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USGS1972

Regional and other general factors bearing on evaluation of earthquake and other geologic hazards to coastal communities of southeastern Alaska

The great Alaska earthquake of March 27, 1964, brought into sharp focus the need for engineering geologic studies in seismically active regions. As a result, nine communities in southeastern Alaska were selected for reconnaissance investigations as an integral part of an overall program to evaluate earthquake and other geologic hazards in most of the larger Alaska coastal communities. This report gives background information on the regional and other general factors that bear on these evaluations. Southeastern Alaska, about 525 miles long and averaging about 125 miles in width, consists of a narrow mainland strip and numerous islands. For the most part, it is a region of rugged relief with numerous glaciers capping many of the higher mountainous areas and with long linear fiords forming the inland waterways. A maritime climate prevails with mild winters and cool summers. The southeastern part of the region receives the highest precipitation in the continental United States. Ketchikan, with a population of 6,994 in 1970, is the largest city. Geology and structure of the area are complex. Igneous, metamorphic, and sedimentary rocks crop out and range in age from Paleozoic to Tertiary. Surficial deposits of Pleistocene and Holocene age mantle many areas. All of southeastern Alaska, except probably the highest peaks, was covered by glacier ice advances of late Pleistocene age. Major deglaciation was well advanced by 10,000 years ago--a time which approximately marks the end of the Pleistocene and the beginning of the Holocene. There followed a period of warm climate called the Hypsithermal, which in southeastern Alaska began 7,000-8,000 years ago and ended about 4,800-3,500 years ago. Glaciers in most places receded back of their present positions. The Hypsithermal was followed by an interval (termed Neoglaciation) of cooler climate and resurgence of glacier ice which continues to the present, although most glaciers are now rapidly receding. During the past 10,000 years worldwide sea level has risen about 100 feet, but during the past 4,000 years it has risen only about 10 feet or about 0.03 inch per year. With sea level used as a datum, the amount of sea-level rise must be added to the apparent uplift of land for the time under consideration to determine the actual amount of land uplift. The widespread presence of emergent marine deposits, several hundred feet above sea level, demonstrates that the land in southeastern Alaska has been uplifted since the last major deglaciation. The greatest known has been uplifted since the last major deglaciation. The greatest known uplift is in the vicinity of Juneau where glaciomarine deposits are present 750 feet above present sea level. Part of southeastern Alaska is presently undergoing one of the most rapid rates of uplift of any place in the world. The fastest emergence is occurring in the Glacier Bay area where the land is being uplifted relative to sea level approximately 3.9 cm per year. Most or all of the uplift appears to be due to rebound as a result of deglaciation. Southeastern Alaska lies within the circum-Pacific earthquake belt, one of the world's greatest zones of seismic activity. During historic time, there have been five earthquakes in the region with magnitudes of 8 or greater, three with magnitudes of 7 to 8, eight with magnitudes of 6 to 7, more than 15 with magnitudes of 5 to 6, and about 140 recorded earthquakes with magnitudes smaller than 5 or of unassigned magnitudes. All of the earthquakes with magnitudes 8 or greater, and a large proportion of the others, appear to be related to the active Fairweather- Queen Charlotte Islands fault system or its western extension, the Chugach-St. Elias fault. Earthquake epicenters on the Denali fault system, the other major fault system in southeastern Alaska, are few in comparison. However, because high microearthquake activity has been recorded recently on this system and earthquakes of moderate size have occurred on some of its segments, the Denali fault system probably should not be dismissed as a relict fault system of no current tectonic importance. There are numerous other known faults, as well as lineaments that may be faults of varying degrees of tectonic activity in southeastern Alaska, adjacent Canada, and eastern Alaska. One of these elements is the Totschunda fault system, which connects with the Denali fault system in eastern Alaska; it has been very active during Holocene time but few historical earthquake epicenters appear to be related to it. Both historical seismicity and geologic conditions, such as frequency and recency of faulting, must be considered together to permit an assessment of the future earthquake probability of an area. Data are too few for both factors for an accurate evaluation to be made of earthquake probability in southeastern Alaska. However, information compiled in the form of strain-release and seismic-zone maps permit some generalizations. Thus, it is tentatively concluded that most, if not all, of southeastern Alaska should be placed in seismic zone 3, a zone in which earthquakes of magnitude greater than 6 will occur from time to time and where there may be major damage to manmade structures. Inferred effects from future earthquakes in southeastern Alaska include: (1) surface displacement along faults and other tectonic land-level changes, (2) ground shaking, (3) compaction, (4) liquefaction in cohesionless materials, (5) reaction of sensitive and quick clays, (6) water-sediment ejection and associated subsidence and ground fracturing, (7) earthquake-induced sub aerial slides and slumps, (8) earthquake induced subaqueous slides, (9) effects on glaciers and related features, (10) effects on ground water and stream flow, and (11) tsunamis, seiches, and other abnormal water waves. Because of the reconnaissance nature of our studies in the coastal communities and the sparsity of laboratory data on physical properties of geologic units in each area studied, the inferred effects must be largely empirical and generalized. Therefore, the inferences are based in large part upon the effects of past major earthquakes in Alaska and elsewhere, particularly upon the well-documented effects of the Alaska earthquake of March 27, 1964. Buildings, highways, bridges, tunnels, harbor facilities, pipelines, canals, and other manmade structures may be severely damaged or destroyed by fault displacement or related tectonic land-level changes in southeastern Alaska. Direct damage from fault rupture would be restricted virtually to structures built directly athwart the fault. In California and Nevada, fault rupture almost always accompanies shocks of magnitude 6.5 or greater. The Alaska earthquake of March 27, 1964, and the Chilean earthquake of May 22, 1960, dramatically illustrated the severe adverse effects that can result from uplift or subsidence over a wide area. The variable most responsible for the degree of shaking at any epicentral distance is the type of ground. Generally, shaking is considerably greater in poorly consolidated deposits than in hard bedrock, particularly if the deposits are water saturated. Severe shaking of alluvial deposits and manmade fill, with resultant heavy damage, is well documented from the records of many past earthquakes. Damage commonly has been heavy as a result of ground settlement caused by compaction of loose sediments by shaking during an earthquake. This has been especially true where compaction was accompanied by tectonic downdrop of land, such as occurred during the Chilean earthquake of 1960 and the Alaska earthquake of 1964. Loosely emplaced manmade fill, deltaic deposits, beach deposits, and alluvial deposits may be susceptible to compaction in southeastern Alaska during a severe earthquake. Liquefaction of sand and silt is a fairly common effect of large earthquakes. It was well illustrated at Niigata, Japan, during the earthquake of June 16, 1964, and resulted in extensive damage. When part of a sloping soil mass liquefies, the entire mass can undergo catastrophic failure and can flow as a high-density liquid. In southeastern Alaska, deltaic deposits probably would be most susceptible to liquefaction. Sensitive and quick clays, which lose a considerable part of their strength when shaken, commonly fail during an earthquake and become rapid earthflows. Extensive studies were made of the sensitivity of the Bootlegger Cove Clay at Anchorage because of the marked loss of shear strength and dramatic failures of the deposits during the Alaska earthquake of 1964. If similar sensitive clays are present in some places in southeastern Alaska, they most likely are in some of the emergent fine-grained marine deposits; supporting data to confirm their presence, however, are largely lacking. Records of some 50 major earthquakes show that in at least half of the instances water and sediment have been ejected from surficial deposits Water-sediment ejection and associated subsidence and ground fracturing commonly cause extensive damage to the works of man. Ejecta may fill basements and other low-lying parts of buildings. Agricultural land can be covered with a blanket of infertile soils, and small ponds can be filled or made shallow. In southeastern Alaska these phenomena are most likely to occur on valley floors, deltas, tidal flats, alluvial fans, swamps, and lakeshores. Earthquake-induced sliding on land generally is confined to steep slopes but may take place in fine-grained deposits on moderately to nearly flat surfaces if the deposits are subject to liquefaction. A large rockslide triggered by the Lituya Bay, Alaska, earthquake of July 10, 1958, generated a wave that surged up the opposite wall of the inlet to a record height of 1,740 feet. During the Hebgen Lake, Montana, earthquake of August 17, 1959, a spectacular rockslide plunged into the Madison River canyon, buried 28 people, dammed the river, and created a large lake. Earthquake-records are replete with accounts of sliding of surficial deposits during moderate to large earthquakes. Most or all of the general factors that favor subaerial landsliding are present in southeastern Alaska. Earthquake-induced subaqueous slides can produce adverse effects both nearshore and some distance offshore. Nearshore sliding may progress shoreward and destroy harbor facilities and other structures, commonly with substantial loss of life. Disastrous large submarine slides occurred along the fronts of deltas in Seward and Valdez during the Alaska earthquake of 1964. In similar fashion, the largest submarine slides in southeastern Alaska likely will be triggered along the larger delta fronts. Sliding farther offshore can constitute a threat to navigation because of changes in water depths. Also underwater sliding can break communication cables. Glaciers were not greatly affected by the Alaska earthquake of 1964 despite the fact that about 20 percent of the area that underwent strong shaking is covered by ice. In contrast, the cataclysmic avalanche of ice and rock that fell from a high glacier-covered peak in Peru during the earthquake of May 31, 1970, produced devastating effects downvalley on man and his works in the form of mudflows. Most towns in southeastern Alaska are sufficiently distant from glaciers so as not be to directly affected. Both the Alaska earthquake of 1964 and the Hebgen Lake, Montana, earthquake of 1959 significantly affected ground- and surface-water regimens. Water levels in some wells declined whereas in others flow increased. Some springs discharged at a rate three times as much as normal; flow of others decreased or stopped. Discharge of many streams increased markedly. Most or all of the effects described above could occur in parts of southeastern Alaska during future large earthquakes. Tsunamis, seiches, and other abnormal water waves associated with large earthquakes commonly cause vast property damage and heavy loss of life. Tsunami effects can be devastating to coastal areas as far as many thousands of miles from their generation source. Seiche effects generally are confined to inland bodies of water or to relatively enclosed coastal bodies of water. Abnormal waves generated by submarine sliding or by subaerial sliding into water generally produce only local effects but may be highly devastating. Tsunami waves resulting from the Chilean earthquake of 1960 inflicted extensive damage and loss of life on coastal communities throughout a large part of southern Chile, and significant runups and damage were recorded in many places throughout the Pacific Ocean area. The tsunami waves generated by the Alaska earthquake of 1964 struck with devastating force along a broad stretch of the Alaska coast and produced heavy property damage and loss of life as far away as Crescent City, Calif. Seiche waves generated by that earthquake reached runup heights of 20-30 feet on some lakes in Alaska, and water-level fluctuations were recorded on streams, reservoirs, lakes, and swimming pools in States bordering the Gulf of Mexico. Waves generated by submarine sliding struck violently at a number of places during or immediately after the quake and were the major cause of loss of life and damage to property. Slide-generated waves probably would have a higher destructive potential in southeastern Alaska than either tsunami waves or seiche waves because of their possibly higher local runups and because they can hit the shores almost without warning during or immediately after an earthquake. Nonearthquake-related geologic hazards, although generally far less dramatic than those related to earthquakes, tend to occur so much more frequently or persistently that their aggregate effects can be significant. Three kinds of geologic hazards of this type are discussed: (1) nonearthquake-induced landsliding and subaqueous sliding, (2) flooding, and (3) land uplift. The potential for nonearthquake-triggered landsliding in southeastern Alaska ranges widely from place to place. Past sliding generally furnishes the clue in the prediction of where and in what materials future sliding will occur. Fast-moving rockslides, debris slides, and mudflows can be expected to occur from time to time on steep slopes and be highly destructive to highways, power plants, pipelines, buildings, and other facilities located on a slope or at its base. Present slow downslope movement of talus can be expected to continue at the same general rate unless conditions are changed by man or there are climatic changes. Snow and debris avalanches can be especially hazardous during winter months. Long-inactive landslides may be triggered into renewed activity or new slides may be created by man-induced modifications. Accelerated slope erosion and debris flows may follow large-scale clearing and cutting of timber. Subaqueous sliding can be expected to occur periodically along fronts of deltas and on other oversteepened underwater slopes. Floods have been common in parts of southeastern Alaska because of heavy precipitation and rapid runoff from steep slopes with resulting heavy damage to roads and other facilities. Continued damage can be expected in the future unless more remedial measures are taken. Current uplift of land in southeastern Alaska, although probably not affecting man significantly in a short period of time, may have some adverse long-term effects. These long-term effects should be borne in mind when facilities such as docks and boat harbors are constructed on or near the shore, where there is a critical relation between height of land and water.

Alaska↗
USGS2026

Preliminary bedrock geologic map of the Port Henry quadrangle, Essex County, New York, and Addison County, Vermont

Introduction The bedrock geology of the 7.5-minute Port Henry quadrangle consists of deformed and metamorphosed Mesoproterozoic gneisses of the Adirondack Highlands unconformably overlain by weakly deformed lower Paleozoic sedimentary rocks of the Champlain Valley. The Mesoproterozoic rocks occur on the eastern edge of the Adirondack Highlands and represent an extension of the Grenville Province of Laurentia. Mesoproterozoic paragneiss, marble, and amphibolite hosted the emplacement of an anorthosite-mangerite-charnockite-granite (AMCG) suite, now exposed mostly as orthogneiss, at approximately 1.18–1.15 Ga (giga-annum). In the Port Henry quadrangle, the AMCG metaigneous rocks (Yhg, Ygb, Yanw) intruded older, mostly metasedimentary rocks of the Grenville Complex during the middle to late Shawinigan orogeny (~1,160–1,150 Ma [mega-annum]). All rocks were subsequently metamorphosed to upper amphibolite to granulite facies conditions during the 1,080–1,050 Ma Ottawan orogeny. New mapping reveals four periods of deformation: (1) D1 produced rarely preserved isoclinal folds in the paragneiss and marble and predates AMCG magmatism. (2) Subsequent D2 deformation produced the dominant gneissic fabric preserved in the rock, recumbent folding, and deformed all the Proterozoic units in the map area. Syn- to late-D2 felsic magmatism resulted in the regionally extensive Lyon Mountain Granite Gneiss, which hosts numerous magnetite ore bodies. (3) Mylonitic extensional shear zones and core complex formation marked the beginning of D3 deformation. Protracted D3 deformation resulted in F3 upright folding, dome and basin formation, pegmatite intrusion, reactivation of the S2 foliation, partial melting, metamorphism, metasomatism, iron-ore remobilization, and intrusion of magnetite-bearing pegmatite both as layer-parallel sills and crosscutting dikes. (4) D4 created northeast- and northwest-trending local high-grade ductile shear zones and boudinage, northwest-trending regional kilometer (km)-wide ductile shear zones, and crosscutting granitic pegmatite dikes. The development of the late-stage regional shear zones (D4) was likely due to the continuation of extensional doming and uplift from upper amphibolite facies conditions at the end of the Ottawan orogeny. The majority of iron-ore deposits in the Port Henry and adjacent Witherbee quadrangles are in the hanging wall of these extensional shear zones. In the Port Henry quadrangle, the km-wide Cheney Mountain shear zone is the result of D4 deformation. Kilometer-scale lineaments readily observed in lidar data are Ediacaran mafic dikes and Phanerozoic brittle faults. The Paleozoic rocks are part of the Early Cambrian to Late Ordovician carbonate bank on the ancient margin of Laurentia. The approximately 1-km-thick Cambrian to Ordovician stratigraphy records a transition from synrift clastics to passive-margin peritidal carbonate buildups to gradually deeper-water subtidal- to shelf-carbonates during foreland basin development associated with the Taconic orogeny. The Paleozoic rocks are weakly folded and block faulted. Large areas of the Champlain Valley are covered by undifferentiated glacial deposits, some of which contain mapped landslides. The map also shows waste rock piles and tailings from historical mining operations. This study was undertaken to improve our understanding of the bedrock geology in the Adirondack Highlands, establish a modern framework for 1:24,000-scale bedrock geologic mapping in the Adirondacks, provide a context for historical iron mines in the eastern Adirondacks, and update the stratigraphy of the Champlain Valley in New York and Vermont. This Open-File Report includes a bedrock geologic map; a description of map units; a correlation of map units; and a geographic information system database that includes bedrock geologic units, faults, outcrops, and structural geologic information.

New York, Vermont↗
USGS1982

Sedimentation and deformation in the Amlia Fracture Zone sector of the Aleutian Trench

A wedge-shaped, landward thickening mass of sedimentary deposits composed chiefly of terrigenous turbidite beds underlies the west-south west-trending Amlia sector (172°20′–173°30′W) of the Aleutian Trench. Pacific oceanic crust dips northward beneath the sector's sedimentary wedge and obliquely underthrusts (30° off normal) the adjacent Aleutian Ridge. The trench floor and subsurface strata dip gently northward toward the base of the inner trench slope. The dip of the trench deposits increases downsection from about 0.2° at the trench floor to as much as 6–7° just above basement. The wedge is typically 2–2.5 km thick, but it is thickest (3.7–4.0 km) near the base of the inner slope overlying the north-trending Amlia Fracture Zone and also east of this structure. Slight undulations and relatively abrupt offsets of the trench floor reflect subsurface and generally west-trending structures within the wedge that are superimposed above ridges and swales in the underlying oceanic basement. The southern or seaward side of some of these structures are bordered by high-angle faults or abrupt flexures. Across these offsets the northern side of the trench floor and underlying wedge is typically upthrown. West-flowing turbidity currents originating along the Alaskan segment of the trench (1200 km to the east) probably formed the greater part of the Amlia wedge during the past 0.5 m.y. The gentle northward or cross-trench inclination of the trench floor and underlying wedge probably reflects regional downbending of the oceanic lithosphere and trench-floor basement faulting and rotation. Much of the undulatory flexuring of the trench wedge can be attributed to differential compaction over buried basement relief. However, abrupt structural offsets attest to basement faulting. Faulting is associated with extensional earthquakes in the upper crust. The west-trending basement offsets are probably normal faults that dip steeply south or antithetic to the north dip of the subducting oceanic crust. Up-to-arc extensional faulting can be attributed to the downbending of the Pacific plate into the Aleutian subduction zone. The rupturing direction and dip is controlled by zones of crustal weakness that parallel north Pacific magnetic anomalies, which were formed south of a late Cretaceous—early Tertiary spreading center (Kula—Pacific Ridge). The strike of these anomalies is fortuitously nearly parallel to the Amlia sector. The up-to-arc fracturing style may locally assist in elevating blocks of trench deposits to form the toe of the trench's landward slope, which is in part underlain by a compressionally thickened accretionary mass of older trench deposits. Compressional structures that can be related to underthrusting are only indistinctly recorded in the turbidite wedge that underlies the trench floor.

Alaska↗
USGS1964

Ground-water resources of the South Platte River Basin in western Adams and southwestern Weld Counties, Colorado

The area described in this report consists of about 970 square miles in western Adams and southwestern Weld Counties in northeastern Colorado. It includes that part of the South Platte River valley between Denver and Kuner, Colo., all of Beebe Draw, and the lower part of the valley of Box Elder Creek. The stream-valley lowlands are separated by rolling uplands. The climate is semiarid, the normal annual precipitation being about 13 inches; thus, irrigation is essential for stable agricultural development. The area contains about 220,000 acres of irrigated land in the stream valleys. Most of the remaining 400,000 acres of land is used for dry farming or grazing because it lacks irrigation water. Most of the lowlands were brought under irrigation with surface water during the early 1900's, and now nearly all the surface water in the area is appropriated for irrigation within and downstream from the area. Because the natural flow of the streams is sometimes less than the demand for water, ground water is used to supplement the surface-water supply. Wells, drilled chiefly since 1930, supply the supplemental water and in some places are the sole supply for irrigation use. Rocks exposed in the area are of sedimentary origin and range in age from Lato Cretaceous to Recent. Those that are consolidated, called 'bedrock' in this report, consist of the Fox Hills sandstone and the Laramie and Arapahoe formations, all of Late Cretaceous age, and the Denver formation and Dawson arkose of Late Cretaceous and Tertiary age. The surface of the bedrock was shaped by ancestral streams, the valleys of which are reflected by the present surface topography. Dune sand, slope wash, and thin upland deposits of Quaternary age mantle the bedrock in the divide areas, and stream deposits ranging in thickness from 0 to about 125 feet partly fill the ancestral valleys. The valley-fill deposits consist of beds and lenses of clay, silt, sand, gravel, cobbles, and boulders. Abundant supplies of ground water for irrigation, municipal, and industrial use are obtained in the principal stream valleys from wells tapping valley-fill deposits beneath the flood plain and bordering terraces. Many domestic and stock wells obtain water from the unconsolidated deposits both on the uplands and in the valleys. The ground water in the valley-fill deposits generally is unconfined but in a few places is under slight artesian pressure. The bedrock formations yield small to moderate supplies of water to municipal, industrial, domestic, and stock wells, but the yields are not sufficient for irrigation. Ground water in the South Platte River valley moves downstream and toward the river and is discharged into the river. The direction of ground-water movement in Beebe Draw and Box Elder Creek valley is nearly parallel to the streams. Beebe Seep, the stream in Beebe Draw, gains water from the groundwater reservoir in some reaches and loses water in others, but Box Elder Creek loses water to the ground-water reservoir throughout its course especially during floods. The shape and slope of the water table are affected chiefly by the permeability of the valley-fill deposits, the location and altitude of the areas of recharge and discharge, and the configuration of the underlying bedrock floor. The depth to water in the South Platte River valley ranges from less than 1 foot beneath the flood plain to as much as 80 feet beneath the terraces. In Beebe Draw the depth to water ranges from less than 1 foot to about 60 feet and in Box Elder Creek valley from about 5 feet to about 40 feet. During the period of record the annual fluctuation of water levels in wells in the area has ranged from 2 to 13 feet. Precipitation within the area and infiltrating water from irrigated tracts, reservoirs, canals, and streams are the principal sources of recharge to the ground-water reservoir; some recharge results from underflow from outside the area. Ground water is discharged by evapotranspiratio

Water Supply Paper↗
USGS2004

Paleozoic sedimentary rocks in the Red Dog Zn-Pb-Ag district and vicinity, western Brooks Range, Alaska: provenance, deposition, and metallogenic significance

Geochemical analyses of Paleozoic sedimentary rocks in the western Brooks Range reveal a complex evolutionary history for strata surrounding the large Zn-Pb-Ag deposits of the Red Dog district. Data for major elements, trace elements, and rare earth elements (REE) were obtained on 220 samples of unaltered and unmineralized siliciclastic rocks from the Upper Devonian-Lower Mississippian Endicott Group (Hunt Fork Shale, Noatak Sandstone, Kanayut Conglomerate, Kayak Shale), the overlying Carboniferous Lisburne Group (Kuna Formation, unnamed drowned shelf facies), and the Pennsylvanian-Permian Siksikpuk Formation. Major base metal sulfide deposits of the region are present only in the Kuna Formation, which in the Red Dog district comprises siliceous black shale and black chert, minor limestone (calcareous radiolarite), and sparse lithic turbidite and bedded siliceous rock. Gray and rare black shales of the Kayak Shale and common black shales of the Kuna Formation are anomalously low in iron (avg Fe/Ti = 6.25 and 6.34, respectively) relative to other Paleozoic shales in the region (9.58-10.6) and to average shales worldwide (10.1-10.5). In contrast, the bedded siliceous rocks contain appreciable hematite (avg Fe/Ti = 35.0) and high U/Ti and REE/Ti ratios that are interpreted to reflect low amounts of detrital material and a major Fe-rich eolian component. Geochemical data (e.g., MnO <0.01 wt %; avg Cr = 317 ppm), sizes of framboidal pyrite grains, and limited bioturbation suggest anoxic and denitrifying depositional conditions for most black shales of the Kuna Formation; low Mo/Ti ratios argue against euxinic (sulfate-reducing) conditions. Organic-rich black shales of the Kuna Formation with up to 8.4 wt percent C organic and gray to black shales of the Kayak Shale with up to 4.1 wt percent C organic typically have only sparse pyrite (<1 wt % S) and very low iron-limited S/C ratios (mostly <0.2). Immobile element plots (e.g., Th-Zr/10-Sc) suggest that source terranes for all of the formations were dominated by one or more felsic-rich continental arcs; a small proportion of recycled sediments is present locally. A minor mafic igneous component also occurs in several shales of the Kuna and Siksikpuk Formations. High average values for the chemical index of alteration [Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O)] &infin; 100 for shales of the Endicott Group (76.4-81.5) imply moderate to intense chemical weathering in source areas of these sediments. A lower average for black shales of the Kuna Formation (73.7) does not require such weathering. Textural and geochemical data record the effects of diagenetic and/or hydrothermal fluid flow in some of the Paleozoic rocks. Mobility of P, F, U, and light REE is documented in black shales of the Kuna Formation by phosphate replacements of carbonate clasts and of matrix material surrounding the clasts. A relatively low average Ce/Ce* value of 0.73 for P-poor black shales of the Kuna Formation ( < 0.05 wt % P 2 O 5 ) and a similar Ce/Ce* value of 0.78 for a siderite concretion in Kayak Shale suggest that these diagenetic fluids were oxidizing. Many shales of the Kuna Formation have high (K 2 O &infin; 100)/(K 2 O + Al 2 O 3 ) ratios of 21.0 to 23.0, which contrast with low ratios of generally <18.0 for shales of the underlying Endicott Group. The high ratios in shales of the Kuna Formation reflect preferential reaction of smectite to illite during the Jurassic-Cretaceous Brookian orogeny, owing to high silica activities in pore fluids that were generated by the dissolution of abundant biogenic silica. The distribution and composition of Paleozoic strata in the western Brooks Range may have played a fundamental role in Zn-Pb mineralization of the Red Dog district. In our model, deposition and early lithification of biogenic chert and bedded siliceous rocks in the upper part of the Kuna Formation served as a regional hydrologic seal, acting as a cap rock to heat and hydrothermal fluids during Late Mississippian base-metal mineralization. Equally important was the iron-poor composition of black shales of the Kuna Formation (i.e., low Fe/Ti ratios), which limited synsedimentary pyrite formation in precursor sediments, resulting in significant H 2 S production in pore waters through the interaction of aqueous sulfate with abundant organic matter. This H 2 S may have been critical to the subsurface deposition of the huge quantities of Zn and Pb in the district. On the basis of this model, we propose that low Fe/Ti and S/C ratios in black shale sequences are potential basin-scale exploration guides for giant sediment-hosted, stratiform Zn-Pb-Ag deposits.

Alaska↗
USGS1978

The marine geology of the eastern Santa Barbara Channel, with particular emphasis on the ground water basins offshore from the Oxnard Plain, Southern California

Marine geophysical investigations provide new data concerning the stratigraphy, tectonic and sedimentary history, and the ground water geology of the southeastern Santa Barbara Channel region. The offshore stratigraphy identified in seismic reflection profiles includes a succession of Neogene to Quaternary strata. The middle Miocene Conejo volcanics form an acoustical basement and the overlying late Cenozoic sedimentary rocks attain a thickness greater than 2,500 m. These sedimentary deposits fill a structurally controlled, physiographic and depositional depression called the Ventura Basin. Structure consists generally of a gently folded, east-trending Tertiary synclinorium bordered on the north by a regional thrust fault and on the south by a steep asymmetrical anticlinal ridge. Most structures show evidence of north-south compression that occurred during early Pleistocene time. Three well-defined unconformities represent widespread erosion in late Miocene, early to middle Pleistocene, and late Pleistocene time. The boundaries of Miocene, Pliocene, and lower Pleistocene strata continue uninterrupted eastward along the southern part of Santa Barbara basin to Hueneme Canyon, where they turn northeast and can be traced to the coast near Port Hueneme. These limits probably represent the south edge of the Santa Barbara basin during Pliocene and Pleistocene time. Fresh water-bearing materials of the Oxnard plain are unconsolidated Quaternary sediment laid down on more consolidated Tertiary rocks. Offshore, the total fresh water-bearing materials distinguished in the seismic reflection profiles attain a thickness of about 356 m and have an areal extent of over 760 km2. Strata that contain the offshore continuation of the five major on-land aquifers (Grimes Canyon, Fox Canyon, Hueneme, Mugu, and Oxnard aquifers) are identified in the seismic reflection profiles. These strata make up the two offshore ground-water basins, the Mound and Oxnard plain ground-water basins, which are separated by the east-west trending Oak Ridge fault. Possible entrance areas for salt water intrusion into fresh water aquifers are found along the walls of the submarine canyons and along the northern slopes of Santa Barbara and Santa Monica basins. Hueneme and Mugu aquifers are probably exposed locally in all five submarine canyons of the Oxnard offshore area and may also crop out along the upper northern slope of Santa Monica basin. In all of these areas, salt water readily intrudes the aquifers. A salinity-temperature-depth study made in April, 1971, does not indicate any great dilution of surface ocean water by fresh water that could be 'leaking' from the exposed aquifers along the walls of Hueneme Canyon and the landward slope of Santa Barbara Channel. Earthquakes in the vicinity of the Oxnard plain suggest that the region is seismically active. Epicenters are widely dispersed over the region. No distinct trend or alignment of earthquake epicenters occurs near the trace of any of the faults, although many epicenters are scattered around the Oak Ridge zone of deformation in the northern part of the region. The largest magnitude earthquake recorded in the area was a magnitude 5.7 that occurred on February 21, 1973, offshore of Point Mugu, south of the Oxnard plain.

California↗
USGS1942

General geology and ground-water resources of the island of Maui, Hawaii

Maui, the second largest island in the Hawaiian group, is 48 miles long, 26 miles wide, and covers 728 square miles. The principal town is Wailuku. Sugar cane and pineapples are the principal crops. Water is used chiefly for irrigating cane. The purpose of the investigation was to study the geology and the ground-water resources of the island. Maui was built by two volcanoes. East Maui or Haleakala Volcano is 10,025 feet high and famous for its so-called crater, which is a section of Hawaii National Park. Evidence is given to show that it is the head of two amphitheater-headed valleys in which numerous secondary eruptions have occurred and that it is not a crater, caldera, or eroded caldera. West Maui is a deeply dissected volcano 5,788 feet high. The flat Isthmus connecting the two volcanoes was made by lavas from East Maui banking against the West Maui Mountains. Plate 1 shows the geology, wells, springs, and water-development tunnels. Plate 2 is a map and description of points of geologic interest along the main highways. Volcanic terms used in the report are briefly defined. A synopsis of the climate is included and a record of the annual rainfall at all stations is given also. Puu Kukui, on West Maui, has an average annual rainfall of 389 inches and it lies just six miles from Olowalu where only 2 inches of rain fell in 1928, the lowest ever recorded in the Hawaiian Islands. The second rainiest place in the Territory is Kuhiwa Gulch on East Maui where 523 inches fell during 1937. Rainfall averages 2,360 million gallons daily on East Maui and 580 on West Maui. Ground water at the point of use in months of low rainfall is worth about $120 per million gallons, which makes most undeveloped supplies valuable. The oldest rocks on East Maui are the very permeable primitive Honomanu basalts, which were extruded probably in Pliocene and early Pleistocene time from three rift zones. These rocks form a dome about 8,000 feet high and extend an unknown distance below sea level. Covering this dome are the Kula volcanics, extruded probably in early and middle Pleistocene time, and characterized by andesites, andesitic basalts, and picritic basalts. They are 2.000 feet thick on the summit and 50 to 200 feet thick at the periphery. They contain a sufficient number of interbedded soils, thin vitric tuff beds, and lava-filled valleys in their upper part to give rise to valuable perched springs in wet areas. The Kula lavas accumulated during a waning volcanic phase which was followed by a quiescence long enough for the erosion of deep amphitheater-headed valleys in the east or wet half of the mountain. Volcanic activity was renewed in middle (?) to late Pleistocene time and continued until Recent time, during which the Hana volcanic series was laid down. The last lava flow was erupted about 1750. The Hana lavas comprise andesitic, picritic, and olivine basalts. They veneered large areas of the east and south slopes, partly filled the deep amphitheater-headed valleys, and deeply buried the smaller valleys in the eastern half of the mountain. The Hana rocks are exceedingly permeable and much rain sinks into them. The oldest rocks on West Maui are the very permeable primitive Wailuku basalts, which were extruded probably in Pliocene and early Pleistocene time from two rifts and from many radial fissures. The basalts form a dome about 5,600 feet high and extend an unknown distance below sea level. Iao Valley is the eroded caldera of this dome. Forming an incomplete veneer over the dome are the Honolua soda trachytes and oligoclase andesites. They were extruded in late Pliocene (?) or early Pleistocene time, chiefly from bulbous domes. The clinker beds carry some water but the rocks are generally too dense to be good aquifers. During early (?) Pleistocene the West Maui volcano was cut by deep amphitheater-headed valleys and then all of Maui was deeply submerged. Four scattered eruptions occurred on West Maui in middle (?) and late Pleistocene time. The cones and lavas cover only small areas and are called the Lahaina volcanic series. The sedimentary rocks of both East and West Maui are chiefly late Quaternary and comprise fans, landslide debris, delta deposits, and valley fills, mostly of poorly permeable and poorly assorted bouldery alluvium. They are overlain on the Isthmus by extensive calcareous dunes of three ages. A mud flow more than 300 feet thick is exposed in Kaupo Valley. During the fluctuations of the ocean in the Pleistocene, the island was emerged and submerged several times. Calcareous fossiliferous marine conglomerates deposited during this period are found up to an altitude of 250 feet on West Maui. The Homomanu, Wailuku, and Kula lavas are the chief aquifers. They supply 28 irrigation wells which yield an average of 170 million gallons a day of basal water. These wells are mine-like shafts with infiltration tunnels and are called Maui-type wells. Well 16 yields 40,000,000 gallons daily with a 22-foot drawdown, which is the largest amount yielded by any well in the Hawaiian Islands. The largest spring (no. 26) on the island is artesian. It yields 10,400,000 gallons daily and issues from Kula lavas near Nahiku. West Maui has numerous perennial streams supplied by springs from a dike complex. Twenty-three tunnels in West Maui recover 20.5 million gallons a day of high-level water, mostly from this dike complex. East Maui has few perennial streams in proportion to its size, and they are chiefly small due to the water sheds being underlain with permeable lavas. Forty tunnels recover 6 million gallons a day of high-level water in East Maui and all from structures other than dikes. It is estimated that about 100 million gallons a day of basal water wastes into the sea from West Maui and about 700 million gallons a day from East Maui. A number of sites are described where wells could be sunk to recover this water. Sites are also described where tunnels could be driven to recover high-level supplies. The hydrology of East and West Maui is conspicuously different in many respects, mainly because of the difference in the stage of dissection, the extensive veneer of very permeable Hann lavas on East Maui, and the comparatively small area of the Lahaina lavas of similar age on West Maui. The only thermal water known in the Hawaiian Islands, except on the active volcano of Kilauea, is in a well in West Maui. The Nahiku area has been mapped and studied in detail. The upper part of the Honomanu volcanic series, exposed in the sea cliffs, in petrographic character is transitional into the overlying Kula lavas, Kula and Hana time were characterized by a long succession of valley-cutting episodes, each valley being filled by lava erupted from the east rift zone. The lavas include olivine basalts, picritic basalts, and basaltic andesites, In the Nahiku area basal ground water occurs largely in the Honomanu basalts. Perched water occurs in many of the later lavas, generally following the axes of buried valleys. The members which perch the water are mostly ashy soil beds, although an unusually extensive, thick layer of much decomposed clinker also appears to be a supporting member. Most of the water travels through the basal clinker members of aa lavas. Artesian water is encountered in the upper, transitional part of the Honomanu volcanic series. The aquifer is permeable porphyritic pahoehoe; the confining members are relatively impermeable nonporphyritic aa. The lavas of East Maui are described according to stratigraphic groups. The oldest or Honomanu lavas are olivine basalts like the primitive lavas in other Hawaiian volcanoes. The later or Kula and Hana lavas include basalts, basaltic andesites, andesites, and picritic basalts. The normative nepheline of analyzed East Maui lavas has not been identified in the mode. The degree of differentiation is inversely proportional to the frequency of eruptions. The lavas of West Maui volcano are divided into the Wailuku volcanic series, consisting largely of olivine basalts with less abundant olivine-poor basalts, hypersthene basalts, and picritic basalts; the Honolua volcanic series, consisting of oligoclase andesites and soda trachytes; and the Lahaina volcanic series, consisting of nepheline basanite and picritic basalts. Coarse-grained gabbros intrude the Wailuku lavas. Differentiation was undoubtedly partly by crystal settling, but the alkali curves of the variation diagram suggest that volatile transfer was of some importance.

Hawaii↗
USGS1963

Ground-water geology of Bexar County, Texas

The investigation in Bexar County was part of a comprehensive study of a large area in south-central Texas underlain by the Edwards and associated limestones (Comanche Peak and Georgetown) of Cretaceous age. The limestones form an aquifer which supplies water to the city of San Antonio, several military installations, many industrial plants, and many irrigated farms. The geologic formations that yield water to wells in Bexar County are sedimentary rocks of Mesozoic and Cenozoic age. The rocks strike northeastward and dip southeastward toward the Gulf of Mexico. In the northern part of the county, in an erosional remnant of the Edwards Plateau, the rocks are nearly flat and free from faulting. In the central and southern parts of the county, however, the rocks dip gulfward at gentle to moderately steep angles and are extensively faulted in the Balcones and Mexia fault zones. Individual faults or shatter zones were traced as much as 25 miles; the maximum displacement is at least 600 feet. In general, the formations are either monoclinal or slightly folded; in the western part of the county the broad Culebra anticline plunges southwestward. Most of the large-capacity wells in Bexar County draw water from the Edwards and associated limestones, but a few draw from the Glen Rose limestone, the Austin chalk, and surficial sand and gravel. The Hosston formation, Glen Rose limestone, Buda limestone, and Austin chalk, all of Cretaceous age, generally yield small to large supplies of water; the Wilcox group and Carrizo sand of Tertiary age yield moderate supplies and alluvium of Pleistocene and Recent age generally yield small supplies. The Edwards and associated limestones are recharged primarily by groundwater underflow into Bexar County from the west, and secondarily by seepage from streams that cross the outcrop of the aquifer in Bexar County. During the period 1934-47 the recharge to the aquifer in Bexar County is estimated to have averaged between 400,000 and 430,000 acre-feet per year. Discharge from the aquifer takes place by means of wells and springs and by underflow into Comal and Guadalupe Counties on the northeast. During the period 1934-47 the estimated average discharge from wells and springs was about 174,000 acre-feet per year. The discharge by underflow out of the county during the same period is estimated to have averaged between 220,000 and 260,000 acre-feet per year. Probably only a small amount of water moves downdip southeast of San Antonio. The presence of highly mineralized water in that area suggests that the circulation of water is poor because of the low permeability of the aquifer. During the period 1934-56 the discharge from the Edwards and associated limestones greatly exceeded the recharge; consequently, water levels in wells declined. The decline was greatest in the northwestern part of the county, where the water levels in wells dropped as much as 100 feet. The decline was progressively less toward the east, averaging 40 feet along the Bexar-Comal County line. The area of the greatest concentration of discharge, which includes San Antonio and extends to the southwest and northeast, coincides with the area of maximum faulting and maximum recorded yields from wells and is not the area of greatest decline. The ability of the Edwards and associated limestones to transmit and store water in the San Antonio area apparently is so great that the discharge from wells results in much smaller declines of water level than do similar or even smaller discharges in other areas. The water from the Edwards is almost uniformly a calcium bicarbonate water of good quality, although hard. In the southern part of the San Antonio area the water is charged with hydrogen sulfide; farther downdip it becomes highly mineralized.

Water Supply Paper↗
USGS1953

Ground-water conditions in the Milwaukee-Waukesha area, Wisconsin

Three major aquifers underlie the Milwaukee-Waukesha area: sandstones of Cambrian and Ordovician age, Niagara dolomite of Silurian age, and sand and gravel deposits of Pleistocene age. The Maquoketa shale of Ordovician age acts as a more or less effective seal between the Pleistocene deposits and Niagara dolomite above and the sandstone aquifer below. Crystalline rocks of pre-Cambrian age form an impermeable basement complex below the Paleozoic sedimentary rocks. The Paleozoic strata dip east at 25 to 30 feet to the mile. There is no evidence that any of the faults and folds known or surmised to be present acts as a barrier to the movement of ground water. The bedrock surface underlying the Milwaukee-Waukesha area is formed by the Niagara dolomite except in northeastern Milwaukee County, where it is dolomite and shale of the Milwaukee formation of Devonian age, and in some of the preglacial valleys where the uppermost bedrock formation is the Maquoketa shale. Unconsolidated deposits of glacial drift cover the surface of the bedrock almost completely. Buried valleys in the bedrock surface contain as much as 150 feet of water-bearing sand and gravel deposits. These deposits, along with the Niagara dolomite, could be developed as sources of water supplemental to that of the sandstone aquifer. The Niagara dolomite is an important aquifer; it yields water from crevices and solution channels. The sandstone aquifer consists of the Galena dolomite, the Platteville limestone, and the St. Peter sandstone of Ordovician age, and the Eau Claire and Mount Simon sandstones of Cambrian age. The St. Peter and Mount Simon are the most productive formations but the others supply some water to wells. Static water levels in the shallow aquifers have not declined to any great extent except in downtown Milwaukee, where a 50-foot-deep cone of depression exists. Static water levels in the sandstone aquifer declined as much as 350 feet in the Milwaukee area from 1880 through 1950. Recharge to the two shallow aquifers occurs locally from precipitation within the area. The major recharge area of the sandstone aquifer lies about 25 miles west of Wauwatosa. Some ground water is recharged to the sandstone aquifer locally by leakage from the Niagara dolomite, mostly through deep uncased wells. Ground water is the source of six municipal supplies and many industrial and commercial supplies in the area. Pumpage from deep wells has increased from about 2.5 mgd in 1900 to about 25 mgd in 1949, of which 6.0 mgd was derived from the shallow aquifers. The total withdrawal from the shallow aquifers, including the 6.0 mgd from the deep wells, was about 19.5 mgd in 1949. Thus the total in 1949 was 38.5 mgd for the area as a whole. Coefficients of transmissibility and storage for the sandstone aquifer were obtained by means of controlled pumping tests at Wauwatosa, Waukesha, Greendale, Town of Lake, and Jefferson. The coefficients were used to calculate the amount of water recharged from the Niagara dolomite to the sandstone aquifer in the area. About 5.5 mgd of the 23 mgd pumped from deep wells in 1950 was supplied locally by leakage from the Niagara dolomite. The coefficients, with corrections for boundaries, were also applied to past records of pumpage to calculate the water-level decline in the sandstone aquifer at Wauwatosa from 1880 through 1950. The calculated decline was 317 feet, and the actual decline was 307 feet. The future water-level decline in the sandstone aquifer at Wauwatosa resulting from estimated future pumping conditions was computed. If the pumpage in the area as ?. whole increases from about 23 mgd in 1950 to about 28 mgd in 1960, the water levels at Wauwatosa will decline as much as 65 feet more by 1960. It is estimated that the available recharge to the sandstone aquifer underlying the Milwaukee-Waukesha area is approximately 60 mgd. It is recommended that new deep wells be located to the west toward the recharge area and that the shallow aquifers be used as auxiliary sources to void excessive lowering of water levels in the sandstone aquifers. Conservation should be practiced by all users of ground water to avoid waste resulting in lower water levels and higher pumping costs.

Wisconsin↗
USGS2006

Hydrostratigraphic characterization of intergranular and secondary porosity in part of the Cambrian sandstone aquifer system of the cratonic interior of North America: Improving predictability of hydrogeologic properties

The Upper Cambrian interval of strata in the cratonic interior of North America has a long history of inconsistent hydrogeologic classification and a reputation for marked and unpredictable variability in hydraulic properties. We employed a hydrostratigraphic approach that requires hydraulic data to be interpreted within the context of a detailed characterization of the distribution of porosity and permeability to arrive at a better understanding of these rocks. As a first step, we constructed a framework of hydrostratigraphic attributes that is a depiction of the spatial distribution of both rock matrix and secondary porosity, independent of hydraulic data such as pumping-test results. The locations of hundreds of borehole geophysical logs and laboratory measurements of rock sample matrix porosity and permeability were mapped on detailed (mostly 1:100,000 or greater), conventional, lithostratigraphic maps. Stratigraphic cross-sections, based on hundreds of natural gamma logs and thousands of water-well records, have provided a markedly improved depiction of the regional distribution of rock matrix hydrostratigraphic components. Borehole, core and outcrop observations of secondary porosity were also tied to detailed stratigraphic sections and interpolated regionally. As a second step, we compiled and conducted a large number of hydraulic tests (e.g., packer tests and borehole flowmeter logs) and analyzed thousands of specific capacity tests (converted to hydraulic conductivity). Interpretation of these data within the context of the hydrostratigraphic attributes allowed us to produce a new hydrogeologic characterization for this stratigraphic interval and gain important insights into geologic controls on hydraulic variability. There are a number of assumptions in herent in most previous hydrogeologic investigations of these strata, such as equivalency of lithostratigraphic and hydrogeologic units and the dominance of intergranular flow in sandstone, that are not consistent with our results. A particularly important outcome of our study is recognition of regionally extensive bedding-plane fracture clusters. Such exceptionally high hydraulic conductivity features dominate the hydraulics of aquifers and confining units in these siliciclastic-dominated strata, including within intervals consisting largely of friable sandstone with high intergranular conductivity. Furthermore, our results provide some measure of fracture predictability, by correlating their abundance and hydraulic importance to specific stratigraphic positions and particular depths of burial beneath younger bedrock. A discrete, consistent stratigraphic interval of fine-grained siliciclastic beds also is apparently resistant to the development of vertically interconnected fractures, making the location of this regionally extensive confining unit predictable. Our more rigorous approach of interpreting typical hydraulic tests as well as relatively new techniques of borehole flowmeter logging, within the context of a hydrostratigraphic framework, results in improved definition of individual aquifers and confining units. It also enables quantification of their hydraulic properties, which leads to improved prediction of groundwater flow paths and time-of-travel. ?? 2005 Elsevier B.V. All rights reserved.

Sedimentary Geology↗
USGS2021

Groundwater hydrology and chemistry of Jamestown Island, Virginia—Potential effects of tides, storm surges, and sea-level rise on archaeological, cultural, and ecological resources

As the site of the first permanent English settlement in North America in 1607, Jamestown Island, Colonial National Historical Park (COLO), Virginia, contains a rich archaeological record that extends from the Paleoindian period (15,000 to 8,000 years ago) through the 20th century. The island is located on the lower James River near the mouth of Chesapeake Bay. Jamestown Island vegetation is dominated by upland forests surrounded by tidal, freshwater-to-oligohaline marshes. Along the Virginia coast, relative sea-level rise was more than 2.5 times the global average during the 20th century. Consequently, the National Park Service (NPS) has identified COLO as one of the 25 national parks most threatened by climate change. Surface waters across the island are hydraulically connected to the laterally continuous Surficial aquifer. The land-surface altitude of the island is low, with two-thirds of the island less than 5 feet (ft) above the North American Vertical Datum of 1988 (NAVD 88). Consequently, sea-level rise, combined with tides and storm surges, threatens the island and its resources as surface-water and groundwater levels rise, saltwater enters the Surficial aquifer, and groundwater chemistry changes. The impact of sea-level rise on the island’s surface-water resources has been well studied, but groundwater effects have been largely ignored. Quantifying the effects of tides, storm surges, and sea-level rise on groundwater levels and chemistry is essential to developing an effective strategy for managing climate-induced changes. The first step in developing a response strategy includes a parkwide general risk assessment for archaeological sites on the island, so that sites can be prioritized for management actions. The U.S. Geological Survey and the NPS began a study in 2015 to develop a long-term groundwater-monitoring program to evaluate this risk and to develop an updated management strategy. The groundwater-monitoring program consists of 45 wells and piezometers in two individual clusters and three transects across the island in different hydrologic and chemical settings. Samples for water quality were collected from the wells and piezometers from October 2015 through September 2018 at variable time intervals. Results of the monitoring identified disparate hydrologic and chemical responses to saltwater intrusion across the island. Specific conductance (an indicator of salinity) of groundwater beneath several marshes responded differently to changes in James River salinity. Groundwater response to changes in James River specific conductance appeared to be controlled by land-surface altitude and slope, differences in lateral and vertical sediment characteristics, distance from surface waters, and the degree of surface water/groundwater connectivity between channels and the aquifer. Groundwater chemistry data from monitoring wells at Black Point, a low-altitude, upland setting, are in contrast with conditions observed in Island House observation wells, a high-altitude, upland setting. Specific conductance (less than 200 microsiemens per centimeter [μS/cm]) and pH (greater than 5.0) of groundwater beneath much of the uplands that characterize the Island House observation wells are typical of groundwater in noncarbonate sedimentary aquifers recharged by precipitation. At Black Point, specific conductance ranged from 2,490 to 15,200 μS/cm, and pH ranged from 3.1 to 6.6 standard units. At the Black Point observation wells, the most saline and dense water was at the water table rather than deeper in the aquifer, causing a density inversion that persisted throughout the study. The density inversion likely resulted from differences in permeability between the shallow clay and fine-grained sands and the deeper coarse-grained sand and gravel. Groundwater with the lowest pH was at the water table. As saline groundwater flows through organic sediment beneath the marshes, bacterial biodegradation of organic matter creates anoxic conditions. Continued biodegradation concomitantly reduces iron-oxide minerals in the sediment and sulfate in saline water. When oxygen is reintroduced into groundwater, iron and sulfur can reoxidize to form sulfuric acid, locally lowering the pH of the water. This report describes the groundwater monitoring network design, rationale for site selection, monitoring approach, and results of monitoring from October 2015 through September 2018. Maps of inundation at selected water-level altitudes are included to identify the risk to archaeological, cultural, and ecological resources. The monitoring results of the hydrology and chemistry data are interpreted, and the different hydrologic and chemical settings are described. The implications of the study results for management decisions are presented, and suggestions for improving the monitoring network are included.

Virginia↗
USGS2016

Mineral thermometry and fluid inclusion studies of the Pea Ridge iron oxide-apatite–rare earth element deposit, Mesoproterozoic St. Francois Mountains Terrane, southeast Missouri, USA

Mineral thermometry and fluid inclusion studies were conducted on variably altered and mineralized samples from the Mesoproterozoic Pea Ridge iron oxide-apatite (IOA)-rare earth element (REE) deposit in order to constrain P-T conditions, fluid chemistry, and the source of salt and volatiles during early magnetite and later REE mineralization. Scanning electron microscopy (SEM)-cathodoluminescence and SEM-backscatter electron images show that quartz and rutile precipitated before, during, and after magnetite and REE mineral growth. Ti-in-quartz and Zr-in-rutile equilibration temperatures range from ≤350° to 750°C in the amphibole, magnetite, hematite, and silicified zones where T increased during magnetite and quartz growth and dropped precipitously after fracturing and brecciation. Late drusy quartz cements within a REE-rich breccia pipe record the lowest T (≤315°–400°C). Liquid-, vapor-rich, and hypersaline (±hematite, calcite) fluid inclusions are common and liquid CO 2 is present locally. Salinities define three populations: saline (10–27 wt % NaCl equiv), hypersaline (34–>60 wt % NaCl equiv), and dilute (0–10 wt % NaCl equiv ). The wide range of eutectic melting temperatures (−67° to −19°C) suggests that saline inclusions trapped variable proportions of a CaCl-MgCl-FeCl-bearing fluid end member and an NaCl-KCl fluid end member. Homogenization temperatures and pressures of these saline inclusions suggest they were trapped when fluids unmixed into brine and vapor at T <350°C, P <15 MPa, and a depth of ~1.5 km. Hypersaline inclusions were trapped at low T and P (~200°C and ~1 MPa) along the V + L + H curve when the system vented to the paleosurface. Data for dilute inclusions in late drusy quartz from the REE-rich breccia pipe are indicative of a boiling epithermal environment. The Na/Cl, Na/K, and Cl/Br ratios of fluid inclusion extracts provide evidence for mixtures of magmatic hydrothermal fluids and evaporated seawater. Extracts from magnetite, hematite, and pyrite plot in the magmatic-hydrothermal field, indicating that Fe was derived from a magmatic source. Their enrichments in Mg and Ca are consistent with a mafic magmatic source. The positive correlation between Na/Mg and Na/Ca ratios may be due to halite saturation or albitization of igneous rocks. Extracts from barite in the REE-rich breccia pipes are enriched in Na and Br and plot near the seawater evaporation trend. He is highly enriched relative to Ne and Ar in fluid inclusion extracts, which precludes air as a source of He. Although the He is mostly of crustal origin, pyrite with a 3 He/ 4 He (R/R A ) of 0.1 contains up to 12% mantle He. Many extracts have low 20 Ne/ 22 Ne ratios due to nucleogenic production of 22 Ne in high F/O minerals such as fluorapatite or F biotite. The arrays of data for 3 He/ 4 He (R/R A ) and 22 Ne/ 20 Ne suggest that volatiles were derived from two sources, a moderate F mafic magma containing mantle He and a high F silicic magma with crustal He. Together with other evidence cited in this report, these data (1) support a magmatic hydrothermal origin for the Mesoproterozoic magnetite-apatite deposit with ore fluids derived from a concealed mafic to intermediate-composition intrusion, (2) suggest that the REE minerals in breccia pipes were either derived from apatite or precipitated in response to decompression and cooling during breccia pipe formation, (3) provide evidence for the influx of basinal brine, magmatic fluids from granitic intrusions, and meteoric water after breccia pipe formation, and (4) show that Pea Ridge was relatively unaffected by the late Paleozoic Mississippi Valley-type (MVT) Pb-Zn system in overlying Cambrian sedimentary rocks.

Missouri↗
USGS2004

Two stages of deformation and fluid migration in the west-central Brooks Range fold and thrust belt, Northern Alaska

The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (~60 and ~45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (~250-300&deg;C) event that was buffered by limestone and a younger, lower temperature (~150&deg;C) event that had distinctly lower &delta; 13 C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160-120 Ma, whereas the lower temperature event probably occurred at about 60-45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by far-traveled allochthons with relatively low structural relief, because it involved a thin (1-4-km [0.6-2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5-10 km (3-6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100-90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Alaska↗
USGS1992

Map showing sediment isopachs in the deep-sea basins of the Pacific continental margin, Point Conception to Point Loma

The U.S. Geological Survey conducted a series of cruises, EEZSCAN 84 (EEZ-SCAN 84 Scientific Staff, 1986), to collect reconnaissance data on the newly proclaimed Exclusive Economic Zone (EEZ), the area out to 200 nautical miles from the coastline of the United States. The cruises systematically surveyed the entire conterminous United States west coast EEZ using the Geological Long-Range Inclined Asdic (GLORIA) side-scan sonar, a 160-in 3 airgun seismic-reflection profiler, a 3.5-kHz high-resolution seismic-reflection profiler, a 10-kHz echo sounder, and a proton-precession magnetometer. The nominal trackline spacing throughout the survey was 30 km. Derivative maps of sediment thickness (I-2089-A, I-2090-A, I-2091-A) and depth to basement (I-2089-B, I-2090-B, I-2091-B) in the basins of the west coast EEZ were compiled from both the sonar-image dala and the deep-penetration seismic data obtained on these cruises. Only EEZ-SCAN 84 data were used for the map compilations because available data from other cruises in this region are sparsely located, have poor navigational control, or were obtained with seismic systems that were not powerful enough to resolve oceanic basement. For this map series, sediment thickness and depth to basement were determined only in the deep-ocean basin regions because the seismic system used on the EEZ-SCAN 84 cruises could not resolve oceanic basement beneath the thick sediments of the continental slope. All the data used to compile the maps are presented in the "Atlas of the Exclusive Economic Zone, Western Conterminous United States" (EEZ-SCAN 84 Scientific Staff, 1986). Basement Outcrops GLORIA imagery was used to locate areas of basement outcrop throughout the region. Where possible, sediment thickness on and immediately adjacent to basement outcrops was measured from seismic data. Where there was no bathymetric or seismic control, a seamount peak was assumed to have no sediment cover. A pattern is used on the map to indicate all basement outcrops. Data Reduction Acoustic basement in the basins, invariably oceanic Layer 2, was observed on all of the seismic records. One-way traveltime was measured from the sea floor to acoustic basement. Because the trackline spacing of about 30 km is relatively large compared to the data density along track, we chose to measure the sediment thickness every 0.5 hour or at an interval of approximately 7.5 km. Water depth was measured with a 10-kHz profiler. Depth to basement was calculated using the sea surface as the zero datum and adding the corrected water depth (Carter, 1980) to the sediment thickness. Acoustic travel times were converted to depths by first calculating a regression equation from the interval velocity versus depth data of Connard and others (1984). Their data base comprises a compilation of all available Deep Sea Drilling Project data plus wide-angle refraction data, which were collected in Cascadia Basin west of Oregon and represents the best data set available for the United States west coast EEZ. The regression equation was integrated to determine sediment thickness as a function of one-way traveltime . The resulting equation is z = 1400t + 0 .5t 2 where z is sediment thickness in meters, and t is one-way traveltime in seconds. Sediment thicknesses calculated using this equation were compared to values calculated from the general equation of Carlson and others (1986). Values for sediment thickness calculated by the two equations differed by no greater than 10 percent throughout the range of travel times. Bathymetry The bathymetry is from Chase and others (1981). The bathymetric data were compiled from a variety of sources, and data quality is inconsistent. Because of differences in navigational precision and density of coverage, the bathymetric contours of a feature may be at a different location than the corresponding isopleths derived from the EEZ-SCAN 84 data, or a feature may not be indicated on the bathymetry at all. These situations occur because either the feature is poorly located in the bathymetric data set or, especially in the outer EEZ, the bathymetric data are too sparse to have defined the existence of a feature. Acknowledgments Topographic digital data bases were corrected and verified by Christina Lief. Gerald Evenden developed the computer software system MAPGEN, used to compose this map. Reviews, suggestions, and technical contributions from Edward C. Escowitz and Florence Wong and advice about cartographic design from Will Stettner substantially improved the quality of this map. References Cited Carlson, R.L., Gangi, A.F., and Snow, K.R., 1986, Empirical reflection-traveltime/ depth and velocity/depth functions for the deep-sea sediment column: Journal of Geophysical Research, v. 91, no. B8, p. 8249-8266. Carter, D.J.T., 1980, Echo-sounding correction tables: Taunton, United Kingdom, Hydrographic Department, Ministry of Defence, 150 p. Chase, T.E., Wilde, Pat, Normark, W.R, Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32&deg; and 49&deg; North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38&deg; latitude, 2 sheets. Connard, G., Couch, R., Keeling , K., Roy, J., and Troseth, S., 1984, Abyssal plain and continental net-objective sedimentary thicknesses, in Kulm, L.D., and others, eds., Western North America continental margin and adjacent ocean floor off Oregon and Washington, Atlas 1 of Regional Atlas Series, Ocean Margin Drilling Program: Woods Hole, Mass., Marine Science International, sheet 7. EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, Western Conterminous United States: U.S. Geological Survey Miscellaneous Investigations Series I-1792, scale 1:500,000, 152 p. index map for I-2089-A See Also "U.S. Pacific West Coast Field Activities" (Paskevich and others, 2011; http://pubs.usgs.gov/of/2010/1332/htmldocs/pc/pc_overview.html). Paskevich, V.F., Wong, F.L., O?Malley, J.J., Stevenson, A.J., and Gutmacher, C.E., 2011, GLORIA sidescan-sonar imagery for parts of the U.S. Exclusive Economic Zone and adjacent areas: U.S. Geological Survey Open-File Report 2010?1332, available at http://pubs.usgs.gov/of/2010/1332/.

IMAP↗
USGS1992

Map showing depth to basement in the deep-sea basins of the Pacific continental margin, Point Conception to Point Loma

The U.S. Geological Survey conducted a series of cruises, EEZSCAN 84 (EEZ-SCAN 84 Scientific Staff, 1986), to collect reconnaissance data on the newly proclaimed Exclusive Economic Zone (EEZ), the area out to 200 nautical miles from the coastline of the United States. The cruises systematically surveyed the entire conterminous United States west coast EEZ using the Geological Long-Range Inclined Asdic (GLORIA) side-scan sonar, a 160-in 3 airgun seismic-reflection profiler, a 3.5-kHz high-resolution seismic-reflection profiler, a 10-kHz echo sounder, and a proton-precession magnetometer. The nominal trackline spacing throughout the survey was 30 km. Derivative maps of sediment thickness (I-2089-A, I-2090-A, I-2091-A) and depth to basement (I-2089-B, I-2090-B, I-2091-B) in the basins of the west coast EEZ were compiled from both the sonar-image dala and the deep-penetration seismic data obtained on these cruises. Only EEZ-SCAN 84 data were used for the map compilations because available data from other cruises in this region are sparsely located, have poor navigational control, or were obtained with seismic systems that were not powerful enough to resolve oceanic basement. For this map series, sediment thickness and depth to basement were determined only in the deep-ocean basin regions because the seismic system used on the EEZ-SCAN 84 cruises could not resolve oceanic basement beneath the thick sediments of the continental slope. All the data used to compile the maps are presented in the "Atlas of the Exclusive Economic Zone, Western Conterminous United States" (EEZ-SCAN 84 Scientific Staff, 1986). Basement Outcrops GLORIA imagery was used to locate areas of basement outcrop throughout the region. Where possible, sediment thickness on and immediately adjacent to basement outcrops was measured from seismic data. Where there was no bathymetric or seismic control, a seamount peak was assumed to have no sediment cover. A pattern is used on the map to indicate all basement outcrops. Data Reduction Acoustic basement in the basins, invariably oceanic Layer 2, was observed on all of the seismic records. One-way traveltime was measured from the sea floor to acoustic basement. Because the trackline spacing of about 30 km is relatively large compared to the data density along track, we chose to measure the sediment thickness every 0.5 hour or at an interval of approximately 7.5 km. Water depth was measured with a 10-kHz profiler. Depth to basement was calculated using the sea surface as the zero datum and adding the corrected water depth (Carter, 1980) to the sediment thickness. Acoustic travel times were converted to depths by first calculating a regression equation from the interval velocity versus depth data of Connard and others (1984). Their data base comprises a compilation of all available Deep Sea Drilling Project data plus wide-angle refraction data, which were collected in Cascadia Basin west of Oregon and represents the best data set available for the United States west coast EEZ. The regression equation was integrated to determine sediment thickness as a function of one-way traveltime . The resulting equation is z = 1400t + 0 .5t 2 where z is sediment thickness in meters, and t is one-way traveltime in seconds. Sediment thicknesses calculated using this equation were compared to values calculated from the general equation of Carlson and others (1986). Values for sediment thickness calculated by the two equations differed by no greater than 10 percent throughout the range of travel times. Bathymetry The bathymetry is from Chase and others (1981). The bathymetric data were compiled from a variety of sources, and data quality is inconsistent. Because of differences in navigational precision and density of coverage, the bathymetric contours of a feature may be at a different location than the corresponding isopleths derived from the EEZ-SCAN 84 data, or a feature may not be indicated on the bathymetry at all. These situations occur because either the feature is poorly located in the bathymetric data set or, especially in the outer EEZ, the bathymetric data are too sparse to have defined the existence of a feature. Acknowledgments Topographic digital data bases were corrected and verified by Christina Lief. Gerald Evenden developed the computer software system MAPGEN, used to compose this map. Reviews, suggestions, and technical contributions from Edward C. Escowitz and Florence Wong and advice about cartographic design from Will Stettner substantially improved the quality of this map. References Cited Carlson, R.L., Gangi, A.F., and Snow, K.R., 1986, Empirical reflection-traveltime/ depth and velocity/depth functions for the deep-sea sediment column: Journal of Geophysical Research, v. 91, no. B8, p. 8249-8266. Carter, D.J.T., 1980, Echo-sounding correction tables: Taunton, United Kingdom, Hydrographic Department, Ministry of Defence, 150 p. Chase, T.E., Wilde, Pat, Normark, W.R, Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32&deg; and 49&deg; North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38&deg; latitude, 2 sheets. Connard, G., Couch, R., Keeling , K., Roy, J., and Troseth, S., 1984, Abyssal plain and continental net-objective sedimentary thicknesses, in Kulm, L.D., and others, eds., Western North America continental margin and adjacent ocean floor off Oregon and Washington, Atlas 1 of Regional Atlas Series, Ocean Margin Drilling Program: Woods Hole, Mass., Marine Science International, sheet 7. EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, Western Conterminous United States: U.S. Geological Survey Miscellaneous Investigations Series I-1792, scale 1:500,000, 152 p. index map for I-2089-A See Also "U.S. Pacific West Coast Field Activities" (Paskevich and others, 2011; http://pubs.usgs.gov/of/2010/1332/htmldocs/pc/pc_overview.html). Paskevich, V.F., Wong, F.L., O?Malley, J.J., Stevenson, A.J., and Gutmacher, C.E., 2011, GLORIA sidescan-sonar imagery for parts of the U.S. Exclusive Economic Zone and adjacent areas: U.S. Geological Survey Open-File Report 2010?1332, available at http://pubs.usgs.gov/of/2010/1332/.

IMAP↗
USGS1993

Map showing sediment isopachs in the deep-sea basins of the Pacific continental margin, Cape Mendocino to Point Conception

The U.S. Geological Survey conducted a series of cruises, EEZSCAN 84 (EEZ-SCAN 84 Scientific Staff, 1986), to collect reconnaissance data on the newly proclaimed Exclusive Economic Zone (EEZ), the area out to 200 nautical miles from the coastline of the United States. The cruises systematically surveyed the entire conterminous United States west coast EEZ using the Geological Long-Range Inclined Asdic (GLORIA) side-scan sonar, a 160-in 3 airgun seismic-reflection profiler, a 3.5-kHz high-resolution seismic-reflection profiler, a 10-kHz echo sounder, and a proton-precession magnetometer. The nominal trackline spacing throughout the survey was 30 km. Derivative maps of sediment thickness (I-2089-A, I-2090-A, I-2091-A) and depth to basement (I-2089-B, I-2090-B, I-2091-B) in the basins of the west coast EEZ were compiled from both the sonar-image dala and the deep-penetration seismic data obtained on these cruises. Only EEZ-SCAN 84 data were used for the map compilations because available data from other cruises in this region are sparsely located, have poor navigational control, or were obtained with seismic systems that were not powerful enough to resolve oceanic basement. For this map series, sediment thickness and depth to basement were determined only in the deep-ocean basin regions because the seismic system used on the EEZ-SCAN 84 cruises could not resolve oceanic basement beneath the thick sediments of the continental slope. All the data used to compile the maps are presented in the "Atlas of the Exclusive Economic Zone, Western Conterminous United States" (EEZ-SCAN 84 Scientific Staff, 1986). Basement Outcrops GLORIA imagery was used to locate areas of basement outcrop throughout the region. Where possible, sediment thickness on and immediately adjacent to basement outcrops was measured from seismic data. Where there was no bathymetric or seismic control, a seamount peak was assumed to have no sediment cover. A pattern is used on the map to indicate all basement outcrops. Data Reduction Acoustic basement in the basins, invariably oceanic Layer 2, was observed on all of the seismic records. One-way traveltime was measured from the sea floor to acoustic basement. Because the trackline spacing of about 30 km is relatively large compared to the data density along track, we chose to measure the sediment thickness every 0.5 hour or at an interval of approximately 7.5 km. Water depth was measured with a 10-kHz profiler. Depth to basement was calculated using the sea surface as the zero datum and adding the corrected water depth (Carter, 1980) to the sediment thickness. Acoustic travel times were converted to depths by first calculating a regression equation from the interval velocity versus depth data of Connard and others (1984). Their data base comprises a compilation of all available Deep Sea Drilling Project data plus wide-angle refraction data, which were collected in Cascadia Basin west of Oregon and represents the best data set available for the United States west coast EEZ. The regression equation was integrated to determine sediment thickness as a function of one-way traveltime . The resulting equation is z = 1400t + 0 .5t 2 where z is sediment thickness in meters, and t is one-way traveltime in seconds. Sediment thicknesses calculated using this equation were compared to values calculated from the general equation of Carlson and others (1986). Values for sediment thickness calculated by the two equations differed by no greater than 10 percent throughout the range of travel times. Bathymetry The bathymetry is from Chase and others (1981). The bathymetric data were compiled from a variety of sources, and data quality is inconsistent. Because of differences in navigational precision and density of coverage, the bathymetric contours of a feature may be at a different location than the corresponding isopleths derived from the EEZ-SCAN 84 data, or a feature may not be indicated on the bathymetry at all. These situations occur because either the feature is poorly located in the bathymetric data set or, especially in the outer EEZ, the bathymetric data are too sparse to have defined the existence of a feature. Acknowledgments Topographic digital data bases were corrected and verified by Christina Lief. Gerald Evenden developed the computer software system MAPGEN, used to compose this map. Reviews, suggestions, and technical contributions from Edward C. Escowitz and Florence Wong and advice about cartographic design from Will Stettner substantially improved the quality of this map. References Cited Carlson, R.L., Gangi, A.F., and Snow, K.R., 1986, Empirical reflection-traveltime/ depth and velocity/depth functions for the deep-sea sediment column: Journal of Geophysical Research, v. 91, no. B8, p. 8249-8266. Carter, D.J.T., 1980, Echo-sounding correction tables: Taunton, United Kingdom, Hydrographic Department, Ministry of Defence, 150 p. Chase, T.E., Wilde, Pat, Normark, W.R, Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32&deg; and 49&deg; North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38&deg; latitude, 2 sheets. Connard, G., Couch, R., Keeling , K., Roy, J., and Troseth, S., 1984, Abyssal plain and continental net-objective sedimentary thicknesses, in Kulm, L.D., and others, eds., Western North America continental margin and adjacent ocean floor off Oregon and Washington, Atlas 1 of Regional Atlas Series, Ocean Margin Drilling Program: Woods Hole, Mass., Marine Science International, sheet 7. EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, Western Conterminous United States: U.S. Geological Survey Miscellaneous Investigations Series I-1792, scale 1:500,000, 152 p. See Also "U.S. Pacific West Coast Field Activities" (Paskevich and others, 2011; http://pubs.usgs.gov/of/2010/1332/htmldocs/pc/pc_overview.html). Paskevich, V.F., Wong, F.L., O?Malley, J.J., Stevenson, A.J., and Gutmacher, C.E., 2011, GLORIA sidescan-sonar imagery for parts of the U.S. Exclusive Economic Zone and adjacent areas: U.S. Geological Survey Open-File Report 2010?1332, available at http://pubs.usgs.gov/of/2010/1332/.

IMAP↗
USGS1993

Map showing depth to basement in the deep-sea basins of the Pacific continental margin, Cape Mendocino to Point Conception

The U.S. Geological Survey conducted a series of cruises, EEZSCAN 84 (EEZ-SCAN 84 Scientific Staff, 1986), to collect reconnaissance data on the newly proclaimed Exclusive Economic Zone (EEZ), the area out to 200 nautical miles from the coastline of the United States. The cruises systematically surveyed the entire conterminous United States west coast EEZ using the Geological Long-Range Inclined Asdic (GLORIA) side-scan sonar, a 160-in 3 airgun seismic-reflection profiler, a 3.5-kHz high-resolution seismic-reflection profiler, a 10-kHz echo sounder, and a proton-precession magnetometer. The nominal trackline spacing throughout the survey was 30 km. Derivative maps of sediment thickness (I-2089-A, I-2090-A, I-2091-A) and depth to basement (I-2089-B, I-2090-B, I-2091-B) in the basins of the west coast EEZ were compiled from both the sonar-image dala and the deep-penetration seismic data obtained on these cruises. Only EEZ-SCAN 84 data were used for the map compilations because available data from other cruises in this region are sparsely located, have poor navigational control, or were obtained with seismic systems that were not powerful enough to resolve oceanic basement. For this map series, sediment thickness and depth to basement were determined only in the deep-ocean basin regions because the seismic system used on the EEZ-SCAN 84 cruises could not resolve oceanic basement beneath the thick sediments of the continental slope. All the data used to compile the maps are presented in the "Atlas of the Exclusive Economic Zone, Western Conterminous United States" (EEZ-SCAN 84 Scientific Staff, 1986). Basement Outcrops GLORIA imagery was used to locate areas of basement outcrop throughout the region. Where possible, sediment thickness on and immediately adjacent to basement outcrops was measured from seismic data. Where there was no bathymetric or seismic control, a seamount peak was assumed to have no sediment cover. A pattern is used on the map to indicate all basement outcrops. Data Reduction Acoustic basement in the basins, invariably oceanic Layer 2, was observed on all of the seismic records. One-way traveltime was measured from the sea floor to acoustic basement. Because the trackline spacing of about 30 km is relatively large compared to the data density along track, we chose to measure the sediment thickness every 0.5 hour or at an interval of approximately 7.5 km. Water depth was measured with a 10-kHz profiler. Depth to basement was calculated using the sea surface as the zero datum and adding the corrected water depth (Carter, 1980) to the sediment thickness. Acoustic travel times were converted to depths by first calculating a regression equation from the interval velocity versus depth data of Connard and others (1984). Their data base comprises a compilation of all available Deep Sea Drilling Project data plus wide-angle refraction data, which were collected in Cascadia Basin west of Oregon and represents the best data set available for the United States west coast EEZ. The regression equation was integrated to determine sediment thickness as a function of one-way traveltime . The resulting equation is z = 1400t + 0 .5t 2 where z is sediment thickness in meters, and t is one-way traveltime in seconds. Sediment thicknesses calculated using this equation were compared to values calculated from the general equation of Carlson and others (1986). Values for sediment thickness calculated by the two equations differed by no greater than 10 percent throughout the range of travel times. Bathymetry The bathymetry is from Chase and others (1981). The bathymetric data were compiled from a variety of sources, and data quality is inconsistent. Because of differences in navigational precision and density of coverage, the bathymetric contours of a feature may be at a different location than the corresponding isopleths derived from the EEZ-SCAN 84 data, or a feature may not be indicated on the bathymetry at all. These situations occur because either the feature is poorly located in the bathymetric data set or, especially in the outer EEZ, the bathymetric data are too sparse to have defined the existence of a feature. Acknowledgments Topographic digital data bases were corrected and verified by Christina Lief. Gerald Evenden developed the computer software system MAPGEN, used to compose this map. Reviews, suggestions, and technical contributions from Edward C. Escowitz and Florence Wong and advice about cartographic design from Will Stettner substantially improved the quality of this map. References Cited Carlson, R.L., Gangi, A.F., and Snow, K.R., 1986, Empirical reflection-traveltime/ depth and velocity/depth functions for the deep-sea sediment column: Journal of Geophysical Research, v. 91, no. B8, p. 8249-8266. Carter, D.J.T., 1980, Echo-sounding correction tables: Taunton, United Kingdom, Hydrographic Department, Ministry of Defence, 150 p. Chase, T.E., Wilde, Pat, Normark, W.R, Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32&deg; and 49&deg; North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38&deg; latitude, 2 sheets. Connard, G., Couch, R., Keeling , K., Roy, J., and Troseth, S., 1984, Abyssal plain and continental net-objective sedimentary thicknesses, in Kulm, L.D., and others, eds., Western North America continental margin and adjacent ocean floor off Oregon and Washington, Atlas 1 of Regional Atlas Series, Ocean Margin Drilling Program: Woods Hole, Mass., Marine Science International, sheet 7. EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, Western Conterminous United States: U.S. Geological Survey Miscellaneous Investigations Series I-1792, scale 1:500,000, 152 p. See Also "U.S. Pacific West Coast Field Activities" (Paskevich and others, 2011; http://pubs.usgs.gov/of/2010/1332/htmldocs/pc/pc_overview.html). Paskevich, V.F., Wong, F.L., O?Malley, J.J., Stevenson, A.J., and Gutmacher, C.E., 2011, GLORIA sidescan-sonar imagery for parts of the U.S. Exclusive Economic Zone and adjacent areas: U.S. Geological Survey Open-File Report 2010?1332, available at http://pubs.usgs.gov/of/2010/1332/.

IMAP↗
USGS1993

Map showing sediment isopachs in the deep-sea basins of the Pacific Continental Margin, Strait of Juan de Fuca to Cape Mendocino

The U.S. Geological Survey conducted a series of cruises, EEZSCAN 84 (EEZ-SCAN 84 Scientific Staff, 1986), to collect reconnaissance data on the newly proclaimed Exclusive Economic Zone (EEZ), the area out to 200 nautical miles from the coastline of the United States. The cruises systematically surveyed the entire conterminous United States west coast EEZ using the Geological Long-Range Inclined Asdic (GLORIA) side-scan sonar, a 160-in 3 airgun seismic-reflection profiler, a 3.5-kHz high-resolution seismic-reflection profiler, a 10-kHz echo sounder, and a proton-precession magnetometer. The nominal trackline spacing throughout the survey was 30 km. Derivative maps of sediment thickness (I-2089-A, I-2090-A, I-2091-A) and depth to basement (I-2089-B, I-2090-B, I-2091-B) in the basins of the west coast EEZ were compiled from both the sonar-image dala and the deep-penetration seismic data obtained on these cruises. Only EEZ-SCAN 84 data were used for the map compilations because available data from other cruises in this region are sparsely located, have poor navigational control, or were obtained with seismic systems that were not powerful enough to resolve oceanic basement. For this map series, sediment thickness and depth to basement were determined only in the deep-ocean basin regions because the seismic system used on the EEZ-SCAN 84 cruises could not resolve oceanic basement beneath the thick sediments of the continental slope. All the data used to compile the maps are presented in the "Atlas of the Exclusive Economic Zone, Western Conterminous United States" (EEZ-SCAN 84 Scientific Staff, 1986). Basement Outcrops GLORIA imagery was used to locate areas of basement outcrop throughout the region. Where possible, sediment thickness on and immediately adjacent to basement outcrops was measured from seismic data. Where there was no bathymetric or seismic control, a seamount peak was assumed to have no sediment cover. A pattern is used on the map to indicate all basement outcrops. Data Reduction Acoustic basement in the basins, invariably oceanic Layer 2, was observed on all of the seismic records. One-way traveltime was measured from the sea floor to acoustic basement. Because the trackline spacing of about 30 km is relatively large compared to the data density along track, we chose to measure the sediment thickness every 0.5 hour or at an interval of approximately 7.5 km. Water depth was measured with a 10-kHz profiler. Depth to basement was calculated using the sea surface as the zero datum and adding the corrected water depth (Carter, 1980) to the sediment thickness. Acoustic travel times were converted to depths by first calculating a regression equation from the interval velocity versus depth data of Connard and others (1984). Their data base comprises a compilation of all available Deep Sea Drilling Project data plus wide-angle refraction data, which were collected in Cascadia Basin west of Oregon and represents the best data set available for the United States west coast EEZ. The regression equation was integrated to determine sediment thickness as a function of one-way traveltime . The resulting equation is z = 1400t + 0 .5t 2 where z is sediment thickness in meters, and t is one-way traveltime in seconds. Sediment thicknesses calculated using this equation were compared to values calculated from the general equation of Carlson and others (1986). Values for sediment thickness calculated by the two equations differed by no greater than 10 percent throughout the range of travel times. Bathymetry The bathymetry is from Chase and others (1981). The bathymetric data were compiled from a variety of sources, and data quality is inconsistent. Because of differences in navigational precision and density of coverage, the bathymetric contours of a feature may be at a different location than the corresponding isopleths derived from the EEZ-SCAN 84 data, or a feature may not be indicated on the bathymetry at all. These situations occur because either the feature is poorly located in the bathymetric data set or, especially in the outer EEZ, the bathymetric data are too sparse to have defined the existence of a feature. Acknowledgments Topographic digital data bases were corrected and verified by Christina Lief. Gerald Evenden developed the computer software system MAPGEN, used to compose this map. Reviews, suggestions, and technical contributions from Edward C. Escowitz and Florence Wong and advice about cartographic design from Will Stettner substantially improved the quality of this map. References Cited Carlson, R.L., Gangi, A.F., and Snow, K.R., 1986, Empirical reflection-traveltime/ depth and velocity/depth functions for the deep-sea sediment column: Journal of Geophysical Research, v. 91, no. B8, p. 8249-8266. Carter, D.J.T., 1980, Echo-sounding correction tables: Taunton, United Kingdom, Hydrographic Department, Ministry of Defence, 150 p. Chase, T.E., Wilde, Pat, Normark, W.R, Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32&deg; and 49&deg; North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38&deg; latitude, 2 sheets. Connard, G., Couch, R., Keeling , K., Roy, J., and Troseth, S., 1984, Abyssal plain and continental net-objective sedimentary thicknesses, in Kulm, L.D., and others, eds., Western North America continental margin and adjacent ocean floor off Oregon and Washington, Atlas 1 of Regional Atlas Series, Ocean Margin Drilling Program: Woods Hole, Mass., Marine Science International, sheet 7. EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, Western Conterminous United States: U.S. Geological Survey Miscellaneous Investigations Series I-1792, scale 1:500,000, 152 p. See Also "U.S. Pacific West Coast Field Activities" (Paskevich and others, 2011; http://pubs.usgs.gov/of/2010/1332/htmldocs/pc/pc_overview.html). Paskevich, V.F., Wong, F.L., O?Malley, J.J., Stevenson, A.J., and Gutmacher, C.E., 2011, GLORIA sidescan-sonar imagery for parts of the U.S. Exclusive Economic Zone and adjacent areas: U.S. Geological Survey Open-File Report 2010?1332, available at http://pubs.usgs.gov/of/2010/1332/.

IMAP↗