to delineate structures that served as conduits for the circulation of metal-bearing fluids. It was used to delineate large crustal structures and is particularly good for delineation of the Battle Mountain-Eureka mineral trend and the Steens lineament, which corresponds to a post-Miocene fault zone. Arsenic distribution patterns also delineated the Black Rock structural boundary, northwest of which the basement apparently consists entirely of Miocene and younger crust. Arsenic is also useful to locate district-sized hydrothermal systems an d clusters of systems. Most important types of hydrothermal mineral deposit in the northern Great Basin appear to be strongly associated with arsenic; this is less so for low-sulfidation epithermal deposits. In addition to individual elements, the distribution of factor scores that resulted from principal component studies of the data was used. The strongest factor is characterized by Fe, Ti, V, Cu, Ni, and Zn and is used to map the distribution of distinctive basalts that are high in Cu, Ni, and Zn and that appear to be related to the Steens Basalt. The other important factor is related to hydrothermal precious metal mineralization and is characterized by Sb, Ag, As, Pb, Au, and Zn. The map of the distribution of this factor is similar in appearance to the one for arsenic, and we used wavelength filters to remove regional variations in the background for this factor score. The resulting residual map shows a very strong association with the most significant precious metal deposits and districts in the region. This residual map also shows a number of areas that are not associated with known mineral deposits, illustrating the utility of the method as a regional exploration tool. A number of these prospective areas are distant from known significant mineral deposits. The deconvolution of the spatial wavelength structure of geochemical maps, combined with the use of large regional geochemical data sets and GIS, permits new possibilities for the use of stream-sediment geochemistry in the study of large-scale crustal features as well as the isolation of mineral-district scale anomalies. ?? 2006 Society of Economic Geologists, Inc.

Economic Geology↗
USGS2013

Tectonic evolution and Cretaceous gold metallogenesis of southwestern Alaska

Cretaceous gold metallogenesis in southwestern Alaska comprises three distinct episodes related to the accretionary evolution of northwestern North America. The oldest mineralizing event is characterized by 112 Ma Cu-Au-Bi-Te porphyry-type(?) veining in the zoned Bonanza and adjacent plutons that intruded rocks of the Nyac terrane. Tectonic reconstructions and limited geological and geochemical data suggest that Cu-Au mineralization in the Nyac district may be related to terminal subduction during accretion of the Togiak-Koyukuk arc. The subsequent 100 to 89 Ma metallogenic event is a product of subduction-related magmatism immediately following accretion of the Peninsular-Alexander-Wrangellia superterrane and includes formation of the giant Pebble porphyry Cu-Au-Mo deposit. Pebble underwent a complex history of highly oxidized magmatism that is isotopically linked to enriched lithosphere or metasomatized mantle sources. Pebble and other porphyryrelated plutons were emplaced as a consequence of changes in plate motion and onset of dextral transpression along the continental margin to the southeast of their present-day locations. The final 75 to 65 Ma metallogenic event is regionally the most extensive. It followed a ~15-m.y.-long magmatic lull and is related to enigmatic subduction-related magmatism in the western part of the Alaska Range and the Kuskokwim basin. This event resulted in the formation of porphyry, reduced pluton-related, orogenic, and possible epithermal Au deposits. In the better-studied relatively low-relief areas of the Kuskokwim basin, many of the mineralized systems are spatially associated with ilmenite-series monzonite to quartz monzonite composite plutons that have isotopic signatures consistent with derivation from crustally contaminated mantle sources. These pluton-hosted Au deposits comprise low-sulfide stockworks, sheeted veins, and/or breccias in the cupolas of moderately differentiated intrusions that contain high Au; anomalous As, Bi, Sb, and/or Te; and have variable, but not uneconomic, Cu concentrations. In addition, Au, Sb, and/or Hg deposits hosted by competent NE-trending granite porphyry dike-sill complexes or along faults in flysch are widespread in the Kuskokwim basin. These deposits, including the giant Donlin Creek Au deposit, formed at shallow crustal levels and are classified here as epizonal orogenic Au deposits and related Hg and Sb lodes. Mesozonal orogenic Au deposits were formed along the margins of the uplifting Willow Creek pluton that is now exposed along the southern side of the Talkeetna Mountains batholith. Recent discoveries in the rugged Alaska Range comprise less well-studied porphyry Au-Cu (Whistler, Island Mountain), epithermal(?) Au (Terra), and reduced pluton-hosted Au deposits (Estelle). The cause of the broad latest Cretaceous magmatism associated with the final metallogenic event is enigmatic, but indicates widespread high heat flow possibly related to flat-slab subduction, lithospheric mantle delamination, or escape tectonics. Plutonism postdated regional folding and coincided with periods of movement along regional faults and formation of the NE-trending structural fabric. Magmatism resulted from initiation of transpressional faulting in more landward positions during oblique subduction of the Kula plate. Crustal contamination of mantle melts, either near their source or during ascent through the thick flysch of the Kuskokwim basin, produced low oxidation state magmas and controlled much of the metallogeny, particularly for those deposits that display similarities to both reduced porphyry Au-Cu deposits and, to a lesser degree, reduced intrusion-related Au systems. High heat flow also induced crustal melting and metamorphic devolatilization to form orogenic Au deposits. A transition to extension at ~60 Ma recorded elsewhere in Alaska temporally corresponds to termination of gold deposit formation in southwestern Alaska. The multiple periods of gold metallogenesis in southwestern Alaska offer a wide variety of targets for exploration within discrete parts of the region.

Society of Economic Geologists Special Publication↗
USGS2009

Stratigraphic framework and estuarine depositional environments of the Miocene Bear Lake Formation, Bristol Bay Basin, Alaska: Onshore equivalents to potential reservoir strata in a frontier gas-rich basin

The Miocene Bear Lake Formation is exposed along the coast and mountains of the central Alaska Peninsula and extends offshore as part of the Bristol Bay Basin. The Bear Lake Formation is up to 2360 m (7743 ft) thick in an offshore well and is considered to have the highest reservoir potential in this gasrich frontier basin. Our new macrofossil and palynological data, collected in the context of measured stratigraphic sections, allow us to construct the first chronostratigraphic framework for this formation. Biostratigraphic age assignments for the numerous, commonly isolated, onshore exposures of the Bear Lake Formation show that deposition initiated sometime before the middle Miocene (15 Ma) and extended to possibly the earliest Pliocene. The bulk of the Bear Lake Formation, however, was deposited during the middle and late Miocene based on our new findings. We interpret the Bear Lake Formation as the product of a regional transgressive estuarine depositional system based on lithofacies analysis. The lower part of the formation is characterized by trough cross-stratified sandstone interbedded with coal and pedogenic mudstone deposited in fluvial and swamp environments of the uppermost parts of the estuarine system. The lower-middle part of the formation is dominated by nonbioturbated, wavy- and flaser-bedded sandstone and siltstone that were deposited in supratidal flat environments. The uppermiddle part of the Bear Lake Formation is characterized by inclined heterolithic strata and coquinoid mussel beds that represent tidal channel environments in the middle and lower tracts of the estuarine system. The uppermost part of the formation consists of tabular, bioturbated sandstone with diverse marine invertebrate macrofossil faunas. We interpret this part of the section as representing the subtidal tract of the lower estuarine system and possibly the adjacent shallow inner shelf. A comparison of our depositional framework for the Bear Lake Formation with core and well-log data from onshore and offshore wells indicates that similar Miocene depositional systems existed throughout much of the Bristol Bay Basin. The documented changes in depositional environments within the Bear Lake Formation are also important for understanding upsection changes in the geometries of potential reservoirs. Copyright ??2009. The American Association of Petroleum Geologists. All rights reserved.

American Association of Petroleum Geologists Bulle↗
USGS1985

Conceptual model for origin of abnormally pressured gas accumulations in low-permeability reservoirs

The largest gas fields in the Rocky Mountain region occur in abnormally pressured reservoirs. These gas accumulations are different from more conventional gas accumulations in that they are commonly located in basin-center positions, they occur downdip from water-bearing rocks, and they are in overpressured or underpressured low-permeability reservoirs. We suggest that overpressured and underpressured gas accumulations of this type have a common origin. In basins containing overpressured gas accumulations, rates of thermogenic gas accumulation exceed gas loss, causing fluid (gas) pressure to rise above the regional hydrostatic pressure. Within the overpressured gas generation zone, free water in the larger pores is forced out of the gas generation zone into overlying and updip, normally pressured, water-bearing rocks. The remaining tightly bound water, at irreducible saturation levels, cannot remove dissolution products, precluding significant permeability and porosity enhancement. Thus, while other diagenetic processes continue, a pore network with very low permeability develops. As a result, gas accumulates in these low-permeability reservoirs at rates higher than it is lost. In basins containing underpressured gas accumulations, rates of gas generation and accumulation are less than gas loss. These basins have typically experienced significant uplift and erosion and/or temporal variations of paleotemperature. Despite these modifications, the basin-center gas accumulation persists, but because of changes in the basin dynamics, the overpressured accumulation evolves into an underpressured system.

American Association of Petroleum Geologists Bulle↗
USGS2020

Apatite trace element geochemistry and cathodoluminescent textures—Acomparison between regional magmatism and the Pea Ridge IOA-REE andBoss IOCG deposits, southeastern Missouri iron metallogenic province, USA

The southeast Missouri iron metallogenic province contains a remarkable wealth of historically important Fe, Cu, Au, and rare earth element (REE) deposits including the Pea Ridge iron oxide-apatite-rare earth element (IOA-REE) deposit and the Boss iron oxide-copper-gold (IOCG) deposit. These deposits are coeval with silicic and intermediate composition magmatism in the St. Francois Mountains terrane. Magmatism, iron-oxide (±Cu, Au, Co) and apatite formation, and REE mineralization overlapped in space and time, but the specific role of regional magmatism in the metallogenesis of these deposits remains unclear and basic petrogenetic models are still debated. We report results from high-spatial resolution textural and geochemical analyses of apatite from regional igneous and ore rocks to elucidate their petrogenetic histories and evaluate deposit models. Backscattered electron and spectral cathodoluminescence imaging of apatite reveal no primary igneous zoning, but show different domains with intricate rims and dissolution/reprecipitation textures, each with distinctive REE patterns in many samples. Apatite from all samples are nearly endmember fluorapatite containing up to ~1.3 wt% Cl and F/Cl ratios span nearly three orders of magnitude. Fresh igneous fluorapatite contain low Na2O (0.15 wt%) while most Pea Ridge ore samples contain higher Na2O (up to ~0.45 wt%), and concentrations of sulfur in fluorapatite of all types are generally moderate to low (0.3 wt% SO3). Significant amounts of Fe (60,000 ppm), Mg (30,000 ppm), Mn (7,000 ppm), and Sr (12,000 ppm) are contained in fluorapatite of all sample types, and they also have moderate amounts of As (4,000 ppm), Ba (2,000 ppm), Th (400 ppm), and U (80 ppm). Fluorapatite show an extraordinarily large range of REE (~0.1-2.0 wt%) and Y (~100-7000 ppm) concentrations. While fresh igneous fluorapatite share many geochemical features with metasomatized igneous fluorapatite and ore-stage fluorapatite from the Pea Ridge IOA and Boss IOCG ore zones, they also have distinct geochemical signatures that are indicative of unique trace element partitioning and substitution mechanisms. These distinguishing textural and geochemical signatures preclude ore-zone fluorapatite genesis directly from a magma (i.e., crystallization directly from a silicate melt) but are permissive of ore-zone fluorapatite formation by magmatic-hydrothermal fluids derived from the regional magmas. Basinal brines may play an important role in the formation of fluorapatite, especially from the Pea Ridge hematite and Boss magnetite-rich zones. Fluorapatite from different ore zones likely formed by crystallization during pulses of hydrothermal fluids with varying Cl-, Na-, and F-contents, which fundamentally controlled the carrying capacity and solubility of REE+Y and generated geochemically distinctive generations of fluorapatite. Exploration geologists using fluorapatite trace element geochemistry to identify IOA and IOCG deposits should proceed with caution, as more high-quality data from these deposits are needed to improve multivariate discrimination analysis. Fluorapatite from IOA/IOCG deposits can be reasonably discriminated from that of other mineral deposit types (e.g., porphyry/epithermal, skarn, orogenic), but no criteria successfully discriminate yet between IOA and IOCG deposits.

Missouri↗
USGS2007

Bald Mountain gold mining district, Nevada: A Jurassic reduced intrusion-related gold system

The Bald Mountain mining district has produced about 2 million ounces (Moz) of An. Geologic mapping, field relationships, geochemical data, petrographic observations, fluid inclusion characteristics, and Pb, S, O, and H isotope data indicate that An mineralization was associated with a reduced Jurassic intrusion. Gold deposits are localized within and surrounding a Jurassic (159 Ma) quartz monzonite porphyry pluton and dike complex that intrudes Cambrian to Mississippian carbonate and clastic rocks. The pluton, associated dikes, and An mineralization were controlled by a crustal-scale northwest-trending structure named the Bida trend. Gold deposits are localized by fracture networks in the pluton and the contact metamorphic aureole, dike margins, high-angle faults, and certain strata or shale-limestone contacts in sedimentary rocks. Gold mineralization was accompanied by silicification and phyllic alteration, ??argillic alteration at shallow levels. Although An is typically present throughout, the system exhibits a classic concentric geochemical zonation pattern with Mo, W, Bi, and Cu near the center, Ag, Pb, and Zn at intermediate distances, and As and Sb peripheral to the intrusion. Near the center of the system, micron-sized native An occurs with base metal sulfides and sulfosalts. In peripheral deposits and in later stages of mineralization, Au is typically submicron in size and resides in pyrite or arsenopyrite. Electron microprobe and laser ablation ICP-MS analyses show that arsenopyrite, pyrite, and Bi sulfide minerals contain 10s to 1,000s of ppm Au. Ore-forming fluids were aqueous and carbonic at deep levels and episodically hypersaline at shallow levels due to boiling. The isotopic compositions of H and O in quartz and sericite and S and Pb in sulfides are indicative of magmatic ore fluids with sedimentary sulfur. Together, the evidence suggests that Au was introduced by reduced S-bearing magmatic fluids derived from a reduced intrusion. The reduced character of the intrusion was caused by assimilation of carbonaceous sedimentary rocks. Tertiary faults dismember the area and drop down the upper part of the mineralizing system to the west. The abundant and widespread kaolinite in oxide ores is relatively disordered (1A polytype) and has ??D and ??18O values suggestive of a supergene origin. The deep weathering and oxidation of the ores associated with exhumation made them amenable to open-pit mining and processing using cyanide heap leach methods. ?? 2007 Society of Economic Geologists, Inc.

Economic Geology↗
USGS2010

Assessment of undiscovered conventional oil and gas resources, onshore Claiborne Group, United Statespart of the northern Gulf of Mexico Basin

The middle Eocene Claiborne Group was assessed for undiscovered conventional hydrocarbon resources using established U.S. Geological Survey assessment methodology. This work was conducted as part of a 2007 assessment of Paleogene-Neogene strata of the northern Gulf of Mexico Basin, including the United States onshore and state waters (Dubiel et al., 2007). The assessed area is within the Upper Jurassic-CretaceousTertiary composite total petroleum system, which was defined for the assessment. Source rocks for Claiborne oil accumulations are interpreted to be organic-rich, downdip, shaley facies of the Wilcox Group and the Sparta Sand of the Claiborne Group; gas accumulations may have originated from multiple sources, including the Jurassic Smackover Formation and the Haynesville and Bossier shales, the Cretaceous Eagle Ford and Pearsall (?) formations, and the Paleogene Wilcox Group and Sparta Sand. Hydrocarbon generation in the basin started prior to deposition of Claiborne sediments and is currently ongoing. Primary reservoir sandstones in the Claiborne Group include, from oldest to youngest, the Queen City Sand, Cook Mountain Formation, Sparta Sand, Yegua Formation, and the laterally equivalent Cockfield Formation. A geologic model, supported by spatial analysis of petroleum geology data, including discovered reservoir depths, thicknesses, temperatures, porosities, permeabilities, and pressures, was used to divide the Claiborne Group into seven assessment units (AUs) with three distinctive structural and depositional settings. The three structural and depositional settings are (1) stable shelf, (2) expanded fault zone, and (3) slope and basin floor; the seven AUs are (1) lower Claiborne stable-shelf gas and oil, (2) lower Claiborne expanded fault-zone gas, (3) lower Claiborne slope and basin-floor gas, (4) lower Claiborne Cane River, (5) upper Claiborne stable-shelf gas and oil, (6) upper Claiborne expanded fault-zone gas, and (7) upper Claiborne slope and basin-floor gas. Based on Monte Carlo simulation of justified input parameters, the total estimated mean undiscovered conventional hydrocarbon resources in the seven AUs combined are 52 million bbl of oil, 19.145 tcf of natural gas, and 1.205 billion bbl of natural gas liquids. This article describes the conceptual geologic model used to define the seven Claiborne AUs, the characteristics of each AU, and the justification behind the input parameters used to estimate undiscovered resources for each AU. The great bulk of undiscovered hydrocarbon resources are predicted to be nonassociated gas and natural gas liquids contained in deep (mostiy >12,000-ft [3658 m], present-day drilling depths), overpressured, structurally complex outer shelf or slope and basin-floor Claiborne reservoirs. The continuing development of these downdip objectives is expected to be the primary focus of exploration activity for the onshore middle Eocene Gulf Coast in the coming decades. ?? 2010 U.S. Geological Survey. All rights reserved.

American Association of Petroleum Geologists Bulle↗
USGS1906

The geography and geology of Alaska; a summary of existing knowledge, with a section on climate, and a topographic map and description thereof

Alaska, the largest outlying possession of the United States, is that great land mass forming the northwestern extremity of the North American continent, whose western point is within 60 miles of the Asiatic coast (PI. II). About one-quarter of this area lies within the Arctic Circle, and from the standpoint of geographic position must be regarded as an arctic province; but the southern seaboard, exposed to the warm winds and waters of the Pacific, gives to the entire southern portion of the territory" a comparatively warm climate. It is not generally realized that the range of climate in Alaska is greater than that between Florida and Maine. At the southernmost point of the Pacific coast the mean annual temperature is not far from that of the city of Washington, the winters being warmer and characterized by less snowfall; the Yukon Valley on the other hand has a winter climate similar to that of northern Montana and Dakota; while in the extreme northern part of the territory the meteorologic conditions are invariably arctic. Though as yet only sparsely settled, Alaska's vast area and great resources make it one of the most important possessions of the United States and promise its rapid development. During the years 1890 to 1900 the population increased from 32,052* to 63,592. The mineral output, which in 1890 was valued at less than $800,000, exceeded $9,000,000 in 1904, and the fisheries show a corresponding growth. This rapid development has attracted public attention and led to urgent demand for explorations, surveys, and other investigations. So actively has this work been pushed, both by public and private enterprise, that exact knowledge of the geography, geology, and mineral resources of the interior has made greater strides within the last eight years than during the preceding thirty-one years since the acquisition of Alaska. The facts regarding the geography and geology, scattered as they are through the many books and reports of this period, are not always readily accessible, and the time seems ripe to present them in a summarized form. The topography of Alaska is varied and complex (see PI. I), and it is not easy to present briefly even the salient features. The limited number of pages here devoted to the subject precludes the possibility of detailed treatment, even if the facts were available. Much of the description has been taken from the results attained by other investigators, the writer being personally familiar with only a part of this large province. A list of the publications consulted is appended. The larger geographic features of Alaska are now fairly well known, though the detailed surveys which are demanded by the development of many localities have hardly been begun. Preliminary surveys have been completed of all but three 8 of the larger rivers. The most important mountain ranges have been at least outlined (fig. 3). Only three large areas remain almost entirely unmapped: One in southwestern Alaska, between Cook Inlet and the lower Kuskokwim, and the others in northern Alaska, embracing the Arctic watershed east and west of the Colville River. Nearly all the surveys of the interior, however, have been of a preliminary and exploratory character, and to meet the requirements of exact geography must be followed by more detailed mensuration. Though the coast line has been fairly well known for more than half a century, knowledge of the interior has been gained chiefly within the last two decades. This has not yet found its way into text-books and has too often been entirely ignored by cartographers. If facts are presented which may seem elementary, it is because even well-informed people have been known to harbor misconceptions in regard to the orographic features, climate, and general character of Alaska. Those who read of the perils and privations of winter travel and explorations are apt to picture a region of ice and snow; others, again, who have personal knowledge of the tourist route of southeastern Alaska, regard the whole district as one of rugged mountains and glaciers. In point of fact, glaciers are now nearly limited to the ranges bordering the Pacific and to the two slopes of the Alaska range; and even during the greatest development of glaciers but a small portion of Alaska was under ice (see map, PI. XXII). As a treatise on geography would hardly be complete without some discussion of the climate, meteorologic data have been compiled by Mr. Cleveland Abbe, jr., but the discussion of this does not pretend to be more than a cursory treatment of the subject. The scope of the paper seems to require also a brief summary of the development of geographic knowledge of Alaska. This subject, with its many ramifications, is of fascinating interest and offers a magnificent field for the trained historian. If the accompanying sketch of discovery and exploration awakens any measure of popular interest the writer will feel amply rewarded for having attacked a theme which hardly falls within the scope of his investigations. When this compilation was begun it was intended to be chiefly a description of the topography of Alaska, as illustrated by the accompanying map (PI. XXXIV, in pocket), which was compiled under the direction of the late R. U. Goode. In the course of the work there accumulated much geologic as well as geographic material which seemed worthy of inclusion in the report. As no comprehensive statement of the geology of Alaska has been made since the modern epoch of investigation was begun, an attempt will be made to give a summary of all results achieved. Since the writer has obtained much of his knowledge of the facts from the work of others, he disclaims any pretense of making an entirely original contribution to geologic science. He feels, however, that a personal familiarity with a considerable part of the province, gained during seven consecutive seasons of field work, will justify Mm in presenting conclusions which may in some cases be at variance with those in the reports on which he must draw for his facts. Throughout this report attempt will be made to credit borrowed material to the source from which it is drawn. Where such matter has been obtained entirely from published reports there is no difficulty in so doing; but as regards investigators of the Geological Survey, with whom the writer has collaborated both in field and in office, the case is somewhat different, for it is not always possible to know whether this or that theory originated with the writer or with one of his colleagues. It will, then, perhaps suffice to state that this report could not have been prepared without the explorations and researches of the geologists, F. C. Schrader, Walter C. Mendenhall, Arthur J. Collier, J. E. Spurr, and Arthur C. Spencer; and the surveys of the topographers, T. G. Gerdine, D. C. Witherspoon, D. L. Reaburn, W. J. Peters, and E. C. Barnard. Each of these men, in the course of from two to six years of field work, has made important contributions to the knowledge of the geography and geology of Alaska, and not all of these results have yet been put in print. In the last season (1903) L. M. Prindle, C. W. Wright, Arthur Hollick, G. C. Martin, F. L. Hess, and Fred H. Moffit have carried on geologic work in Alaska, and the writer has made use of their work now in course of publication. He has also been fortunate in having access to the manuscript reports of Walter C. Mendenhall and F. C. Schrader on the Copper River basin, to which references will be made. The matter here presented should be credited in a measure to all of these investigators, but for many of the theories advanced the writer alone is responsible. As this manuscript goes to press there has been opportunity to incorporate some of the results of the field work of 1904. As far as possible these have been embodied in the text, but in some instances it has been found advisable to add them only as footnotes. During the past summer F. E. and C. W. Wright extended the geologic reconnaissance in southeastern Alaska. In southwestern Alaska G. C. Martin and T. W. Stanton have determined the general Mesozoic section, while F. H. Moffit has made a reconnaissance of the northern part of the Kenai Peninsula. A. J. Collier has mapped the geology of the Cape Lisburne region, and L. M. Prindle and F. L. Hess have made contributions to the knowledge of the metamorphic terranes of the Yukon-Tanana district. It is the writer's purpose to describe in nontechnical language the larger geographic features and discuss their relation as far as the data available will permit. In the treatment of the geology, however, less effort will be made to make the matter acceptable to the lay reader. It is hoped, however, that a brief summary of the salient features of the geologic history' may be not without interest to the general public. If this paper serves in some measure to dispel the popular fallacies regarding Alaska and to disseminate more accurate knowledge of its geographic and geologic features, the purpose of its publication will be accomplished.

Alaska↗
USGS2011

Stratigraphy and depositional environments of the upper Pleistocene Chemehuevi Formation along the lower Colorado River

The Chemehuevi Formation forms a conspicuous, widespread, and correlative set of nonmarine sediments lining the valleys of the Colorado River and several of its larger tributaries in the Basin and Range geologic province. These sediments have been examined by geologists since J. S. Newberry visited the region in 1857 and are widely cited in the geologic literature; however their origin remains unresolved and their stratigraphic context has been confused by inconsistent nomenclature and by conflicting interpretations of their origin. This is one of the most prominent stratigraphic units along the river below the Grand Canyon, and the formation records an important event or set of events in the history of the Colorado River. Here we summarize what is known about these deposits throughout their range, present new stratigraphic, sedimentologic, topographic, and tephrochronologic data, and formally define them as a lithostratigraphic unit. The Chemehuevi Formation consists primarily of a bluff-forming mud facies, consisting of gypsum-bearing, horizontally bedded sand, silt, and clay, and a slope-forming sand facies containing poorly bedded, well sorted, quartz rich sand and scattered gravel. The sedimentary characteristics and fossil assemblages of the two facies types suggest that they were deposited in flood plain and channel environments, respectively. In addition to these two primary facies, we identify three other mappable facies in the formation: a thick-bedded rhythmite facies, now drowned by Lake Mead; a valley-margin facies containing abundant locally derived sediment; and several tributary facies consisting of mixed fluvial and lacustrine deposits in the lower parts of major tributary valleys. Observations from the subsurface and at outcrops near the elevation of the modern flood plain suggest that the formation also contains a regional basal gravel member. Surveys of numerous outcrops using high-precision GPS demonstrate that although the sand facies commonly overlies the mud facies where the two are found together, contacts between the two occur over a range in elevation, and as a consequence, the sand and mud facies are similarly distributed both horizontally and vertically throughout the valley. Collectively, the outcrops of the formation lie below a smooth elevation envelope that slopes 50 percent more steeply than the historic (pre-Hoover Dam) valley, from nearly 150 m above the historic flood plain near the mouth of the Grand Canyon to less than 30 m above the flood plain at the head of the flood plain near Yuma, Arizona. The steepness of the valley at the peak of aggradation probably represents a depositional slope. Layers of fine grained volcanic tephra have been found below and within the Chemehuevi Formation at five widely separated sites, one of which is now submerged beneath Lake Mead. Major element geochemistry of glass shards from the four accessible tephra sites were analyzed. Three of the sampled tephra layers are interbedded within the Chemehuevi Formation, and a fourth tephra conformably underlies the formation. The three interbedded tephra layers are similar enough to one another that they are probably from the same eruptive unit, hereafter referred to as the Monkey Rock tephra bed. The other sample, which locally underlies the formation, is similar enough to the Monkey Rock tephra bed to suggest it is from the same volcanic source area; however, it may not be from the same eruption, and thus may not be the same age. On the basis of the stratigraphic contexts of chemically similar tephra layers found elsewhere in the Basin and Range, we suspect that the source area is the Mammoth Mountain dome complex in Long Valley, east-central California. Two samples of proximal Mammoth Mountain pumice were analyzed and produced geochemical signatures similar to all four of the Chemehuevi Formation tephra, supporting Mammoth Mountain as a possible source area. The Mammoth Mountain volcanic center produced eruptions between about 111±2 and 57±2 ka and was most active in the later part of this time interval, during Marine Oxygen Isotope (MOI) stage 4 (between 74 and 59 ka ago). Chemically similar tephra in cores from Owens Lake and Walker Lake are approximately 70 and 74 ky old, based on age models of those cores. Other lines of stratigraphic evidence from nine tephra-containing sections in the Basin and Range are also consistent with an age assignment for the Monkey Rock tephra of ~72 ky, near the beginning of MOI stage 4. We propose to designate the Chemehuevi Formation as a formal lithostratigraphic unit, and propose as the type section a well exposed outcrop near the ranger station at Katherine Landing, Arizona, in the Lake Mead National Recreation Area. This exposure shows the two dominant facies, an example of one of the four known tephra layers, and interbedded lenses of locally derived gravel. In the type section, as in many of the other examples of the formation, the sand facies overlies the mud facies on a conspicuous, abrupt erosional surface; however, nearby is a contiguous section demonstrating that the mud and sand facies interfinger. In addition to the type section, measured reference sections compiled here illustrate other important lithologic and stratigraphic features of the formation. Our preferred interpretation of the Chemehuevi Formation is that it contains the remnants of deposits formed during a single major episode of fluvial aggradation, during which the Colorado River filled its valley with a great volume of dominantly sand-size sediment. This would reflect an increase in the supply of sand-size sediment, and(or) a reduction in transport capacity below the mouth of Grand Canyon. The most likely cause for the aggradation is an extraordinary increase in sand supply, likely due to widespread climatic change. However, other explanations have not been ruled out. Other aggradation events predated the Chemehuevi Formation, and some smaller events may have postdated the formation. However, the Chemehuevi Formation contains the remnants of the most recent large magnitude (>100 m) aggradation of the Colorado River.

California;Nevada;Arizona↗
USGS1995

Geologic framework of a transect of the central Brooks Range: Regional relations and an alternative to the Endicott Mountains allochthon

This paper evaluates the geologic framework and tectonic development of the central Brooks Range based on a transect through the range and Arctic foothills. A geologic cross section constructed through the transect is confirmed by comparing the retrodeformed section with the regional distribution of lithofacies in the central Brooks Range. Stratigraphic relations in the retrodeformed section are further explained by comparing them to similar stratigraphic relations in the Ikpikpuk-Umiat basin under the Arctic coastal plain. The structural framework of the central Brooks Range and Arctic foothills consists of fold nappes, thrust faults, and detached folds that sole in decollements and late-stage high-angle faults. In the central Brooks Range, shortening is by north-directed thrust faulting and folding of mostly Paleozoic rocks, and transport of any individual thrust sheet relative to underlying rocks is less than 30 km. In the middle of the range, imbricate blocks of lower Paleozoic basement are exposed in the core of the Doonerak anticline, and thrust sheets of stratigraphically higher Paleozoic rocks that overlie basement are exposed in the limbs of the anticline. In the northeast part of the anticline, the Amawk thrust emplaces Silurian and Upper Devonian rocks on a succession of Lower Mississippian an stratigraphically higher rocks that have been detached from the underlying basement along the Blarney Creek thrust. The Slatepile fault system, a system of high-angle faults in the north limb of the Doonerak anticline, drops the core and part of the north limb of the anticline down, giving the impression that the succession of Lower Mississippian and stratigraphically higher rocks that lie on basement south of the system high-angle faults extends under the Upper Devonian rocks that extensively crop out north of the high-angle faults. In the Arctic foothills, the mostly Paleozoic rocks of the north-central Brooks Range extend under Lower Cretaceous rocks of the North Slope foreland basin, and blind thrusts that sole in the Paleozoic rocks ramp up into the Lower Cretaceous and stratigraph cally higher rocks. Also in the Arctic foothills, a thrust sheet that contains the Arctic foothills assemblage overlies rocks of the north-central Brooks Range and Lower Cretaceous rocks of the North Slope foreland basin. Thrust transport of the Arctic foothills assemblage more than 40 km from south of the Doonerak anticline took place during the Early Cretaceous, but thrusting that deformed rocks of the North Slope foreland basin took place during the early Tertiary, with the vertical uplift of the Doonerak anticline being a late-formed feature. Conclusions based on the retrodeformed cross section contrast significantly with previous work in which the Upper Devonian and stratigraphically higher rocks north of the Doonerak anticline are considered part of the Endicott Mountains allochthon, a regional allochthon that extends the breadth of the Brooks Range. In these models, Upper Devonian and younger rocks in the north-central Brooks Range have been thrust-transported 90 or 200 km from south of the Doonerak anticline, and emplacement of the allochthon could reflect as much as 885 km of tectonic shortening. The Lower Mississippian and stratigraphically higher rocks together with the underlying basement in the northeast part of the anticline are considered to be in a window in the Endicott Mountains allochthon and to extend northward beneath allochthonous Upper Devonian and stratigraphically higher rocks in the north-central Brooks Range. Lithofacies patterns in rocks in the central Brooks Range are consistent with the retrodeformed cross section and imply plausible Upper Devonian and Carboniferous depositional systems. Thick Upper Devonian and Lower Mississippian(?) clastic prisms were deposited in basins north of the Doonerak anticline. Mississippian carbonate rocks that overlie these clastic prisms were deposited in differentially subsiding shelf environments that included rocks in the Doonerak anticline. Restored across the Blarney Creek thrust, the Mississippian shelf carbonate rocks that presently lie north of the Doonerak anticline are those that were deposited on basement in the anticline. A carbonate ramp at the south edge of these shelf deposits extends east-southeast across the central Brooks Range and in th retrodeformed section lies south of the Doonerak anticline where Upper and Middle(?) Devonian shaly rocks thicken to the south. Unrestored, the ramp would extend across the Doonerak anticline. Restored Late Devonian and Carboniferous lithofacies patterns in the central Brooks Range also are plausible from a regional perspective and have implications for exploration of basins under the Arctic coastal plain. The Late Devonian to Early Mississippian(?) basins in the north-central Brooks Range are part of a system of Early(?) Devonian to Early Mississippian(?) clastic basins that extend the length of the Brooks Range and include basins under the Arctic coastal plain. These basins are a template for depositional patterns in overlying rocks. Marine shelves between these basins where Mississippian strata unconformably lie on basement, such as in the northeastern Brooks Range and the Doonerak anticline, have depositional histories that are in contrast to areas that overlie the basi s. The resulting stratigraphic framework, together with the structural framework in the basement rocks that controlled the basins, has had a profound effect on the structural style of the fold belt, the salient effect being folds and thrust faults that are not orthogonal to the direction of structural transport. Stratigraphic relations exposed in the fold belt, especially the distribution of potential source rocks, likely model little-explored basins that underlie the North Slope foreland basin.

Alaska↗
USGS1965

Ordovician, Silurian, and Devonian biostratigraphy of east-central Alaska

A predominantly shale and chert sequence has been mapped from the Canadian border at Latitude 65°00′ N. to the Nation River about 25 miles northwest. It has Ordovician and Silurian graptolites in the lower half, and Middle Devonian corals and Upper Devonian spores in the upper half. The lower half of the sequence consists of about 400 to 900 feet of predominantly dark gray graptolitic shale with lesser amounts of grayish black laminated chert. This unit has been traced by Green and Roddick from the international border southeastward into the Dawson area of the Yukon Territory and northeastward into the Ogilvie and Richardson Mountains where it has been mapped by Jackson & Lenz, Norris, and others as the Road River Formation. In the Tatonduk-Nation Rivers area the Road River Formation rests disconformably on a sequence of Middle and Upper Cambrian limestone that locally may include Lower Ordovician (Tremadoc) beds at its top. The sequence of graptolite faunas from the Road River Formation in Alaska is essentially the same as the graptolite zones established by Elles and Wood in Great Britain. All the standard series of the Ordovician and Silurian are probably represented in the formation with the exception of the Tremadoc and possibly the Arenig and Wenlock. Overlying the Road River Formation is a formation, 200 to 800 feet thick, consisting predominantly of thinly bedded and laminated light gray to black chert and siliceous shale that is here named the McCann Hill Chert. This new formation disconformably overlies the Road River Formation, and its lower boundary is at the base of a distinctive limestone and shale member. The basal limestone and shale member of the McCann Hill Chert contains a remarkably varied fauna including corals, brachiopods, trilobites, conodonts, ostracods, and fish of Eifelian (early Middle Devonian) age. Spores from the upper part of the formation resemble those from the overlying Nation River Formation and indicate a Late Devonian age. The McCann Hill Chert is conformably overlain by the Nation River Formation—a thick succession of inter-bedded graywacke, chert conglomerate, and silty shale of Late Devonian age. In the vicinity of Jones Ridge, only 7 miles north of exposures of the Road River Formation, Upper Cambrian limestone is overlain by a pure limestone section containing Ordovician, Middle Devonian, and perhaps Silurian shelly faunas contemporaneous with the Road River Formation and the thinner basal limestone and shale member of the McCann Hill Chert. The rapid change from graptolitic shale to limestone probably reflects changes in conditions of sedimentation, but could be a result of thrust faulting. The Tatonduk-Nation Rivers area seems to lie in a transitional zone between an early Paleozoic eugeosynclinal belt south of the Yukon River and a contemporaneous miogeosynclinal belt along the Porcupine River. One puzzling feature of the lower Paleozoic sequence in the Tatonduk-Nation Rivers area is its thinness in contrast to the much thicker volcanic and detrital sequence on the south and the considerably thicker carbonate sequence on the north.

Alaska↗
USGS2017

U.S. Geological Survey Karst Interest Group Proceedings, San Antonio, Texas, May 16–18, 2017

Introduction and Acknowledgments Karst aquifer systems are present throughout parts of the United States and some of its territories, and have developed in carbonate rocks (primarily limestone and dolomite) and evaporites (gypsum, anhydrite, and halite) that span an interval of time encompassing more than 550 million years. The depositional environments, diagenetic processes, post-depositional tectonic events, and geochemical weathering processes that form karst aquifers are varied and complex. These factors involve biological, chemical, and physical changes that when combined with the diverse climatic regimes in which karst development has taken place, result in the unique dual- or triple-porosity nature of karst aquifers. These complex hydrogeologic systems typically represent challenging and unique conditions to scientists attempting to study groundwater flow and contaminant transport in these terrains. The dissolution of carbonate rocks and the subsequent development of distinct and beautiful landscapes, caverns, and springs have resulted in the most exceptional karst areas being designated as national or state parks. Tens of thousands of similar areas in the United States have been developed into commercial caverns and known privately owned caves. Both public and private properties provide access for scientists to study the flow of groundwater in situ . Likewise, the range and complexity of landforms and groundwater flow systems associated with karst terrains are enormous, perhaps more than for any other aquifer type. Karst aquifers and landscapes that form in tropical areas, such as the cockpit karst along the north coast of Puerto Rico, differ greatly from karst landforms in more arid climates, such as the Edwards Plateau in west-central Texas or the Guadalupe Mountains near Carlsbad, New Mexico, where hypogenic processes have played a major role in speleogenesis. Many of these public and private lands also contain unique flora and fauna associated with these karst hydrogeologic systems. As a result, numerous federal, state, and local agencies have a strong interest in the study of karst terrains. Many of the major springs and aquifers in the United States have developed in carbonate rocks, such as the Floridan aquifer system in Florida and parts of Alabama, Georgia, and South Carolina; the Ozark Plateaus aquifer system in parts of Arkansas, Kansas, Missouri, and Oklahoma; and the Edwards-Trinity aquifer system in west-central Texas. These aquifers, and the springs that discharge from them, serve as major water-supply sources and form unique ecological habitats. Competition for the water resources of karst aquifers is common, and urban development and the lack of attenuation of contaminants in karst areas due to dissolution features that form direct pathways into karst aquifers can impact the ecosystem and water quality associated with these aquifers. The concept for developing a platform for interaction among scientists within the U.S. Geological Survey (USGS) working on karst-related studies evolved from the November 1999 National Groundwater Meeting of the USGS. As a result, the Karst Interest Group (KIG) was formed in 2000. The KIG is a loose-knit, grass-roots organization of USGS and non-USGS scientists and researchers devoted to fostering better communication among scientists working on, or interested in, karst science. The primary mission of the KIG is to encourage and support interdisciplinary collaboration and technology transfer among scientists working in karst areas. Additionally, the KIG encourages collaborative studies between the different mission areas of the USGS as well as with other federal and state agencies, and with researchers from academia and institutes. To accomplish its mission, the KIG has organized a series of workshops that have been held near nationally important karst areas. To date (2017) seven KIG workshops, including the workshop documented in this report, have been held. The workshops typically include oral and poster sessions on selected karst-related topics and research, as well as field trips to local karst areas. To increase non-USGS participation an effort was made for the workshops to be held at a university or institute beginning with the fourth workshop. Proceedings of the workshops are published by the USGS and are available online at the USGS publications warehouse https://pubs.er.usgs.gov/ by using the search term “karst interest group.” The first KIG workshop was held in St. Petersburg, Florida, in 2001, in the vicinity of the large springs and other karst features of the Floridan aquifer system. The second KIG workshop was held in 2002, in Shepherdstown, West Virginia, in proximity to the carbonate aquifers of the northern Shenandoah Valley, and highlighted an invited presentation on karst literature by the late Barry F. Beck of P.E. LaMoreaux and Associates. The third KIG workshop was held in 2005, in Rapid City, South Dakota, near evaporite karst features in limestones of the Madison Group in the Black Hills of South Dakota. The Rapid City KIG workshop included field trips to Wind Cave National Park and Jewel Cave National Monument, and featured a presentation by Thomas Casadevall, then USGS Central Region Director, on the status of Earth science at the USGS. The fourth KIG workshop in 2008 was hosted by the Hoffman Environmental Research Institute and Center for Cave and Karst Studies at Western Kentucky University in Bowling Green, Kentucky, near Mammoth Cave National Park and karst features of the Chester Upland and Pennyroyal Plateau. The workshop featured a late-night field trip into Mammoth Cave led by Rickard Toomey and Rick Olsen, National Park Service. The fifth KIG workshop in 2011 was a joint meeting of the USGS KIG and University of Arkansas HydroDays, hosted by the Department of Geosciences at the University of Arkansas in Fayetteville. The workshop featured an outstanding field trip to the unique karst terrain along the Buffalo National River in the southern Ozarks, and a keynote presentation on paleokarst in the United States was delivered by Art and Peggy Palmer. The sixth KIG workshop was hosted by the National Cave and Karst Research Institute (NCKRI) in 2014, in Carlsbad, New Mexico. George Veni, Director of the NCKRI, served as a co-chair of the workshop with Eve Kuniansky of the USGS. The workshop featured speaker Dr. Penelope Boston, Director of Cave and Karst Studies at New Mexico Tech, Socorro, and Academic Director at the NCKRI, who addressed the future of karst research. The field trip on evaporite karst of the lower Pecos Valley was led by Lewis Land (NCKRI karst hydrologist), and the field trip on the geology of Carlsbad Caverns National Park was led by George Veni. This current seventh KIG workshop is being held in San Antonio at the University of Texas at San Antonio (UTSA). This 2017 workshop is being hosted by the Department of Geological Sciences’ Student Geological Society (SGS), and student chapters of the American Association of Petroleum Geologists (AAPG) and Association of Engineering Geologists (AEG), with support by the UTSA Department of Geological Sciences and Center for Water Research. The UTSA student chapter presidents, Jose Silvestre (SGS), John Cooper (AAPG), and Tyler Mead (AEG) serve as co-chairs of the 2017 workshop with Eve Kuniansky of the USGS. The technical session committee is chaired by Eve Kuniansky, USGS, and includes Michael Bradley, Tom Byl, Rebecca Lambert, John Lane, and James Kaufmann, all USGS, and Patrick Tucci, retired USGS. The logistics committee includes Amy Clark, Yongli Gao, and Lance Lambert (Department Chair), UTSA Department of Geological Sciences; and Ryan Banta and Allan Clark, USGS, San Antonio, Texas. The field trip committee is chaired by Allan Clark and includes Amy Clark, Yongli Gao, and Keith Muehlestein, UTSA; Marcus Gary, Edwards Aquifer Authority and University of Texas at Austin; Ron Green, Southwest Research Institute; Geary Schindel, Edwards Aquifer Authority; and George Veni, NCKRI. Additionally, two organizations have assisted the UTSA student chapters in hosting the meeting by donating funds to the chapters: the Edwards Aquifer Authority, San Antonio, Texas, and the Barton Springs Edwards Aquifer Authority, Austin, Texas. Additionally, Yongli Gao, Center for Water Research and Department of Geological Sciences, UTSA, helped develop sessions on cave and karst research in China for this workshop. These proceedings could not have been accomplished without the assistance of Lawrence E. Spangler as co-editor who not only has subject matter expertise, but also serves as an editor with the USGS Science Publishing Network. We sincerely hope that this workshop continues to promote future collaboration among scientists of varied and diverse backgrounds, and improves our understanding of karst aquifer systems in the United States and its territories. The extended abstracts of USGS authors were peer reviewed and approved for publication by the USGS. Articles submitted by university researchers and other federal and state agencies did not go through the formal USGS peer review and approval process, and therefore may not adhere to USGS editorial standards or stratigraphic nomenclature. However, all articles had a minimum of two peer reviews and were edited for consistency of appearance in the proceedings. The use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The USGS Water Availability and Use Science Program funded the publication costs of the proceedings.

Scientific Investigations Report↗