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J. E. Case

Publications and source records attributed to J. E. Case.

36 records · Page 2Linked to original sources
USGS1981

Gravity anomaly and interpretation map of the Chignik and Sutwik Island quadrangles, Alaska

The gravity field of the Chignik and Sutwik Island quadrangles near the center of the Alaska Peninsula represents a complex series of transitions between probable continental crust on the north, probable oceanic crust on the south, sedimentary basins on each side of the peninsula, and a central structural high and volcanic arc. The resulting gravity field may be generalized as a central southwest- to northeast-trending gravity high bordered on both sides by flanking gravity lows over sedimentary basins underlying the lowlands and continental shelf. All three gravitational features show discontinuities and magnitude variations that reflect the complex geologic setting. Data concerning this complex gravity field have accumulated slowly and intermittently since an initial measurement was made at Port Heiden more than 25 years go (Thiel and others, 1958, 1959). A few reconnaissance measurements by R. V. Allen and an offshore traverse conducted by the U.S. Coast and Geodetic Survey R/V Surveyor were made in the early 1960's (Barnes and others, 1966; Barnes, 1967). At about the same time, Gulf Oil Corp. contracted for a relatively detailed survey of the lowlands on the north shore of the peninsula; the map prepared from this survey is now available (Gulf Oil Corp., Port Moller Development Contract Report, available at U.S. Geological Survey, Anchorage, Alaska). In the early 1970's, a few new land measurements were made by P. L. Dobey, of the Alaska Division of Geological and Geophysical Surveys, and an offshore traverse was completed by the R/V C. Greene on contract to the U.S. Geological Survey (Fisher, 1979, and M. A. Fisher, written commun., 1977). The most recent measurements, the basis for this map, were made by M. E. Yount, D. L. Detra, D. R. Jefferis, and J. E. Case during the mineral assessment of the two quadrangles. D. F. Barnes, R. L. Morin, D. R. Jefferis, and R. F. Sikora have shared in the data compilation. Geologic background for the text was provided by R. L. Detterman, T. P. Miller, E. Young, and F. H. Wilson (1979, 1981).

Alaska↗
USGS1979

Geologic interpretation of the gravity anomaly map of the Seward and Blying Sound quadrangles, Alaska

For more than 20 years, gravity surveys have been conducted in the vicinity of Prince William Sound, the epicentral region of the great Alaska earthquake of March 27, 1964. Some of the major regional gravity anomalies were outlined by Thiel, Ostenso, Bonini (1960), and additional anomalies were defined by pre- and post-earthquake gravity studies reported by Barnes and Allen (1965) and Case, Barnes, Plafker, and Robbins (1966). Another gravity anomaly map was prepared by personnel of the U.S. Coast and Geodetic Survey (now the National Oceanic and Atmospheric Administration) (Wood, 1966, p. 131; Rice, 1969, p. 5-20). Anomalies shown by Rice (1969) are broadly similar to those shown by Case, Barnes, Plafker, and Robbins (1966); Rice's interpretations concerned mainly gravity changed related to the earthquake, and the data were not interpreted in terms of regional geology. New gravity stations were established in 1976 as part of the mineral resource appraisal of the Seward and Blying Sound quadrangles. This report incorporates the new data obtained in 1976, reinterprets some anomalies previously discussed by Case, Barnes, Plafker, and Robbins (1966), and interprets other anomalies not previously discussed. Descriptions of the major rock units and structural features are found in the companion report by Tysdal and Case (in press). An interpretation of the aeromagnetic map of the region has been prepared by Case, Tysdal, Hillhouse, and Gromme (1979).

Alaska↗
USGS1979

Geologic interpretation of aeromagnetic map of the Seward and Blying Sound quadrangles, Alaska

An aeromagnetic survey of the Seward and northern part of the Blying Sound quadrangles was flown by Geometrics, Inc., during 1975-77 to provide magnetic data to aid in an appraisal of the mineral resources. Background details of the regional geology and major structures are described in a companion report by Tysdal and Case (1979), and interpretation of the gravity data are described in a report by Case and others (1979). Preliminary reports of a paleomagnetic investigation have been reported by Hillhouse and Gromme (1977), and a more complete analysis of the results is in preparation. The aeromagnetic survey was flown at a nominal height of 300 m above the surface and flight lines were spaced about 1.7 km apart. Total magnetic field was measured by a proton-precession magnetometer. The International Geomagnetic Reference Field (IGRF), updated to 1976, was removed, and the residual values were computer-contoured to produce the aeromagnetic map (sheet 1). Detailed aeromagnetic maps at scale 1:63,360 and a map at scale 1:250,000 have been released as Open File Reports 78-1080 through 1083 (U.S. Geological Survey, 1978). Magnetic anomalies in the Seward-Blying Sound region are somewhat less complex than in other parts of south-central Alaska, primarily because of the huge volume of relatively nonmagnetic flysch that constitutes the Valdez and Orca Groups. Several rather isolated groups of mafic-ultramafic rocks are significantly more magnetic than the flysch assemblages, and cause distinctive patterns of magnetic anomalies. A few of the granitic plutons are magnetic, but most appear to have little or no magnetic expression.

Alaska↗
USGS1977

Sheeted dikes, gabbro, and pillow basalt in flysch of coastal southern Alaska

A Paleocene to Eocene(?) mafic sequence of igneous rocks on Knight Island and a Cretaceous mafic and ultramafic sequence of the Resurrection Peninsula in coastal southern Alaska are characterized by pillow basalts, sheeted dikes, and gabbro intrusions. At both localities, pillow basalts are interbedded with flysch, and the gabbros intrude both the sheeted dikes and the immediately overlying sedimentary rocks. At the Resurrection Peninsula, small bodies of serpentinized dunite are present in the gabbro, and overlying low-grade metasedimentary rocks and mafic tuffs are interbedded with sedimentary rocks. At Knight Island the sheeted dikes intrude Paleocene and Eocene(?) sedimentary rocks. These igneous rocks have petrographic features similar to those of oceanic tholeiites and have some of the characteristics of ophiolites. They apparently were formed near the continental margin, because both sequences intrude and are interlayered with flysch. We believe the igneous rocks were intruded along faults (perhaps leaky transform faults) of ocean floor that happened to be near the continental margin; the faults were oriented parallel to the depositional strike and subsequent deformational strike of the Valdez(?) and Orca Groups.

Alaska↗
USGS1977

The Alaskan Mineral Resource Assessment Program; background information to accompany folio of geologic and mineral resource maps of the McCarthy Quadrangle, Alaska

The McCarthy 1? by 3? quadrangle, in eastern south-central Alaska, contains potentially significant resources of copper and possibly of a few other commodities. This circular and a companion folio of maps represent results of integrated field and laboratory studies in the disciplines of geology, geophysics, geochemistry, and satellite imagery that are designed to provide a modern mineral resource assessment of the quadrangle. The maps are accompanied by descriptive texts, explanatory material, pertinent references, and by a few auxiliary tables and diagrams. This circular provides background information for the mineral resource assessment and integrates the component maps. It also includes a master list of references (see 'Bibliography') relevant to the geology and mineral deposits of the quadrangle.

Circular↗
USGS1974

Oceanic crust forms basement of eastern Panamá

Basement rocks of parts of eastern Panamá include tholeiitic pillow basalt and diabase overlain by sedimentary rocks typical of deep oceanic environments. Both paleontologic and stratigraphic evidence indicate that some of these rocks are of Late Cretaceous age or older. Regional Bouguer anomalies over the basement terrane exceed +120 mgal, indicating that eastern Panamá is a raised block of oceanic crust. Age relations in the Caribbean region apparently preclude an “Atlantic” or single intra-Caribbean origin for eastern Panamá and the southern Caribbean basin, but multiple intra-Caribbean origins of the basaltic basement rocks are permitted by the age data. An in situ origin of the oceanic basement of Panamá at a position which later became a Cenozoic island arc is likewise consistent with the available geologic and geophysical data. Seismic horizon B″ in the Caribbean is correlative with or overlain by deep-sea sedimentary rocks of Coniacian to Campanian age. This horizon forms the top of the apparent basement of eastern Panamá and, thus, the eastern isthmus is a horstlike block that has been elevated or obducted a minimum of 6 km since Coniacian-Maastrichtian time, partly caused by northeastward underflow of the Pacific plate but possibly related to southwestward movement of the Caribbean plate beneath the isthmus.

GSA Bulletin↗
USGS1973

Trans-Andean geophysical profile, southern Colombia

Negative Bouguer anomalies (−80 mgals) near the Pacific coast of southern Colombia define the position of the Tertiary Bolívar trough. Values increase eastward to a huge positive anomaly (+75 mgals) over Mesozoic “eugeosynclinal” rocks of the western Andes. This anomaly is part of the West Colombian gravity high, which extends from Panamá into western Ecuador and is caused by shallow mafic crust. Bouguer anomalies are strongly negative (−220 mgals) over pre-Mesozoic(?) metamorphic rocks, Mesozoic(?) granitic bodies, and Tertiary to Holocene volcanic rocks of the central Andes between Pasto and Ipiales. The steep gravity gradient between the West Colombian gravity high and the negative anomaly of the central Andes represents the transition between mafic crust to the west and continental crust to the east. This zone parallels the Romeral-Cauca megashear system. East of the Andes, Bouguer anomalies range from −50 to −120 mgals over a Mesozoic-Tertiary basin of the Putumayo district, indicating that the crust there is thinner or denser than it is beneath the central Andes. Models derived from gravity data suggest that the crust is about 45 km thick under the south-central Colombian Andes. If this is correct, the crust must thicken southward along the strike of the Andes, as thicknesses of 70 km have been reported in the Andes of southern Perú, Bolivia, and northern Chile by Lomnitz (1962) and James (1971a). Such differing crustal thicknesses may reflect different intensities of tectonic activity, greater crustal thickness indicating more intense or rapid growth of the volcano-plutonic arc or foreshortening of an existing crustal section.

Geological Society of America Bulletin↗
USGS1973

Regional gravity anomalies and crustal structure in northern Colombia

The central range of the Colombian Andes gives way northward to a series of Cenozoic fault-bordered basins and uplifts near the Caribbean Sea. Pre-Cenozoic structures exposed in the uplifts curve increasingly toward the east to become parallel to the continental margins along the south side of the Caribbean. Major Cenozoic faults, with large vertical and horizontal displacements, cut across older structures which include Permian-Triassic(?) and Late Cretaceous to early Tertiary metamorphic zones, Precambrian gneiss, and Jurassic batholiths. Gravity anomalies have large amplitudes in the Santa Marta area. Bouguer anomalies rise to +130 mgals over the crystalline rocks of the high Santa Marta massif. Over adjacent Cenozoic basins, they range down to −80 mgals over the Lower Magdalena basin and to −65 mgals over the Baja Guajira basin. Steep gravity gradients characterize the Santa Marta and Oca faults on the west and north sides of the massif, respectively. In the Guajira Peninsula region, Bouguer anomalies increase to +105 mgals over a serpentinite zone at Cabo de la Vela and to +55 mgals over Cretaceous volcanic rocks in the southern peninsula. Two smaller basins on the peninsula are characterized by negative Bouguer anomalies. Steep gradients characterize many of the major Cenozoic faults, and two concealed faults are postulated on this basis. Though useful for evaluating the relative vertical displacements, which may exceed 10 km along faults bounding the Santa Marta massif, the gravity data yield no definitive information on the large horizontal displacements postulated along some of the major faults of the area. The Bouguer anomalies do indicate, however, the extension of some of the Cenozoic basins into offshore areas. Strong positive Bouguer anomalies of the Santa Marta massif and its great relief, which exceeds 9 km relative to the floor of the adjacent Caribbean, indicate thin continental crust, lack of isostatic balance, and relatively recent uplift for the massif. After corrections are made for the gravitational effects of Tertiary sedimentary basins in the Guajira Peninsula, most of the peninsular region also has positive anomalies, suggesting a relatively thin continental crust and a lack of isostatic balance. A mechanism of overthrusting, in relatively recent time, of the continental margin over the adjacent Caribbean upper mantle and crust to the northwest can account for the observations.

Geological Society of America Bulletin↗
USGS1966

The Alaska earthquake, March 27, 1964: regional effects

This is the third in a series of six reports that the U.S. Geological Survey published on the results of a comprehensive geologic study that began, as a reconnaissance survey, within 24 hours after the March 27, 1964, Magnitude 9.2 Great Alaska Earthquake and extended, as detailed investigations, through several field seasons. The 1964 Great Alaska earthquake was the largest earthquake in the U.S. since 1700. Professional Paper 543, in 10 parts, describes the regional geologic effects.

Alaska↗
USGS1966

Gravity survey and regional geology of the Prince William Sound epicentral region, Alaska

Sedimentary and volcanic rocks of Mesozoic and early Tertiary age form a roughly arcuate pattern in and around Prince William Sound, the epicentral region of the Alaska earthquake of 1964. These rocks include the Valdez Group, a predominantly slate and graywacke sequence of Jurassic and Cretaceous age, and the Orca Group, a younger sequence of early Tertiary age. The Orca consists of a lower unit of dense-average 2.87 g per cm 3 (grams per cubic centimeter) pillow basalt and greenstone intercalated with sedimentary rocks and an upper unit of lithologically variable sandstone interbedded with siltstone or argillite. Densities of the clastic rocks in both the Valdez and Orca Groups average about 2.69 g per cm 3 . Granitic rocks of relatively low density (2.62 g per cm 3 ) cut the Valdez and Orca Groups at several localities. Both the Valdez and the Orca Groups were complexly folded and extensively faulted during at least three major episodes of deformation: an early period of Cretaceous or early Tertiary orogeny, a second orogeny that probably culminated in late Eocene or early Oligocene time and was accompanied or closely followed by emplacement of granitic batholiths, and a third episode of deformation that began in late Cenozoic time and continued intermittently to the present. About 500 gravity stations were established in the Prince William Sound region in conjunction with postearthquake geologic investigations. Simple Bouguer anomaly contours trend approximately parallel to the arcuate geologic structure around the sound. Bouguer anomalies decrease northward from +40 mgal (milligals) at the southwestern end of Montague Island to -70 mgal at College and Harriman Fiords. Most of this change may be interpreted as a regional gradient caused by thickening of the continental crust. Superimposed on the gradient is a prominent gravity high of as much as 65 mgal that extends from Elrington Island on the southwest, across Knight and Glacier Islands to the Ellamar Peninsula and Valdez on the northeast. This high coincides with the wide belt of greenstone and pillow basalt of the Orca Group and largely reflects the high density of these volcanic rocks. A large low in the east-central part of the sound is inferred to have a composite origin, and results from the combined effects of low-density sedimentary and granitic rocks. The Prince William Sound gravity high extends southwest-northeast without major horizontal offset for more than 100 miles. Thus the belt of volcanic rocks causing the high constitutes a major virtually continuous, geologic element of south-central Alaska.

Alaska↗