Valley Graben Research Articles

Use of Digital Potential Field, Geologic, and Remote Sensing Data in Studies of Structure of the Mid-Continent: ABSTRACT

A great deal of digital potential field, geologic, and remote sensing data exists for the Mid-Continent region of the United States. These data sets are fundamental to the understanding of the End_Page 1168------------------------------ structure of the crust. As the data are geographically oriented and in digital format, standard image-processing and analysis techniques can be applied. For instance, we have used a spatial filtering technique to interpolate between station locations of gravity and magnetic data to produce gray or color-coded images with continuous coverage throughout the Mid-Continent. The images contain more information than found in standard contour maps, because they can have many more contour intervals, and because they preserve local details of anomaly patterns. As an example of applicability of these techniques, gravity images registered and overlayed with magnetic, geologic, and remote sensing data lead to the identification of a Precambrian rift structure that begins at a break in the Mid-Continent gravity high in southeast Nebraska, extends across Missouri in a northwest-southeast direction, and intersects the Mississippi Valley graben. The rift structure is about 435 mi (700 km) long and 75 to 100 mi (120 to 160 km) wide. It is expressed in gravity images as a low with a Bouguer amplitude of about -34 milligals below regional values. Some of the discrete positive magnetic anomalies in Missouri are located along the borders of the gravity low. The gravity feature cuts across a major age boundary within the Precambrian basement. Finally, digitally enhanced thermal infrared images show a distinct alignment of linear structures with the gravity feature. The linears in some places correspond to mapped high-angle normal faults, to drape folds over relief within the Precambrian basement, and in some places, to extensions of mapped structures. End_of_Article - Last_Page 1169------------

Model of the southeastern margin of the Mississippi Valley graben near Memphis, Tennessee, from interpretation of truck-magnetometer data

The geometry and magnetic properties of the southeastern margin of the Mississippi Valley graben ate studied by analyzing detailed magnetic-anomaly profiles that are based on measurements made along roads near Memphis, Tennessee. The closely spaced measurements were obtained with a truck-mounted magnetometer system developed by the U.S. Geological Survey. Modeling results suggest that the graben margins represent both structural boundaries and conduits for ascending magma. About 2 km of vertical offset associated with normal faulting occurs within an interpreted 5.5-km-wide zone in which magnetic basement has an average dip of 20° NW into the graben. This zone may contain several large normal faults formed during a stage of rifting in late Precambrian or early Paleozoic time. Subsequent erosion of the scarps probably aided in forming the gentle northwestward slope of the buried Precambrian surface across the fault zone. The high apparent susceptibility (about 3 × 10 −3 emu) of magnetic basement associated with this fault zone and with the upblock suggests either that ascending magma intruded the upblock or that the two blocks differed lithologically prior to the formation of the graben.

Petroleum Exploration in Pedregosa Basin, Southwestern New Mexico: ABSTRACT

In southern Hidalgo and Grant Counties, New Mexico, the northwestern end of the Pedregosa basin has a high petroleum potential. Paleozoic rocks, dominantly shallow-marine carbonates, are over 11,000 ft (3,350 m) thick. Of the 11 formations, ranging in age from Cambrian to Permian, 7 contain favorable oil- or gas-source units. Mesozoic rocks, generally shallow-marine limestones and deltaic sandstones-mudstones, are nearly 10,000 ft (3,050 m) thick. Two of the three Lower Cretaceous formations contain favorable source units. Gas-prone kerogens are more abundant than the oil-prone types. At normal depths of burial, organic matter in the source units has reached thermal maturity, and some in the older formations is overmature. In the lower plate of a major Laramide thrust fau t, the Lower Cretaceous units are overmature and the older Paleozoic units may be thermally metamorphosed. Best reservoir objectives are the porous dolostones, totaling 484 ft (148 m) in thickness, in the upper part of the Horquilla Formation (Pennsylvanian-Permian). They are located at the shelf margin of a deep-marine basin. Other favorable reservoir units are indicated in surface exposures. In this frontier area, only 11 exploration wells have been drilled to Precambrian, Paleozoic, or Mesozoic rocks. Shows of oil or gas were reported in 6 of the wells. None of the wells have tested the better reservoir objectives in the deeper parts of the graben valleys, where commercial accumulations of petroleum are likely to be preserved. Tertiary-intrusive and volcanic-cauldron complexes have thermally metamorphosed older sedimentary units only locally. Basin and Range faulting has disrupted subsurface fluid systems in many parts of the area. Despite the challenging risks, the high potential encourages further exploratory drilling on selected petroleum prospects. End_of_Article - Last_Page 248------------

The Pocatello Valley (Idaho-Utah border) earthquake sequence of March to April 1975

abstract An earthquake sequence during March and April 1975, which included a main shock of magnitude ( M L ) 6.0 on 28 March, was centered in Pocatello Valley on the Idaho-Utah border. A foreshock of magnitude ( M L ) 4.2 preceded the main shock by 22 hr and was followed by at least 156 microearthquakes that behaved as aftershocks of the first event. Accurate locations of 587 aftershocks that occurred during the first 20 days after the main shock define irregular zones of concentrated activity beneath the south-central, western, and northern parts of Pocatello Valley. The main shock is interpreted to have involved nonuniform dipslip rupture, without surface faulting, on a previously unrecognized normal fault that dips NW, obliquely transecting the north-trending Pocatello Valley graben. The speace-time pattern of aftershocks and composite focal mechanisms indicate subsidiary faulting and the interaction of diverse fracture trends resulting in a mixture of normal, strike-slip, and oblique faulting. Details of the earthquake sequence imply a complex episode of Basin and Range graben subsidence that may typify similar source regions elsewhere in Utah9s Wasatch Front area.

Model for the tectonic evolution of the Mississippi embayment and its contemporary seismicity

Aeromagnetic and gravity surveys of the northern Mississippi embayment have delineated a northeast-trending basement depression about 70 km wide and more than 300 km long. The feature, the Mississippi Valley graben, is remarkably linear over its known length, has nearly parallel sides, and has estimated depths relative to surrounding basement of more than 2 km. Both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben. Estimated magnetization directions for the plutons indicate a Mesozoic age. The graben, of probable earliest Paleozoic age, underlies the northwest side of the embayment and contains the area of major seismicity. In particular, much of the instrumentally located contemporary seismicity and the estimated locations of the large-magnitude 1811–1812 earthquakes are in the central part of the graben. We interpret the early Paleozoic graben, the Mesozoic plutonism, and the sedimentation cycles as evidence of stretching events. Seismicity is attributed to strain in and below the central graben area, which would appear to be a focal point of deformation in the stretching model. We suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in Pre-cambrian time. The postulated rotating stress field is presumably controlled by the system that drives the global plates.

The significance of fission-track ages of apatite in relation to the tectonic history of the Front and Sawatch Ranges, Colorado

Fission-track ages of apatite in the Mount Evans area of the Front Range show that the Late Cretaceous 100 °C isotherm, which was 4 km below the sea floor, is now at an altitude of 3.5 km. Assuming a geothermal gradient of 25 °C per kilometre, the projected level of the latest Cretaceous sea floor, represented by the Fox Hills Sandstone, is at an altitude of 7.5 km, about 3.2 km above the top of Mount Evans, and the structural relief from that projection of the Fox Hills on the crest of the Front Range uplift to the deepest part of the Denver Basin to the east is 6.5 km. Ages of apatite in Precambrian rock along the Elkhorn fault on the west margin of the Front Range uplift show that in the area of maximum stratigraphic displacement the fault must have a very low dip and that the east margin of the Paleocene South Park basin must have been nearby. Ages in the Sawatch Range suggest that significant uplift occurred during or after Eocene time. Ages on the east side of the range reflect development of the Arkansas Valley graben and concomitant uplift of the east margin of the modern Sawatch Range in Miocene time.

Petroleum Potential of Basin and Range Province, Western United States: ABSTRACT

Five oil fields have been discovered within the Basin and Range province of the western United States. They are Eagle Springs (1954), Trap Spring (1976), and Currant (1978) fields in Railroad Valley graben of east-central Nevada, and in the Great Salt Lake area of Utah, Rozel Point (circa 1904) and West Rozel (1978) fields. Rozel Point and Currant fields are non-commercial accumulations. Reservoirs are either fractured Oligocene ignimbrites, Eocene lake sediments, or fractured Miocene-Pliocene basalts. Accumulations occur in truncation-fault traps or in drape over faulted structure. The source of the oil is believed to be Tertiary lake deposits and/or Chainman Shale of Mississippian age. End_Page 707------------------------------ Exploration for Tertiary accumulations is carried out by: (1) mapping source rocks in basins with proper depth for maturity, (2) establishing presence of reservoir rocks, and (3) delineation of traps by photogeologic-geomorphic techniques, gravity surveys, and seismic shooting. Numerous shows of oil and gas have been recorded in wells drilled in various basins, both in Paleozoic and Tertiary rocks. Other oil and gas indications include the Bruffey oil and gas seeps (Pine Valley, Nevada), the Wells oil seep (west of Wells, Nevada), an asphaltite dike in Mississippian sediments (Pinon Range, east of Pine Valley), the West Brigham City and Farmington gas areas (east of Great Salt Lake, Utah), and the Fallon gas area (Carson Sink, Nevada). Oil source units include Cretaceous to Tertiary lake deposits (Sheep Pass Formation, Elko Shale, Kinsey Canyon Formation, Newark Canyon Formation, and King Lear Formation), Mississippian Chainman Shale, Devonian Pilot Shale, and Ordovician Vinnini Shale. Several Paleozoic plays exist in Nevada, including the Mississippian Diamond Peak (Illipah, Scotty Wash) sandstone pinch-outs. Reef buildups may be present in the Silurian and Devonian section. Exploration in the Basin and Range province should result in significant discoveries of oil and gas in the future. End_of_Article - Last_Page 708------------

Oil and Gas Exploration Wells in Pedregosa Basin: ABSTRACT

In the Pedregosa basin and adjoining areas covering 49,500 sq mi (110,700 sq km) in southeastern Arizona, southwestern New Mexico, northwestern Chihuahua, and northeastern Sonora, 37 petroleum-exploration wells have penetrated Paleozoic and/or Precambrian rocks. several shows of oil and gas have been reported, but no commercia production has been found to date. Many of the wells have been drilled on basin and range uplifts where reservoirs tend to be flushed with meteoric water. The best remaining prospects lie below the deeper parts of graben valleys where preservation of petroleum is more likely. The highest ranking objective of the region is in Upper Pennsylvanian-Lower Permian rocks at the margin of the Alamo Hueco basin where shallow-marine dolostone reservoirs are juxtaposed to deep-marine, organically rich, limestone and mudstone source rocks. A regional isopach and facies map of the Pennsylvanian shows that the basin axis trends generally southeastward from southern Hidalgo County, New Mexico, across the Ascension-Villa Ahumada area of Chihuahua. Several other petroleum-exploration objectives are indicated in the Paleozoic and Mesozoic rocks. End_of_Article - Last_Page 450------------

Recent vertical crustal movement along the east coast of the United States

The first detailed profile of apparent vertical crustal movement along the east coast of the United States, extending from Calais, Maine to Key West, Florida, has been produced from the results of precise leveling by the National Geodetic Survey. Comparison of this profile with one obtained from secular trends in sea level reveals a serious systematic disagreement between the two methods, as large as 19 mm/year. Estimates of normal measurement error can account for some, but not all, of the misclosures indicated. Thus at least one of the methods must be seriously affected by some systematic non-tectonic influence. The cause of the discrepancy is presently unknown, but the comparisons presented here make possible an estimate of its magnitude, an important step toward identification of the cause and elimination of its effect. The two data sets have been adjusted using different assumptions as to the source of their disagreement to produce several possible versions of a crustal movement profile along the east coast. Comparison of these profiles with topography, gravity and seismicity indicate some weak correlations. However, there are several striking correspondences between apparent movements and tectonic and geomorphic features on a regional scale; in particular, the Connecticut Valley graben, the Chesapeake embayment, the Cape Fear arch and the Cape Canaveral prominence correspond to marked changes in the crustal movement profile. These correlations persist in all of the adjustments and it is unlikely that they can be attributed to chance. The implication is, therefore, that these four features are active elements in the contemporary tectonic fabric of the eastern United States.

Geologic Exploration of Taurus-Littrow: Apollo 17 Landing Site

Apollo 17 landed in a deep graben valley embaying the mountainous highlands southeast of the Serenitatis basin. Impact-generated breccias underlie the massifs adjacent to the valley, and basalt has flooded and leveled the valley floor. The dark mantle inferred from orbital photographs was not recognized as a discrete unit; the unusually thick regolith of the valley floor contains a unique high concentration of dark glass beads that may cause the low albedo of much of the surface.

Application of electrical resistivity and gravimetry in deep geothermal exploration

The electrical resistivity method has been proven applicable to geothermal exploration because of the direct relationship between fluid and rock temperatures on the one hand electrical conductivity on the other. The problem of exploitation of a surface technique, such as resistivity, to the determination of geothermal gradients or ‘hot spots’ is complicated by the other geological parameters which affect resistivity: porosity, fluid salinity, cementation factor and clay content. However, by rational application of common geological techniques it is possible at times to isolate electrical resistivity changes due to temperature alone, and calculate the geothermal gradients. Nomograms are provided for calculation of some of the parameters which affect resistivity. Under some geologic conditions, gravimetry provides a useful clue as to the areas of abnormal heat flow, due to the metamorphism of sediments by circulating plumes of mineralized fluids. A case history of the application of the above methods to the geothermal exploration of the Imperial Valley graben of southern California is described showing how the above techniques were employed to detail the geothermal characteristics of portions of the valley.

Preliminary observations of the hydromechanics, nutrient cycles and eutrophication status of Lake Kinneret (Lake Tiberias)

With the establishment of a new limnological laboratory at Tabgha, on the shores of the Lake Kinneret, a number of projects have been undertaken recently to study different aspects of the lake. As well as a detailed routine survey, we have initiated studies on topics relating to the hydrodynamics, and the chemical and biological dynamics of the water and sediments. Lake Kinneret is located in the Jordan Valley graben at -209 m below MSL. It is 22 km long on the N.S. axis and 12 km wide. The maximum depth is 42 m and the mean depth is 24 m. The extreme temperatures of the water mass are 14° C and 30° C. The waters are stratified from May to December and the hypolimnion is depleted of oxygen during most of this period.

Role of Membrane Hyperfiltration on Origin of Thermal Brines, Imperial Valley, California: ABSTRACT

Unique saturated Na-Ca-K Cl thermal brines (380°C.) are recovered from geothermal wells near the Salton Sea, Imperial Valley, California. The reservoir chamber is fractured metamorphic rock (mainly greenschist facies) at 3,900-8,000 feet. The shallow waters are relatively dilute NaHCO3-Cl, high in B; NH4, I, and F are present; the Na/K ratio is less than in the brines; CO2 gas is abundant. The waters of these brines are dominantly meteoric, as evidenced by End_Page 454------------------------------ the previously reported deuterium content of the brines and local surficial waters. The most critical geochemical questions concern the mechanism for brine concentration, the origin of the Cl ion, and the surprisingly high Ca/Mg, K/Na, and Ca/K ratios of the brine. The arkosic sedimentary fill of the Imperial Valley graben contains ample material to provide by solution every chemical in the brines except for the Cl ion. Models of the brine column versus original material contained in the encompassing rock column suggest a deficiency of Cl ion. A high degree of interchange between the host rock and the thermal waters exists, as evidence by previously reported data (18O and B content) and the similarity of the Rb/K ratios of the brines to those of arkosic materials. The Cl ion must be either juvenile in origin, or a result of the concentration of meteoric interstitial water of the sedimentary fill, or derived from Cl absorbed onto silicate surfaces. The similarity of the Br/Cl ratio of the brines to local meteoric surface and ground waters, and its complete dissimilarity to those of Cl evaporites, suggest that the brines are dominantly interstitial meteoric water concentrated manyfold. Doubtless some of these brine halogens also were obtained by desorption of absorbed material at high temperatures. Hyperfiltration of relatively dilute hydrothermal solutions through electrostatic semi-permeable membranes composed of abundant montmorillonitic and illitic clays in the sedimentary fill provides the best mechanism for concentrating the brines within the proposed thermal convection cell and of affecting the relative composition of the brine and the relatively dilute waters underlying the thermal anomaly. In particular, this mechanism best explains the Ca/Na ratios of the brine; the relative abundance of Sr, Ca, and Mg within the brines possibly may be a result of this mechanism; the increase of HCO3/Cl, F/Cl, and B/Cl ratios in the dilute overlying waters, which would be effluent to the proposed membrane system, probably is a result of such hyperfiltration. High-temperature metastable equilibria between the thermal brine and its enclosing rocks strongly affect the specific composition of the brine. Such reactions probably control completely the trace-element metal content of the brines. The relative abundance of the alkali-metals appears to be strongly influenced by such rock-water reactions as well as by relative hyperfiltration. Experimental investigations are needed to understand further the origin of these waters. End_of_Article - Last_Page 455------------

Tectonic Development of Idaho-Wyoming Thrust Belt

Three stages are evident in the tectonic development of southeastern Idaho and western Wyoming. First are the changing patterns of tectonic elements during deposition; second, development of northward-trending folds and thrust faults; and third, development of block faults that produced horst ranges and graben valleys. During Paleozoic time about 50,000 feet of marine sediments, mostly limestone and dolomite, were deposited in a miogeosyncline and about 6,000 feet of mixed marine sediments were deposited on the shelf to the east. Detritus came from both east and west from Cambrian time on. Starting in Mississippian time, the belt between shelf and miogeosyncline, where thicknesses increase markedly, shifted progressively eastward. During Mesozoic time about 35,000 feet of marine and continental sediments were deposited in the western part of the region and about 15,000 feet in the eastern part, with terrestrial deposits becoming increasingly dominant. Western positive areas became the chief source of detritus. The belt of maximum thickening and the site of maximum deposition were relocated progressively eastward; maximum thicknesses of succeeding geologic systems are not superposed. In Late Triassic a belt on the west rose and the miogeosyncline started to break up. As Mesozoic time progressed the western high spread eastward, until by the end of the Jurassic the miogeosyncline gave way to intracratonic geosynclinal basins that received thick deposits, particularly in Cretaceous time. Cenozoic sedimentary rocks are products of orogeny in the region. The second stage, which overlapped the first, produced folds overturned toward the east and thrust faults dipping gently west in a zone, convex to the east, 200 miles long and 60 miles wide. Stratigraphic throw on many larger faults is about 20,000 feet; horizontal displacement is at least 10 to 15 miles. Lack of metamorphism and mylonite along the faults is striking. From west to east, the thrust faults cut progressively younger beds, have progressively younger rocks in their upper plates, and are estimated to be successively younger. Thrusting started in the west in latest Jurassic and ended in the east perhaps as late as early Eocene time; detritus shed from emergent upper plates is preserved in coarse terrestrial strata of corresponding ages. West of the thrust belt is a northwestward-trending area underlain mostly by lower Paleozoic rocks and flanked on east and west by upper Paleozoic and Mesozoic rocks. Scattered pieces of eastward-dipping thrust faults have been reported west of the older rocks. This central area of old rocks has been interpreted as: (1) part of a large continuous thrust sheet moved scores of miles from the west; or (2) an uplifted segment of the earth's crust from which thrust sheets on the east and west were derived. Both interpretations have defects; relative thrust ages are difficult to explain under the first; a large positive gravity anomaly, expectable under the second, is apparently absent. Block faulting, the third stage of tectonic development, started in Eocene time. Faulting has continued to the Recent, as indicated by broken alluvial fans, displaced basalt flows less than 27,000 years old, and earthquakes. North-trending and east-trending fault sets are recognized. Old east-trending steep faults in the Bear River Range may be tear faults genetically related to thrusting. Movement along many faults has been recurrent. Patches of coarse Tertiary gravel on the flanks and crests of ranges, for which there is no provenance with present topography, may record reversed vertical movement along some north-trending faults. Present topographic relief of basins and ranges is tectonic.

Structural Geology of Aconcagua Province and Its Relationship to the Central Valley Graben, Chile

Research Article| June 01, 1965 Structural Geology of Aconcagua Province and Its Relationship to the Central Valley Graben, Chile W. D CARTER; W. D CARTER U. S. Geological Survey, Washington, D. C. Search for other works by this author on: GSW Google Scholar LUIS AGUIRRE LE B LUIS AGUIRRE LE B Institute de Investigacions Geológicas, Santiago, Chile Search for other works by this author on: GSW Google Scholar GSA Bulletin (1965) 76 (6): 651–664. https://doi.org/10.1130/0016-7606(1965)76[651:SGOAPA]2.0.CO;2 Article history received: 23 Nov 1964 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. D CARTER, LUIS AGUIRRE LE B; Structural Geology of Aconcagua Province and Its Relationship to the Central Valley Graben, Chile. GSA Bulletin 1965;; 76 (6): 651–664. doi: https://doi.org/10.1130/0016-7606(1965)76[651:SGOAPA]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Aconcagua Province is herein divided into three major structural provinces which, for the sake of simplicity, are named the Coastal Cordillera, Central Valley graben, and Andean Cordillera structural provinces to correspond to the three geomorphic provinces recognized farther south.The coastal structural province includes the Coastal Cordillera which is underlain mainly by layered sedimentary and effusive rocks that strike north and dip homoclinally to the east, range from Triassic to Late Cretaceous in age, and are intruded by Cretaceous granodioritic and dioritic rocks. Igneous and metamorphic rocks largely of Paleozoic age comprise the western coastal margin, Along the eastern edge of the province is the Los Angeles fault zone, a wide, poorly defined band of semiparallel, arcuate faults which show downward displacement to the east and which appear to have resulted mainly from intrusion and uplift on the west. The Coastal Cordillera, therefore, may be considered a large horst with an intrusive granodiorite core.The Central Valley graben is bounded on the west by the Los Angeles fault zone and on the east by the Pocuro fault zone. Between these two fault zones is an area 20–30 km wide in which volcanic rocks of Late Cretaceous age are flat lying to gently folded and block faulted. In places, pipelike stocks of andesitic to dioritic igneous rock intrude the area. The Pocuro fault zone is a prominent lineament that marks the eastern limit of the Central Valley at Santiago and has been traced northward for 150 km. It may, however, have a mappable length of more than 1200 km. Vertical displacement downward to the west has been measured near Los Andes to be at least 2000 m. Consequently, this fault zone may rank among the major faults of the world.The Andean structural province is subdivided into the Las Ollas and Juncal subprovinces. The Las Ollas is a mountainous front range that lies east of the Pocuro fault and extends eastward for about 25 km. Upper Cretaceous volcanic strata are gently warped into broad, open, north-trending folds, with local sharp flexures and faulted mainly by normal faults. The eastern part of the subprovince is cut by a narrow belt of Tertiary plutonic rocks, some of which are associated with porphyry copper deposits. The Juncal structural subprovince extends eastward into Argentina. It is typified by close, overturned folding, vertical bedding, and thrust faulting. In general, structural deformation increases in intensity from west to east, with imbricate overthrusting to the east in Argentina.Present evidence indicates that major graben formation began during early Tertiary (pre-Miocene) time as a result of tensional stress developed by strain release of earlier compressional forces that folded the Andes. Post-Miocene uplift of the central Andean region renewed tensional stress and opened deep-seated, north-striking fractures and reinitiated volcanism. Most of the active volcanos of Chile may be aligned along such fractures. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic Development of Idaho-Wyoming Thrust Belt: ABSTRACT

Three stages are evident in the tectonic development of southeastern Idaho and western Wyoming. First, the changing patterns of tectonic elements during deposition; second, development of northward-trending folds and thrust faults; and third, development of block faults that produced horst ranges and graben valleys. During Paleozoic time about 50,000 feet of marine sediments, mostly limestone and dolomite, were deposited in a miogeosyncline and about 6,000 feet of mixed marine sediments were deposited on the shelf to the east. Detritus came from both east and west recurrently from Cambrian time on. Starting in Mississippian time, the belt between shelf and miogeosyncline, where thicknesses increase markedly, shifted progressively eastward. During Mesozoic time about 35,000 feet of marine and continental sediments were deposited in the western part of the region and about 15,000 in the eastern, terrestrial deposits becoming increasingly dominant. Western positive areas became the chief source of detritus. The belt of maximum thickening and the site of maximum deposition shifted progressively eastward; maximum thicknesses of succeeding geologic systems are not superposed. In Late Triassic a belt to the west rose and the miogeosyncline started to break up. As Mesozoic time progressed the western high spread eastward, until by the end of the Jurassic the miogeosyncline gave way to intracratonic geosynclinal basins that received thick deposits, particularly in Cretaceous time. Cenozoic sedimentary rocks are products of oroge y in the region. The second stage which overlapped the first, produced folds overturned to the east and thrust faults dipping gently west in a zone, convex to the east, 200 miles long and 60 miles wide. Stratigraphic throw on many larger faults is about 20,000 feet; horizontal displacement is at least 10 to 15 miles. Lack of metamorphism and mylonite along the faults is striking. From west to east, the thrust faults cut progressively younger beds, have progressively younger rocks in their upper plates, and are estimated to be successively younger. Thrusting started in the west in latest Jurassic and ended in the east perhaps as late as Early Eocene time; detritus shed from emergent upper plates is preserved in coarse terrestrial strata of corresponding ages. West of the thrust belt is a northwestward-trending area underlain mostly by lower Paleozoic rocks and flanked on east and west by upper Paleozoic and Mesozoic rocks. Scattered pieces of eastward-dipping thrust faults have been reported west of the older rocks. This central area of old rocks has been interpreted as (1) part of a large continuous thrust sheet moved scores of miles from the west, or (2) an uplifted segment of the earth's crust from which thrust sheets to the east and west were derived. Both interpretations have defects: relative thrust ages are difficult to explain under the first; a large positive gravity anomaly, expectable under the second is apparently absent. Block faulting, the third stage of tectonic development, started in Eocene time. Faulting has continued to the Recent, as indicated by broken alluvial fans, displaced basalt flows less than 27,000 years old, and earthquakes. North-trending and east-trending fault sets are recognized. Old east-trending steep faults in the Bear River Range may be tear faults genetically related to thrusting. Movement along many faults has been recurrent. Patches of coarse Tertiary gravel on the flanks and crests of ranges, for which there is no provenance with present topography, may record reversed vertical movement along some north-trending faults. Present topographic relief of basins and ranges is tectonic. End_of_Article - Last_Page 1878------------

Regional Gravity Survey of the Northern Great Salt Lake Desert and Adjacent Areas in Utah, Nevada, and Idaho

From 1957 to 1961 a regional gravity survey was made over the northern part of the Great Salt Lake Desert and adjacent areas in Utah, eastern Nevada, and southeastern Idaho. A total of 1040 stations were taken over an area of about 7000 square miles. The results were compiled as a Bouguer gravity anomaly map with a contour interval of 2 mgal. The Bouguer values ranged from a high of about —120 mgal over the outcrop areas to a low of about —196 mgal over the alluvium-covered graben areas. The gravity high over the Raft River Mountains apparently corresponds with the Raft River Mountains anticline. A belt of gravity contours, with a total relief of 15–20 mgal, extending for 40 miles between the Wildcat Hills and the southern part of the Grouse Creek Mountains and beyond, is interpreted provisionally as caused partly by abrupt thickening and/or downwarping of the rocks of late Paleozoic age in a general northwestward direction, perhaps to form a foredeep (part of the Butte-Deep Creek trough of Steele, 1960) south of the Raft River Mountains; however, this anomaly could also be partly caused by overthrusting. Many Basin and Range faults, grabens, and horsts are indicated by the gravity data. In the northern part of the surveyed area, Junction Valley, Upper Raft River valley, and Curlew Valley are indicated to be grabens. The Newfoundland Range, in the northeastern part of the Great Salt Lake Desert, is a horst flanked by a graben on each side. The northwestern margin of the Great Salt Lake Desert comprises a complex pattern of Basin and Range fault blocks, large and small, that lie along a generally northward-trending belt or zone. The Silver Island-Pigeon fault block, which comprises the Silver Island Mountains, the Little Pigeon Mountains, and Pigeon Mountain, forms an elongate, arcuate horst that is flanked by a belt of grabens on the west (Pilot Valley, Lucin, and Grouse Creek grabens) and east (Wendover, Crater Island, Little Pigeon, and Pigeon grabens). A major fault zone is indicated along the east margin of the Pilot Range. The Pilot-Grouse Creek rift belt, at least 90 miles long, extends northward between Pilot Valley and the Upper Raft River valley and constitutes a major lineament in the earth9s crust along which the graben blocks were displaced downward relative to the adjacent mountain blocks. In the southern part of the rift belt, the grabens are separated by blocks (the Lemay and Lucin horsts) that were probably downfaulted relative to the large mountain blocks, but became lodged at intermediate height; in the northern part, however, the blocks (for example the Junction Valley graben), apparently merely broke from the main crustal unit along this belt of weakness. The indicated thickness of the valley fill of Cenozoic age in some of the graben areas ranges up to about 6000 feet.

Geologic Reconnaissance of Alturas Area, Northeastern California: ABSTRACT

To augment the California Division of Mines State Geologic Map project, photogeologic mapping, End_Page 255------------------------------ groundchecked along main roads, was conducted in 1957-1958 on the new Army Map Service Alturas Sheet, scale 1:250,000, in the northeast corner of California (41°-42° North Latitude, 120°-122° West Longitude). Previous maps in the area, such as Anderson's in the Medicine Lake Highland, Peacock's and Powers' in the Modoc Lava Beds Quadrangle, and R. J. Russell's in the Warner Range were incorporated, with alterations to fit State Map units, newly available topographic detail, and the authors' geologic observations. The oldest rocks in the area occupy about 100 square miles in the southwest corner of the area, and include Triassic sedimentary and metasedimentary rocks (Modin, Brock, Hosselkus, and Pit units); Jura-Trias metavolcanic rocks (Bagley andesite); Jurassic marine sediments (Potem formation); and Eocene arkosic sandstone (Montgomery Creek formation). Miocene to Recent basaltic flow rocks cover most of the remainder of the area, with a several thousand foot thick, uplifted section of Oligocene to Pliocene agglomerates, tuff-breccias, and pumiceous tuffs in the eastern part of the map (Warner Range); and a thousand feet or more of Miocene to Pleistocene diatomaceous, ash-rich, lake-laid sediments exposed beneath near-horizontal lava cover in large basins and river cuts in various parts of he sheet. Some of the units, such as Cedarville series, Warner basalt, and Alturas formation, previously assigned and widely used to designate many of these rocks, appear to be subject to redefinition and subdivision, although the present authors have not completed this project. Structurally, the eastern part of the area is dominated by the uplifted and tilted fault block of the Warner Range, flanked on the east by the Surprise Valley graben. The remainder of the area is mainly a dissected volcanic terrain of the Modoc Plateau, with many north-northwest-trending faults of slight displacement. Many basaltic shield volcanoes and cinder cones occur along the southern edge and in the northwestern corner of the area. End_of_Article - Last_Page 256------------

Occurrences of Shales Partially Altered to Pyrophyllite

AbstractExamination of samples from shale outcrops on the periphery of the Utah Valley graben has revealed an unusual association of disseminated pyrophyllite with various clay minerals. Numerous samples were obtained from exposures in brick clay pits located in the Manning Canyon formation of Mississippian-Pennsylvanian age and from the Long Trail member of the Great Blue formation of Mississippian age. Associated clay minerals in these beds include illite, illite-montmorillonite mixed-layer clays, 7 Å chlorite, 14Å chlorite, kaolinite and sericite. Associated nonclay minerals include quartz, calcite, and small amounts of dolomite. Associated secondary minerals are calcite, aragonite, jarosite, gibbsite and gypsum.Three possible explanations of genesis have been considered in the present study: deposition of detrital pyrophyllite, surface weathering under special conditions, and hydrothermal or pneumatolytie activity. The third alternative explanation, hydrothermal or pneumatolytie activity, is believed to be the most acceptable. It is hypothesized that magnesium, iron, and interlayer cations have been removed from some of the original 2 : 1 layer clays in the shale by solutions localized along fault zones.

Wasatch Plateau Gas Fields, Utah

Another geologic province in Utah, the rugged, mountainous Wasatch Plateau, has recently been proved to have gas fields of importance at Clear Creek and Flat Canyon. Important carbon dioxide gas reserves were previously discovered at Gordon Creek. The producing areas of the plateau are located on a regional uplift, herein named the Monument Peak, which consists of two major, northeast-southeast zones of folding. These are considered marginal bands of folding to the adjacent San Rafael swell, an ancient positive element. These flexures represent the culmination of at least three periods of movement: middle Upper Cretaceous, late Upper Cretaceous, and post-middle Eocene. Extensive north-south graben faulting subsequent to the folding has cut across the flexures, thus segmenting them into many fault blocks. Although situated on the crests of two different anticlines, the Clear Creek and Flat Canyon fields are both contained in the same closed fault block which lies west of, and is closed against, the Pleasant Valley graben. Strata ranging from middle Upper Cretaceous to middle Eocene occur on the surface. A deep well at Gordon Creek has penetrated strata as old as early Permian. A major part of the area appears to be underlain by Jurassic salt-bearing beds, the Arapien formation, whose instability has probably contributed to the extensive faulting in the surface strata. The Upper Cretaceous Ferron sandstone is the principal producing formation although the Dakota sandstone is also productive at Flat Canyon.