Continental Terrace Research Articles

Tectonic regime of the southern Kurile Trench as revealed by multichannel seismic lines

The analysis of the two multichannel seismic reflection records P-855 and P-856 reveals the complexity of the tectonic structure of the Kurile margin. The absence of trench fill and the relatively constant thickness of the subducted sediments on top of the oceanic basement shows that little or no accretion is occurring at present. The lower slope is characterized by active thrusting within a small accretionary prism. The toe of the middle slope is a more consolidated unit that constitutes an efficient buttress. A piggy back basin, possibly filled by sediments that have drifted down slope, has formed on top of this unit. A narrow high marks the transition to the edge of the continental terrace. Furthermore, the upper slope reveals an unconformity marking the top of the basement. The similarity of this unconformity and its sedimentary cover with the well documented data acquired around DSDP Legs 56–57 suggests that northern Japan and the southern Kurile forearcs may have experienced a similar Cenozoic vertical history.

Continental slope processes off northern Israel and southernmost Lebanon and their relation to onshore tectonics

The continental terrace of Israel and Lebanon consists of a 1–2 km thick Pliocene-Quaternary sediment wedge that narrows, steepens and deepens from south to north. The continental terrace is the product of (1) Nile-derived sediment accumulation, whose bulk and rate of deposition decrease along the active Levant continental margin with increasing distance from the Nile mouths, and (2) of the vertical tectonic uplift of Israel and Lebanon versus the subsidence of the adjacent submarine Levantine Basin which increases to the north. Collapse of the entire seaward-facing part of the continental terrace off northern Israel, caused by large-scale halokinetic rotational slumping, has further accentuated its steep and narrow wedge form in the study area. Intensive mass-wasting and canyon cutting over the steep continental slope have shaped its present furrowed morphology. Canyon incision is fault controlled, the faults being submarine extensions of the continental fault system effected by drag of the Sinai sub-plate along the Dead Sea-Jordan-Beqa'a transform that has controlled the tectonics and the structure of Israel and Lebanon since the Late Miocene. The canyon heads at the continental shelf lie opposite the mouths of the continental rivers. Canyon cutting processes are not, however, connected to fluvial erosion of the emergent continental shelf during the Last Glacial lowered sea-level stand. Canyon incision started on the continental slope and advanced downslope and upslope toward the shelf. Embryonic slope valleys, canyons confined to the continental slope, a canyon in the process of breaching the shelf break and mature canyons that indent the continental shelf to nearshore distances were mapped and are described.

Organic matter variations in sediments from DSDP sites 362 and 532: evidence of changes in the Benguela Current upwelling system

Abstract Diatom-rich nannofossil oozes from DSDP Sites 362 and 532 on the Walvis Ridge continental terrace contain light-dark cycles which are related to fluctuations in upwelling intensity over these sites from Middle Miocene to Holocene times. The dark intervals in the alternations result from dissolution of carbonates and addition of clastic components. Oxidation of marine-derived organic matter is important to dissolution of carbonates in these sediments. Concentrations of organic matter in the dark sections are enhanced by larger contributions of both marine and land-derived material than in light sections. The dark sections appear to correlate with periods of greater Antarctic ice volumes, suggesting that regional upwelling was intensified and that transport of organic matter from Africa by winds and possibly rivers was increased during these times.

Geometry, evolution, and tectonic framework of the Skeena Fold Belt, north central British Columbia

The Intermontane Belt of the Canadian Cordillera has long been viewed as a passive, relatively rigid block between two metamorphic‐plutonic belts, the Coast Plutonic Complex and the Omineca Belt. However, the Skeena Fold Belt, which spans most of the width of the northern Intermontane Belt, exhibits shortening comparable with that in the Rocky Mountain Fold and Thrust Belt. The Skeena Fold Belt has many features of thin‐skinned fold and thrust belts, such as thrust faults which sole into a detachment, a wide variety of structural styles which depend on rock type, a foreland basin which was cannibalized by continued deformation, a frontal triangle zone, and a hinterland of metamorphic and plutonic rocks (Coast Plutonic Complex). The Skeena Fold Belt thus is comparable with the Rocky Mountain Fold and Thrust Belt, but rather than deforming a continental terrace wedge, it developed in a terrane (Stikinia) which had accreted to North America in the early Mesozoic, and in Jurassic and Cretaceous clastic successions (Bowser Lake and Sustut groups) which overlie Stikinia. Structural and stratigraphic relationships show that the earliest deformation occurred between Oxfordian and Albian time and that the last folds developed in latest Cretaceous or early Tertiary time. As much as 160 km of northeastward shortening in the Skeena Fold Belt was broadly contemporaneous with crustal thickening in the Coast Plutonic Complex and Omineca Belt, with dextral strike‐slip faulting east of the Skeena Fold Belt, and with shortening in the Rocky Mountain Fold and Thrust Belt. Therefore, between latest Jurassic and early Tertiary times, horizontal shortening occured across most of the width of the northern Canadian Cordillera. Concurrent shortening across the Cordillera suggests that a common detachment (or detachments) fed all of these zones as far east as the Rocky Mountain Fold and Thrust Belt.

Benthic foraminiferal assemblages from the eastern Weddell Sea between 68 and 73°S: Distribution, ecology and fossilization potential

Surface sediment samples taken with a vented ☐ corer from the eastern Weddell Sea on four profiles perpendicular to the continental margin have been investigated for their benthic foraminiferal content. The live fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Based on the dead assemblages, potential fossil assemblages were calculated to facilitate the analogy with late Neogene core material. Five distinct live assemblages inhabit the continental margin today. Six dead assemblages and five potential fossil assemblages, respectively, correspond to these biocoenoses. A predominantly calcareous live fauna dominated by Trifarina angulosa is correlated with strong bottom currents and sandy sediments at the shelf break and on the uppermost continental slope. Below this, on the upper slope down to 2000 m water depth, the predominantly calcareous Bulimina aculeata assemblage coincides with the core of warm (>0°C) Weddell Deep Water and with fine and more organic-rich sediments. These calcareous live assemblages completely change composition during early diagenesis because of calcite dissolution within the uppermost sediment, which depends largely on the grain size distribution of the sediment and the fluxes of organic matter. Therefore, a still calcareous T. angulosa-dominated fossil assemblage indicates the sandy substrates on the shelf break and the upper slope, whereas the deeper slope with hemipelagic calm sedimentation and with high fluxes of organic matter is indicated by Martinottiella nodulosa, the characteristic arenaceous fossil remnant of the former predominantly calcareous live B. aculeata fauna. On a continental terrace between 2500 and 3500 m water depth Cribrostomoides subglobosus dominates the live fauna, but because of rapid disintegration of the empty tests of this agglutinated species a predominantly calcareous fauna characterized by Oridorsalis umbonatus and Epistominella exigua comprises the dead assemblage and the potential fossil assemblage, respectively. On the lower continental slope, between the carbonate lysocline (3500 m) and the carbonate compensation depth (4000 m), tests of Nuttallides umbonifer are the characteristic dead and potential fossil remnants of a former predominantly arenaceous live fauna, which is associated with the lower part of the Antarctic Bottom Water (AABW). This corroborates earlier investigations suggesting a relationship between the carbonate-corrosiveness of water masses and the distribution of N. umbonifer. This is important for inferring paleo-routes and estimates of paleo-production rates of AABW during the Neogene.

Cover gneisses of the Monashee Terrane: a record of synsedimentary rifting in the North American Cordillera

The Monashee Terrane of southeastern British Columbia is composed of Lower Proterozoic basement gneisses unconformably overlain by cover gneisses. The latter constitute a thick (> 2000 m) and laterally extensive (> 150 km) upper-amphibolite-grade succession of metasedimentary rocks, locally intercalated with minor intrusive and extrusive units. This succession is interpreted as reflecting initial broad, amagmatic subsidence and sedimentation on a cratonic platform (basement gneisses), most likely of North American affinity. Throughout most of the terrane, syndepositional magmatism is first marked by a laterally extensive (> 100 km) stratiform pyroclastic carbonatite, which is part of intermittent (long-lived?) alkaline magmatism. One alkaline body was intruded at ca. 740 ± 36 Ma (U–Pb zircon), suggesting that it may be part of initial Windermere rifting. Post-pyroclastic-carbonatite syndepositional extensional tectonism is further evidenced at the north end of the terrane by interlayered mature and immature siliciclastic sediments, with rapid facies changes, intercalated with ultramafic and mafic sills and flows, plus minor felsic pyroclastic deposits. All of these later deposits lie above stratigraphy correlated with strata hosting a stratiform Pb–Zn deposit with an Early Cambrian galena Pb-isotope age and, therefore, may be correlative with a rift–drift transition recorded in Hamill Group strata to the east. Rift tectonism recorded in cover gneisses may reflect one or more documented rift and rift–drift events recorded in Upper Proterozoic to Lower Cambrian strata of the western North American continental terrace prism.

The Lower Jurassic phosphorites of southeastern British Columbia and terrane accretion to western North America

Sinemurian phosphorites in southeastern British Columbia and southwestern Alberta conform with the "West Coast type" phosphorite depositional model. The model indicates that they were deposited on or near the Early Jurassic western cratonic margin, next to a sea or trough from which cold water upwelled. This suggests that the allochthonous terrane Quesnellia lay well offshore in Sinemurian time. The sea separating Quesnellia from North America was partly floored by oceanic crust ("Eastern Terrane") and partly by a thick sequence of rifted, continental terrace wedge rocks comprising the Purcell Supergroup and overlying Paleozoic sequence. This sequence must have been depressed sufficiently that access of upwelling deep currents to the phosphorite depositional area was not impeded.

Ironstones in the Mesozoic passive margin sequence of the Tethys Himalaya (Zanskar, Northern India): sedimentology and metamorphism

Summary The sedimentary succession of the Zanskar continental terrace comprises Triassic (Quartzite Series), Jurassic (Ferruginous Oolite) and Cretaceous (Giumal Sandstone) shelf siliciclastic units, all of which contain either chamositic ironstones or glauconitic greensands associated with reworked phosphorites. Ironstones are found above major unconformities at the top of shoaling arenaceous sequences, and generally mark the rapid transition from shallow-marine sands to highstand offshore pelites or pelagic foraminiferal limestones. Petrographical and sedimentological features of condensed intervals indicate deposition by transgressive fronts. During sea-level rise, high-energy waves mixed ferruginous and phosphatic grains formed in the course of earlier starved stages at low sedimentation rates, with detritus from shoreline and paralic sources reworked during stepwise coastal retreat. The timing of ironstone deposition apparently coincides with breakup stages affecting the Indian continental margin. Ironstones are also important metamorphic markers in the Tethyan sedimentary zone, which has undergone intense fold-thrust deformation at very low to low metamorphic grade during the Tertiary Himalayan orogeny. In the Triassic iron oolites of central Zanskar, the occurrence of stilpnomelane suggests upper anchimetamorphic conditions ( c. 300° C). In the Cretaceous greensands, however, K-rich glauconite is only peripherally replaced by incipient stilpnomelane growth, pointing to metamorphic conditions comparable to lower prehnite-pumpellyite facies ( c. 260° C).

Rôle des modifications des niveaux quaternaires des mers Caspienne, Noire et Méditerranée dans la formation du relief de leurs marges montagnardes

Effects of the quaternary sea levels' changes of the Caspian Sea, Black Sea and Mediterranean Sea upon the relief's formation of their mountainous margins. This papier gives first a statement of the main stratigraphical records along the coasts in relation with numerous marine or continental terraces whose the different stages are examinated with references to the quaternary coastal stratigraphy of the Mediterranean Sea. It's necessary to explain the lack of synchronism of the sea levels between the three basins ans the geomorpho- logical consequences, to show also the effects of the neotectonical deformations upon the vertical spreading of the coastal terraces. Climatic, eustatic and tectonical controls are mixed. The holocene terraces containing many archeological traces and the man made sea-level fluctuations of the Caspian Sea are so called up.

Sedimentary record of the northward flight of India and its collision with Eurasia (Ladakh Himalaya, India)

— Stratigraphic and petrographic analysis of the Cretaceous to Eocene Tibetan sedimentary succession has allowed us to reinterpret in detail the sequence of events which led to closure of Neotethys and continental collision in the NW Himalaya.During the Early Cretaceous, the Indian passive margin recorded basaltic magmaüc activity. Albian volcanic arenites, probably related to a major extensional tectonic event, are unconformably overlain by an Upper Cretaceous to Paleocene carbonate sequence, with a major quartzarenite episode triggered by the global eustatic sea-level fall at the Cretaceous/Tertiary boundary. At the same time, Neotethyan oceanic crust was being subducted beneath Asia, as testified by calc-alkalic volcanism and forearc basin sedimentation in the Transhimalayan belt.Onset of collision and obduction of the Asian accretionary wedge onto the Indian continental rise was recorded by shoaling of the outer shelf at the Paleocene/Eocene boundary, related to flexural uplift of the passive margin. A few My later, foreland basin volcanic arenites derived from the uplifted Asian subduction complex onlapped onto the Indian continental terrace. All along the Himalaya, marine facies were rapidly replaced by continental redbeds in collisional basins on both sides of the ophiolitic suture. Next, foreland basin sedimentation was interrupted by fold-thrust deformation and final ophiolite emplacement.The observed sequence of events compares favourably with theoretical models of rifted margin to overthrust belt transition and shows that initial phases of continental collision and obduction were completed within 10 to 15 My, with formation of a proto-Himalayan chain by the end of the middle Eocene.

Light-dark cycles in opal-rich sediments near the Plio-Pleistocene boundary, DSDP Site 532, Walvis Ridge continental terrace

Samples taken at 10 cm intervals from DSDP Core 532B-17 contain variations in carbonate, opal, organic carbon, and terrigenous components that correlate with light-dark cycles in sediment color. The core site, at 1300 m water depth, is well above the CCD, yet the color variations appear to result largely from cyclical fluctuations in carbonate dissolution, which was greater during glacial periods. Higher concentrations of organic carbon and of terrigenous sediment components correlate with enhanced carbonate dissolution, but opal concentrations inversely correlate and suggest that biological productivity at this site diminished during glacial periods. A complicated glacial-interglacial picture emerges from the data. In interglacial times, upwelling associated with the Benguela Current produced abundant opaline material, organic matter was fairly well preserved, and carbonate was only moderately dissolved. In glacial times, the upwelling core shifted as sea level fell and winds intensified. Productivity in the waters over Site 532 decreased, but lateral supply of oxidizable organic matter enhanced carbonate dissolution, giving rise to light-dark cycles in these sediments.

Sound velocity as a function of depth in marine sediments

Additional data from sonobuoys and the Deep Sea Drilling Project (DSDP) justify separating sound-velocity-depth functions and velocity gradients (in the first layer of soft marine sediments) into some geographic areas and sediment types. Based on sonobuoy and core measurements (where V is sound velocity in km/s, and h is depth in sediments in km), the following data are obtained: continental shelf basins off Sumatra and Java—V=1.484+0.710h−0.085h2; U. S. Atlantic continental rise—V=1.513+0.828h−0.138h2; deep-sea terrigenous sediments—V=1.519+1.227h−0.473h2; and siliceous sediments of the Bering Sea— V=1.509+0.869h−0.267h2. Selected DSDP data (through leg 74) in similar areas yield: continental terrace silt–clays—V=1.505+0.712h; deep-sea terrigenous sediments—V=1.510+1.019h; and deep-sea siliceous sediments—V=1.533+0.761h. Computed velocity gradients from sonobuoy measurements are generally supported by the DSDP gradients. Only DSDP data give the following: hemipelagic sediments—V=1.501+1.151h; deep-sea calcareous sediments—V=1.541+0.928h; and deep-sea pelagic clay—V=1.526+1.046h. Where fast sediment accumulation occurs, there has not been enough time to reduce sediment pore spaces under overburden pressure; areas of slow accumulation may have relatively high sediment structural strength. Both cases have lower velocity gradients because higher porosities and consequent lower velocities persist to deeper depths.

Refined Names for Brookian Elements in Northern Alaska: ABSTRACT

The major negative element of the North Slope has been called the geosyncline, the trough, the foredeep, or the basin, whereas the positive element coupled to the north has been universally called the The name Colville is most consistent with the apparently compound nature of this foreland successor element. We also recommend that Barrow be substituted for Barrow as the name for the positive element or structural hinge that formed between middle Cretaceous deposits in the basin and those along the continental margin. The term aptly describes the weak reversals in regional dip that mark this feature, and constrains the sense of either active uplift or a preexisting high, which has evo ved with current usage of arch. The markedly asymmetric basin consists of: deformed and thickened middle Cretaceous flyschoid deposits lying on earlier Cretaceous allochthons of the ancestral Brooks Range; a greater than 10-km thick belt of deposits that is incoherent on seismic records but is floored by poor reflectors, presumably of earliest Cretaceous and older age; and a foreland flank that slopes gently from within a kilometer of the surface at Point Barrow, about 200 km to the north. Seismic profiles show this flank to have been an abyssallike plain after Barrovia, the northern provenance of Ellesmerian deposition, had been replaced by the Arctic Ocean early in Cretaceous time. The flank was more than 1 km deep and stretched broadly southward from the new continental edge that now seems to be nearly 3 km north-northeast of Barrow. The plain was progressively loaded and depressed, first by the downlapped distal edge of the flysch prism in the south and then by the shelf of molassoid deposits that prograded from the Brookian orogen during the middle Cretaceous. The inflection denotes reversals in regional dips from less than 2° to the south to approximately 1° to the north. The axes of reversals subparallel the present coastline between and the Arctic National Wildlife Refuge and plunge eastward at a rate of approximately 1 km/100 km. Inflections on successive stratigraphic horizons do not stack vertically as in parallel folds; dip reversals in the lowest Brookian strata, for example, occur several kilometers south of the inflection on the basement surface. The structure appears to have formed as the prograding middle Cretaceous deposits (Nanushuk Group) made the foundation for the present continental terrace; subsidence of the continental margin beneath that load reversed dips along the former north edge of the Colvi le basin. Minor and relatively passive upward bowing likely occurred along this hinge between the negative (depressed) elements. The positive feature clearly is an arch neither in the sense of a broad anticlinal fold nor in the sense of a high such as the Cincinnati It had no north flank before the Late Cretaceous, and it could not have been an element of pre-Brookian (Ellesmerian) geology. End_of_Article - Last_Page 664------------

Wave-built shelves on some low-energy coasts

A nearshore terrace has formed in unconsolidated deposits of the low-energy coast of the Strait of Georgia. Scale-modelling concepts have been used to draw similarities between this small-scale terrace and the continental shelf. The beach grades seaward onto an upward-concave shelf; the outer edge of the shelf is marked by an abrupt increase in gradient on the steep offshore slope. The morphology of the shelf varies systematically with the degree of exposure to wave energy; the width of the shelf and depth of water over the shelf break increase with effective fetch length. Sediments grade seaward across the shelf with a transition from sand to mud at the shelf break. Calculated near-bed velocities under waves, recorded at six sites of differing fetch length within the Strait, show the shelf break has formed in a transitional zone between high and low “wave effectiveness”. The profile form, shelf sediments and nearshore dissipation of wave energy suggest that the shelf evolves as an equilibrium form in response to contemporary wave processes. Similar terraces have developed on sheltered lake shores and on the Fiordland coast of New Zealand suggesting a general model of terrace formation at effective fetch lengths up to 50 km. Data from the Great Lakes indicate that the model may be extended up to 100 km, but break down at larger fetch lengths where effective fetch is probably an unreliable indicator of wave conditions. Similarities between the restricted fetch model and the continental terrace suggest there may be a continuum of shelf forms from small lakes through to proto-continental shelves.

The Western Canada sedimentary basin

The Western Canada Sedimentary Basin, a simple northeasterly tapering wedge of sedimentary rocks more than 6 km thick, extends southwest from the Canadian Shield into the Cordilleran foreland thrust belt. Its internal structure and the lateral variations in its shape reflect a long and complex history of development involving a foreland basin that was superimposed on a cratonic platform and continental terrace wedge. This history, which is inextricably linked to the evolution of the Canadian Cordillera, can be outlined succinctly with reference to the unconformity-bounded transgressive-regressive stratigraphic sequences established by Sloss ( Bull. geol. Soc. Am . 74, 93 (1963)), each of which has a distinctive character in Western Canada. The continental terrace wedge was established with the deposition of the Proterozoic Purcell (1500-1350 Ma) and Windermere (850-600 Ma) sequences, but the first record of the platformal phase is the early Palaeozoic transgressive onlap of the early Proterozoic ( > 1750 Ma) crystalline basement by the Sauk sequence. Early Palaeozoic subsidence of the margin of the craton may have been due to cooling of the lithosphere after renewed stretching at the ancient rifted western margin of the Precambrian craton, and to isostatic flexure of the lithosphere under the weight of the sediment that had accumulated at the margin in the oceanward prograding continental terrace wedge. During a subsequent Middle Ordovician to Middle Jurassic phase, the cratonic platform became differentiated into an intersecting network of epeirogenic arches with intervening basins. Development of the basins was as much a result of erosion and uplift of the arches between transgressive-regressive cycles as it was a result of differential subsidence of the basins during the cycles. The cause of the long (> 300 Ma) episode of intermittent epeirogenic movements that produced the basins and arches is a major unsolved problem. The foreland basin developed in two stages, in Middle Jurassic to early Cretaceous and late Cretaceous to Palaeocene time, as a result of collisions between North America and two pieces of a tectonic collage of oceanic terranes that were accreted to its western margin. During these two collisions, the continental terrace wedge, which had accumulated outboard from the rifted margin of the continental craton, was compressed and displaced over the western margin of the craton. Part of the supracrustal cover was scraped off the craton and accreted to the overriding mass to form a wedge of imbricate thrust fault slices that was tectonically prograded over the margin of the continental craton. Isostatic flexure of the continental lithosphere in response to the tectonic loading imposed on it by the displaced continental terrace wedge and the accretionary wedge of thrust slices produced the migrating moat in which the outwash of clastic detritus from the evolving thrust belt was trapped to form the foreland basin.

Evolution of sedimentary basins in the Canadian Arctic

Since middle Proterozoic time, two long-lasting phases affected the Canadian Arctic Archipelago, each forming a different sedimentary basin. The Franklinian Basin, which was floored by continental or quasi-continental crust, received 10 km or more of clastic, carbonate and volcanic rocks from the mid-Proterozoic to Devonian. Internal parts of the basin were deformed, intruded and metamorphosed locally, and external parts were folded and thrust cratonward by compressional episodes of the Ellesmerian Orogeny, which culminated in the late Devonian. This marked the end of a phase, at which time the entire region may have been emergent. The nature of plate interactions that produced Ellesmerian deformation are unknown. The second phase began in the early Carboniferous, when plate movements of the Boreal Rifting Episode created the proto-Canada Basin by left-hand transform motion of a plate along the modern continental margin and the location of the Kaltag Fault of northern Alaska. As a marginal side effect of that motion, the Sverdrup Basin developed as a peri-cratonic incipient rift. From the Carboniferous to late Cretaceous the basin received about 13 km of cratonic-derived clastic detritus. From late Cretaceous to early Tertiary time, the Arctic Archipelago was disrupted by the interference of two plate movements originating in the Arctic and North Atlantic regions. Those events had three main effects: the craton was extended and a graben-filled depression formed in the southeastern part of the archipelago; the eastern and central parts of the Sverdrup Basin were compressed and uplifted (Eurekan Orogeny); and resultant elastics prograded northwestward toward the Canada Basin, to form the Arctic continental terrace wedge.

Evolution of a Mixed Carbonate/Terrigenous Platform: Lower Cambrian Continental Terrace Wedge of Mackenzie Mountains, Northwest Territories, Canada: ABSTRACT

Rocks of the Sekwi Formation and underlying siltstones of G.S.C. units 10A and 13 exposed in the western Mackenzie Mountains define a continental terrace wedge that evolved during the Early Cambrian on the northwestern flanks of the North American continent. Growth of the continental terrace wedge can be categorized by three phases of platform up-, out- and in-building. Phase 1--Continuous out- and up-building, and evolution from a ramp to a platform configuration. Growth culminated in middle and upper Nevadella zone rocks with prominent karst surfaces, abundant intraclast pebbles in oolite shoal deposits and a switch in composition of slope and fan deposits from carbonates to terrigenous clastics. Phase 2--Coincided with transgression of the shelf near the Nevadella and onnia-Olenellus zone boundary. Maximum platform out-building occurred at this time and climaxed with subaerial exposure of mid-Bonnia-Olenellus zone rock. Slope deposits changed from carbonate to terrigenous sedimentation. Phase 3--Renewed slow up- and in-building that led to eventual drowning of the platform by latest Bonnia-Olenellus zone time. Sedimentation patterns on this platform are reciprocal. Periods of sea-level highstand are characterized by abundant limestones, whereas periods of sea-level lowstand are marked by terrigenous influxes. These two stages of sea-level stand are highlighted in compositional differences of carbonate breccia deposits. During sea-level highstands, breccias contain predominantly ribbon and nodular limestone clasts. They reflect conditions of early lith fication of the sea floor and non-equilibrium compaction of these deposits with subsequent sliding and downslope transport. In contrast during sea-level lowstands breccias consist preferentially of shelf and shelf-margin derived clasts. End_of_Article - Last_Page 590------------

Crustal structure and velocity anisotropy beneath the Beaufort Sea

Crustal-scale seismic refraction data obtained in the Beaufort Sea during 1976 reveal a 4–5 km thick sedimentary layer overlying an oceanic crust that thickens rapidly as it approaches the continental terrace of Alaska. A synthetic seismogram analysis of multiple reflected, water-wave events indicates that the upper sedimentary layer has a compressional velocity of 1.8 km/s and a shear velocity of about 0.2 km/s. An oceanic layer 2A of 4.3 km/s, a layer 3A of 6.6 km/s, and a layer 3B of 7.6 km/s overlying an anisotropic (3%) upper mantle with a median velocity of 8.3 km/s are interpreted. The direction of maximum upper-mantle velocity appears to be approximately north–south in the area surveyed, suggesting that a rotation of the Northwind Ridge–Chukchi Plateau away from the Barrow–Martin Point sector of Alaska may have occurred.

Late phanerozoic structures, Canadian Arctic Archipelago

Late Cretaceous and Tertiary structures in the northern and eastern parts of the Canadian Arctic Archipelago have been used as evidence supporting some concepts of plate motions of Arctic North America and environs. For these relationships to be acceptable, the concepts must consider the constraints imposed by the positions, ages and styles of the structures. Halokinetic diapirs, mafic dikes and sills and faults and folds associated with those elements developed during Mesozoic evolution of Sverdrup Basin. The northeastern margin of the basin was segmented in Late Cretaceous (Campanian or Maastrichtian); the southwestern part sagged northward and became part of the Arctic continental terrace. Large arches, possibly basement cored, developed in the eastern part of the archipelago in Early Tertiary. About the same time, part of the Arctic Platform was fragmented by large normal faults. Middle Eocene and older rocks in the eastern part of the archipelago were folded and faulted before (probably Early) Miocene. These structures provide the only clear evidence of the regional Cenozoic compression in the archipelago. They are distributed about a salient of Pre-cambrian basement in southeastern Ellesmere Island. Relict alluvial fans and normal faults indicate some local uplift took place in Miocene and possibly later.

Geology and tectonic development of the continental margin north of Alaska

The continental margin north of Alaska, as interpreted from seismic reflection profiles, is of the Atlantic type and consists of three sectors of contrasting structure and stratigraphy. The Chukchi sector, on the west, is characterized by the deep late Mesozoic and Tertiary North Chukchi basin and the Chukchi Continental Borderland. The Barrow sector of central northern Alaska is characterized by the Barrow arch and a moderately thick continental terrace build of Albian to Tertiary clastic sediment. The terrace sedimentary prism is underlain by lower Paleozoic metasedimentary rocks. The Barter Island sector of northeastern Alaska and Yukon Territory is inferred to contain a very thick prism of Jurassic, Cretaceous and Tertiary marine and nonmarine clastic sediment. Its structure is dominated by a local deep Tertiary depocenter and two regional structural arches. We postulate that the distinguishing characteristics of the three sectors are inherited from the configuration of the rift that separated arctic Alaska from the Canadian Arctic Archipelago relative to old pre-rift highlands, which were clastic sediment sources. Where the rift lay relatively close to northern Alaska, in the Chukchi and Barter Island sectors, and locally separated Alaska from the old source terranes, thick late Mesozoic and Tertiary sedimentary prisms extend farther south beneath the continental shelf than in the intervening Barrow sector. The boundary between the Chukchi and Barrow sectors is relatively well defined by geophysical data, but the boundary between the Barrow and Barter Island sectors can only be inferred from the distribution and thickness of Jurassic and Cretaceous sedimentary rocks. These boundaries may be extensions of oceanic fracture zones related to the rifting that is postulated to have opened the Canada Basin, probably beginning during the Early Jurassic.