Geology topics
J. E. Case
Publications and source records attributed to J. E. Case.
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.
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.
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.
Complete Bouguer anomaly gravity map of the Albuquerque-Grants area, New Mexico
No abstract available.
Principal facts for gravity stations in the La Sal Mountains area, Grand and San Juan Counties, Utah, and Mesa and Montrose Counties, Colorado
Observed gravity values, station locations, terrain corrections, and Bouguer gravity data are provided as computer printout tabulations for the La Sal Mountains area, Utah-Colorado.
Preliminary geologic map and engineering geologic information, Oakland and vicinity, California
No abstract available.
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.
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.
Aeromagnetic map of parts of Marquette, Dickinson, Baraga, Alger, and Schoolcraft Counties, Michigan, and its geologic interpretation
The U. S. Geological Survey, in cooperation with the Geological Survey Division of the Michigan Department of Conservation, is conducting a comprehensive restudy of the iron-bearing districts of Michigan. The use of aeromagnetic data is one of the fastest and most reliable methods for outlining the areal distribution of magnetic iron-formation and thus is of material value in assessing the iron resources of the United States. As part of this program of restudy, aeromagnetic surveys of approximately 4,500 square miles of the Northern Peninsula have been made. The results of several of these surveys have been presented by Balsley and others (1949) and by Wier and others (.1953). This report is about a survey made in 1950, over an area of about 1,400 square miles in Marquette County and ad-· joining strips in northern Dickinson and northeastern Baraga Counties, and a later survey flown in 1961, over about 500 square miles in parts of Marquette, Alger, and Schoolcraft Counties. The Marquette iron range extends in an east-west direction across approximately the west-central part of the area, and iron districts of lesser importance are located at Republic, in the southwestern part of the area, and near Gwinn, in the south -central part of the area. Aeromagnetic surveys were flown along north-south lines spaced at intervals of one-quarter mile in the western part of the area and at intervals of from about 1 to 3 miles in the eastern part. Lines were flown at approximately 500 feet above the surface or at elevations of 1,100 to 2,400 feet above sea level. The flight path of the aircraft was recorded by a gyrostabilized continuous-strip camera, and elevation was continuously recorded by a radio altimeter. Magnetic measurements were made by an AN/ASQ-3A fluxgate magnetometer. Compilation of aeromagnetic data was by standard methods, described by Balsley ( 1952 ), under the supervision of John Kirby and Frank Petrafeso. The aeromagnetic contours are relative to an arbitrary datum. Magnetic properties of rocks were measured by W. E. Huff. At the time the earlier aeromagnetic survey was flown, the only base maps available were planimetric maps such as the Helena NW quadrangle and county road maps. Small errors, therefore, may exist in the location of flight lines and specific aeromagnetic a nomalies. The major magnetic anomalies and broad areas that have characteristic magnetic patterns are correlated herein with geology as determined from published reports and from mapping done since 1957 by the U. S. Geological Survey in the eastern part of the Marquette iron range. The geology north of the Marquette iron range and west of the line between R. 25 W. and R. 26 W. is based mainly on a compilation made by Robert Reed of the Geological Survey Division of the Michigan Department of Conservation from data in the files of the State Survey. In addition some aeromagnetic features have been checked by ground magnetometer in selected localities and correlated directly with the rock types. A few oriented samples have been collected for measurement of the magnetic properties of the major rock units, but many hundreds or thousands of samples would be required to obtain a full range of values of the magnetic susceptibility and remanent magnetization of the rocks over such a large area. Although the geology of much of the area is still imperfectly known, the available aeromagnetic data together with the best obtainable geologic data are presented at this time, rather than awaiting the eventual extension or completion of the present mapping program, because the combined data provide much information on the extent and location of magnetic iron-formation and other major rock units where concealed beneath glacial, vegetative, and alluvial cover.
Principal facts for gravity stations in the Wind River Basin area, Wyoming
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Aeromagnetic and geologic map of part of the Silver City mining region, Grant County, New Mexico
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Regional geophysical investigations in the La Sal Mountains area, Utah and Colorado
No abstract available.