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Pangaean divergent margins caused by the breakup by rifting of that supercontinent display three distinct evolutionary stages. In their youth or rift stage, divergent margins display a synsediment fault style that has considerable tectonic instability. During the mature or drift stage (seafloor spreading) continental margins are relatively stable and are dominated by thermal subsidence. These two stages (rifting and drifting) are separated in many areas by an unconformity coeval with the oldest associated oceanic crust — the breakup unconformity; in some areas the onset of seafloor spreading is marked by intense magmatic activity. Sediment facies the rifting stage range from continental clastics landward of the continental basement hinge and along the flanks of the basin to saline deposits and more open-water carbonates in the axial zone of the rift. Lithologies emplaced during the rift or mature stage of margin development range from shallow clastics/carbonates to deep-water carbonate/siliceous oozes, turbidites and contourites, some of which were deposited in an anoxic environment. Deposition then is controlled by changes in configuration of basins, changes in sea level, and plate migration through several climatic belts. During the final, or old-age, of margin development, divergence gives way to convergence. This stage, which generally terminates with continental collision, is characterized by tectonism, magmatic activity, and crustal shortening. Mesozoic divergent margins along the east-west trending former Tethys Ocean have reached this stage in their evolution.

Limestones of Oligocene age obtained from a submersible at two sites about 3,000 m deep on the Oriente Slope and from two shallow drill holes on nearby Grand Cayman Island were deposited in shallow marine waters. All samples contain calcareous algae and shallow-water foraminifera, but no deep-water remains were found. The limestone on the slope was lithified in fresh water, according to its petrography and stable isotopes. A third dive encountered massive beds presumed to be outcrops of limestone that dip southward down the slope. We conclude that the region began to subside along a hinge line parallel to the Oriente Slope during Oligocene time and soon became too deep to generate a shallow-water carbonate deposit. This subsidence was associated with plate movements along the Cayman Trough.

Flint pebbles and nodules from the Upper Cretaceous chalks of Europe occur offshore and at many seaports along the Atlantic coast of North America, where they were brought as ship's ballast. Isolated pieces imported from Europe as gunflints also are present.

Contours of the Arctic Basin were based on a compilation of all known soundings, largely from recent Russian expeditions. Additional general inferences regarding the bottom topography were derived from indirect considerations, such as distribution of Pleistocene glaciers, character of land topography, positions of earthquake epicenters, and sources and composition of the bottom sediments. It is concluded that the floor of the basin is far more irregular than is indicated by the present sparse soundings. Off Alaska and Canada the basin is deepest and least well sounded.

Three 24-hour multiple-ship surveys, each ship lowering a bathythermograph at 15-minute intervals, revealed the presence of internal waves off southern California. The waves had the form of vertical fluctuations of temperature of up to 90 feet over a wide range of frequencies dominated by an approximate semidiurnal period. They appeared to be roughly parallel to the top of the continental slope and to move shoreward at a speed of about 7 knots in deep water and less than 1 knot over the shallow shelf. There may be evidence of internal surf as the waves approach the coast.

Dredging operations on banks, on escarpments, and on walls of submarine canyons off southern California have shown that nodular phosphorite is the most abundant type of rock in these nondepositional environments. Approximately one-third of all the rock recovered is phosphorite. Petrographic and megascopic examination reveals that the nodules are largely formed by direct precipitation, but that they enclose some replaced material. Examination also shows that the phosphorite was probably deposited essentially in situ . Miocene Foraminifera have been identified in the nodules from many of the stations, but Recent or Pleistocene faunas have also been found in some of the phosphorite. Whereas the enclosed Miocene fauna suggests a Miocene age for most of the nodules, significant nonpaleontological data indicate that this fauna has probably been reworked into more recent deposits before enclosure in the nodules.

Geophysical traverses across flat or nearly flat mud bottom at depths between 2000 and 5500 meters off western Africa permitted the making of extensive shallow-penetration recordings at 3.5 kHz. The recordings reveal the common presence of alternating dark and light triangular features whose internal structure and acoustic properties may be due to local centers of cementation by gas hydrates, or clathrates. Probably the features are widely distributed in the fine-grained sediments of continental rises and abyssal plains of the world ocean, but are not detected by the usual echo sounding at 12 kHz or seismic reflection profiling at 20 to 100 Hz.

In his attempts to understand the world that he occupies, man has had two sets of questions: what? and how? Prior to recorded history and throughout nearly all of the time since then man has asked who created the universe around him. Much later, and especially during the past century, the question has been how was the universe created. To answer how requires knowledge of the subsets what and when, whose difficulty was so great that they were neglected and by default the question who long dominated. Eventually, the question why may be asked.

Continental margins can be classified according to their stages of development as judged from published continuous seismic reflection profiles. Mapping of the stages of initial, youth, maturity, and old age (= destruction) shows that their distributions are related to plate movements and to sediment supply. The main information gaps are in the Arctic Sea, off Antarctica, in the open Indian Ocean, and along parts of eastern Asia. Even in these regions the stages of development can be inferred from related regions having available profiles and from general knowledge of the topography and structure. The distribution for the entire earth is about 6% initial, 48% youth, 25% maturity, and 21% old age. Best petroleum prospects are believed to be mature margins and thick basin fi ls within youthful margins.

About 30,750 line-km of geophysical traverses (seismic reflection and refraction, magnetics, and gravity) were made in the Gulf of Guinea and vicinity aboard R/V Atlantis II during 1972 and 1973 as part of the International Decade of Ocean Exploration program. These traverses, supplemented by about 50,000 line-km of previous ones by other ships, provide a basis for mapping and understanding the geologic structure, history, and origin of the region. The deep indentation of the outline of western Africa is paralleled by a similar bend of the Mid-Atlantic Ridge and by the prominent bulge of northeastern Brazil. These sharp bends are the result of left-lateral offsets by many transform faults in a belt of equatorial fracture zones. Some of the fracture zones continue eastward and intersect the entire length of the east-west coast of the Gulf of Guinea and penetrate the continent at the Benue trough or graben. The valleys of the fracture zones have been sites of sediment deposition, whereas the ridges have served as dams that force the sediment to move westward. Where enormous quantities of sediment have been delivered to the ocean by the Niger-Benue Rivers, a large delta has deeply buried the irregular topography of the fracture zones. In this entire belt of fractured ocean floor the structure, physiography, and stratigraphy have been controlled by lateral movement, or translation, of the ocean floor with respect to the continent. In contrast, the regions southeast and northwest of the belt of equatorial fractures have fewer large fracture zones, smoother topography, and simpler sediment wedges. These two regions owe their origin to simple divergence during sea-floor spreading, when new oceanic basement added at the Mid-Atlantic Ridge increased the distances between the African continent, the Mid-Atlantic Ridge, and the American continents. Deposition of sediments along the margins of the originally narrow Atlantic Ocean was dominated early by coarse-grained and largely nonmarine sediments. South of the Gulf of Guinea these deposits were followed by evaporites as products of restricted water circulation in a long narrow arm of the ocean. There was little flow of water across the equator because of the sliding-v lve nature of the region of translation between the two regions of divergence. As spreading continued, the ocean widened, and thick prisms of marine sediments were deposited on the continental margins. Large deltas in western Africa first began at the south, with the now submerged deltas of the Orange and the Congo Rivers being chiefly Mesozoic in age and having no present coastal projection. The Niger delta farther north is mostly Cenozoic in age. Petroleum prospects appear to be far greater in the Niger delta and the region of divergence south of it than in the entire region west of the delta. The favorable continental margin contains thicker sediments, large ancient and modern deltas, and salt and mud diapirs. End_Page 2209

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.

Quantitative estimates of undiscovered resources of oil and gas beneath land areas have been prepared by many organizations and individuals on the basis of exploratory test wells and the presumed similarity of undrilled sedimentary basins to well-known ones. These estimates span a wide range of reliability. Undiscovered oil and gas resources beneath continental shelves probably are larger per unit area than those beneath the land, because the shelves are primarily marine depositional features, but the estimates of quantities beneath the shelves are even less reliable owing to fewer drillholes per unit area than on the land. Least known of all potential oil and gas environments are the deep-water areas that include deep marginal basins, continental slopes, continental rises, and the deep ocean floor. Some analogs with the marginal basins are provided by similar basins that have been filled with sediments to form part of the continental shelf or even the adjacent land, but uplifted and accessible unmetamorphosed examples of continental slopes, continental rises, and abyssal plains are rare, if present at all. Marine geophysical data are available for these deep-ocean sedimentary environments, but drillhole samples do not exist other than those from the JOIDES Deep Sea Drilling Project, which intentionally avoided sites of oil potential to avoid possible pollution of the ocean. In addition, few of these dri lholes penetrated as deep as 1,000 m into the bottom. Lack of suitable drillhole data means that quantitative estimates of undiscovered oil and gas resources of the deep ocean floor are meaningless--and, therefore, so are those for the whole world.

About 22,000 km of continuous seismic-reflection, magnetic, and gravity profiles, 118 radiosonobuoy recordings, 98,000 km of geophysical profiles from previous investigations, 15 deep-sea-drilling logs, and many dredge samples served to reconstruct the history of the continental margin and adjacent deep-ocean floor between Senegal and Portugal. Initial structures of the margin south of Morocco formed by divergence when Africa and North America separated 180 m.y. ago. The margin off western Portugal had a similar origin when the Iberian Peninsula and North America separated about 80 m.y. ago. Between these two divergent segments the area of the Strait of Gibraltar formed by a combination of translation from 180 to 72 m.y. ago and plate convergence from 63 m.y. ago to the p esent, with convergence becoming more intense during the past 10 m.y. Oldest sedimentary rocks atop basement include an evaporite of Late Triassic to Early Jurassic age. When the apron deposited by upbuilding and outbuilding became thick enough, mobility of the evaporites deformed the overlying sediments especially north of the Canary Islands. Except off Morocco and the Strait of Gibraltar and possibly off southern Senegal the sedimentary blanket is dominantly calcareous, reflecting the general lack of fluvial influx. Included is a middle-Late Jurassic algal reef that constitutes the lower continental slope off Morocco. During Aptian-Cenomanian time the deep ocean off much of northwestern Africa had only sluggish bottom circulation, recorded by organic-rich sediments. A major hiatus in deep-ocean sedimentary rocks and in three prominent sedimentary ridges (off Madeira, near Agadir canyon, and north of Conception bank) probably was caused by temporarily intensified circulation. Tertiary tectonics modified the divergent margin south of Morocco by folding of shelf strata off the western High Atlas, emplacement of the Canary Island Ridge, folding of slope strata off Spanish Sahara, and uplift of the Cape Verde plateau. These orogenies also may have uplifted oceanic basement beneath the upper rise and formed the volcanic seamounts along this ridge. Maximum modification by Tertiary diastrophism occurred on the margin of translation-convergence near the Strait of Gibraltar. There, the convergence phase caused uplift of Gorringe bank. Plate convergence also deformed sediments atop oceanic basement, aided by the mobility of Triassic-Jurassic evaporites. More recently, probably because of uplift of the Iberian Peninsula during the Pliocene, well-stratified Miocene and younger deposits atop the deformed lower unit slid oceanward away from the peninsula, with the megaslide coming to rest against the Moroccan continental slope. Associated folding also involved the lower deformed sequence.

Isotopic dates of anorthosites from the Moon and meteorites indicate that the age of the Solar System is about 4.6 b.y. Modal lead ages of volcanic rocks and ores suggest that the age of the Earth is 4.55 to 4.6 b.y.; thus the major melting and crustal-forming events closely followed accretion of the Earth and the other planetary bodies. Until 1.0 b.y. ago plate growth may have been accommodated mostly by vertical or horizontal displacements and buckling and shearing of plates (Kroner, 1981). According to Condie (1982), continental rifting began about 2.0 b.y. ago and became widespread about 1.0 b.y ago, when continental fragmentation occurred and oceanic basins were produced and destroyed by sea-floor spreading. This cycle was important for the Pan-African system in northern Africa, Brazil, and other areas around the North Atlantic Ocean. However, most of the Pan-African mobile belts are ensialic (only sial), suggesting that this was a time of transition between dominantly ensialic tectonics and modern plate tectonics. Continental fragmentation followed by sea-floor spreading was the dominant tectonic style with the opening of the North Atlantic (Iapetus oceanic basin) 700 to 500 m.y. ago (Fig. 101). This Paleozoic sea-floor spreading episode was followed by closing of the Iapetus basin and then by renewed sea-floor spreading that initiated the present ocean-floor cycle during the Mesozoic Era (Fig. 101, 103) and produced the present structural configuration of the Atlantic Ocean. The position and morphology of the transition from continental to oceanic crust was controlled by the construction of the mega-continent Pangaea, and the subsequent breakup of this landmass.

The commonest and most stable position of the empty valves of large pelecypods on the wave-washed zone of beaches is concave down; this position is due to the presence of fast currents and the absence or inadequacy of bioturbation. In contrast, most large pelecypod valves on the surface of the continental shelf lie in a concave up position in response to the action of carnivores and scavengers and to powerful bioturbation and the absence of fast currents. These observations of preferred position can be applied to paleoecological studies of ancient strata.

One of the most important and interesting geological questions awaiting solution is that of the origin of continental shelves and slopes. Just as for other geological features, more than a single origin is involved for different areas or for different times. Data on structure, composition, and topography provide the clues for interpretation of geological history and thus of origin, but such clues are presently so incomplete that interpretations are uncertain. Common to many areas is the presence of a downwarped basement under the slope and (or) a topographic depression beyond the slope--but is this cause or effect? Also common is the composition: marine sediment--but what can we infer about the precise environment of deposition of these sediments? Are they neritic under t e inner part of the shelves and bathyal beneath the slopes? How important are turbidites for the continental rise beyond? How similar are the Mesozoic and Tertiary sediments to those of the Pleistocene and Recent? The present topography is reasonably well known--but is it similar to ancient topography? In many areas the present shelf has been shaped by processes unique to the Pleistocene--does this mean that continental shelves did not exist before the Pleistocene? A comparison of the results of the field work by different workers in large, but widely separated, areas will provide a much needed fresh approach to the question of the origin of continental shelves and slopes.

The structural framework of the southeastern Bahamas has been reconstructed from seismic profiler, magnetic, and gravity data. The Bahama Escarpment that marks the boundary between the southeastern Bahamas and the deep-sea floor may follow an ancient fracture zone. The geophysical data suggest that the crust beneath the southeastern Bahamas has a thickness (20 km) intermediate between those of the crusts of continents and ocean basins, and that this basement is partly volcanic in origin. The Bahamas may have formed by subsidence of a continental crust and carbonate accretion. If the southeastern Bahamas were formed in this manner, the crustal foundation must be very thin because the carbonate apron may be as thick as 10 km. Another explanation is that the Bahamas are unde lain by oceanic crust, in which case the Bahamas could have been formed in two ways. The northwestern Bahamas may be located at the site of a trough formed before or at the time the Atlantic Ocean was open. After this trough was filled nearly to sea level with terrigenous sediment, carbonate deposition was initiated. The southeastern Bahamas, on the other hand, may be located along a fracture zone that was formed during the opening of the Atlantic. The sedimentary section may be entirely carbonate. As the continents separated, the sediment-filled trough and the fracture zone subsided with carbonate accretion keeping pace with subsidence. The interpretation of the southeastern Bahamas being built on oceanic crust eliminates the problem of its overlap onto Africa in continental drift recon tructions.

One of the most important and interesting geological questions awaiting solution is that of the origin of continental shelves and slopes. Just as for other geological features, more than a single origin is involved for different areas or for different times. Data on structure, composition, and topography provide the clues for interpretation of geological history and thus of origin, but such clues are presently so incomplete that interpretations are uncertain. Common to many areas is the presence of a downwarped basement under the slope and (or) a topographic depression beyond the slope--but is this cause or effect? Also common is the composition: marine sediment--but what can we infer about the precise environment of deposition of these sediments? Are they neritic under t e inner part of the shelves and bathyal beneath the slopes? How important are turbidites for the continental rise beyond? How similar are the Mesozoic and Tertiary sediments to those of the Pleistocene and Recent? The present topography is reasonably well known--but is it similar to ancient topography? In many areas the present shelf has been shaped by processes unique to the Pleistocene--does this mean that continental shelves did not exist before the Pleistocene? New data will be presented by the different speakers, all of whom have been active in field studies. We, perhaps, will find that a comparison of the results of their field work in large, but widely separated, areas will provide a much needed fresh approach to the question of the origin of continental shelves and slopes. End_of_Article - Last_Page 356

The sediments that were deposited on the continental shelves during and immediately after the latest glacial stage of the Pleistocene Epoch are unrelated to their present environments. The rise of sea level during the past 19,000 yrs has caused previously deposited subaerial, lacustrine, and paludal sediments to become submerged in shallow-marine waters, and later to become deeply submerged. Land-laid or shallow-marine sediments which occur in deep water, such as those near the seaward edge of the continental shelf, are properly termed "relict." At present, the sediments exposed atop about 70 percent of the area of the continental shelves of the world can be identified as relict. Eventually, new detrital sediments that are contributed to the ocean by streams or shore eros on should prograde entirely across the continental shelf and bury the relict sediments. This burial should be hastened if the rise of sea level remains as slow as it has been during the past few thousand years; nevertheless, several tens of thousands of years probably will be required before most of the relict sediment becomes buried.

During mid-1967 two cruises of the Woods Hole Oceanographic Institution's R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.

Continuous seismic-reflection profiles were recorded along 15 lines across the continental margin of the Gulf of Mexico. The steep slopes off Florida and Yucatan (Florida and Campeche escarpments) appear to have been formed by construction of coral or algal reefs during the Cretaceous Period. The adjacent upper continental slope and continental shelf were formed by prograding and upbuilding of largely calcareous sediments after the death of the Cretaceous reef-building organisms. Off Louisiana, Texas, and part of Mexico the upper continental slope and the continental shelf were formed by progradation and upbuilding of terrigenous sediment contributed largely by the Mississippi River during the Tertiary Period. These sediments buried deeply much of the Early Cretaceous ree . Considerable alteration of the general structure off Louisiana and Texas resulted from contemporaneous intrusion of salt diapirs. Sediments in the basins between the diapirs were derived partly from the tops of the nearby salt intrusives. The Sigsbee escarpment at the seaward edge of the sedimentary prism off Louisiana and Texas is bordered by a broad ridge of diapirs, to which it may owe its origin. Although diapiric structures are best known north of the Sigsbee escarpment, others are present on the south, at the center of the Gulf basin, and as far south as Golfo de Campeche. During Early Cretaceous time the reef once almost surrounded the Gulf of Mexico, probably leaving only one narrow connection with the open ocean at the southeast. The narrow opening and the low relative sea level, indicated by reef structure and calcareous algae nearly 3,000 m below present sea level, suggest that water circulation was restricted during Early Cretaceous time. If it had been somewhat more restricted earlier, conditions could have been suitable for the widespread deposition of Late Triassic to Middle Jurassic salt that later fed the diapiric structures.

New data from marine dredgings off Monterey, California, correlated with wells and outcrops on land indicate that Monterey, Carmel, and Soquel submarine canyons are related closely to the geological history of the continent. Upper Pliocene siltstone and sandstone bodies in Monterey Canyon correlate with the Pomponio Member of the Purisima Formation in the Santa Cruz Mountains. Granodiorites exposed along the south slope of Monterey Canyon and the east slope of Carmel Canyon correlate with lower Upper Cretaceous Santa Lucia granodiorite on land. Siliceous siltstone, highly-indurated micritic limestone, and metamorphic rocks comprise the west slope of Carmel Canyon. These siltstone and limestone beds contain silicoflagellates, radiolarians, and middle Miocene diatoms, but no Foraminifera. Quartzite from Carmel Canyon probably correlates with the pre-Cretaceous Sur series. A seaward extension of Sur (Nacimiento) and Palo Colorado faults separates sedimentary and metamorphic rocks from igneous ones in Carmel Canyon; it probably continues northward to connect with the San Gregorio fault in the Santa Cruz Mountains. Monterey fault in Monterey Canyon separates upper Pliocene rocks on one side from granodiorites on the other. The sedimentary rocks in Monterey Canyon contain bathyal foraminifers stratigraphically 400 ft higher than shallow-water Pliocene rocks in the lower Salinas Valley. These relations suggest vertical movement of more than 1,000 ft on a northward extension of the Reliz fault which separates the two areas. A seaward extension of the Carmel Valley fault controls the nearshore east-west trend of Carmel Canyon and the Carmel Canyon fault (seaw rd extension of the Sur and Palo Colorado faults) controls the northwest trend of Carmel Canyon. Canyon erosion occurred in zones of poor induration and/or weakness along faults and contacts between sedimentary and igneous rocks having diverse induration. Erosion was effected by turbidity currents, by mass movement of sediment down canyon axes, and perhaps was aided by alteration of granodiorite to clay. The middle Miocene Salinian deformation resulted in more than 15,000 ft of subsidence in Monterey graben, which underlies the shelf at the north side of Monterey Bay. Additional structural features such as Monterey, Gabilan, Carmel Canyon, and Carmel Valley faults either formed at this time or were reactivated. Geomorphic features termed the "Elkhorn erosion surface" and "Pajaro gorge" were formed also at this time. Late Miocene, Pliocene, and early Pleistocene drainage from the Great Valley of California debouched at Monterey Bay via Elkhorn Slough at the head of the present Monterey Canyon. All canyon heads were cut or modified subaerially to an approximate depth of 300 ft below present sea level. Monterey and Soquel Canyons were cut during late Pliocene or early Pleistocene time. Carmel Canyon and Ascension Canyon initially may have been eroded earlier; they may have funneled sediments seaward before Pleistocene time. Monterey Canyon and Elkhorn Slough lie directly above the buried middle Miocene Pajaro gorge. The older canyon is not ancestral to Monterey Canyon, but differential compaction in the gorge fill may have directed rivers to reach the sea above the gorge. A sediment trap may have been eroded at the mouth of the slough whereby both littoral-drifted and river-transported sediments were deflected seaward to aid in the erosion of Monterey Canyon.

A geophysical survey of the East China Sea, Yellow Sea, and Ryukyu Island arc and trench was conducted during October-November 1968, by the U. S. Naval Oceanographic Office with participation by scientists from ECAFE member nations. More than 12,000 km of continuous seismic reflection profiling, magnetic, and bathymetric data were recorded. The distribution of sedimentary strata appears to be controlled by a series of NE-SW trending ridges that dam and separate large sediment-filled depressions. The sediments were derived mainly from the large section of Chinese mainland drained by the Yellow and Yangtze Rivers. The Fukien-Reinan massif (between southeastern Korea and the mouth of the Yangtze River) consists of Precambrian to Mesozoic rocks uplifted during the Mesozoic Era. It dams a thick sequence of Paleogene and Neogene sedimentary rocks in the Yellow Sea. A ridge of folded sedimentary and igneous rock near the edge of the continental shelf is probably the sea-floor expression of the Taiwan-Sinzi folded zone which began to be active during Paleogene time. Approximately 1 million cu km of Neogene sedimentary strata (at aining a thickness of probably at least 5 km) and some Paleogene sedimentary rocks are dammed by this ridge. An extensive unconformity separates Neogene from Paleogene strata beneath the Yellow Sea and the continental shelf. The Neogene unit is generally undeformed, whereas the Paleogene unit shows evidence of structural deformation followed by erosion. Beyond the continental shelf the Ryukyu Ridge, composed of volcanic rocks, folded Paleogene and faulted Neogene strata, and Paleozoic and Mesozoic igneous and metamorphic rocks, has dammed a belt of sedimentary strata of probable Neogene age in the Okinawa Trough. Sediment fill in the trough exceeds 1.2 km in thickness and contains many internal reflectors that may be turbidite sand layers. Abundant faults and complex folds beneath the si e slopes suggest fault origin of the Okinawa Trough. Another dam halfway down the eastern edge of the Ryukyu Ridge has trapped additional sediments to form a broad terrace. The Ryukyu Trench at the foot of the Ryukyu Ridge contains zero to about 600 m of sediment and terminates against Taiwan in a trench-transform fault junction. Sedimentary strata beneath the continental shelf and the Yellow Sea appear to have a high potential for oil and gas. Much of the continental shelf north of Taiwan has sedimentary thickness exceeding 2 km, most of which is believed to be of Neogene age, as are the oil-producing strata on Taiwan. Present information suggests that the strata are dominantly shale but with some interbedded sandstone zones that serve as acoustic reflectors. Numerous anticlines, faults, and unconformities were recorded. Our reconnaissance study indicates that detailed seismic surveys are warranted and that eventual test drilling will be required to establish the potential for oil and gas in the region.

Continuous seismic reflection profiles accompanied by total-field magnetic measurements were made in the China basin by civilian survey ships on contract to the U.S. Navy Oceanographic Office between 1967 and 1969. The results show the presence of stratigraphic units similar to those previously found in the adjacent East China Sea and South China Sea: (1) acoustic basement in the southern part of the basin that may be a continuation of the igneous and metamorphic rocks beneath the adjacent shelf which were peneplaned during Late Cretaceous-early Cenozoic time, and much more irregular basement in the northern part of the basin that may be oceanic basement (Layer 2); (2) predeformational sediment, conformably with the surface of much of the acoustic basement (probably Paleo ene); and (3) postdeformational sediment, largely turbidites deposited in deeper areas (probably Neogene to present) after the main episode of deformation. During the deformation a series of northeast-trending ridges was folded along the floor of the China basin. Similar ridges underlie the basin side slopes, and these served as dams to trap sediments brought to the ocean by streams from the adjacent land areas. Another ridge separates the Manila Trench and the West Luzon Trough, and extends northeastward as the Central Range of Taiwan. Oil potential appears to be greatest beneath the shelf between Taiwan and Hainan off mainland China, but the basin ridges that are surmounted by banks and islands also warrant further investigation.

A 16,000-km series of seismic and geomagnetic traverses in the Gulf of Thailand, northern Sunda Shelf, and the adjacent deep-sea floor was completed during 1969. The results show the presence of three large sediment-filled basins separated by swells or ridges. Westernmost is the Gulf of Thailand basin that extends almost from Bangkok to Singapore. A buried ridge separates this basin from two other basins on the northeast, the Mekong and the Brunei-Saigon. Both of the latter basins reach the adjacent land areas, and the Brunei-Saigon basin appears to be the main part of the Northwest Borneo geosyncline. The Mekong and Brunei-Saigon basins are limited on their northeastern sides by a narrow ridge that lies beneath the outer continental shelf and the upper continental slope, a feature termed the "Peripheral Ridge." Sediments in all three basins are thicker than 2 km and they may be thicker than 4 km. They contain tectonic folds, unconformities, faults, and at least a dozen structures believed to be diapiric intrusions. The deep-sea floor in the northeast beyond the continental slope contains an abyssal plain and also a broad comparatively level plateau that is dotted with seamounts, some of which are capped with coral atolls. Sediments in this deep region, the China basin, are of two distinct ages separated by a widespread unconformity. Inferences from geologic studies on land suggest that the basin in the Gulf of Thailand was formed during Late Cretaceous time, and the present form of the ones beneath the Sunda Shelf was acquired subsequently. Sediments have accumulated in them ever since, with interruption by tectonic activity and erosion. Prospects for oil and gas appear to be favorable, as also is indicated by the intense exploration and test drilling off Borneo and in the southern part of the Gulf of Thailand by international companies. Other parts of the sediment-filled basins deserve attention as well.

Negligible success of investigators in relating tide gauge records of the west coast of North America to eustatic changes of sea level is the result of tectonic movements of the land reference level beneath the tide gauges. The vertical tectonic movements are caused by horizontal movements of oceanic crustal plates. Sinking of 12 mm/yr at Cordova, Alaska, is associated with intense subduction at the east end of the Aleutian Trench; rising by 6 mm/yr at Skagway may be caused by resistance to lateral crustal movement toward the filled trench; rising of 1 mm/yr in southern Alaska (uplift above slow subduction); little vertical movement between Sitka and Mendocino Fracture Zone (translation and slow subduction including fracture zones and thick sediments); sinking of 1 mm/yr west of the San Andreas fault (translation); and sinking of 2 mm/yr along northwestern Mexico (effects of cooling of late rifted crust and only diagonal subduction). Superimposed on this tectonism due to plate motion is broad scale isostatic adjustment (both rise and fall) resulting from deglaciation. Removal of rigid plate motion deltas helps resolve subplate tectonic processes. Higher-frequency (0.05–0.5 cpy) sea level changes as well as large-scale pressure fluctuations and wind stress events are associated with El Nino/Southern Oscillation. An apparent increase in relative sea level rise near 1935, also observed in other regions, remains to be explained.