Dale S. Sawyer

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.

Gulf of Mexico Plate Reconstruction by Palinspastic Restoration of Extended Continental Crust: ABSTRACT

A number of recently published Gulf of Mexico plate reconstructions are strikingly dissimilar. There are no sea floor magnetic lineations, and the sizes and shapes of the continental blocks are not well-defined. Perhaps the only common feature of the several reconstructions is that they ignore the role of continental crust extension during rifting. In this study, total tectonic subsidence analysis was used to estimate the mount of crust extension in the Gulf of Mexico to determine its effects on the proposed plate reconstructions. This involves the calculation and mapping of the sediment-unloaded basement depth from observations of the basement depth, water depth, and sediment compaction properties. The well-known depth-age relation for oceanic crust and a model for the subsidence of extended continental crust allowed within the limits of available data the identification and mapping of crust type and the amount of extension of transitional crust. The zone of extended continental crust under the northern margin of the Gulf is extraordinarily wide, more than 800 km (500 mi) in a cross section through east Texas. The zone of extended crust to the south is much narrower, about 150 km (90 mi) on the margin of the Yucatan Block. Palinspastic restoration shows that the total 950 km (590 mi) of extended and thinned continental crust corresponds to 490 km (300 mi) of continental crust of original thickness. Therefore 460 km (280 mi) of crustal extension occurred during rifting and prior to ocean crust formation. The 460 km (280 mi) of extension along this cross section, and the results of similar calculations on other cross sections, must be accounted for properly when reconstructing the prerift configuration of the Gulf of Mexico. End_of_Article - Last_Page 525------------

Exploration History, North U.S. Atlantic Margin: ABSTRACT

The Baltimore Canyon Trough is the site of 26 exploration wells and two stratigraphic tests. As of November 1981, six dry holes had been drilled on the Great Stone Dome. This structure appeared to be the largest and most promising in the basin. Seventeen wells have been drilled along the edge of the continental shelf with significant hydrocarbon shows reported from five wells. Combined daily flow rate is 90 mmcf. This flow is approximately one-half the amount required to warrant construction of a production platform and pipeline. Georges Bank basin is characterized by an older thick carbonate and evaporite sequence (0 to 8 km) of Late Triassic-Early Jurassic age; a middle sequence of interbedded limestone, sandstone, mudstone, and red shale of Middle Jurassic to Early Cretaceous age (0 to 2.5 km); and a thin sequence (middle Cretaceous and younger) of transgressive shelf limestone and regressive claystone and siltstone (0.5 to 2 km). Elevated patch reefs beneath the shelf and a massive reeflike carbonate buildup under the slope form potential hydrocarbon traps. The patch reefs, which are elongate to circular and as much as several kilometers across, have caused a broad arching of younger strata. They may be built on salt swells or elevated basement blocks. A two-dimensional, finite-difference simulation of the main basins thermal history of crustal stretching and subsidence suggests that some of the oldest sedimentary sections over the seaward part of rift-stage crust and extending out to oceanic crust are thermally mature for oil generation. End_of_Article - Last_Page 545------------

Distribution of Oceanic Versus Transitional Crust in Deep Gulf of Mexico Basin--Implications for Early History: ABSTRACT

Regional studies of seismic reflection and refraction data in the deep Gulf of Mexico basin outline in considerable detail the distribution of oceanic vs. transitional crust. Oceanic crust forms a narrow east-west belt up to 300 km wide across the deep Gulf. Most current models for early Gulf evolution suggest the belt was emplaced in the Late Jurassic following widespread deposition of salt on rifted and attenuated continental crust (transitional crust). The southern boundary is defined by a zone of prominent salt structures along the northern margin of the Sigsbee salt basin. The northern boundary is obscured below the Texas-Louisiana slope, but is inferred from the distribution of large vertical salt structures. The eastern boundary is clearly marked by onlap and pinch out of thick Jurassic sedimentary sequences. This distribution is corroborated by regional magnetic and gravity data and total tectonic subsidence analysis, and provides constraints for early Gulf basin reconstructions. An appropriate reconstruction must account for plate motion accommodated by ocean crust formation and extension of continental crust. The data seem most consistent with a model in which the Yucatan block moved generally south and rotated somewhat counterclockwise. This reconstruction implies very little lateral displacement along transform faults between Yucatan and Florida during early basin history. This is supported by seismic stratigraphic studies and DSDP drilling in the southeastern Gulf. End_of_Article - Last_Page 241------------

Distribution of Crust and Early History, Gulf of Mexico Basin: ABSTRACT

ABSTRACT Studies of an extensive regional grid of multifold seismic data and refraction data in the deep central Gulf of Mexico combined with studies from surrounding regions provide a generalized picture of the geologic framework and tectonic setting of the basin. Various criteria, including seismic reflection data, seismic refraction data, depth-to-basement, gravity, magnetics, distribution of Jurassic salt and sediments, and total tectonic subsidence analysis, provide a basis for characterizing and mapping the distribution of oceanic crust, thin transitional crust, and thick transitional crust or continental crust. This distribution of crust provides constraints for a reconstruction of the Gulf area, which involves closing up, first, oceanic crust and then transitional crust. This reconstruction provides room to accommodate any overlap of South America with Yucatan and supports a counter-clockwise rotation of Yucatan out of the northern Gulf. It is compatible with other published reconstructions that treat Yucatan as a separate block independent of South America, particularly ones that rotate Yucatan out of the Gulf in a counter-clockwise manner. This reconstruction also is compatible with a general model for the early evolution of the Gulf basin that includes 1) a Late Triassic to Middle Jurassic rift stage and formation of transitional crust, culminating with the widespread deposition of evaporites, 2) a brief Late Jurassic period of oceanic crust formation in the deep central Gulf, 3) a Late Jurassic through Early Cretaceous period of cooling and subsidence of the crust and buildup of extensive carbonate platforms surrounding a deep basin, and 4) formation of a widespread middle Cretaceous unconformity (MCU).