Showing papers on "Continental margin published in 1970"

Mountain belts and the new global tectonics

Abstract: Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault-bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc-alkaline and basaltic magmas. Cordilleran-type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high-temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision-type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision-type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present-day oceans, island arcs, and continental margins.

Reconstruction of Pangaea: Breakup and dispersion of continents, Permian to Present

Abstract: We present a new continental drift reconstruction of the universal continent of Pangaea in the Permian plus a series of five world maps to depict the breakup and dispersion of continents with each subsequent geologic period, Triassic to Recent. Plate tectonics and sea-floor spreading are accepted as the guiding rationale. Also utilized are the morphologic fitting of continental margins and paleomagnetic pole positions. Rigor is imposed by the geometric requirements involved in presenting continental drift dispersion on maps in orderly time sequence and by following certain assumed rules of plate tectonics. The reconstructions were first made on a globe and then transferred to an Aitoff world projection. In the Permian, the Atlantic and Indian oceans were closed so that all the continents were configured into the universal landmass of Pangaea. The reconstruction is based largely on the morphologic best fit of continental margins to the 1000-fathom isobath, except for India, the east coast of which is placed against Antarctica, as dictated by plate tectonics. In the Triassic the breakup of Pangaea commenced. The southwest Indian Ocean rift was created, which split West Gondwana (South America and Africa) away from East Gondwana while a Y junction lifted India off Antarctica. An independent North Atlantic–Caribbean rift also formed, which lifted Laurasia (North America and Eurasia) off of South America and the bulge of Africa. In the Jurassic, northward and westward sea-floor spreading further opened the central North Atlantic and the Indian oceans. At the end of the period, a new rift incipiently split South America away from Africa. The Walvis mantle thermal center or ‘hot spot’ formed, which would subsequently provide an absolute geographic reference point for subsequent continental drift. In the Cretaceous, the motions already established continued. The North Atlantic rift grew northward, blocking out the Grand Banks and the western margin of Greenland. Spain rotated sinistrally, forming the Bay of Biscay. An offshoot rift split Madagascar from Africa, dropping off this subcontinent from Africa, which continued its northern flight. The northward trek of India continued, and Australia incipiently split away from Antarctica. During the Cenozoic, Antarctica rotated further westward. Australia experienced a remarkable flight northward, and New Zealand was split away from its east coast. The North and South Atlantic oceans continued to open; the rift that formerly passed west of Greenland now switched to the east and split Greenland away from northern Europe and extended through the Arctic Ocean. Africa moved slightly northward, continuing sinistral rotation. The Tethyan megashear became dextral for the first time, India collided with and underran Asia.

Lithosphere Plate-Continental Margin Tectonics and the Evolution of the Appalachian Orogen

Abstract: In terms of plate tectonic theory, and by analogy with modern continental margins, the Appalachian orogen evolved through a sequence of interrelated sedimentation-deformation-metamorphism patterns within a tectonic belt situated along the eastern margin of the North American continent. As exemplified by the northern part of the orogen, Appalachian stratigraphic-tectonic zones and deformation sequences are related to Late Precambrian to Ordovician expansion, followed by Ordovician through Devonian contraction, of a Proto-Atlantic ocean. This oceanic opening and closing was achieved by initial extensional necking of a single North American/African continent and by lithosphere plate accretion, followed by contractional plate loss along a trench, or complex of trenches, marginal to the drifted North American continent. A lithosphere plate model for the evolution of the orogen incorporates spatial and chronologic relations within and between bulk stratigraphic units and tectonic events. Pre-orogenic Appalachian sedimentation patterns were essentially the same as those found along modern continental margins; that is, shelf/slope/rise/abyss. Appalachian tectonic patterns are also analogous with modern tectonic patterns of continental margins, island arcs, and trenches, and involved continent-ward driven thrust sheets and ancillary exogeosynclines.

Mechanism of the Chilean Earthquakes of May 21 and 22, 1960

Abstract: The Chilean earthquake sequence of May 21–22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level. The region involved is more than 200 km wide and about 1000 km long, and lies along the continental margin between latitude 37° and 48° S. Significant horizontal strains accompanied the vertical movements in parts of the subsided zone for which triangulation data are available. Displacements were initiated near the northern end of the deformed region during the opening earthquake of the sequence (M s ≅ 7.5) on May 21 at 10h 02m 50s GMT and were extended over the remainder of the region during the culminating shock (M s ≅ 8.5) on May 22 at 19h llm 17s GMT. During the latter event, sudden uplift of adjacent portions of the continental shelf and much or all of the continental slope apparently generated the destructive tsunami that immediately followed the main shock. Available data suggest that the primary fault or zone of faulting along which displacement occurred probably is a complex thrust fault roughly 1000 km long and at least 60 km wide; it dips eastward at a moderate angle beneath the continental margin and intersects the surface on the continental slope. Dip slip required to satisfy the surface displacements is at least 20 m and perhaps as large as 40 m. There is some evidence that there was a minor component of right-lateral slip on the fault plane.

The Uralides and the Motion of the Russian and Siberian Platforms

Abstract: The Uralides—the late Precambrian and Paleozoic orogenic terrane between the Russian and Siberian Platforms—in part are exposed in the Ural Mountains, in the central Soviet Arctic, along the west edge of the Siberian Platform, and in southern Siberia and Kazakhstan, and in part are buried beneath the fill of the West Siberian Lowlands and other basins. Paleomagnetic orientations suggest that the Russian and Siberian Platforms were far apart during the early Paleozoic, converged during the middle Paleozoic, and collided in the Permian or Triassic. The geology of the Uralides accords with the concept that the two subcontinents approached and collided as the intervening oceanic plate slid beneath them along subduction (Benioff) zones. The medial eugeosyncline of the Uralides consists largely of what may be oceanic material scraped off against the edges of the opposed subcontinents. Basalt-and-spilite belts may represent ocean-floor abyssal tholeiite, and the manganiferous cherts and other sediments upon them may be pelagic oozes. Andesite belts may have formed as island arcs within the ocean, swept subsequently against the continents. Fossil subduction zones are recorded by great faults soled by, or containing tectonic injections of, mafic and ultramafic rocks from the lower oceanic crust and upper mantle, and containing high-pressure metamorphic rocks. Granitic and silicic-volcanic rocks may have formed above the subduction zones in the accreted parts of the continental plates. Both these continental-margin magmatic rocks and the island-arc complexes display ratios of potassium to silicon that vary across strike and so indicate the directions of dip of the subduction zones. From the distribution of such indicators of various ages, a history of the continental margins can be deduced. An active subduction zone dipped beneath the Siberian Platform during at least parts of late Precambrian and early, middle, and late Paleozoic time. The late Precambrian and Cambrian history of the Russian side is unclear, but in the Ordovician and Silurian the Russian continental margin was stable, while somewhere offshore an island arc was present whose trench was on the Russian side; the last of the intervening oceanic plate vanished down the subduction zone in about the Early Devonian, and the island arc became part of the continental margin. During the remainder of the Devonian and during the Carboniferous and Early Permian, a subduction zone was present along the margin of the enlarged Russian continent and dipped beneath it. Each subcontinent grew oceanward as oceanic material was accreted against it, and the subduction zones stepped oceanward correspondingly. The continental magmatic zones migrated oceanward behind the accreting edges of the continental plates, so the tectonic and magmatic progression with time at any one place is analogous to the variations present across the entire orogenic belt at any one time. Severe right-lateral deformation of the Uralides, the Russian side having moved northward relative to the Siberian side during Mesozoic and early Cenozoic time, is inferred from structural and magnetic-anomaly patterns. The deformation was accomplished by oroclinal folding, strike-slip faulting, and tensional thinning of the crust. The Uralides may have been continuous in early Mesozoic time with the Ellesmerides of North Greenland and the Canadian Arctic islands. The Cenozoic (and late Mesozoic?) opening of the Arctic Ocean was accomplished by spreading of the Eurasia Basin, and by opening of the Canada Basin behind a counterclockwise-rotating Alaska.

Continental Rise off Eastern North America

Abstract: 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.

Peru-Chile Trench Sediments and Sea-Floor Spreading

Abstract: The hypotheses of sea-floor spreading and plate tectonics require the removal of sediment from oceanic trenches either by crustal underthrusting or by folding against the base of a continental or insular margin. Accordingly, over a period of time the volume of sediment removed by way of spreading must be equal to the difference between the observable volume of undeformed terrigenous deposits in a trench and the volume contributed to it by continental erosion. To assess possible sediment loss from the central Chilean segment (23°–44° S.) of the Peru-Chile Trench, we have compared the volume of terrigenous deposits overlying the land, the continental margin, and filling the trench with that expected from continental denudation. Our data indicate that an episode of sediment removal occurred at the base of the margin and adjacent deep-sea floor in Late Cretaceous and perhaps earlymost Tertiary time and may imply spreading. Nearly 100 × 10 3 km 3 of deposits of Tertiary age, chiefly Eocene to Pliocene, have accumulated on the margin, and perhaps an additional 5 × 10 3 km 3 in the trench. This amount of offshore sediment could be supplied by fairly low rates (3 cm/10 3 yrs) of Tertiary erosion. However, many uncertainties in our denudation-sedimentation budget make it impossible to determine whether or not sediment reaching the base of the margin was removed tectonically in Tertiary time. Between 27° and 44° S., the trench contains nearly 70 × 10 s km 3 of turbidite deposits that we believe accumulated during late Cenozoic periods of glacially lowered sea level. The volume of turbidites in the trench is virtually equal to that expected from continental erosion, which is estimated to have probably been no greater than 5 cm/10 3 yr for the arid region between 27° and 31°, and 50 cm/10 3 yr for the humid and partially glaciated region from 36° to 42°. During this time of rapid erosion and trench filling, magnetic data indicate that convergence of lithospheric plates was taking place below the trench at a rate between 5 and 10 cm/yr. If turbidite deposits were swept from the trench at these rates, then continental denudation must have been exceedingly rapid: 20–40 cm/10 3 yr for the arid zone, and 110–165 cm/10 3 yr for the partially glaciated region. If more conventional estimates of erosion are valid, then either (1) late Cenozoic underthrusting has not taken place (or at a rate much slower than that implied by geophysical data), or (2) underthrusting at the prescribed rates has not involved the removal of a significant volume of sediment from the trench.

The tide‐swept continental shelf sediments between the shetland isles and france

Abstract: SUMMARY Extensive data obtained by side-scan Asdic and echo-sounder equipments, viewed in conjunction with bottom notations on navigational charts and some seabed samples, show the widespread occurrence of sand ribbons, sand waves and sand patches on the continental shelf west of the British Isles and France. The close similarity of their directional trends with those of the strongest tidal currents strongly suggests a causal relationship which applies to much of the continental shelf, parts of the upper continental slope and also to off-lying shoals such as Rockall Bank. The sand transport paths broadly parallel the coasts in the epicontinental seas and on the open shelf west of the British Isles. In contrast, the paths are substantially normal to much of the coast of western France.

Continuous deep sea salt layer along North Atlantic margins related to early phase of rifting.

Abstract: Deep seismic reflexion surveys have revealed diapiric structures in deep water off Labrador, Newfoundland, Mauritania, Morocco, Portugal, Spain and Ireland as well as in the Mediterranean. Many of these diapirs are similar to the Gulf of Mexico salt domes, and it is suggested that there is a continuous deep sea salt layer, off the continental margin, which is related to the early phase of rifting.

Plate tectonics of the Red Sea and East Africa.

Abstract: I WOULD like to comment on some of the assumptions and results of a recent letter by McKenzie et al.1. They reconstruct the pre-movement position of Arabia and Africa by fitting the two coast lines of the Red Sea, assuming that the entire space between the coasts is occupied by newly formed oceanic crust. This assumption ignores the existence of the Danakil horst, which consists of continental crust (Pre-Cambrian, Jurassic) and which is some 80 km wide, in between these two coast lines in the southern Red Sea depression. It seems impossible to close the gap of the Red Sea without leaving the space required for this continental block.

Sea level at -175 m off the Great Barrier Reef 13,600 to 17,000 year ago.

Abstract: THE shelf around Australia, like many other continental shelves1, has its edge chiefly2–5 at depths of 120 to 130 m and is marked locally by terraces and notches, which register low stands of the sea during the Quaternary. From radiometric dates of shallow water fossils recovered from terraces in different parts of the world the lowest eustatic level of the sea during the past 35,000 yr is estimated to be −130 m, 16,000 yr ago6. More recent information, principally from Australian waters, suggests that the sea level may have stood much lower during this period. Dill7 and Conolly4 found submerged terraces with shallow water fossils and sediments between depths of 175 to 238 m in 39 of 78 narrow beamed echo sounder profiles made all around Australia. The continuity and consistent depth of the terraces indicate that there has been little tectonic warping of the continental margin since the terraces were formed. Similar features are reported off southern and Baja California7, and a shallow water mollusc from a deep terrace off Baja California has a radiocarbon date of 14,380±190 yr BP. Another record of shallow water fossils of similar age and depth relates to the south-eastern Caribbean Sea8, where fragments of algae and hermatypical corals at a depth of 157 m (Station index 1,203) have radiocarbon dates of 13,590±270 and 13,800±330 yr BP, and algae at 187 m (Station index 1,202) a date of 14,220±350 yr BP. These organisms did not necessarily live at sea level6 but, being reef dwellers, probably indicate shallow water. To date, the evidence from Australia is only morphological. Here we describe the occurrence and radiometric dates of two shallow water specimens collected in situ from deep terraces off the Great Barrier Reef during descents by J. J. V. in the Japanese research submersible Yomiuri in February 1969.

Continental Margins from Viewpoint of the Petroleum Geologist

Abstract: Major features of the earth's surface are its continents and its ocean basins--which in turn reflect fundamental differences between its continental crust and its oceanic crust. The broad zone of contact between continental and oceanic domains has been called the continental margin. Geomorphically, the margin is expressed by the continental slope, but also includes the seaward part of the submerged continental shelf and the landward part of the continental rise. Geologically, the margin includes the zone of lateral change in the lithosphere marking the oceanward limits of characteristic continental crust. Many of the most exciting events in the history of our planet have taken place at this contact zone, between continental and oceanic crust, and between continents and oceans; the continental margin represents the stage where, throughout earth history, this drama has been played. Important elements of the continental margin are the outer shelf, the borderland, the marginal plateau, the slope, the base of the slope, the rise, and the marginal trench. The nature and the origin of these features and their sediments are of vital importance to petroleum geology. Of particular interest to the petroleum geologist are the sediment-rich, semi-enclosed basins or seas associated worldwide with the continental margin; the structural barriers and reefs commonly developed near the rim of the continental slope; the numerous manifestations of gravity tectonics and diapirism on the slope; and the growing evidence for impressive vertical movements associated with continental margins. Great advances in our understanding of the processes active at the continental margins have come from the subsea geologic and geophysical studies of the last decades, and rapid additional progress may be expected from the stimulus of "the new global tectonics"; but current hypotheses are still largely in a developmental stage. Factual data are still woefully inadequate, and the need for additional continuous refraction seismic profiles across the margin is critical. Moreover, continuing studies are needed, not only to better know the margins of the present, but also to better identify the margins of the past. Of special interest are the extent and age of transmarginal features. For the petroleum geologist, it is significant that through the ages the continental margin has been the great mixing bowl in which has been brewed most of the world's petroleum and from which most of its petroleum production to date has been derived. The continental margin should be the fruitful meeting ground of the petroleum geologist, the geologist of the oceans, and the student of earth history.

Sur-Nacimiento Fault Zone of California: Continental Margin Tectonics

Abstract: The Sur-Nacimiento fault zone extends northwest through the southern and central Coast Ranges of California, and presumably continues offshore on the Continental Shelf. In part, or perhaps in its entirety, it forms the approximate boundary between the Franciscan trench(?) assemblage on the southwest and the granitic and regionally metamorphosed basement rocks of the Salinian block on the northeast. The Sur-Nacimiento fault zone includes the Sur fault zone, the Nacimiento fault, and a number of other faults of various kinds and various ages. There has been a long history of recurrent activity characterized by sequential changes in the types of movement. The Sur fault zone is here considered to have originated at the former margin of the continent during latest Cretaceous or Early Tertiary time as the culmination of processes that operated during the entire Late Mesozoic. Throughout the latest Jurassic, Early Cretaceous, and mid-Cretaceous, Franciscan deposits were probably carried into a trench and pushed against or partly beneath the crust to the east, by ocean-floor spreading. The Franciscan assemblage has a melange-like aspect which is appropriate to this kind of disturbance. Some rocks within the assemblage are about the same age (Early or mid-Cretaceous) as the adjacent Salinian granite, but they are unaffected by the granite. The postulated trench containing the Franciscan assemblage was probably far offshore during the time of granitic intrusion, but westward drift of North America narrowed the intervening distance. Conveyor-belt action of eastward-moving oceanic crust impinging against and beneath the edge of the continent may have ripped off and carried downward blocks of sialic crust. Perhaps this was the mechanism that eventually brought the Franciscan assemblage against the Salinian granite, creating the ancestral Sur fault zone. The Franciscan terrain is locally tectonically covered by allochthonous masses of Great Valley-type Late Mesozoic clastic sedimentary rocks predominantly of two ages: (1) Tithonian-Valanginian, and (2) Campanian-Maestrichtian(?). It is tentatively suggested that these allochthonous rocks were deposited between the continent and the trench in which the Franciscan accumulated, and that they were underthrust by the Franciscan during the same action that produced the Sur fault, in latest Cretaceous or Early Tertiary time. In the Tertiary, normal faulting occurred along the Sur-Nacimiento zone, following the annihilation of the active trench system. There are indications that strike-slip faulting and reverse faulting ensued at various times.

Physiographic Features on the Outer Shelf and Upper Slope, Atlantic Continental Margin, Southeastern United States

Abstract: Both erosional and constructional processes appear to have formed physiographic features near the shelf break along the southeastern United States, as indicated by extensive echosounder profiles, rock-dredge material, and bottom photographs. Between Cape Hatteras, North Carolina, and Fort Lauderdale, Florida, four distinct physiographic areas are delineated, each having characteristic morphologies and lithologies. The ridges and well-defined troughs on the outer shelf and upper slope (depths of about 50 to 150 m) between Cape Hatteras and Cape Fear may be related largely to earlier Gulf Stream erosion, and the rocks (algal limestones and sandstones) and sediments dredged from these features probably are mainly Holocene, relict shallow-water deposits forming a thin veneer over this erosional surface of the sea floor. Relatively rapid accumulation of pre-Holocene sediments may account for the general absence of pronounced physiographic features on the outer shelf and upper slope from Cape Fear to Cape Kennedy . Ledges, small terraces, and rises (depths of 50 to 110 m) in this area are probably Holocene features eroded into, or constructed on the pre-Holocene sediments, which are covered by transgressive Holocene algal limestones and sandstones similar to those collected to the north. The lithology, together with radiocarbon dates of rock material, indicate that well-defined ridges in depths of 70 to 90 m between Cape Kennedy and Palm Beach are relict oolitic ridges or “dunes” formed during the Holocene transgression; these features are now covered by modern Oculina sp. coral debris. From Palm Beach to Fort Lauderdale, where the continental shelf is narrow and shallow, a small ridge present at the shelf break (15 to 30 m) is thought to be an “inactive” coral reef.

Kermadec arc—New Zealand tectonic confluence

Abstract: The transition in tectonic style from that of the Kertnadec island arc to that of continental crust in New Zealand has been ascribed, in global tectonic schemes, to the reduction of “shortening” rate as the pole of rotation between the Australian and Pacific crustal blocks is approached. However, the recognition of a west-facing island arc south of New Zealand and of widespread Quaternary crustal shortening within the New Zealand continental crust leads to the conclusion that the differing tectonic styles reflect instead the variation in mechanical response of oceanic and continental crust. The Alpine Fault is not considered to be a trench-trench transform because it parallels and co-exists with island arc features. Displacement of the early Tertiary Vening Meinesz fracture zone by the Alpine Fault indicates 70 km of late Tertiary-Quaternary horizontal motion along this fault. It is possible that the rest of the 500 km displacement along the South Island trace occurred during the late Mesozoic wh...

Atlantic Continental Shelf and Slope of the United States - Shallow structure

Abstract: -----------------------------------------Introduction---------------------~----------------Acknowledgments _____________________________ _ Topographic setting ___________________ --_____ ---__ Methods of study __________________________________ _ Scotian Shelf area _________________________________ _ Gulf of Maine and Bay of FundY-------------~------­ Northeast and Great South Channels----------------Georges Bank _____________________________________ _ Long Island, Block Island, Rhode Island, and Vineyard Sounds, and Buzzards Bay ________________________ _ P11ge Pllge 11 Continental shelf from Cape lioa to Virginia___________ 113 1 Continental slope from· Georges Bank to Cape Hatteras_ 15 2 Continental shelf from Cape Hatteras to Cape Romain___ 19 2 Blake Plateau area__________________________________ 19 2 Straits of Florida ___________ :... _______________ :._______ 30 5 Geologic maP-------------------------------------31 5 Isopach maps_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 33 9 Conclusion___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 38 11 Nature of the con tin en tal shelf and slope_ _ _ _ _ _ _ _ _ 38 Formation of the continental terrace-_____________ 38 12 References cited____________________________________ 41

Comparison of Continental Margins of Eastern North America at Cape Hatteras and Northwestern Africa at Cap Blanc

Abstract: The opposing continental margins of Cape Hatteras and Cap Blanc appear broadly symmetric with respect to (1) distribution of physiographic provinces, with the exception of the lower continental rise hills which have not been observed off Cap Blanc, (2) early and middle Paleozoic, Mesozoic, and Cenozoic stratigraphic frameworks, (3) late Precambrian, Paleozoic, and Mesozoic tectonic frameworks, with certain qualifications, and (4) certain offshore residual magnetic anomalies, including a positive anomaly associated with the continental shelf edge and the boundary between smooth and rough magnetic fields. The third point must be qualified because (a) the absence of late Paleozoic (post-Devonian) strata in the Mauritanides fold belt of northwest Africa, as opposed to their i volvement in the Appalachian fold belt of eastern North America, limits resolution of the respective orogenic movements and (b) the existence of a system of Triassic fault-block basins parallel with the northwest African continental margin, corresponding to the system in eastern North America, has not been well documented. The opposing continental margins appear broadly asymmetric with respect to Cenozoic tectonic frameworks. Sedimentary strata underlying the coastal plain, continental terrace, and continental rise near Cape Hatteras are predominantly undeformed, except by superficial gravitational displacement processes acting above Horizon A (Late Cretaceous-Eocene). Sedimentary strata underlying the corresponding physiographic provinces off Cap Blanc are deformed by deep structural processes including (1) compressional folding related to Alpine diastrophism, (2) tensional faulting along west-northwest fracture trends, coincident with eastward projections of fracture zones which cross the Mid-Atlantic Ridge, and along inferred north-northeast fracture trends which regionally parallel the continental m rgin, (3) volcanism, at least of Oligocene to Holocene age, concentrated along the fracture zones, and (4) diapirism, probably produced by rock salt deposits beneath the continental terrace and continental rise correlative with Late Triassic and Jurassic evaporites in the northwest African coastal basins. Mesozoic and Cenozoic mean rates of subsidence and sequences of gross lithology generally correlate between the opposing continental margins. Similarities in the stratigraphic records of the opposing continental margins and the adjacent ocean basin indicate that the continental margins have behaved as if vertically, as well as horizontally, coupled to the ocean basin. Mesozoic and Cenozoic mean subsidence rates of the opposing continental margins (1-9 cm/1,000 year) are about 10-3 of inferred mean spreading rates of the intervening sea floor (1-4 cm/year). The mean subsidence rates vary in unison with the independently inferred mean spreading rates, to a first approximation. The limited data suggest that the epeirogenic subsidence of the opposing continental margins and the inferred spreading of the intervening sea floor are related genetically. The apparent symmetry in space and time of tensional rifting, orogenic compression, and epeirogenic subsidence on the opposing continental margins is consistent with a hypothesis of sea-floor spreading. These findings imply that geologic conditions relevant to the occurrence of petroleum can be predicted between the opposing continental margins.

Deep Sedimentary Basins proved in the Shetland–Hebridean Continental Shelf and Margin

Abstract: A REGIONAL geophysical survey of the continental shelf and margin west of the Orkney and Shetland Isles was completed during cruises of RRV John Murray in 1967 and 1968. These surveys yielded about 9,000 km of continuous surface ship gravimeter and magnetometer track across the shelf and margin, including ten seismic reflexion (sparker) traverses. This report gives a preliminary interpretation of the resulting Bouguer anomaly map (Fig. 1).

Geosynclinal Development of the British Isles during the Silurian Period

Abstract: The British geosynclinal complex was bounded on both northwest and southeast by continental margins during the Lower Paleozoic. The northern margin, in Scotland and the northern parts of Ireland, consisted of a mountain range, the Scottish Highlands, which was technically active and was positioned between a stable continental foreland to the northwest, and an uncoupled oceanic crustal plate to the southeast. The southern margin, in Wales and the Welsh Borderland, was characterized by a horst and graben structure and was gradually subsiding during most of the Silurian period. The Scottish margin produced tremendous quantities of immature graywacke sands which accumulated in a trench adjacent to the source area, on the site of the present Southern Uplands. A pattern developed about the middle of the Silurian in which deposits in this trench became deformed and uplifted shortly after deposition and, in turn, contributed sediments to the trench which was progressively displaced to the south toward the Lake Di...

Distribution of the Toquima-Table Head (Middle Ordovician Whiterock) Faunal Realm in the Northern Hemisphere

Abstract: Discovery of a Whiterock trilobite assemblage in the Albany Mudstone, Girvan District, southwestern Scotland, led to an assessment of the distribution of Middle Ordovician brachiopod and trilobite faunas previously assigned to the White-rock Stage of Cooper (1956). These faunas lie within a belt designated as the Toquima-Table Head Faunal Realm. This realm is closely related to the position of the transition from miogeosynclmal to eugeosynclinal facies, presumed to indicate the position of Ordovician continental margins. In Middle Ordovician time North America, parts of Ireland, Scotland, Norway, Sweden, and northeastern Asia may have constituted a single continental mass.

Shallow Structure of the Continental Margin, Southern Brazil and Uruguay

Abstract: The shallow structure of two coastal basins or embayments along the continental margin of southern Brazil and Uruguay was investigated by geophysical techniques during a cruise of the U.S. Coast and Geodetic Survey Ship Oceanographer in late 1966. Deformation of the continental shelf and slope sediments in the first of these basins, the Sao Paulo Embayment, situated between Rio de Janeiro and Florianopolis, suggests the presence of diapiric intrusions. Grabens developed above diapirs are reflected as topographic valleys on the continental slope and as zones of contemporaneous faulting on the outer shelf where sedimentation has kept pace with deformation. The Sao Paulo Embayment subsided sufficiently from Late Cretaceous through Pleistocene time to accommodate a series of three prograded sedimentary wedges with a cumulative stratigraphic thickness of at least 4 km. Each wedge of prograded sediment is presumed correlative with the onset of block-faulting, uplift, and rejuvenation of a mountain complex which borders the landward edge of the Sao Paulo Embayment. The axis of maximum sediment accumulation has migrated landward during the Tertiary. In any reconstruction of pre-rift South America and Africa, the Sao Paulo Embayment lies immediately to the south of the Cuanza Basin of Angola. These two coastal basins have Mesozoic similarities but, beginning in the Tertiary, their patterns of marine sedimentation diverge; this fact is consistent with proposed Cretaceous rifting of this portion of the two continents. The Pelotas Basin may be traced from beneath the coastal lagoons in southern Brazil and Uruguay part-way across the continental shelf. The continental slope off the Pelotas Basin consists of a large conical apron of sediment; the deeper strata are arched mildly and younger sediments thin over structural highs. The relationship of the cone to the Pelotas Basin is not clear. Although submarine canyons are numerous and well-developed north and south of the study area, only fault-controlled valleys are known along the southern Brazil margin. Outcrops of continental shelf strata are rare along the slope; progradation and slope-conformable sedimentation are the rule.

Magnetic anomalies in the northeast Atlantic between the Canary and Cape Verde Islands

Abstract: During 1968 about 7500 km of new magnetic data Were recorded by the USNS J. W. Gibbs between the Canary and Cape Verde islands, from the continental shelf to approximately 30°W. These data, together with an equal amount of data from other sources, reveal major magnetic features. The magnetic boundary between relatively undisturbed and disturbed magnetic zones is delineated near the middle of the northwest African continental rise. A sequence of linear north-south-trending anomalies immediately seaward of the magnetic boundary comprise a band about 300 km wide and can be correlated from about 15°N at the Cape Verde Islands to about 26°N just south of the Canary Islands. The band of magnetic anomalies appears to have a right lateral offset of about 100 km near 25°N where intersected by a west-northwest-trending fault near the eastward projection of the Atlantis fracture zone from the mid-Atlantic ridge. At least one prominent positive magnetic anomaly is associated with the northwest, African continental shelf. The magnetic disturbance boundary and the associated band of linear magnetic anomalies are nearly mirror images of similar anomalies associated with the continental margin off eastern North America.

Atlantic Continental Shelf and Slope of the United States - Gravels of the northeastern part

Abstract: Gravel is concentrated mainly on the glaciated part of the continental margin-the Gulf of Maine, Scotian Shelf, and northern part of Georges Bank. Most coarse detritus in the Gulf of Maine is exposed on ledges and shallow banks as well as on the hummocky topography between basins. It is a very poorly sorted mixture of gravel, sand, silt, and clay. The fragments are subrounded to .angular, and some gravels have multimodal grain-size distribuition. Rock types are varied, and the detritus seems t.o be derived fl'lom local bedl'lock. On Georges Bank and Nantucket Shoals, the gravel is better 'Sorted, more quartzose, and better rounded. It is associated with sand waves afld tidal ridges in both areas. Gravel is both coarse and abundant in the exit channels that lead s?award from the Gulf ·of ::\Iaine across the continental shelf; the abundance of gravel in these channels indicates that they were occupied by lobes of glacial ice during the Pleistocene. Gravel on the Scotian Shelf resembles that of both the Gulf of Maine and Georges Bank in that well-sorted sanely gravel is closely associated with till-like mixtures of gravel and finer sediment. As in the Gulf of Maine, basin sediment is silty clay or clayey silt. Echosounding records suggest that these fine-grained sediments mask the gravel and were deposited during the Holocene rise in sea level. Scattered occurrences of gravel are found on the continent slope as far ~outh as Hudson Canyon. The gravel fraction on the slope is a minor part of the sediment (most is silt and clay) and shows a wide range in size and roundness. On the nonglaciated shelf south of Xew England and Long Island, gravel is distributed sporadic-ally; largest concentrations are associated with the drowne!d Hudson ChannE'l east of New .Jersey. The gravel is moderately sorted quartzose, and commonly in a bimodal grain-size distribution with sand. Interpretation of the areal distribution and properties of gravel allows us to (1) infer the bedrock geology for most of the Gulf of :Maine and Scotian Shelf, (2) fix the approxima.te limits of glaciation on the continental shelf, and (3) list the agents that dispersed the gravel. Sedimentary rock of probable Triassic age contributed detritus to much of the northeastern Gulf of Maine and Bay of Fundy and probably underlies these areas. Sedimentary rock ,of Cretaceous and younger :age was a ~ource for rock fragments in the southern Gulf and some of the "vein" quartz pebbles so abundant on Georges Bank and NantuckE't Shoals. Granite and felsi,te clasts are abundant off 1 Contribution 2107 of the Woods Hole Oceanographic Institution, based on work done under a program conducted jointly by the U.S. Geological Survey and the Woods Hole Oceanographic Institution and financed by the U.S. Geological Survey. the central 1\Iaine coast, southeasitern New England, and the Scotian Shelf. Spotted schist and mica gneiss arE' concentrated southwest of Nova Scotia; along will Basin) are a mixture of all rock types. Resistant rock types ("vein" quartz, quartzite, and chert) are concentrated mainly south of the Gulf of Maine and probably represent a mixed provenance; some fragments were probably brought in from crystalline bedrock to the north by glaciers. Some also may have come from quartzose conglomerates in strata of Cretaceous age, which are thought to underline Georges Bank and the southern Gulf of 1\Iaine. Utilizing the concentration of coarse detritus to mark the sea ward extension of ice, we find that the boundary extends enstward as a lobate line from glacial moraines on Nantucket and :Martha's Yineyard, across Great South Channel and the northern part of Georges Bank. It continues across the seaward terminus of No,rtheast Channel and along the seaward edge of the Scotian Shelf. 1\Iost of Georges Bank was subaerially exposed during low ,stands of st>a level, so that melt-water ~.treams drained south to the shelf edge, where they clumped detritus into the many submarine canyons 'that incise d1e southern part of Georges Bank. The boundary indicates that ice c>xtended at least to the edge of the Scotian Shelf, where it formed a floating, calving margin in the sea. Glaciers moving southward from New England and Canada sculptured the northern continental margin and contributed the poorly sorted till-like mixtures of gravel, sand, silt, and clay. They dumped debris along northern Georges Bank, probably, as moraines and outwash plains. Hence, the moderate sorting, better roundness, and increase in resistant rock types noted in gravels on Georges Bank, Nantucket Shoals, and parts of the Scotian Shelf reflect some current transport by melt-water streams 'and by marine bottom currents. Coarse dt>bris was rafted by floating ice to the continental slope and rise; this is reflected in the wide variation in rock types and roundness (pointing to multiple sources) and in the "tacked on" nature of the gra Yel fraction to the main part of the grain-size distribution. l\Iost of the shelf off New England, Long Island, and New Jersey is mantled by sand and lesser amounts of gravel in amounts probably sufficient to constitute an economic asset. A drowned river terrace on the shelf soutlwast of New York City and isolated glacial gravelly sands offshore from Boston are promising deposits meriting further detailed study. Other deposits are off Rhode J,sland, Cape Cod, and Long Island. A few

Mineralogy and geochemistry of continental shelf sediments of Washington-Oregon coast

Abstract: An analysis of the mineralogy and geochemistry of continental shelf sediments offshore of the Washington-Oregon Coast indicates that three depositional areas are present on the continental shelf; these consist of a nearshore area including the lower portion of the Columbia River estuary and a main shelf area, both characterized by poorly sorted sediments, and a third area seaward of 80 fm which is characterized by glauconitic sediments. Within each area the mineral and chemical compositions of the sediments appear distinctive and characteristic for the area. The glauconite in the continental shelf sediments is generally found in the deeper shelf areas. It is derived from glauconitic-bearing Tertiary sedimentary rock units exposed in the area, some may be transported north from shelf areas off central and southern Oregon and some may be forming in place. Boron is associated with the glauconite and may be indicative of the presence of glauconite or as an indicator of the marine environment.

Aeromagnetic and Gravity Investigations of the Coastal Area and Continental Shelf of Liberia, West Africa, and Their Relation to Continental Drift

Abstract: An aeromagnetic survey has shown the existence of several basins in which magnetic basement depths are greater than 5 km on the continental shelf off Liberia Magnetic diabase of 176 to 192 my (Jurassic) in age intruding the Paleozoic (?) rocks and overlain by younger rocks onshore requires the distinction between “magnetic basement” and “basement” Several lines of evidence suggest that the Paleozoic(?) rocks are less than 1 km thick; this implies that the diabase does not introduce a large error in depth-to-basement estimates The dikes or their extrusive equivalents are traceable, on the basis of the magnetic data, beneath the younger sedimentary rock in the basins to the edge of the continental slope The magnetic data also delineate a second zone of diabase dikes 90 km inland, parallel to the coast, which cross the entire country The intrusion of the younger dikes probably coincides with rifting at the beginning of the separation of Africa and South America, and the associated magnetic anomaly zones appear to be parallel with and continuous into the anomaly bands in the Atlantic A major northeast-trending break in the magnetic fabric intersects the coast near 9° W and is associated with Eburnean age rocks (about 2000 my) to the southeast as contrasted with Liberian-age rocks (about 2700 my) to the northwest Change in magnetic fabric direction inland from northeast to northwest in the coastal area allows recognition of a boundary between the Liberian-age rocks inland and Pan-African-age (about 550 my) rocks in the coastal area northwest of about 9° 209W Sets of north-northwest-and west-northwest—trending faults of 1 to 2 km vertical displacement cut the Cretaceous sedimentary rocks onshore and can be traced into the offshore basins Vertical displacements of several kilometers in the magnetic basement underlying the continental shelf suggest a pattern of block faulting all along the coast and continental shelf Negative Bouguer anomalies exist over two Cretaceous basins in the coastal area; a negative Bouguer anomaly exists over one of the basins southwest of Monrovia, as shown by a marine traverse, suggesting that Cretaceous or younger sedimentary rocks fill these basins also A 50 to 60 mgal positive Bouguer anomaly area exists along the coast from Sierra Leone to Ivory Coast This anomaly correlates with mafic granulites in the Monrovia region, where the gradient is too steep to be entirely due to crustal thickening at the continental margin and may be related to tectonic activity associated with the basins The only major break in this positive anomaly above basement rocks along the entire coast of Liberia is over granite gneiss adjacent to (and presumably underlying) the only onshore basins on the Liberian coast Three seismic reflection profiles support the interpretation of a substantial section of sedimentary rock offshore A suggested sequence of events indicates tectonic activity in the periods about 2700, about 2000, and about 550 my BP; uplift and exposure of deep crustal rocks; deposition of Paleozoic sediments; intrusion of diabase dikes in inland zones; intrusion of 176 to 192 my-old dikes and sills accompanying separation of Africa and South and North America; block faulting along coast and continental shelf, and active sea-floor spreading; filling of basins in Cretaceous and Tertiary(?) time; basaltic extrusion on spreading sea floor and sedimentation on continental shelf and slope

The Phuket Group, Peninsular Thailand: a Palaeozoic ?geosynclinal deposit

Abstract: The Phuket Group is the oldest rock unit in the Phuket-Takua Pa-Krabi region of the Thai Peninsula. The Group comprises two Formations: a Lower Formation of Ordovician, or possibly Cambrian, to Lower Permian age, exceeding 3 km in thickness, and an Upper Formation of early Permian age between 100 and 200m thick.Facies analysis shows that the Lower Formation consists of mass flow deposits and sandstones comparable to the deposits of modern continental rises and continental slopes. The Upper Formation was deposited in a shallow marine and probably deltaic environment. The Phuket Group and litho-logically similar sedimentary rocks to the north and south occupy an elongate belt extending from the Shan States of Burma to the Langkawi Islands in Malaysia. The sediments, which are rich in quartz, were derived from a continental area and deposited on its margin. In contrast to previous interpretations of the Palaeozoic history of Peninsular Thailand, it is considered that the continental source lay to the east. The continental margin resembled modern margins of either Atlantic or Japan Sea type.