Abstract Three fundamental styles of structural deformation have formed the margins of the continents since late Paleozoic time. The structural and sedimentary histories of the shelves and slopes appear to be genetically related to either a single simple structure style or to multihistory deformation in which two or even three different styles have been operative. The following discussion relates specifically to the sedimentary sequences occurring at the margins of our continents; not to the intracratonic rift basins, sinks, and sags (i.e. North Sea, Sirte Basin, etc.) that indent margins of many continents or to “drowned cratons” (i.e. The Sunda Shield, etc.). One of these styles is characterized by extensional tectonics, a result of the “break-up” of the supercontinents of Gondwana and Laurasia which occurred primarily during the Mesozoic and early Cenozoic eras. In contrast, during middle and late Cenozoic times plate movements have deformed many plate boundaries by “shear-zone tectonics.” This second style of margin has been subjected to divergent extension and/or convergent compression as components of the synthetic shearing. Subduction, occurring at the “leading edges” of plates in motion, is the third basic style of continental margin deformation. Within this tectonic framework may be found both frontal and marginal basins. The structural, sedimentary, and volcanic histories of these subduction margins involve complex geologic relationships and pose extremely difficult exploration problems. Direct subduction, oblique subduction, island-arc collision, continental collision, and “flipped” Benioff zones are some of the scenarios depicting such margins. In spite of the great thicknesses of both marine and

Abstract Existing geological and geophysical information shows that the physiographic components of continental margins (continental shelves, continental slopes, and continental rises) in different regions have characteristics that depend upon their tectonic and sedimentary histories. These characteristics also determine whether significant oil and gas accumulations probably are present, may be present, or probably are absent. Most promising are the oil and gas prospects of divergent continental margins whose sediment accumulations are large and less disturbed than those of convergent margins; translation margins are intermediate. Better knowledge of all continental margins, particularly in their middle parts, awaits exploration by deep drilling. Without such information estimates of the Earth's total oil and gas resources must remain very incomplete.

Abstract Active continental margins and the active flanks of island arcs lie in the forearc regions of arc-trench systems generated by plate consumption. Arc-trench systems are initiated by contractional activation of previously rifted continental margins, by reversal of subduction polarity following arc collisions, and as island arcs within oceanic regions. The varied configurations of shelved, sloped, terraced, and ridged forearcs arise partly from differences in initial geologic setting, but mainly from differences in structural evolution during subduction. Forearc terranes enlarge during subduction through linked tectonic and sedimentary accretion of deformed ocean-floor sediments and igneous oceanic crust, uplifted trench-floor and trench-slope sediments, and the depositional fills of subsiding forearc basins. Trench inner slopes typically are underlain by growing subduction complexes composed of imbricate underthrust packets of ocean-basin, trench-floor, and trench-slope sediments in thrust sheets, isoclines, and melanges. The Structure of subduction complexes is governed by the thickness and nature of oceanic layers rafted into the subduction zone, variable thicknesses of trench and slope sediments, and the rate and obliquity of plate convergence. Forearc basins between the magmatic arc and the trench axis include (a) intramassif basins lying within and resting upon basement terranes of the arc massif, (b) residual basins resting upon oceanic or transitional crust trapped between the arc massif and the site of initial subduction, (c) accretionary basins resting upon accreted elements of the growing subduction complex, and (d) composite basins resting upon more than one of the foregoing basement types. Strata deposited in forearc basins are typically immature clastic sediments composed of unstable clasts derived from rapid erosion of volcanic mountains or uplands of plutonic and metamorphic rocks within the arc massif. In equatorial regions reef carbonate associations are also common. Facies patterns of turbidites, shelfal sequences, and fluvio-deltaic complexes within forearc basins are governed by the elevation of the basin thresholds, the rate of sediment delivery, and the rate of subsidence of the substratum. Petroleum prospects in forearc regions typically are limited by small, obscure structures within the subduction complex, scarcity of good reservoirs in the forearc basin, often immature source beds, and low geothermal gradients except within the arc massif where heat flux is commonly excessive.

Abstract Continental slopes and rises are often the sites of high organic productivity because of nutrients supplied by deep water upwelling and river runoff. This marine organic matter has a much higher petroleum convertibility than terrestrial material which predominates on continental shelves. Incorporation of organic matter into sediments prior to oxidation is essential and requires reducing conditions as occur where irregularities in bottom topography result in closed anoxic basins. Slope sediments average 0.6 to 1.0 weight percent organic carbon and are typically the most organic-rich continental margin deposits. Conversion of organic matter to petroleum requires appropriate time-temperature conditions influenced by the geothermal gradient, the sedimentation rate, and the age of the source section. Deep wells on continental shelves have shown that on pull-apart margins, 2 to 4 km of burial is required for oil generation and 3 to 7 km for gas generation. Warmer compressional margins need less burial, but this is compensated in part by much younger rocks. Most of the sedimentary section under present day slopes and rises is not mature enough to have generated petroleum and many of the deeper intervals were deposited in unfavorable source environments. The section beneath slopes and rises contain oil and gas source beds only where organic content and maturity requirements are met. The most effective generation and expulsion occur in areas of rapid burial or high geothermal gradients. Migration and accumulation are most efficient where reservoir sequences prograde over deeply buried source intervals and in areas of structural complexity. Source beds for large amounts of oil and gas do exist on slopes and rises, but they are the exception rather than the rule. The challenge is to find them.

Abstract Potential hydrocarbon reservoir rocks may be emplaced on or under the outer continental margin by several different processes or combinations of processes: I. Normal processes of distribution of sediments are reasonably well understood from studies of the modern oceans, but the lessons of these studies cannot be applied indiscriminately to interpretation of pre-Quaternary sediments and continental margins because of the profound effects of Quaternary sea level fluctuations. II. The products of these processes of deposition may be displaced relative to present sea level by several mechanisms. These displaced facies may constitute prospective reservoir rocks only if they could otherwise have been favorable before displacement and if their reservoir characteristics are not adversely affected during the displacement processes. IIA. Eustatic sea level changes of the past may have resulted in accumulation of shore zone sediments, for example, as much as a few hundred meters below the present shore zone. IIB. Crustal thinning and block faulting are common during early rifting stages of formation of what become intraplate continental margins, followed by subsidence of porous and permeable continental, shore zone, and shallow marine sediments to abyssal depths, where they may be covered by pelagic sediments and/or prograding wedges of normal continental margin facies. IIC. The process of subduction may be accompanied by transfer of sediments from the underthrusting plate to the leading edge of the overriding plate, followed by uplift in imbricate thrust sheets to the crest of a non-volcanic ridge or edge of a marginal plateau lying outboard of a volcanic arc or Andean Type continental margin. This phase of uplift may then be followed by subsidence under the forearc basin. IID. Sediment accumulations of outer shelf or slope environments may be displaced into deeper water base-of-slope environments by submarine sliding. Such slides or olistostromes may occur on either intraplate or plate edge margins. Slope instability may result from either depositional or tectonic over-steepening and may be triggered by either oceanographic or tectonic disturbance.

Abstract The depositional facies on slopes and rises depend in large part on the tectonic processes and sea level changes, as well as rate and type of sediment supply. Seismic examples of slope and rise depositional facies from divergent, convergent, and strike-slip continental margins are presented and discussed in terms of sea level changes and sediment supply. Seismic sequence and seismic facies analyses are effective methods for recognition of depositional facies on slopes and rises and studying the interrelationships between tectonic processes, sea level changes, and sediment supply. Seismic sequence analysis is based on the identification of stratigraphic units composed of a relatively conformable succession of genetically related strata termed depositional sequences, Fig. 1. The upper and lower boundaries of depositional sequences are unconformities or their correlative conformities. The time interval represented by strata of a given sequence may differ from place to place, but the range is confined to synchronous limits marked by ages of the sequence boundaries where they become conformities. Depositional sequence boundaries are recognized on seismic data by identifying reflections caused by lateral terminations of strata termed onlap, downlap, toplap, and truncation, Fig. 1. The depositional sequences, because they consist of genetically related strata having Chronographic significance, provide an ideal stratigraphic interval for seismic facies analysis. Seismic facies analysis is the deliniation and interpretation of reflection geometry, continuity, amplitude, frequence, and interval velocity, as well as the external form and associations of seismic facies units. Once the seismic facies parameters are described and mapped, an interpretation of

Abstract The earth's crust is a thin dynamic shell which is constantly changing in both thickness and composition resulting from sub crustal processes and the lateral interaction of crustal plates. Continental margin basins can be related to a cycle of construction and destruction of continental crust. Such processes form depressions of various types within the earth's crust which hoast continental margin basins. The type of crust which underlies a continental margin basin determines the basin's physical framework, manner of structuring and bottom configuration at the time of deposition. This, in turn, controls the condition of sedimentation throughout the life of the basin. Schematic models depicting the various basin types demonstrate the basic tectonic and stratigraphic processes involved in their respective development.

Abstract A brief review of exploration history of continental shelves and slopes is followed by a discussion of the different methods of assessment of exploitable hydrocarbons. The first step in deep water exploration now underway is for subsided portions of former continental shelves. Exploration techniques and methods in these regional are the same as those used in unexplored basins of the continental shelf. Far more important by size and volume are the large sedimentary wedges in the oceanic portions of marginal basins below the continental rise. Accumulation in these poorly structured basins must be predominantly in stratigraphic traps. In order to make these potential resources exploitable, the exploration risk must be reduced by improving and developing methods and tools which allow localizing hydrocarbon accumulations and estimating recoverable volumes by means of geophysics and geochemistry prior to drilling.

Abstract Since 1972, the U. S. Geological Survey has been carrying on an extensive program of geophysical and geological investigations of the structure of the Atlantic continental margin of the United States. Results of the work in the Baltimore Canyon Trough area are based on profiles covering a distance of about 5,000 km of 24-and 48-channel CDP reflection seismic data, a high-sensitivity aeromagnetic survey, and gravity, and seismic refraction data integrated with subsurface geologic information from the approximately 5-km deep COST hole B-2, several 300-m cores from the USGS Atlantic margin drilling program (1976) and other core information. Sedimentary rock thicknesses as much as 15 km underlie the continental shelf east of New Jersey. Relatively high seismic velocities (3½ to 5 km/sec) increase with depth from 2½ to 15 km. These velocities are associated with rocks marked by moderately low amplitude (−20 mgal) gravity anomalies and indicate the presence of higher density older rocks underlying the continental margin in the area compared with the Gulf of Mexico. Acoustic and magnetic basements appear to be coincident near the coast at a depth of 1 km. They deepen seaward to 10 km about 50 km offshore, further seaward they diverge. In the area of the upper slope, the deepest magnetic horizons (about 6 to 9 km) are shallower than the deepest seismic reflectors, suggesting magnetic contrasts within the sedimentary rock section. There is no evidence in the CDP seismic data for the previously postulated “basement ridge” beneath the outer shelf. The best available recent data suggests the possibility of a reef (?) buried about 5 km beneath the upper slope. The East Coast magnetic anomaly is interpreted as the boundary between oceanic and continental crust. A domal structure (−39°23′N, 73°05′W) 20 to 30 km in diameter is marked by magnetic and gravity anomalies and prominent seismic reflectors. An intrusive body of probable Cretaceous age which has its top at a depth of about 3½ km is the inferred source or these geophysical anomalies.

Abstract Immediately before the rifting and disruption of Pangaea, that great continent lay athwart the Triassic equator. The continental interior was arid, and isostatic adjustments following rifting produced drainage flowing away from the newly-formed elongate troughs. These climatic and physiographic conditions led inevitably to the deposition of evaporites as the sea entered the rift zones whose margins now form the edges of the continents around the Atlantic. As a result, the sediment sequence in the Atlantic coastal basins shows a change from continental to evaporitic to normal marine conditions, except where evaporites were deposited directly onto newly-formed oceanic crust. The age of these evaporites gives a chronology for the establishment of a permanent body of water between the separting fragments of Pangaea. The North Atlantic appeared in the north in late Triassic time and became established further south by Middle Jurassic. The South Atlantic became established during Aptian time. The Gulf of Mexico came into existence in Middle Jurassic time by the subsidence of continental material. The union of the two parts of the incipient ocean came about during or later than Albian time and the resulting free circulation of water in the Atlantic eliminated the restriction necessary for the continued accumulation of evaporite deposits. No such deposits are known, therefore, from the northern coast of Brazil or from the northern side of the Gulf of Guinea.

Abstract Great thicknesses of Triassic and younger sediments are observed seismically in the Blake Plateau-Bahama area. Basement is thought to be from 7 to 14 km deep below high velocity carbonates. Ridge-like reflectors along the Blake Escarpment are found by drilling to be Cretaceous reefal rim-complexes. Deep wells indicate that this rim facies extends southeast along the Bahama Platform margin. The Gulf Stream apparently has swept the continually subsiding Blake Plateau during much of the Cenozoic creating this deeper water area; while contour currents eroded the Blake-Bahama Escarpment and deposited muds to form the Blake-Bahama Outer Ridge. Deep water Bahamian channels developed in the Cretaceous and Cenozoic between the accreting carbonate banks; while Jurassic salt has apparently domed in Exuma Sound. The crustal type, oceanic vs. continental, beneath the Blake Plateau-Bahamas is still problematic. Other geophysical data indicate that the igneous basement is about 10 km deep and that the crust is of intermediate seismic velocity and density. Red Sea - Afar Triangle crustal types might be the modern analog, agreeing with Plate-tectonic reconstructions and the Atlantic Margin rifting in Triassic-Jurassic. The Blake-Bahama area has great petroleum potential. Fault structures, regional tilting over reef-complexes, and possible salt domes are good for traps. Porous dolomite horizons and cavernous limestones have been drilled, but flushing of these excellent reservoirs is unfavorable for Cretaceous and younger rocks. Jurassic carbonate targets offer potential reserves with carbonate sources being likely. Mexican Golden Lane - Poza Rica fields and Smackover analogies are possible.

Abstract The sedimentary evolution and petroleum potential of the Gabon Coastal Basin are closely tied with plate tectonics. Two kinds of objectives associated with the two main phases can interest the petroleum geologists: The proto-oceanic rift - some detrital deposits in the rift have a great extension as the “gres de base” and the GAMBA Formation, while Lucina and Dentale formations have only a local extension. All of them can constitute good reservoir beds. Source rocks are proved to be in situ. They have to be located in the organic rich levels of the same formations. Traps are associated with extensional tectonics of the rift. They are situated on the fringes of old moles and horts. Evolution and embankment of the Continental Margin of the growing ocean. Salt deposit occurred at the end of the rift phase and/or the very beginning of the oceanic opening. Structural features of the overlying series are conditioned mainly by salt tectonics. The progradation of the continental edge is done by a succession of overlapping lenses. The BATANGA, ANGUILLE and Tertiary formations are supposed to be deep sea fans. These hydrocarbon bearing formations have been fed by surrounding marine shale source rocks.

Abstract The rapidly converging Nazca and South American plates produce a complex geotectonic framework along the Peru continental margin. Rupture of oceanic layer 2 within segments of the Peru-Chile Trench forms large scale basalt ridges and disturbed axial turbidite basins adjacent to the margin. The shallow dipping (10–15°) oceanic slab extends tens of kilometers beneath the continental slope as a coherent, but often faulted feature, becoming less coherent landward. The relation between this slab and a high velocity (5.7–6.2 km/sec) metamorphic block which is the foundation for continental shelf basins is unclear. Prominent upper continental slope basins contain up to 2 km of sediment (1.6–3.0 km/sec). A metamorphic block forms the basement of the landward portions of the upper slope basins, whereas a thick highly diffracting section (>3.0 km/sec) underlies the seaward part. Smaller basins may occur in the middle slope region. Landward migration of the deposition centers in upper slope basins suggests uplift of the outer margin and therefore the accretion of trench sediments. Upper slope deposits are either complexly faulted or essentially undisturbed. Large ocean plate features, namely the Nazca Ridge, appear to inhibit the development of slope basins. Although there is no clear indication of imbricate thrust sheets within the continental slope seismic reflection sections, the ruptured oceanic slab, the migrating basins, and the thick diffracting section are suggestive of the imbricate thrust model. Continual movement along new and older imbricate thrusts within the continental slope may produce the ubiquitous diffracting section sandwiched between the oceanic slab and slope basins and may disturb existing basin deposits in some areas.

Abstract The continental margin off Kodiak Island has formed at converging lithospheric plates as indicated by the eastern Aleutian Trench, the Aleutian volcanic chain, and the well-defined Benioff zone; hence, its structure should reflect the tectonic features of a subduction zone. Igneous oceanic crust seaward of the trench is overlain by at least 35 m of lower Miocene pelagic sediment coarsening upward to Miocene and Plio-Pleistocene turbidite sequences. Downbowing of this oceanic crustal sequence has formed the trench, which is filled by a wedge of turbidites 0.8 to 1.0 km thick and less than 1 m.y. old. The structure of the adjacent continental slope is expressed morphologically by a rough lower slope, a poorly developed mid-slope terrace, and a relatively smooth upper slope. Core samples from the lower slope yielded tectonically deformed and highly compacted Plio-Pleistocene sediment, similar to those that would be expected in a subduction zone. However, the original environment of deposition cannot be determined from lithologic and paleontologic data to establish whether these samples are oceanic-basin material accreted to the continent or deformed slope deposits. Seismic records across this drill site show few coherent reflecting horizons owing to the deformation. In contrast, records from an adjacent area show coherent, little-deformed reflections of the oceanic-basin sequence that can be traced beneath the slope, landward from the trench to about the mid-slope area. Such variability in intensity of deformation along the leading edge of a subduction zone is surprising and difficult to explain. The upper slope's smooth morphology is due to thick terrigeneous sediment filling older structural irregularities, a circumstance that suggests relatively less intense late Cenozoic tectonism than has affected the lower slope. The shelf break at the top of the slope, is formed by an alignment of short, relatively broad anticlines, and where they are breached by erosion, Miocene and younger sediment has been recovered. Growth of the anticlines has helped to pond sediment landward in a broad, deep shelf basin. The seismic and drilling data allow a rough comparison between the minimum volume of oceanic sediment that was on the subducted plate, and the maximum accreted material at the continental margin of equivalent late Tertiary age. Assuming the rate of subduction was 5 cm/yr, these volumes appear similar suggesting that all subducted sediment may have been accommodated in the margin. The data suggest that rapid underthrusting occurs mainly along the lower continental slope in a zone 30–40 km wide adjacent to the trench. The intensity of tectonism diminishes landward just above this zone so that the shelf basin has been only mildly affected by subduction.