Northeastern Segment Research Articles

Structural Traps in Duplexes Within the Pine Mountain Thrust System, Southern Appalachians: ABSTRACT

The Rose Hill and Ben Hur oil fields in the Pine Mountain thrust system are classic examples of structural traps formed by duplexes in the southern Appalachian thrust belt. Integration of surface, seismic, and well data has enabled the construction of eight balances cross sections and an interpretation of the three-dimensional geometry and kinematic development of structural traps in the area. In the southwestern and northeastern segments of the Pine Mountain thrust block, the structure consists of a simple ramp-related anticline, with the shortening decreasing from 70,000 to 10,000 ft toward the northeast. In the central segment, the Pine Mountain thrust is folded by one or more footwall thrusts, resulting in a system of overlapping ramp anticlines (class III duplex), with the total shortening varying between 54,000 and 28,000 ft. Structural traps in the Rose Hill and Ben Hur oil fields occur within Ordovician and Silurian units in the crest of the Bales I thrust sheet. These traps formed in areas with the maximum structural relief prior to refolding by subsequently formed footwall thrusts. The porosity and permeability of the reservoirs are enhanced by fold-related longitudinal and transverse fractures at high angles to bedding. Faults are typically characterized by amore » few inches of gouge surrounded by a brecciated and imbricated permeable zone several feet or tens of feet wide. This suggests that while the faults acted as impermeable barriers between different thrust sheets, they also provided effective pathways for communication within each sheet during the migration of hydrocarbons.« less

Thermal implications for the evolution of the spitsbergen transform fault

Heat flow taken between Svalbard and Greenland reveal three thermal provinces: 1. (1) the Molloy Ridge within the Spitsbergen Transform, 2. (2) the Yermak Plateau 3. (3) the northeastern margin of Svalbard (Nordaustlandet). The Molloy Ridge is a short spreading segment and the average heat flow is much above the Sclater et al. (1971), cooling curve but agrees with values from the Norwegian-Greenland Sea. An additional zone of intrusion identified by heat flow lies to the northwest of the Molloy Ridge. It straddles both the visible fracture zone and part of the Yermak Plateau. A thermal boundary lies between the warm western segment of the Yermak Plateau and the shelf off Nordaustlandet. If the thermal subsidence of the western Yermak Plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m.y. (approximately 20 m.y. younger than the northeastern segment of the plateau). Plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7. We postulate that the original transform is associated with the Hornsund Fault zone. In response to deviatoric stress across the oblique ridge-transform system, the Nansen Ridge propagated southwestward aborting the old transform trace, and shifted to its present position. It is suggested that this propagation and migration of the ridge-transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the Western Yermak Plateau. The propagation phenomenon coincides with large-scale Tertiary volcanic activity on Svalbard. Readjustment and migration of the oblique transform is still taking place. As the transform-ridge system is liberated from continental constraints, the migration rate will diminish as orthogonality is approached.

Geophysical investigations in the Dead Sea

Bathymetric, magnetic, and continuous seismic measurements were made along approximately 540 km of track in the Dead Sea in July and August of 1974. A new precision bathymetric map was prepared, using these measurements, together with all earlier soundings. The northern basin is shown to be shallower than previously thought, and almost flat bottomed over nearly 40% of its area. The maximum depth, relative to Mediterranean sea level, is 730 m (i.e. a water depth of 332 m below the Dead Sea level at the time of the survey), rather than the 798 m reported for Lt. Lynch's 1848 survey. The basin is asymmetric, with western slopes generally averaging 7° and eastern slopes averaging 30° except in the vicinity of the Arnon River delta where slopes are around 8°. The western flank of the basin is quite corrugated, and along the northwestern flank of the basin the topography becomes steeper, following a linear trend of N27°E which is probably fault controlled. Sediments from the Jordan River delta have built up a smooth and almost planar ramp sloping 3° to depths of 650 m (below MSL), and 1° thereafter into the deep basin. Revised hypsometric calculations show inflection points in the area curve at 425 and 725 m, reflecting the flat-floored nature of the northern basin. The volume of water enclosed with sea level at 400 m is 147 km 3, an increase over earlier estimates. The total field magnetic anomaly map with 5 gamma contour interval shows several interesting features, the anomalous field has an eastward dip with gradients of 5 to 20 gammas per kilometer. Magnetic anomalies (10 to 20 gamma amplitude) were detected east of Ein Gedi, Kallia, and the Jordan River delta as well as in other locations. The E—W trending magnetic anomaly north of Ein Gedi may be an extension of a similarly trending magnetic anomaly noted by Folkman and Yuval (1976) across the Hebron mountains, while the pronounced eastward magnetic gradient may reflect a transition from a less continental type of crust on the west to a more continental type of crust east of the Rift. Diapiric structures induced by the rock salt masses of the Sedom Formation were detected all over the northern basin. They are mainly concentrated along the major western submarine N—S trending border fault, and are probably genetically associated with Mt. Sedom. Similar structures, although smaller in size and fewer, are also found along the steep eastern border fault. Two large, dome-shaped diapiric structures have pierced through the bottom of the median trough: one off Ein Gedi (the Ein Gedi Diapir) and the other at the northeastern segment of the Jordan Delta (the Jordan Delta Diapir). They seem to be genetically related to the diapir of the Lisan Peninsula and to others which were detected south of it in the southern basin. The diapirism processes have been active since deposition of the Amora (Foothills) Formation and especially during deposition of the Lisan Formation (Late Pleistocene). Great thicknesses of younger sediments have been synchronously accumulated in the associated ring depressions. Almost complete desiccation occurred at the end of the Lisan Formation (some 15,000 to 10,000 years B.P.). The rate of deposition which is mostly controlled by the huge quantities of chemical precipitates, have compensated for and exceeded the rate of diapirism. The Jordan Delta was built out and accumulated only since the end of the desiccation phase. The erosional drainage pattern generated during and just after the desiccation phase, was partially masked by the younger Holocene sediments. The eastern border faults exhibit a systematic pattern: a major N—S trending linear fault from which secondary faults periodically branch off to the NNE. The trends of the individual faults in the western border fault system are less regular and uniform. They comprise, however, a system of step-faults. Two NW trending faults border the Lisan Peninsula on its north, and south. No E—W trending fault was found beneath the Jordan Delta to mark the northern edge of the northern basin.

Geological framework of the Brazilian continental margin

Based on distinctive stratigraphic and/or structural characteristics, the Brazilian continental margin can be divided into two main provinces: (1) The southeastern-eastern province, extending from the Pelotas to the Recife-Joao Pessoa Basin, presents a tensional tectonic style of Late Jurassic-Early Cretaceous age, paralleling the structural alignements of the Precambrian basement, except in the north-eastern segment where the Mesozoic faults of the Recife-Joao Pessoa Basin cut across the east-west basement directions. The basin-fill, Upper Jurassic through Recent, consists, where complete, of three stratigraphic sequences, each of a distinct depositional environment: (a) a lower clastic non-marine sequence; (b) a middle evaporitic sequence, and (c) an upper clastic paralic and open marine sequence. (2) The northern province, extending from the Potiguar Basin to the Amazon Submarine Basin, displays both tensional and compressional tectonic styles of Upper Jurassic (?) to Upper Cretaceous age either paralleling or cutting transversally the basement alignments. The stratigraphic column differs from the southeastern-eastern province in lacking the Lower Cretaceous evaporitic rocks.

Soft X-ray profile and spectrum of the supernova remnant IC 443

The spectrum and emission profile of IC 443 are presented in the 0.5 to 3 keV band. The data are fitted by power-law and bremsstrahlung emission models with interstellar absorption. A characteristic temperature of 10(+18, -4) -10/sup 6/ K with a hydrogen column density of 3.5(+2, -1.7) -10/sup 21/ atoms cm/sup -2/ is obtained. The emission profile is presented for a scan along the northwest-southwest axis with 9' resolution. The X-ray emission is centered at ..cap alpha.. (1950) =6/sup h/13/sup m/45/sup s/ +- 15/sup s/, delta=22degree29' +- 4', with the strongest emission found in the northern part of the remnant. The results indicate a gross correlation with the enhanced radio and optical activity in the northeastern segment. Interpretation within the standard shock-wave model yields an age of 4000(+2500, -1800) years, in conflict with some other determinations. A possible reconciliation exists if the X-ray emission arises primarily in shocked intercloud medium, as suggested by McKee and Cowie, and the optical and radio emission arises primarily in shocked clouds. In this scenario the pulsar PSR 0611+22 may not be associated with the remnant. The constraints on the soft X-ray absorption to the remnants are used to set a lower limit of 1more » kpc for the distance to the remnant. (AIP)« less

Tectonic Setting of the Northeastern Reading Prong

The Reading Prong is one of the major areas of exposure of Precambrian crystalline rocks in the north-central Appalachians. The extent of its participation in early Paleozoic tectonic activity has long been controversial. Although work by Drake (1969) has shown that the southwestern portion of the prong is characterized by a series of allochthonous Precambrian slices that overlie lower Paleozoic rocks, no evidence to support significant overthrusting of Precambrian rocks is found in the northeastern segment of the prong. In this area, geophysical and geological data indicate that the tectonic setting of the Precambrian rocks is a result of several periods of large-scale basement uplift along northeast-trending, high-angle faults. These data further suggest that the regional configuration of the northeastern Reading Prong was established prior to the Taconic orogeny.

Patterns of Coastal Sedimentation: Carbonate Muds of Florida Bay: ABSTRACT

Muddy carbonate sediments of Florida Bay have accumulated in response to hydraulic processes characteristic of coastal environments. These processes are reflected in faunal distribution as well as physiography of the accumulations. The frequently encountered coastal sedimentary pattern of lakes, and mainland veneer is expanded laterally in Florida Bay because of topography of the underlying Pleistocene rock surface. In Florida Bay the dominant physiographic pattern consists of circular lakes of deeper water surrounded by curvilinear banks and islands. The banks, composed predominantly of mud sediment, reach within a foot or so of mean sea level and are largest in the western bay nearest the open Gulf of Mexico. The northeastern (interior) segment of the bay is characterized by narrower banks, in many places exposed subaerially as islands. Spitlike accretion is apparent from growth lines on islands and some banks. This indicates locally directed currents; however, overall randomness of orientation and circular patterns of sediment distribution suggest that significant currents develop in all directions. The larger submerged banks of the outer bay display prominent accretion lines and are in addition elaborately channeled. The channeling follows a distinctive cycle of establishment and decline that seems closely related to bank growth. Current control of deposition of muddy sediments is reflected also in the ancient sedimentary record, notably the Pennsylvanian Virgil mounds near Alamogordo, New Mexico, and Pennsylvanian Lansing mounds in southeastern Kansas. Sediment-baffle processes previously proposed for the construction of mound-topography appear unneeded inasmuch as current processes may achieve similar results. End_of_Article - Last_Page 632------------

Geology of West Bastian Bay Field, Plaquemines Parish, Louisiana: ABSTRACT

West Bastian Bay field, one of the largest gas fields in the south Louisiana Gulf Coast area, produces from beds of late Miocene age between 6,950 and 15,500 ft. Structurally, the field is an anticline on the southern downthrown side of a fault whose throw exceeds 3,000 ft. The center of uplift moved progressively northwestward for a distance of 2 mi. from its X Sand position near the southeastern edge of the field. The fairly recent development of a new locus of uplift near the eastern edge of the field has given the field a double closure at the level of the uppermost Miocene beds. A small, long-quiescent fault with 60-100 ft. of throw across the axis of the fold and the eastern part of an axial fault separate the deep reservoirs in the northeastern segment of the fie d from the deep reservoirs in the other segments. The fact that gas columns in this area are thinner than elsewhere along the apex of the fold is believed to be the result of the presence of the nearby East Bastian Bay field and the eastward shaling out of the W and X Sands. End_of_Article - Last_Page 2323------------

The Simpson tunnel

With ongoing Alpine convergence and deformation during exhumation there is a general progression from ductile to brittle behaviour on the Periadriatic Fault and kinematically related faults farther north (e.g. the Simplon–Rhone Fault and a diffuse, discontinuous zone of generally dextral strike–slip movement on the southern boundary of the Aar massif). K–Ar dating of fine grained illite from clay fault gouges provides a reliable method for establishing the maximum age of this transition to brittle faulting and, for short-lived faults, the approximate time of faulting itself. The new results establish that brittle faulting on the northeastern segment of the Canavese Fault (i.e. the part of the Periadriatic Fault southwest of Lago Maggiore) occurred around 20 Ma, with south-side-up kinematics. A K–Ar age of ca. 17 Ma for the crosscutting Giudicarie Fault in the Eastern Alps is effectively identical with an already published pseudotachylyte age and places a lower limit on major and continuous strike slip movements of the Periadriatic Fault. The age of brittle faulting farther north, on the southern border of the Aar Massif, is from 13.6 ± 0.3 to 8.3 ± 1.1 Ma, consistent with the younger cooling and exhumation in this area. The dominantly dextral brittle faulting becoming younger to the north reflects the continued indentation and anticlockwise rotation of Adria as Alpine units become exhumed and progressively welded to the southern block. In contrast to the Eastern Alps, there is no field evidence for (westward) lateral extrusion of Alpine units relative to Adria and the European foreland.