John A. Grow

Regional Geologic Framework Off Northeastern United States

Six multichannel seismic-reflection profiles taken across the Atlantic continental margin off the northeastern United States show an excess of 14 km of presumed Mesozoic and younger sedimentary rocks in the Baltimore Canyon trough and 8 km in the Georges Bank basin. Beneath the continental rise, the sedimentary prism thickness exceeds 7 km south of New Jersey and Maryland, and it is 4.5 km thick south of Georges Bank. Stratigraphically, the continental slope--outer edge of the continental shelf is a transition zone of high-velocity sedimentary rock, probably carbonate, that covers deeply subsided basement. Acoustically, the sedimentary sequence beneath the shelf is divided into three units which are correlated speculatively with the Cenozoic, the Cretaceous, and the Jurassic-Triassic sections. These units thicken offshore, and some have increased seismic velocities farther offshore. The uppermost unit thickens from a fraction of a kilometer to slightly more than a kilometer in a seaward direction, and velocity values range from 1.7 to 2.2 km/sec. The middle unit thickens from a fraction of a kilometer to as much as 5 km (northern Baltimore Canyon trough), and seismic velocity ranges from 2.2 to 5.4 km/sec. The lowest unit thickens to a maximum of 9 km (northern Baltimore Canyon), and velocities span the 3.9 to 5.9-km/sec interval. The spatial separation of magnetic and gravity anomalies on line 2 (New Jersey) suggests that in the Baltimore Canyon region the magnetic-slope anomaly is due to edge effects and that the previously reported free-air and isostatic gravity anomalies over the outer shelf may be due in part to a lateral increase in sediment density (velocity) near the shelf edge. The East Coast magnetic anomaly and the free-air gravity high both coincide over the outer shelf edge on line 1 (Georges Bank) but are offset by 20 km from the ridge on the reflection profile. Because the magnetic-slope-anomaly wavelength is nearly 50 km across, a deep source is likely. In part, the positive free-air gravity anomaly likewise may represent the significant lateral density increase within the sedimentary section to ard the outer edge of the shelf.

Crustal structure beneath the southern Appalachians: Nonuniqueness of gravity modeling

Gravity models computed for a profile across the long-wavelength paired negative-positive Bouguer anomalies of the southern Appalachian Mountains show that the large negative anomaly can be explained by a crustal root zone, whereas the steep gradient and positive anomaly east of the root may be explained equally well by three different geometries: a suture zone, a mantle upwarp, or a shallow body. Seismic data support the existence of a mountain root but are inadequate to resolve differences among the three possible geometries for the positive anomaly. The presence of outcropping mafic and ultramafic rocks in the southern Appalachians and the inferred tectonic history of the Appalachian orogen are most consistent with the suture-zone model. Crust similar to continental crust probably exists beneath the Coastal Plain and inner continental shelf where the gravity anomalies return to near-zero values.

IPOD-USGS multichannel seismic reflection profile from Cape Hatteras to the Mid-Atlantic Ridge

A 3,400-km-long multichannel seismic-reflection profile from Cape Hatteras to the Mid-Atlantic Ridge was acquired commercially under contract to the National Science Foundation and the U.S. Geological Survey. These data show evidence for massive erosion of the continental slope, diapirs at the base of the continental slope, and mantle reflections beneath the Hatteras Abyssal Plain.

Seismic refraction study of the continental edge off the eastern united states

Abstract Three long, strike-parallel, seismic-refraction profiles were made on the continental shelf edge, slope and upper rise off New Jersey during 1975. The shelf edge line lies along the axis of the East Coast Magnetic Anomaly (ECMA), while the continental rise line lies 80 km seaward of the shelf edge. Below the unconsolidated sediments (1.7–3.6 km/sec), high-velocity sedimentary rocks (4.2–6.2 km/sec) were found at depths of 2.6–8.2 km and are inferred to be cemented carbonates. Although multichannel seismic-reflection profiles and magnetic depth-to-source data predicted the top of oceanic basement at 6–8 km beneath the shelf edge and 10–11 km beneath the rise, no refracted events occurred as first arrivals from either oceanic basement (layer 2, approximately 5.5 km/ sec) or the upper oceanic crust (layer 3A, approximately 6.8 km/sec). Second arrivals from 10.5 km depth beneath the shelf edge are interpreted as events from a 5.9 km/sec refractor within igneous basement. Other refracted events from either layers 2 or 3A could not be resolved within the complex second arrivals. A well-defined crustal layer with a compressional velocity of 7.1–7.2 km/sec, which can be interpreted as oceanic layer 3B, occurred at 15.8 km depth beneath the shelf and 12.9 km beneath the upper rise. A well-reversed mantle velocity of 8.3 km/sec was measured at 18–22 km depth beneath the upper continental rise. Comparison with other deep-crustal profiles along the continental edge of the Atlantic margin off the United States, specifically in the inner magnetically quiet zone, indicates that the compressional wave velocities and layer depths determined on the U.S.G.S. profiles are very similar to those of nearby profiles. This suggests that the layers are continuous and that the interpretation of the oceanic layer 3B under the shelf edge east of New Jersey implies progradation of the shelf outward over the oceanic crust in that area. This agrees with magnetic anomaly evidence which shows the East Coast Magnetic Anomaly landward of the shelf edge off New Jersey and with previous seismic reflection data which reveal extensive outbuilding of the shelf edge during the Jurassic and Lower Cretaceous, probably by carbonate bank-margin accretion.

Seismic stratigraphy and tectonic development of Virgin River depression and associated basins, southeastern Nevada and northwestern Arizona

Virgin River depression is a Neogene basin, with a surface area that exceeds 1,500 km 2 in the Basin and Range structural province of south-eastern Nevada and northwestern Arizona. The depression formed within the foreland of the Sevier orogenic zone, a region that was characterized in Paleogene time by a flat-lying section of Cambrian to Cretaceous platform strata about 5 km thick. Well data from Mobil Virgin 1A on Mormon Mesa reveal 2,000+ m of Neogene basin fill that consists mostly of the Muddy Creek Formation (4-10 Ma), the red sandstone unit (10-12 Ma) of Bohannon (1984), and the Lovell Wash Member of the Horse Spring Formation (12-13 Ma). Seismic reflection data from six primacord and two vibroseis lines show that the Muddy Creek Formation uniformly fills Virgin River depression to a depth of 1-2 km. Two older and less-extensive basins, the Mormon and Mesquite, lie beneath the Muddy Creek and are separated from one another by a complex buried ridge. The basins are mostly filled with rocks of the red sandstone unit and Lovell Wash Member to depths locally exceeding 6 km. Two older members of the Horse Spring Formation, the Rainbow Gardens and Thumb, also occur in Mormon basin (the western one), where they rest disconformably on the pre-Tertiary strata. The basins are east-tilted half grabens that are bounded on the east and southeast by large listric normal fault systems. The faults that bound Mormon basin are buried by the Muddy Creek Formation, but the Piedmont fault, on the east side of Mesquite basin, cuts Quaternary alluvium. The Virgin River depression formed in three stages. The period from 24-13 Ma is characterized by slow subsidence in Mormon basin and little noticeable deformation of the basin substrate. The Mormon and Mesquite basins became fully differentiated during the period from 13-10 Ma. This stage is associated with large displacements on the normal faults bounding both basins and the buried ridge. Proterozoic crystalline rocks were exposed locally, providing a source for part of the red sandstone unit deposited in the basins. Tectonic denudation during the 13-10 Ma stage locally removed large amounts of the pre-basin section. By 10 Ma, most of the fault activity had ceased, the ridge between the basins was overlapped, and Virgin River depression began to subside uniformly over a wide area. This stage lasted until the commencement of the modern period of dissection associated with the Colorado River. Our structural analysis suggests that upper crustal extension within the basin, mostly during the 13-10 Ma stage, might have exceeded 60%. The basin subsidence was partly due to extension in the upper crust and partly due to viscous flow in the deeper crust beneath the basin. It is not clear to us what caused the uplifts that flank the depression, but isostatic rebound due to tectonic denudation remains aviable possibility.

The gravity field of the U.S. Atlantic continental margin

Abstract Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from −20 to −80 mgal occurs along the base of the continental slope, except for a −140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW—SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod. Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between −10 and −30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10–20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the relationship of isostatic gravity anomalies to the magnetic anomalies and the ocean—continent transition is variable. Two-dimensional gravity models have been computed for two profiles off Cape Cod, Massachusetts and Cape May, New Jersey, where excellent reflection, refraction and magnetic control appear to define 10 and 12 km deep sedimentary basins beneath the shelf, respectively and 10 km deep basins beneath the rise. The basins are separated by a 6–8 km deep basement ridge which underlies the ECMA and appears to mark the landward edge of oceanic crust. The gravity models suggest that the oceanic crust is between 11 and 18 km thick beneath the ECMA, but decreases to a thickness of less than 8 km within the first 20–90 km to the southeast. In both profiles, the derived crustal thickness variations support the interpretation that the ECMA occurs over the ocean-continent boundary. The crust underlying the sedimentary cover appears to be 12 to 15 km thick on the landward side of the ECMA and gradually thickens to normal continental values of greater than 25 km within the first 60 to 110 km to the northwest. Multichannel seismic profiles across platform areas, such as Cape Hatteras and Cape Cod, indicate the ocean-continent transition zones there are much narrower than profiles across major sedimentary basins, such as the one off New Jersey.

Jurassic Seismic Stratigraphy and Basement Structure of Western Atlantic Magnetic Quiet Zone

The basement structure and acoustic stratigraphy of Jurassic sedimentary rocks within the western Atlantic Jurassic magnetic quiet zone north of lat. 31°N have been examined in multichannel and single-channel seismic reflection profiles. East of the Blake Spur magnetic anomaly, the basement relief is dominated by northwest-trending ridges and troughs associated with fracture zones. The only prominent basement features paralleling the magnetic lineations generated by seafloor spreading are west of the Blake Spur magnetic anomaly. A large scarp, downdropped to the west, is parallel with and just west of the Blake Spur magnetic anomaly. Both the scarp and the Blake Spur magnetic anomaly are present only south of lat. 36°N. Three flat-lying conformably acoustic hori ons have been identified within the Jurassic sedimentary rocks below horizon s. The uppermost horizon, J1, onlaps oceanic basement in the vicinity of magnetic anomaly M-23 (age 142 m.y.). The next lower horizon, J2, is west of and terminates against the scarp associated with the Blake Spur magnetic anomaly (inferred age 165 m.y.). The lowest horizon, J3, extends from the East Coast magnetic anomaly halfway to the Blake Spur anomaly. Sedimentation rates for the sedimentary units below these three horizons are estimated at 18 m/m.y. between horizons J1 and J2, and greater than 300 m/m.y. for the units below horizon J2.

Structure of the northern Brazilian continental margin

Results from two-ship seismic refraction profiles and supplementary data from several sonobuoys show that typical oceanic crust underlies both the landward and seaward sides of the North Brazilian Ridge. The Ceara Rise is also an oceanic structure, probably formed by local tectonic activity. Sediments of the upper Amazon Cone are more than 11 km thick, but they thin both landward and seaward. Owing to the great sedimentary thickness, refraction arrival times from oceanic basement could not be identified, but basement is assumed to extend under the cone.

Structure of the Békés Basin Inferred from Seismic Reflection, Well and Gravity Data

The Bekes basin (areal extent 3900 km2) is a northwest-trending, Neogene basin located in southeast Hungary. The basin contains over 6500 m of synrift and postrift sedimentary fill. Middle Miocene synrift deposits are relatively thin and no Paleogene rocks have been reported to be present. The prerift section (basement) is composed of Mesozoic carbonate and clastic rocks and Paleozoic and older volcanic, igneous and metamorphic rocks. The Mesozoic rocks represent dominantly shallow-water environments and are up to 5000 m thick in the Bekes-Doboz Mesozoic trough and 2000 m thick in the Battonya-Pusztafoldvar Mesozoic trough.

Subsidence, Crustal Structure, and Thermal Evolution of Georges Bank Basin

A geophysical study of Georges Bank basin defines a deep crustal structure that is interpreted in terms of the basins tectonic and thermal history. Gravity models along three basin cross sections delineate two zones of crustal thinning at the basement hinge zone and oceanic crustal margins. These two zones bound rift-stage crust (about 25 km thick) which underlies the central portion of the basin. Subsidence analysis of the basin, using data from multichannel seismic reflection lines and two COST wells, suggests a rifting and (uniform) extensional origin. Two-dimensional finite difference modeling of the basin defines a crustal structure that concurs with the gravity and subsidence studies. The resulting isotherms show no major changes in the thermal structure since the ate Jurassic. In some areas of the basin, temperatures sufficient for oil generation are determined from maturation studies of Jurassic sediments. Hydrocarbon generation is questionable, however, because of the probable lack of proper and sufficient kerogen in the Jurassic deposits.