Abstract This volume is about the evolution of the North America Plate. It also briefly discusses the evolution of the Pacific Ocean and the Caribbean as they relate to North America. We will briefly sketch the present outlines of the North America Plate, review the plate-tectonic development of North America in a global context, and offer an overview of the major tectonic (Fig. 1) and geomorphic elements of our continent. Earthquakes are primary indicators of present plate boundaries and of plate motions that are related to these boundaries. The outline of the North America Plate is shown clearly on Figure 2, which is based only on the distribution of earthquakes occurring from 1977 to 1987. Note also the good definition of the Caribbean and Cocos Plates. In the Arctic regions (Fig. 3), earthquakes clearly outline the plate boundary along the mid-ocean ridge up to the north coast of Siberia. However, from there southward, across northeastern Siberia, the margin of the North America Plate is quite diffuse. Substantial intraplate earthquakes (Fig. 2) have occurred particularly within the United States. The 1811 to 1812 New Madrid (Missouri) and the 1866 Charleston (South Carolina) earthquakes exceeded magnitude 7 and occurred in areas that are still active today (Seeber and Armbruster, 1988; Hinze and Braile, 1988). New England is another area of intraplate seismic activity. According to Zoback and others (1986), the midplate stress of the North America Plate is compressive, with a maximum horizontal principal stress oriented northeast to east northeast. The stress field extends from the Rocky Mountain front to within 230 km of the Mid-Atlantic Ridge.

Abstract Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface. Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant. Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time. The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the "real world" in their teaching. The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.

ABSTRACT Broadly interrelated assemblages of geologic structures constitute the fundamental structural styles of petroleum provinces. These assemblages generally are repeated in regions of similar deformation, and their associated hydrocarbon traps can be anticipated prior to exploration. Styles are differentiated on the basis of basement involvement or detachment of sedimentary cover. Basement-involved styles include wrench-fault structural assemblages, compressive fault blocks and basement thrusts, extensional fault blocks, and warps. Detached styles are decollement thrust-fold assemblages, detached normal faults ("growth faults" and others), salt structures, and shale structures. These basic styles are related to the larger kinematics of plate tectonics and, in some situations, to particular depositional histories. Most styles have preferred plate-tectonic habitats: (1) wrench faults at transform and convergent plate boundaries; (2) compressive fault blocks and basement thrusts at convergent boundaries, particularly in forelands and orogenic belts; (3) extensional fault blocks at divergent boundaries in all stages of completion and certain parts of convergent boundaries; (4) basement warps in a variety of plate-interior and boundary settings; (5) decollement thrust-fold belts in trench inner walls and foreland zones of convergent boundaries; (6) detached normal faults, usually in unstable, thick clastic wedges (mostly deltas); (7) salt structures primarily in interior grabens that may evolve to completed divergent boundaries; and (8) shale structures in regions with thick overpressured shale sequences. Important differences in trend arrangements and structural morphologies provide criteria for differentiation of styles. These differences also result in different kinds of hydrocarbon traps. Wrenchrelated structural assemblages are concentrated along throughgoing zones and many have en echelon arrangements. The basic hydrocarbon trap is the en echelon anticline, in places assisted by closure directly against the wrench fault itself. Compressive and extensional fault styles typically have multiple, repeated trends, which combine to form zigzag, dogleg, or other grid patterns. Their main trap types are fault closures and drape folds above the block boundaries. Basement warps (domes, arches, etc.) are mostly solitary features and commonly provide long-lived positive areas for hydrocarbon concentration in broadly flexed closures.

Abstract Figure 1 (left) is a common-shot-point gather of synthetically made seismic traces using a random number generator. These data would represent the zero end point of the signal-to-noise ratio spectrum -a ratio commonly used to measure the reflection signal strength as compared to random ambient noise. Figure 2 (center) is a final stacked section made from a collection of these random field files. The processing sequence that resulted in this figure is common to the industry (elevation static corrections, sort to common-depth-point gathers, normal velocity moveout corrections, etc.). These data will necessarily still be random, but an interpreter who is extremely hardened to working in poor record areas may see some chance alignments in this display. Perhaps this can be more clearly seen by viewing the section obliquely. Figure 3 (right) results when these final stacked traces are mathematically manipulated by a new modern processing technique. In essence, this figure results when severe trace mixing is the end result of a process that is more sophisticated at face value. Here, the chance alignment is enhanced by the mixing process. Whereas this particular method would not be misused by the scrupulous processing geophysicist, Figure 3 should alert anyone who commonly works with seismic data in poor record areas to the possible dangers of signal enhancement beyond reasonable limits.

ABSTRACT Migration is recognized as the essential step in converting seismic data into a representation of the earth's subsurface structure. Ironically, conventional migration often fails where migration is needed most - when the data are recorded over complex structures. Processing field data shot in Central America and synthetic data derived for that section demonstrates that time migration actually degrades the image of the deep structure that lies below a complicated overburden. In the Central American example, velocities increase nearly two-fold across an arched and thrustfaulted interface. Wavefront distortion introduced by this feature gives rise to distorted reflections from depth. Even with interval velocity known perfectly, no velocity is proper for time migrating the data here; time migration is the wrong process because it does not honor Snell's law. Depth migration of the stacked data, on the other hand, produces a reasonable image of the deeper section. The depth migration, however, leaves artifacts that could be attributed to problems that are common in structurally complicated areas: (1) departures of the stacked section from the ideal, a zero-offset section; (2) incorrect specification of velocities; and (3) loss of energy transmitted through the complex zone. For such an inhomogeneous velocity structure, shortcomings in common midpoint stacking are directly related to highly non-hyperbolic moveout. As with migration velocity, no proper stacking velocity can be developed for these data, even from the known interval-velocity model. Proper treatment of nonzero-offset reflection data could be accomplished by depth migration before stacking. Simple ray-theoretical correction of the complex moveouts, however, can produce a stack that is similar to the desired zero-offset section. Overall, the choice of a velocity model most strongly influences the results of depth migration. Processing the data with a range of plausible velocity models, however, leads to an important conclusion: although the velocities can never be known exactly, depth migration is essential for clarifying structure beneath complex overburden.

Abstract Seismic sections almost always display significant examples of primary structures, that is structures that are primarily the product of sediment deposition or have an igneous origin, and that were not affected by later structural deformation. We, therefore, solicited a number of examples illustrating structurally undisturbed stratigraphic features, igneous structures, and the layering of the lower crust. Section 112 of this atlas complements the many publications which have recently appeared on the subject of seismic stratigraphy. The reader should particularly refer to AAPG Memoir 26, edited by C.E. Payton, 1977. This classic volume will serve for some time as the foremost introductory text on the matter. In particular, we refer to the series of articles published by Gil et al (1977) on seismic stratigraphy and global changes of sea level. Additional material that highlights some of the geophysical aspects is offered by Anstey (1980), Neidell (1980), and Sheriff (1980). Section 121 contains some high frequency profiles that offer a high resolution of stratigraphic and structural details. The reader is advised to initially focus on the paper by Bouma et al, because that paper provides an overview of the differences obtained by applying different high frequency techniques. Section 122, on unconformities, 123 on illustrating sequences, 124 on carbonates, and 125 on clastics offer perhaps a somewhat artificial subdivision of various aspects of seismic stratigraphy. Note that a number of profiles occurring in other sections of this atlas also contain very fine examples of seismic stratigraphy. This applies particularly to a number of papers in section 222 on rifts, 223 on passive margins, 224 on cratonic basins (atlas Volume 2), and some of the foredeep profiles included in section 341 on decollement tectonics (atlas Volume 3). We obtained a number of profiles across various igneous structures (section 13) and would like to point out that there is an additional profile across a volcanic seamount in the paper by Lehner et al on the Tonga Trench (section 342, atlas plume 3). Section 14 contains a profile across a presumed impact structure. We would like to obtain more examples of similar features, because of their importance for the evaluation of the current hypothesis on mass extinction (for a summary see McLaren, 1983, and Silver, 1982). Section 15 includes a number of crustal profiles that were not easily included under some of the other subdivisions. But it should be noted that additional crustal profiles are presented in section 221 (atlas Volume 2), crustal profiles across extensional provinces, and section 321 (atlas Volume 3), crustal profiles across compressional provinces.

Abstract Single channel, shallow penetration, high-resolution seismic reflection profiling systems are commonly used by research geologists and by the offshore service industry. The increased resolution produced by high-resolution seismic systems allows for more detailed interpretation of geological phenomena in the upper sedimentary column, which is of paramount importance for platform design and pipeline routes. In many cases, the seismic data are collected as analog records, although industry is increasing its use of digital acquisition systems. Rather than direct analog paper records only, most nondigital users apply analog taping because it permits replaying of rough input data at different filter settings to remove part of the acoustic noise, enhance some of the resolution, and increase penetration. Many groups use more than one acquisition system simultaneously; the different frequencies result in differences in penetration and resolution. The disadvantages of using a multisensor seismic system are acceptable and normally only result in cross talk between the systems. Most of the common high-resolution reflection systems fit into the following categories (Sieck and Self, 1977; Sylwester, 1982): 1) tuned transducer-frequency range 3.5 to 7.0 kHz, subbottom penetration to 30 m (100 ft); 2) electromechanical-frequency range 0.8 to 5.0 kHz, subbottom penetration to 120 m (390 ft); 3) sparker-frequency range 0.04 to 0.150 kHz, subbottom penetration to 1000 m (3,300 ft); and 4) airgun-frequency range 0.02 to 0.50 kHz, subbottom penetration to 3000 m (10,000 ft). Heavier sparkers and larger airguns are used for medium and deeper penetration (3 to 5 seconds two-way traveltime). In this chapter, we present examples with our interpretation of geological phenomena of Upper Pleistocene and Holocene deposits from the Gulf of Mexico continental margin off Texas and Louisiana and from the Mississippi Fan collected with 3.5 kHz, minisparker, and small airgun systems (Figure 1, Table 1). To demonstrate the differences in penetration and resolution, each geologic phenomenon is presented as it is recorded by the individual systems.

Abstract Over 260,000 sq km (96,525 sq m) of the outer continental margin of southwest Africa have been affected by Neogene slump structures. They occur along the whole of the margin between the eastern Walvis Ridge and the southern tip of the African continent (about 200S to 370S) North of about 300S, four discrete slump zones have been recognized, but to the south, the margin has been cut by several (at least four) coalescing features which form an almost continuous zone over 850 km (527 mi) long. Three seismic profiles and interpretative sections are shown to illustrate the geometry and associated structures of these slumps (Figure 1) - two from the large feature (Chamais Slump) immediately north of 300S (sections A and B) off southern Namibia, and one from the complex zone southwest of Cape Town (section C). Further details of the geological setting and overall geometry of these and similar large allochthonous structures on the nearby southeast continental margin can be found in Emery et al (1975), Dingle (1977, 1979), Bolli et al (1978), Summerhayes, Bornhold, and Embley (1979), and Dingle, Seisser, and Newton (1983). Figure 2 diagrammatically shows the main morphological features that have been used by various workers to identify large slumped masses from seismic profiles. In the following account of the seismic profiles A-C, vertical scales are shown in seconds of traveltime (two-way time), which in the interpretive sections have been converted to water depths at 1,500 m/sec (4,921 ft/sec) (meters on the left, fathoms on the right). In the discussion, thicknesses of strata are quoted in meters, based on a nominal P-wave velocity of 2,000 m/sec (6,562 ft/sec) in the sediment. On the interpretive sections, thick lines indicate glide planes, and opposed arrows show directions of relative movement along some of the larger glide planes. Slope inclinations are calculated from the horizontal, and quoted as a ratio (for example, 1 in 10 =1:10).

Abstract Figure 1 shows a conventional "Vibroseis" data acquisition effort across a stratigraphic channel sand. A 58-12 Hz sweep was used with the data recorded for 24-fold stack. The data have been processed to maximally emphasize the objective zone at a depth of 4,700 ft (1,432 m). This objective depth is marked on the right side of the seismic section at about 0.9 seconds with a label identifying the known sand channel location. Figure 2 shows high frequency data recorded along the same line. Again, the zone of interest is shaded gray in Figure 3, a marked version of 2, at a depth of 4,700 ft (1,432 m). The data was recorded with a 90-25 Hz sweep, and excellent 909 Hz penetration was obtained at the objective depth. The increased frequency content is apparent and provides adequate resolution to map 30 ft (9 m) sands. The limits of the sand channel are easily seen on the high-frequency data, whereas extensive interpretation is required to locate the channel on the conventional data.

Abstract The seismic line in Figure 1 is a high-frequency data example applied to a coal exploration problem. The location of the line is shown on the map of the Illinois basin. The line was recorded with two objectives in mind. First, to seismically map a shallow coal seam at a depth of 530 ft (161 m). Second, to map sand channels in the coal seam, before mining, in order to plan an efficient mine layout. The data was recorded using a sweep of 260-50 Hz, and as is certainly apparent, the line met both objectives. On the right side of Figure 2 (a marked version of Figure 1) the reflection from the low velocity (8,000 ft/sec, 2,438 m/sec) coal seam is uniform and moderately high in amplitude. As the low velocity is replaced by high velocity (13,000 ft/sec, 3,962 m/sec) sandstone, the coal event i reduced in amplitude, virtually disappearing on most of the left half of the line. The data does an excellent job of defining the 12-ft (3.6-m) coal seam.

Abstract Figure 1 is a migrated high-frequency seismic section from the Arkoma Basin, Oklahoma. Figure 2 is an interpreted version of Figure 1, with two objective zones marked by arrows. The first zone, a shallow gas sand, exists at depth A (1,067 m, 3,500 ft) and the second zone, a faulted horizon, is at depth B (2,438 m, 8,000 ft). Figure 3 is the same high-frequency seismic section before seismic migration. The high frequency content in both Figure 1 and 3 gives excellent vertical resolution. However, the lateral resolution in Figure 3 is quite poor as evidenced by faults that are not easily detected. In contrast, the faults are readily identified in Figure 1. The difference between Figure 1 and Figure 3 illustrates quite dramatically how lateral resolution of seismic data may be greatly improved by seismic migration. Furthermore, this generalization applies not only to structurally complex areas but also to areas with seemingly monotonous "layer-cake" stratigraphy. Frequency content at reflection A objective (Figure 2) is approximately 100 Hz with sufficient bandwidth to uniquely map the limits of the producing sand. Further interpretive work with inter active modeling supported gas sand thickness estimates from seismic data. Frequency content at the deeper horizon B is about 80 Hz. The 20 Hz of high-frequency loss between A and B objectives is apparently due to attenuation in the thick, dominantly shale section between these two seismic markers. This shale section is approximately 1,371 m (4,500 ft) thick. When coupled with migration frequency content at the deeper zone is certainly sufficient to accurately map faults as shown in Figure 2. The fault on the right exhibits about 35 msec or 53 m (175 ft) of vertical displacement.

Abstract The reflections from beds below the unconformity surface terminate abruptly at various eroded portions of the profile. The chaotic reflection pattern immediately above the unconformity is possibly due to the disorder of the reflecting surfaces. The reflections being fragmentary and of low amplitude suggest that the strata were deposited in variable settings of relatively high energy. The depressed parts of the unconformity surface could be channels of various depths.

Abstract For a complete discussion of the horizons present in this area of the margin, see the Jaunich paper (Part I) and composite seismic line in section 223 (in plume 2 of the atlas). This is an example of buried paleotopography in the Tertiary sequence and also shows clearly the R-P drift-onset unconformity. Topography in this case was probably caused by slumping.

Abstract Along the edge of the interior salt basin in the southeastern United States there is a high relief unconformity. This unconformity was developed during deposition of the Jurassic Louann Salt and was buried by Jurassic sediments during the subsequent marine transgression related to the divergence of the South and North American continents and subsidence of the basin. The morphology of the unconformity may be characteristic of early stages of continental plate divergence. A seismic line near the edge of the salt basin in south Alabama demonstrates the acoustic response along the unconformity. The area described in this paper is located in the southeastern United States along the edge of the interior salt basin (Figure 1). The unconformity is Jurassic in age and can be identified from Florida to Texas. Because it is regionally extensive, it truncates a wide variety of older rocks. The major units it truncates are Paleozoic sediments of the Appalachian and Ouachita orogenic belts, metamorphic rocks associated with these orogenic events, and Mesozoic continental sediments of the Neward group which were deposited during the early stages of divergence. There were four Jurassic units younger than the unconformity in the area shown on the cross section (Figure 2) and interpreted record section (Figure 3). These are the Norphlet, Smackover, Haynesville, and Cotton Valley. In Alabama, the Norphlet is a shallow marine and eolian sandstone; the Smackover is a shallow marine carbonate; the Haynesviller is a prograding sabkha and continental sequence; and the Cotton alley is a continental sandstone sequence. There are unconformities at top of Haynesville and Cotton Valley which modified the topography of the older unconformity where is was not buried. The seismic section (Figures 3, 4, 5) is located in south Alabama just north of the updip edge of the salt. It is a 24-fold, common-depth-point (CDP) dynamite line shot in 1980. The line parallels regional dip and is 5 mi (8 km) in length. Well control indicates that the rocks beneath the unconformity are granulites and mica-rich metamorphic rocks. The area is interpreted to be the subsurface equivalent to the Brevard zone of the Appalachians.

Abstract The use of seismic profiles to recognize and map potential stratigraphic traps is becoming an increasingly important exploration tool, especially within the more mature hydrocarbon provinces of the world. Seismic stratigraphy often requires not only high resolution seismic data, but also considerable geologic input to interpret features indicative of possible stratigraphic traps. Fortunately, one of the most readily recognized traps of this nature, the simple stratigraphic pinchout, does not require specially acquired seismic data. The composite line illustrated in this text shows such a feature. Although this "pinchout" may not seem overly impressive, it demonstrates the trapping mechanism for the giant East Texas field located on the east side of the East Texas Tyler) basin in northeast Texas. This field was discovered in 1930, by random drilling, long before seismic was considered an effective exploration tool. It is obvious, however, that the trap is easily recognized on reasonably good seismic data, and could readily be mapped with such data. The trap is basically a stratigraphic pinchout of the Upper Cretaceous Woodbine formation. The Woodbine sand-shale sediments were truncated by erosion on the west flank of the elevated Sabine Uplift. These sediments are unconformably overlain by the Austin Chalk which serves as the main top seal for this trap. The bottom seal is the open marine shales and limestones of Lower Cretaceous age. The illustrations provided with this text show the geologic setting and parameters associated with this classic stratigraphic trap. The relatively low velocity sand-shale section of the Woodbine overlying the high velocity, predominantly open-marine, carbonates of the Lower Cretaceous results in an excellent seismic interface mapable over most of the East Texas basin. Velocities associated with the Austin Chalk are usually sufficiently fast enough to also result in a well defined seismic interforce at the base of the Chalk. The velocity contrast between these three lithologic sequences is great enough to allow these

Abstract The effects of strong circulation for the generation of erosional surfaces in the deep sea are observed in the region of the Rio Grande Gap, Western South Atlantic. Conspicuous unconformities are observed on multichannel seismic lines shot by the University of Texas Institute of Geophysics (UTIG) research ship R/V Fred Moore in July 1979, while surveying the Rio Grande Gap and the Brazil basin for future locations of DSDP sites. The Rio Grande Gap, a low basement area with an average width of 150 km (93 mi), is located between the Rio Grande Rise and the basement high to the west (Figure 1). The Rio Grande Gap is the major connection between the Argentine basin to the south and the Brazil basin to the north. The 600-km (372-mi) long Vema Channel stretches along the western limit of the Gap (Figure 1). This channel allows significant quantities of northward-flowing Antarctic Bottom Water (AABW) to enter the Brazil basin. Abroad terrace extends from the Vema Channel to the base of the Rio Grande Rise. Regional seismic studies of this part of the South Atlantic (for example, Le Pichon et al, 1971; Gamboa, 1981) have revealed a complex depositional history in the area, mainly related to the onset and fluctuations of the Antarctic Bottom Water circulation. The purpose of this paper is to present some examples of these erosional and depositional events identified on seismic lines across the Rio Grande Gap and the southern portion of the Brazil basin. Analyses of UTIG multichannel seismic data in the Rio Grande Gap allow us to distinguish four major seismic sequences within the sedimentary cover of the region. The sediments in the Rio Grande Gap are about 1.2 km (.7 mi) thick and the sequences are designated by letters A to D from the base to the top (Figures 2 and 3). Sequence A lies on a strong reflector inferred to be top of oceanic crust. In general, the basement reflector is fairly smooth, but in places considerable relief is observed, which appears to indicate offset by faulting. Sequence A is characterized by weak (relatively low amplitude) but continuous subparallel internal reflectors. The lower part of this sequence onlaps and fills the relief on the basement. The upper limit of Sequence A is defined by a prominent regional unconformity (unconformity A) which truncates this sequence at several places. This unconformity is a fairly smooth and level surface and probably marks a major change in the bottom-water circulation through this area. Sequence B is acoustically transparent, showing only few discontinuous reflectors. Sequence B thins and pinches out locally beneath the axis of the Vema Channel. The upper boundary of Sequence B is a prominent regional reflector, unconformity B. Sequence B probably represents a regime of restricted sedimentation controlled by deep sea currents, which began to affect the Rio Grande Gap area. Initiation of these currents probably eroded sequence A to produce unconformity A. Sequence C forms the major part of the terrace to the east of the Vema Channel. This sequence is characterized by dipping (prograding) and contorted internal reflectors (Figures 2 and 3). In cross section Sequence C is somewhat similar in geometry to an alluvial terrace (Figures 1, 2 and 3). This sequence represents a striking change in the sedimentation pattern in the Rio Grande Gap area. Its dipping layers indicate a progradation of sediments transported along the bottom, which filled the gap and formed the terrace to the east of the Vema Channel.

Abstract For a complete discussion of the horizons present in this area of the margin, see the Jaunich paper (Part I) and composite seismic line in section 223 (in plume 2 of the atlas). This is an example of buried paleotopography in the Tertiary sequence and also shows clearly the R-P drift-onset unconformity. Topography in this case was probably caused by slumping.

Abstract The Albo-Aptian extension creates tilted, rapidly subsiding, fault blocks filled with thick marly sediments. Reefs are located on high fault blocks (Figures 2a and b). During the Upper Cretaceous, and related to the orogeny of the Pyrenees to the south, a foredeep filled with flysch formations is being formed. Toward the north this foredeep merges into a carbonate platform that is located on a flexure associated with older Albo-Aptian fault systems. The accentuation of the Pyrenees orogeny during the Eocene leads to the northward migration of the flysch trough which remains bounded to the north by the carbonate biothermal shelf edge, with a restricted marine platform to the east. After filling of the flysch trough by mid-Eocene time, sedimentation in the region is dominated by Eocene-Oligocene fluvio-deltaic deposits. The southern portion of the profile shows the structurally complex north front of the Pyrenees with associated "injections" of Triassic salt.