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Two-and three-dimensional (3-D) seismic reflection data acquired over oceanic crust in the deepwater west Niger Delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading. Using the 3-D seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the Moho that are interpreted as thrusts. Thrust propagation results in the development of associated hanging-wall anticlines and footwall synclines. These structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them. The Charcot Ridge is located 140 km south of these thrusts and is a significantly larger structure. It is a triangular-shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the NE--SW-oriented Charcot Fracture Zone. Two interpretations are possible for the role of the fracture zone in the development of the Charcot Ridge: (1) A thin-skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward, with detachment occurring close to the level of the Moho. The ridge forms as a result of translation and folding above a crustal-scale ramp-flat thrust geometry. (2) A thick-skinned model where there is no detachment close to the Moho, with the thrust fault being much steeper, penetrating the crust and probably the mantle lithosphere. In this interpretation the structure formed owing to the compressional reactivation of the fracture zone. Approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 Ma ago. The Charcot Ridge represents one of the largest thrust structures to be identified in a passive margin setting. Many other compressional folds with the same orientation formed to the northeast in the Benue Trough, probably during the Santonian, as a result of a change in the spreading direction during South Atlantic rifting. We speculate that the same causal mechanism applies for the formation of the Charcot Ridge.

Three-dimensional seismic analysis is used to describe the detailed morphology of the Silverpit crater, a proposed marine impact crater located in the southern North Sea, UK. We examine and constrain for the first time the geometry, extent and age of the post-crater sediment units that define the crater fill. A circular bowl-shaped depression has been mapped and interpreted to represent the upper limit of impact deformation and the geometry of the crater at the end of cratering processes. Reflections exhibit a parallel onlap fill onto the bowl-shaped crater floor, indicating that accommodation space was created instantaneously without any contemporaneous sedimentation. Using regional lithostratigraphic markers constrained by microfossil and calcareous nannofossil analysis, we have dated the first onlapping sediments onto the crater. The age of the onlapping sediments allows us to constrain the crater formation to be Middle Eocene in age. This date is c. 10--15 Ma younger than the previous ascribed age for the crater formation and rules out the Silverpit crater as a K--T boundary impact. The age of crater formation considerably predates regional folding and accompanying salt flowage, effectively ruling out a salt withdrawal origin for the Silverpit crater.

Three-dimensional seismic reflection surveying is one of the most exciting technology-driven developments in the Earth Sciences over the past century. Three-dimensional seismic data present the geoscience community with an extraordinary ability to map structures and stratigraphic features in 3D detail to a resolution of a few metres over thousands of square kilometres. This resolving power has been a major factor in the recent recognition of a number of previously unsuspected phenomena in sedimentary basins and has led to some significant insights in basin analysis. This paper reviews some of the recent advances in basin analysis made using the medium of 3D seismic data, focusing on the broad field of fluid--rock interactions in sedimentary basins. Several recently discovered phenomena are reviewed, including polygonal fault systems and large-scale fluid flow pipes. Three-dimensional seismic data have also been invaluable in documenting the spatial variability of diagenetic fronts. The potential for future developments in this field of geophysical interpretation is considerable, and a number of potential areas for future research are highlighted.

Postdepositional sediment mobilization is a common phenomenon that occurs over large areas of sedimentary basins, usually within mudstone-rich successions; however, defining the exact mechanism that initiates this process has proved elusive. Here we describe a number of large-scale clastic intrusions (injectites), 50 to >300 m in height, hosted within fine-grained biosiliceous upper Eocene to Pliocene strata in four sedimentary basins. Within these basins diagenetic conversion of opal A into opal CT (cristobalite and tridymite) causes a marked reduction in sediment porosity and the liberation of bound water, often at relatively shallow burial ( Keywords: injectites, pore fluid, opal A, opal CT, overpressure.

We use 3D seismic data to describe the 3D geometry of a large igneous intrusion, the Solsikke Compound Sill, and address a number of issues related to sill emplacement. The Solsikke Compound Sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer-shaped depressions and linear discontinuities. One of the saucer-shaped sub-elements of the compound sill, the Solsikke Lobate Sill, has a previously unrecognized morphology. It has a basal feeder and consists of a bifurcating network of interlinked lobe-shaped sill segments. We propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments. The lobate pattern indicates that the Solsikke Lobate Sill was led at its deepest point and adopted its geometry through outwards and upwards propagation. The feeder location is coincident with a fault intersection, suggesting that magma transport from the underlying source exploited the zone surrounding the intersection.

Three-dimensional seismic interpretation of Tertiary sills within the northeast Atlantic margin demonstrates that shallow-level (20 km long. The fill of the minibasin is dated bio-stratigraphically as 54.6-55 Ma, which fits well with radiometric dates for the timing of intrusion of sills around this basin (ca. 53-55 Ma). The recognition and dating of onlap-fill seismic reflector packages that are delimited by forced folds form a new and useful method for dating shallow-level igneous intrusions in sedimentary basins, a method that provides independent constraints on radiometric dating techniques. Similar forced folds are recognized from other basins that have shallow-level sills, suggesting that the technique presented here may be similarly applicable in comparable geologic settings. Keywords: igneous intrusions, sills, dating technique, deformation, sediment-water interface.

This paper describes large-scale hummock structures from an Oligocene-Miocene succession in the Faeroe-Shetland Trough, off the northern coast of the United Kingdom. These hummocks have a wavelength of 1-2 km, and an amplitude of about 50 m; they are characterized by a polygonal planform geometry. Two successive depositional packages in this probably pelagic-dominated interval have this form of structural expression, but troughs of the younger set are directly superposed on crests of the older set. They occur at the present-day subsea-bottom depth of about 1000 m, and cover an area of approximately 6000 km(super.2). This extraordinary structural configuration is attributed to a protracted depositional and deformational history in which a density inversion was established, hummock formation was initiated in response to differential loading above an irregular polygonal fault system, and then subsequent collapse produced syndepositional troughs in the overlying units. These structures are similar in geometry to hummocks described from near-surface sediments in the Norway Basin by P.R. Vogt and are the largest type of density inversion deformation structure yet described from a clastic sedimentary succession.

This paper describes some simple graphical techniques for analyzing the kinematic evolution of growth faults and presents a case study of the late Pleistocene - recent history of a group of 17 faults from offshore Texas. Throw versus depth plots derived from depth-converted high-resolution seismic data were used to define growth histories. The faults exhibit polycyclic kinematic behavior, with at least three cycles of active fault growth separated by periods of inactivity. This cyclic behavior correlates broadly with three transgressive-regressive cycles along this part of the Gulf Coast. In detail, however, we find that activity of closely spaced faults in the array can either be in phase or out of phase with neighboring faults. This complex group behavior is attributed to the geometry of the detachment surface, pore-fluid pressure distribution, loading, and friction. This study demonstrates that although a general correlation with sediment loading may exist, individual fault activity is unlikely to correlate with periods of maximum sediment accumulation.