Showing papers on "Continental margin published in 1975"

Potash-Depth (K-h) Relations in Continental Margin and Intra-Oceanic Magmatic Arcs

Abstract: Plots of potash content against depth from volcanoes to inclined seismic zones for active magmatic arcs reveal slightly different relations for (a) continental margin arcs containing full or excessive thicknesses of continental crust overlying presumably old continental lithosphere, and (b) migratory intra-oceanic arcs that bound interarc basins of marginal seas and lack any known continental basement rocks. Detached intra-oceanic arcs containing continental basement rocks and stationary intra-oceanic arcs where back-arc spreading is absent display intermediate or equivocal relations. The significance of the difference in K- h correlation for continental margin and intra-oceanic magmatic arcs is uncertain but should be taken into account by theories for the petrogenesis of arc magmas.

The Neogene calcalkaline volcanism of Central Anatolia: geochronological data on Kayseri—Nigde area

Abstract: The evolution of Neogene volcanic activity in the Central Taurus is investigated; stratigraphical and radiometric age data from the Urgup (Kayseri) basin idicate that volcanism in the area began at least as early as Upper Miocene, lasting up to prehistoric times. The volcanism maintained throughout this time interval a calcalkaline character. The diachronous end of calcalkaline volcanism along the Taurus margin is tentatively related to the differential collision between the Afro-Arabian and the Anatolian plates, probably due to an original irregular shape of the Anatolian continental margin.

Marine Geology of the Rockall Plateau and Trough

Abstract: The Rockall Plateau is an extensive shallow water area located south of Iceland and west of the British Isles: it is separated from the British Isles by the 3000 m deep Rockall Trough. Rockall Island, composed of 52 ± 9 Ma aegirine-granite is the sole subaerial expression. The Rockall Plateau is interpreted as a continental fragment or microcontinent isolated during the sea floor spreading evolution of the North Atlantic Ocean. A geological reconnaissance of the Rockall Plateau and Trough has been made by using a 650 cm 3 (40 in3) seismic reflexion profiling system, supplemented by sparker (8 kJ) profiles on Rockall Bank and arcer (60 kJ) profiles across the margin west of the British Isles. Stratigraphic interpretation of these profiles has been aided by deep sea drilling data, bottom sampling on Rockall Bank and by the relation between the various reflecting horizons and oceanic basement dated by oceanic magnetic anomaly identifications. Analysis of the microtopography of the area has given information on Post-Palaeogene sedimentation processes. Three major sedimentary basins are present in the area. The Hatton-Rockall Basin is developed in thinned continental crust on Rockall Plateau. The Rockall Trough is developed on continental crust and includes oceanic crust believed to have been generated in Late Jurassic-Early Cretaceous time. The Porcupine Seabight may be developed on thinned continental crust. All three basins have a faulted basement and exhibit a history of progressive and/or intermittent subsidence. The subsidence phases correlate closely with estimated changes in sea-floor spreading rate. This correlation and the regional pattern of uplift and subsidence is discussed with reference to the effects of thermal subsidence and differential loading of the lithosphere beneath continental margins. Post Upper Eocene sedimentation throughout the area was characterized initially by widespread chert deposition and subsequently by differential deposition of Early Miocene to Recent oozes. The onset of widespread differential deposition in the Early Miocene indicates the present near bottom-circulation was established at this time and may be related to subsidence of the Iceland-Faeroes Ridge. The relation between differential deposition, topography and circulation is discussed in terms of flow around obstacles.

Thermal and mechanical models of continent‐continent convergence zones

Abstract: The thermal regimes of continent-continent convergence zones are modelled by a finite difference technique, assuming that there is some subduction of continental crust. Gravity and heat flow profiles are generated from the thermal models. Subducted crust and slab remain cool except at the upper surface where frictional heating is important. Crustal rocks may be metamorphosed or melted by friction while the plates are converging or by radioactive self-heating more than 30 m.y. after the plates have stopped. In the former case, melting requires frictional shear stresses between 500 and 2000 bars at a plate velocity of 5 cm/yr. At lower velocities the upper limit of frictional stress is greater. The total work performed in continental subduction may be minimized if the angle of underthrusting becomes more shallow, changing the location of subduction. A model for the geometry of oceanic and continental slabs in the Zagros Mountains is presented which satisfies gravity, heat flow, seismic, and geologic-constraints. Continental underthrusting is beginning along a new fracture at the edge of the Persian Gulf, isolating the Arabian continental shelf from the subduction process. Slippage along this fault is Pleistocene and probably does not exceed 30 km. Subduction of continental crust at the crush zone is probably insignificant.

Quaternary Sedimentation on the Amazon Continental Margin: A Model

Abstract: Sedimentation on the Amazon continental margin occurred in two distinct patterns during the Quaternary Period. During interglacials, when sea level was high (as at present), most of the sediment contributed by the Amazon River was deposited near the river mouth, then transported northwest along the innermost shelf by longshore currents. Apparently, little Amazon sediment escaped the nearshore environment. As sea level fell in response to glacial advance, the sedimentation pattern remained roughly similar, resulting in construction of an extensive prograding shoreline. Once the sea fell more than −60 to −80 m relative to present sea level, however, most Amazon sediment was channeled directly to the deep sea. Shoreline accretion stopped, but prograded sediments were preserved as a mud wedge that presently occupies the inner shelf seaward of the Amazon River. This model also explains the accumulation of large quantities of arkosic sands throughout the western Equatorial Atlantic deep sea during Pleistocene glacial epochs. These arkosic sediments appear to have been derived mostly from the Andes Mountains via the Amazon River.

Continental Margin Off Western Africa: Angola to Sierra Leone

Abstract: 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------------------------------

Framework mineralogy and chemical composition of continental margin-type sandstone

Abstract: Compositional data for modern deep-sea sand and ancient graywacke were used by Crook to postulate that Atlantic, Andean, and western Pacific continental margin types could be distinguished on the basis of unique composition—a progressive decrease in framework quartz, total SiO 2 , and K 2 O/Na 2 O ratio. A consideration of the major source areas generated by plate tectonics mechanisms suggests that the premise is valid and can be expanded to include sandstone varieties other than gray-wacke, but more data on modern sand are needed to refine the concept. The potential usefulness of such contrasts in sandstone mineralogy and chemistry is demonstrated by comparing the composition of several ancient sandstone units (mainly from the Appalachian-Caledonian belt) for which plate tectonics basin models have been proposed.

Plate Tectonics and the Structural Evolution of the Aleutian–Bering Sea Region

Abstract: The general aspects of the structural evolution of the Aleutian-Bering Sea region can be described in terms of plate tectonics. Involved in this model is the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time. The ridge is presumed to have formed in response to a southward relocation in the convergence zone of the Pacific oceanic plate, a shift away from the Beringian continental margin connecting Alaska and Siberia to an oceanic location at the Aleutian Trench. Prior to the formation of the ridge, Pacific crust is presumed to have directly underthrust the northeast-trending Koryak-Kamchatka coast. The middle and late Mesozoic eugeosynclinal or thalassogeosynclinal masses that underlie this segment of the Pacific fold belt are highly deformed, thrust faulted, and intruded by ultramafic bodies-characteristics that can be ascribed to the mechanical and magmatic consequence of plate underthrusting. This model implies a similar orogenic process for the formation of the stratigraphically and structurally similar Mesozoic rocks underlying the northeast-trending continental margin of southern Alaska. Less intense underthrusting may have occurred along the northwest-trending Pribilof segment of the Beringian margin connecting Alaska and Siberia. This margin may have been more parallel to the approximate direction of relative motion between the oceanic and continental plates. Nonetheless, fold belts, possibly intruded by ultramafic masses, formed along this segment of the Beringian continental margin in Late Cretaceous and perhaps earliest Tertiary time. The folds have since subsided below sea level-their eroded tops presently underlying as much as 3 km of virtually undeformed Cenozoic deposits. Our model relates pre- and postorogenic deposits underlying the Beringian margin and adjacent coast to the time of formation of the Aleutian Ridge, which marked the cessation of continental underthrusting and the beginning of island-arc underthrusting. Our model also implies that the ridge formed near or at its present location and that oceanic crust of late Mesozoic age underlies the Aleutian Basin of the Bering Sea. Since formation of the ridge this basin has received from 2 to 10 km of sedimentary fill. Although the model we suggest broadly explains the observed changes in tectonic style, magmatic history, and sedimentation for the Aleutian-Bering Sea region, it also implies that thousands of kilometers of oceanic crust underthrust the Kamchatka, Beringian, and Alaskan margins between Late Triassic and Late Cretaceous time, and hundreds of kilometers underthrust the Aleutian Ridge in Cenozoic time. The enormous masses of pelagic and volcanic offscrapings that would be indicative of extensive or long-term crustal underthrusting are not apparent as mappable units. Thus, while our model may be stylistically adequate, it paradoxically predicts quantities of rocks and structures that we are not able to find. Presumably they have been subducted.

Continental Margin Off Western Africa: Cape St. Francis (South Africa) to Walvis Ridge (South-West Africa)

Abstract: Approximately 17,000 km of continuous gravity, magnetic, and seismic-reflection profiles were recorded to determine the structure of the continental margin from Cape St. Francis to Walvis Ridge, and of the adjacent Agulhas and Cape deep-ocean basins. These and previous sea-floor and land data suggest that basement structures are the result of the breakup of Gondwanaland and the dispersion of the fragments to their present positions. This breakup may have been initiated as early as the Carboniferous Period, but most of the dispersion has taken place since Middle Jurassic. Igneous activity during the early phase may have led to the emplacement of ridges along the continental margin. Later, block faulting and volcanism along the fracture zones that delineate the flow lines of the drifting continents produced Walvis Ridge, Cape Rise, and the Agulhas Plateau. One of these fracture zones, the Agulhas fracture zone, dominates the structural grain of the continental margin and deep-ocean floor off the African southern coast. Sediments as thick as 7 km buried the fragmented continental basement and adjacent oceanic basement off the west coast and formed a broad continental rise and abyssal plain within Cape basin. The source of much of this clastic debris is believed to be the Orange River. In contrast, sedimentation off the southern coast since the breakup of Gondwanaland has been very limited, mostly being restricted to the narrow St. Francis basin atop the shelf. The adjacent continental slope and Agulhas basin have only a thin sediment cover. Much of the sediment that was present on the slope has slumped into the narrow Agulhas fracture zone at the base of the slope. Numerous swells on the southwestern end of the Walvis Ridge, undulating topography of the ocean floor in the western part of the Cape basin, swells on the upper continental rise, and the rough topography of the Agulhas Plateau were formed by the movement of the South Atlantic Bottom Water that enters Cape basin on the west side, flows along the southern flank of Walvis Ridge, and then is deflected southward by the continental slope. Pleistocene eustatic changes in sea level considerably modified the shelf, upper slope, and the eastern end of Walvis Ridge by wave action, turbidity currents, and the Benguela Current.

New radiometric ages and seismic data from Fuerteventura (Canary Islands), Maio (Cape Verde Islands), and Sao Tome (Gulf of Guinea)

Abstract: The early magmatic phase on the islands of Fuerteventura, Maio and Sao Tome ranges from Eocene to Middle Miocene. This conclusion is based on K-Ar datings, field observations and reflection seismic data which are all in good agreement. The three islands occur in different geological settings. The magmatic events are roughly synchronous and appear related in time to major unconformities on the continental margin of western Africa. They are also contemporaneous with major alpine orogenic events and cannot be connected with early opening stages of the Atlantic. A change in stress regime along the West African margin is probably in causal relationship with the collision of African and Eurasian continental masses. Deep-seated crustal deformation may have triggered off the early magmatic phase.

Cenozoic studies in northernmost Chile

Abstract: The Cordillera de la Costa is constructed from a block-faulted Paleogene landscape. The range is cut by canyons and bordered by a high cliff and a zone of marine terraces. To the east the longitudinal depression is infilled by Neogene deposits which were laid-down over the Paleogene surface and have since been locally structurally disturbed. In the Andes stratovolcanoes rise above the Andean plateau and are surrounded by internal drainage basins. In northernmost Chile subsidence of the submarine continental margin has been complemented by uplift of the Andes. Ingestion of sial and sima in the trench area seems to have led to magma production, and there has been an eastward movement in both igneous and structural events. The present plate motions have produced a compressive tectonism, but local distension has occurred at the edge of the continental plate. The longitudinal depression is believed to be a tectonically neutral zone west of the eastward-dipping reverse faulting of the Andean region. Wave-action on a subsiding coast is considered responsible for carving the high cliff, whereas littoral terraces reflect a recent zonally-atypical uplift phase. N-S strike-slip faulting is attributed to ancient plate-closure patterns. Problematic approximately E-W faults could reflect the adjustment of the continental margin to the stresses generated during recent relative plate motions. Early Tertiary erosion produced a Paleogene pediplain. The resulting large quantities of Paleogene sediment are considered to have been subducted during plate convergence. Tectonic movements formed the longitudinal depression and other smaller basins in the late Oligocene. Towards the south, Neogene planation has eroded the Paleogene pediplain, although major pediplanation and aggradation had everywhere ceased by the Upper Miocene when canyon formation commenced. Ignimbritic eruption waned in the Upper Miocene, and gave way to andesitic stratovolcano production that modified the Altiplano internal drainage. Post-Miocene capture of some of this drainage has occurred by headward erosion along Andean flank channels. Canyons across the coastal mountains were established at about the same time, although some of the long channels in the south are of greater antiquity.

Early Paleozoic continental margin sedimentation, trilobite biofacies, and the thermocline, western United States

Abstract: The Upper Cambrian Whipple Cave Formation and Lower Ordovician lower House Limestone of eastern Nevada contain algal stromatolites, flat-pebble conglomerate, lime grainstone, and other carbonate rocks inferred to represent shoal-water depositional environments. Coeval parts of the Hales Limestone, 170 km to the west, consist of dark-colored lime mudstone and wackestone interbedded with coarse-textured alloch-thonous gravity-flow and slump deposits interpreted to have formed in a deeper water slope environment. The Whipple Cave Formation and some allochthonous gravity-flow deposits of the Hales Limestone contain trilobite genera typical of the North American Faunal Province, whereas in situ deeper water limestones of the Hales contain trilobite taxa that are widespread in northwestern and southern China. An abrupt biofacies change between shelf and deeper water sites may be analogous to major changes with depth shown by Holocene marine isopod crustaceans in low latitudes, where the permanent thermocline is a major barrier to faunal dispersal. Thus, caution should be exercised in using faunal resemblance data for early Paleozoic plate-tectonic reconstructions unless the habitat and marine climatic preferences of the faunas are known.

Structure and development of the southern margin of australia

Abstract: Off the southern coast of Australia, exploration results and deep-water reconnaissance seismic data support the concept of an aseismic Atlantic-type continental margin. Characteristic is a sedimentary wedge which extends from the shelf to the abyssal plains and includes crustal elements of continental and oceanic origin. Oceanward, a continuous level of diffractions ascribed to the top of oceanic crust can be observed on seismic records, steeply dipping beneath the continental rise towards a smooth, flat, often faulted reflector which is correlated with top Precambrian or Palaeozoic continental basement. The sedimentary wedge which overlies the block-faulted and collapsed continental basement is subdivided by unconformities into: (a) a continental Lower Cretaceous unit and a fluviodeltaic unit of Upper Cretaceous-Danian age which are taken to represent rift valley stages of deposition controlled by extensional tectonics and (b) a post-breakup sequence of Tertiary units representing regional collapse and out-building of the shelf. The Upper Cretaceous sequence is missing along much of the continental edge where Tertiary sediments appear to rest directly on the Lower Cretaceous unit. Our interpretation suggests that a prolonged period of uplift took place along the axis of the rift valley prior to continental break-up. On the basis of palaeomagnetic data and biostratigraphic analysis the breakup phase started in the Upper Paleocene. From the continent outward several structural zones can commonly be recognised: (a) a zone of shallow basement with a thin Lower Cretaceous cover normally faulted and overlain by thin gently dipping Tertiary beds, (b) a zone of faulted and landwards tilted basement blocks and Lower Cretaceous sediments overlain (sometimes with clear unconformity) by thick Upper Cretaceous sediments, (c) a zone of thick, moderately deformed Tertiary sediments whose axis of deposition is generally offset to the south of the Upper Cretaceous basinal axis, (d) a zone of rotational faults and associated toe thrusts affecting the Cretaceous sediments and apparently related to the time of margin collapse, (e) an area of little disturbed Cretaceous and Tertiary sediments overlying continental basement. This zone extends into the "magnetic quiet zone" which is therefore believed to be, at least in part, a collapsed portion of the continental margin adjacent to oceanic crust. The interpretation of the geological evolution of the southern Australian margin based on the stratigraphic and structural data presently available can be related to current theoretical models on continental margin development.

Foraminiferal Life and Residue Assemblages from Cretaceous Slope Deposits

Abstract: Distinctive foraminiferal assemblages and dissolution patterns are associated with Cretaceous clastic deep-water sediments along the eastern North Pacific continental margin. These sediments were deposited in bathyal, low-oxygen chemically reducing environments, as is evidenced by their foraminiferal composition, lithology, sedimentary constituents, organic-carbon content, and associated organisms. In species composition and morphology, the foraminiferal assemblages closely resemble modern faunas from low-oxygen environments on the continental slope and in deep-water basins along the eastern North Pacific Ocean. The Cretaceous life assemblage is characterized by species of Praebulimina , the subfamily Chilostomellinae, and several agglutinated and nodosariid genera, among others. In southern California this assemblage is restricted to the laminated mudstone facies of the Upper Cretaceous Point Loma Formation, which exhibits limited bioturbation and contains large amounts of organic detritus and pyrite. By comparison with modern depositional environments, bottom conditions probably were oxygen deficient but not anaerobic. Differential preservation of the foraminiferal fauna in the Point Loma Formation resulted from diagenic dissolution in the chemically reducing sediments. The subsequent residue assemblage is enriched in resistant forms such as thick-walled, compact calcareous and agglutinated benthic species; it is impoverished in planktonic and less resistant benthic species. Selected species are ranked according to their susceptibility to dissolution by noting their successive stage of preservation. These data illustrate the complex interactions between environmental and diagenetic processes that strongly influence interpretations of paleontologic age and environment of deposition.

Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure

Abstract: A seismic-refraction study of the sedimentary structure of the South West African continental shelf was carried out between lat 17"s and 24"s using expendable sonobuoys. Striking differences exist both in the topography and sedimentary structure between the shelf north and south of the Walvis Ridge. South of the ridge, as far as lat 23"S, the shelf consists of a prograded series, whereas north of the ridge, at least as far as lat 17"S, east-trending canyons cut the shelf sedimentary cover. The steep northem scarp of Walvis Ridge can be traced eastward under the sediment of the continental margin. The southem Bank of the ridge is buried under a thicker sedimentary cover and could only be traced eastward to long. tOOE on seismic-reflection records. This flank probably parallels the northem scarp under the continental margin. Two-dimensional structural models, built with the help of seismic-retlection and seismic-refraction results and based on the hypothesis of local isostatic equilibrium, account for the obsewed gravity profiles. A compensating root consists of light material (density 2.95 g/cm? and reaches a depth of about 25 km. Gravity results also suggest that the Walvis Ridge does not constitute a superimposed load on the lithosphere; rather, the ridge and its underlying compensating mass were created at approximately the same time as the adjacent ocean basins. The creation of the two aseismic ridges of the South Atlantic - the Rio Grande Rise and Walvis Ridge - by a mantle hot spot and plume is accepted; this theory seems to explain most of the peculiar features of the Walvis Ridge. However, it is probable that the surface expression of the mantle hot spot was controlled by the presence of weak zones in the lithnsphere such as transform

Triassic seaways and the Jurassic Tethys Ocean in the central Mediterranean area

Abstract: IN the central Mediterranean area phases of middle Triassic and middle Liassic rifting are recognisable. The former produced only relatively deep pelagic basins founded on thinned continental crust; the latter, however, marked the opening of the central Tethys. The Porphyrit-Hornstein and the Diabas-Hornstein formations1 are the results of such phenomena. During Dogger and Malm times the spreading continued and a large amount of oceanic crust was formed. The Tethys Ocean opened parallel to the long axes of some Triassic deep sea basins and across the long axes of others. Consequently, the positions of Triassic seaways along the southern continental margin of the Tethys ‘geosyncline’ are defined by the effects of at least two main palaeotectonic events.

Geophysical Studies in the Gulf of Mexico

Abstract: The Gulf of Mexico is a small ocean basin which, for the last 20 years, has been subject to intensive geological and geophysical investigation. Much has been learned about its framework, but a host of complex problems remains unsolved.

The Petrology and Origin of Some Glauconitic and Glauco-Conglomeratic Phosphorites from the South African Continental Margin

Abstract: Glauconitic and glauco-conglomeratic phosphorite rocks are areally widespread on the South African continental margin. The glauconitic phosphorites ( 18% P2O5) have packstone or wackestone textures and are composed of sand-sized grains of glauconite, with lesser amounts of microfossils and terrigenous grains set in a matrix of collophane-francolite and micrite. The glauco-conglomeratic phosphorites ( 17% P2O5) are composed of variably phosphatized limestone pebbles set in a matrix very similar in texture and composition to the glauconitic phosphorites. Fifteen representative samples were selected for major element analysis and the relatively low P2O5 concentrations (15-21%) determined for these rocks reflect the diluent effects of allogenic grains. Substitution of CO32- for PO43- is indicated by the relatively high F/P2O5 ratios and the high apatite CO2 levels determined. Coupled substitution of Na+ and S6+ for Ca2+ and P5+ is also indicated, and high Fe2O3, K2O and MgO levels reflect the presence of glauconite. Many of the sedimentary features exhibited by these texturally heterogeneous rocks are incompatible under normal hydrodynamic conditions, suggesting an unusual depositional environment. It is proposed that the matrix cement was originally lime mud and that under the influence of a regressive coastline/shallowing sea, tidal and storm wave currents were responsible for the accumulation of the proto-phosphorite sediments in file relatively shallow and flat continental shelf areas. In deeper and steeper continental slope areas it is suggested that sediment accumulation resulted from the action of turbidity currents and mudflows. Lithification of the sediments is considered to have resulted from the phosphatization of lime mud by interstitial waters rich in phosphates. A shallow water lago nal/estuarine environment is indicated for the diagenesis of those rocks found in shallow continental shelf areas, whereas an open ocean environment is indicated for those rocks found in deeper water on the continental slope.

Geologic Framework of the Alaskan Continental Terrace in the Chukchi and Beaufort Seas

Abstract: Seismic, magnetic and gravity data indicate that the Chukchi and Beaufort epicontinental seas off northern Alaska overlie three sedimentary basins, or provinces, separated by structural highs of regional extent. The basins trend west to northwest and their enclosed sediments become increasingly marine from south to north. The Chukchi-Beaufort continental margin is similar to those of Atlantic type. Hope Basin, in the southern Chukchi Sea, overlies strongly deformed Paleozoic to mid-Cretaceous rocks of the Brooks Range orogen. The basin is inferred to contain nonmarine and marine clastic sedimentary rocks in a one km-thick Upper Cretaceous (?), a 1.5 km-thick Paleogene (?), and a 1.5/0.75 km-thick Neogene(?) sequence. A large anticline and many faults and smaller folds disrupt mainly the older sequences. The Hope Basin sedimentary units onlap Herald Arch, which trends northwestward from Cape Lisburne in the central Chukchi Sea. At the Herald fault zone Brooks Range rocks in the arch are thrust eastward or northeastward over Mississippian to Jurassic shelf carbonate and clastic rocks of the Arctic Alaska (Ellesmerian) basin and overlying Cretaceous flysch and molasse of the Colville Geosyncline. These Mississippian to Cretaceous rocks underlie the northeast Chukchi Sea and reportedly are about 10 km thick near the Herald fault zone on the Lisburne Peninsula. The great Chukchi syntaxis in western Brooks Range rocks and structures is thought to result from intersection of the west-trending Brooks Range orogen with the northwest-trending Herald fault zone. The Mississippian to Jurassic shelf sequence thins northward, and onlaps the Barrow Arch, which trends northwest from Point Barrow to 161° W. Long., thence west-southwest to the Herald fault zone. The Colville Geosyncline sequence oversteps both the pre-Cretaceous rocks and the Barrow Arch to form the North Chukchi Basin west of 161° W. Long, and the progradational Beaufort continental terrace to the east. The North Chukchi Basin may contain about 6 km of probable Cretaceous and Tertiary section, which may be deltaic in origin. Diapirs (of Cretaceous shale?) pierce the gently northward-dipping strata of this basin, in places reaching the sea floor. Thick Tertiary marine and nonmarine clastic rocks of the Camden Basin overlie the Cretaceous rocks of the North Slope and inner Beaufort continental terrace east of the Colville River delta. These rocks dip gently seaward west of 146° W. Long, but are deformed into long, high-amplitude, east-northeast-striking folds to the east.