North Atlantic Continental Margins Research Articles

Characterization of Carbonate Crust from a Recently Discovered Methane Seep on the North Atlantic Continental Margin of the USA

This study is focused on mineralogical and chemical characterization of an authigenic carbonate rock (crust) collected at a recently discovered cold seep on the US North Atlantic continental margin. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the carbonate rock is composed of microcrystalline aragonite cement, white acicular aragonite crystals (AcAr), equant quartz crystals, small microcrystalline aluminosilicates, and trace amounts of iron sulfide microcrystals. Element/calcium ratios were measured with laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) using a calcite standard, which was prepared by annealing USGS certified carbonate powder (MACS-3). The occurrence of microscopic, non-carbonate inclusions precluded evaluation of trace elements in the aragonite cement, but allowed for in situ analysis of AcAr crystals. Carbon and oxygen isotopes were analyzed via isotope ratio mass spectrometry (IRMS) and expressed as δ13C and δ18O. Low δ13C values suggest that aragonite grew as a result of anaerobic oxidation of methane and observed δ18O values indicate that the temperature of aragonite crystallization was 1.7–1.9 °C.

Patterns of distribution of deepwater demersal fishes of the North Atlantic mid-ocean ridge, continental slopes, islands and seamounts

Basin-scale spatial and depth-related distribution patterns of deepwater demersal fishes were analysed using bottom trawl datasets from the North Atlantic continental margin, slopes of oceanic islands and seamounts, and the mid-Atlantic Ridge. Depth-stratified presence-absence data for 593 species were compiled from fisheries-independent trawl studies with full species lists. The datasets comprised trawls conducted on the upper continental slope (200m) to abyssal depths, and 750m wide depth strata were used. Number of species and families declined with depth in all areas. Species number was highest in the western North Atlantic, significantly lower on the mid-Atlantic Ridge and eastern North Atlantic. Observed species numbers are also low in southern areas (Bahamas, NW Africa, southerly seamounts), but the sampling effort in these waters has been much less than in northern sites. Fish assemblages vary by depth, latitude and longitude, and the study corroborates earlier suggestions that assemblages are broadly distributed in relation to regional circulation and watermass features. The mid-Atlantic Ridge assemblages between Iceland and the Azores are most similar to those on eastern North Atlantic slopes and rises, rather dissimilar to all others, including western Atlantic, Greenland, northwest Africa and Azorean seamount and island assemblages. Across the North Atlantic differences between subareas are strongest at slope depths, much less pronounced at the less speciose rise and abyssal depths. Demersal fish biomass estimates suggest that the American slope (New England) has low biomass compared with Newfoundland and European areas, and that the supposedly oligotrophic mid-Atlantic Ridge has a level of biomass similar to or higher than the European margin.

The composition and structure of volcanic rifted continental margins in the North Atlantic: Further insight from shear waves

Imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non-volcanic rifted and convergent margins. Massive volcanism occurred during break-up at 70% of the passive margins bordering the Atlantic Ocean, the causes and dynamics of which are still debated. This paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four-component ocean bottom seismometers along two North Atlantic continental margin profiles. This traveltime tomography was performed using two different approaches. The first, a flexible layer-based parameterisation, enables the quality control of traveltime picks and investigation of the crustal structure. The second, with a regularised grid-based parameterisation, requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via Monte Carlo analyses. The velocity models indicate high lower-crustal velocities and sharp transitions in both velocity and Vp/Vs ratios across the continent–ocean transition. The velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins. Interpretation of the high quality seismic reflection profile on the Faroes margin confirms that this mixing is through crustal intrusion. Converted shear wave data also provide constraints on the sub-basalt lithology on the Faroes margin, which is interpreted as a pre-break-up Mesozoic to Paleocene sedimentary system intruded by sills.

Identification and inversion of converted shear waves: case studies from the European North Atlantic continental margins

Wide-angle shear wave arrivals, converted from compressional to shear waves at crustal interfaces, enable crustal Vp/Vs ratios to be determined which provide valuable constraint on geological interpretations. Analysis of the converted shear wave phases represents the next logical step in characterizing the crustal structure and composition following multichannel seismic structural imaging and tomographic inversion of the wide-angle compressional wave phases. In this offshore study across two passive margins extending from stretched continental to fully oceanic crust, the high-data density (2–10 km ocean bottom seismometer, OBS, spacing) and a consistent, efficient conversion interface produced shear wave data sets suitable for traveltime inversion. The shear waves were recorded by three orthogonal geophones in each OBS. Arrival phases, visible to 180 km offset, were identified using their arrival times, moveout velocities and particle motions. Across the North Atlantic volcanic rifted continental margins studied, breakup was accompanied by the eruption of large volumes of basalts of the North Atlantic Igneous Province. The interface between post-volcanic sediments and the top of the basalts provides the dominant conversion boundary across the oceanic crust and the continent—ocean transition. However, the shear wave data quality was significantly diminished at the continental ends of the profiles where the thick basalt flows and hence this conversion interface feathers out and crustal attenuation increases. Initial modelling of the converted shear wave phases was carried out using a layer-based approach with arrivals converted on the way up used to constrain the Vp/Vs ratio of the post-volcanic sedimentary sequence beneath each OBS. To produce a model with continuous crustal S-wave velocities, the compressional wave velocities beneath the sediment-top basalt interface were transformed into starting shear wave velocities using a constant value of Vp/Vs and the inversion carried out by specifying the appropriate ray path. Once the data set had been fully interpreted, correction of the traveltimes to effective symmetric ray paths enabled us to apply a regularized grid inversion. Such inversions are less subjective than the layer-based approach and yield more robust minimum structure results with quantifiable errors, except in the vicinity of a known subbasalt low-velocity zone encountered on the Faroes margin. Monte Carlo analyses were performed for this approach; the average model from multiple inversions using randomized starting models and traveltimes shows the structure required by the traveltimes and the model standard deviation gives an estimate of uncertainty. Model and inversion parametrizations were fully tested and optimum parameters chosen for compressional and shear wave inversions. This allows, after appropriate model smoothing, an estimate to be made of the spatial variation of the Vp/Vs ratio within the crust. There are marked gradients in Vp, Vs and Vp/Vs ratio across the continent—ocean transition, which may result from intrusion of high magnesium mafic igneous material into the crystalline continental crust. The Vp/Vs ratio, used in conjunction with Vp, also provides constraints on the subbasalt lithologies forming the low-velocity zone. We conclude from such an analysis that this zone is unlikely to be composed entirely of igneous hyaloclastite material; some proportion of clastic sedimentary rocks is likely to be present. The Vp/Vs and Vp properties of the units underlying the low-velocity zone are inconsistent with crystalline continental basement and this unit is likely to represent a sill-intruded Mesozoic sedimentary sequence from a pre-breakup sedimentary basin.

A biogenic origin for anomalous fine‐grained magnetic material at the Paleocene‐Eocene boundary at Wilson Lake, New Jersey

The Paleocene‐Eocene Thermal Maximum, which occurred ∼55.5 Ma, was caused by a massive release of carbon, as indicated by an ∼3‰ negative carbon isotope excursion recorded in the marine, atmospheric, and terrestrial reservoirs. One suggested source for the carbon, a cometary impactor, is based on the sudden appearance and high concentration of single‐domain (SD) magnetite in Paleocene‐Eocene (P‐E) boundary cores from the North Atlantic continental margin. We evaluate the potential sources of SD magnetite at the P‐E boundary by presenting new magnetic hysteresis, low‐temperature magnetic remanence, and transmission electron microscopy data from the North Atlantic coastal ocean. Our results show a similar increase in SD material but demonstrate that the magnetic material has a biogenic origin. These findings indicate that the high concentrations of SD magnetite immediately above the P‐E boundary are the result of unusual accumulations and/or preservation of magnetotactic bacteria. Such bacteria typically occupy the oxic‐anoxic transition zone near the sediment‐water interface or in the water column. The high abundances of SD magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of SD magnetite. It may also indicate that the oxic‐anoxic redox boundary shifted into the water column. The latter explanation implies transient eutrophy of the coastal ocean in this region, most likely due to seasonally enhanced runoff, and increased stratification and nutrient loading.

Palaeothermal and compaction studies in the Mochras borehole (NW Wales) reveal early Cretaceous and Neogene exhumation and argue against regional Palaeogene uplift in the southern Irish Sea

The North Atlantic continental margins have been subject to widespread exhumation, although the timing, magnitude and causes of this uplift and erosion are debated. Exhumation is thought to be most severe across the Irish Sea basin system (Western UK), and recent years have seen an increasing emphasis on the role of Palaeogene underplating as the primary mechanism of uplift. We present a study of palaeothermal and compaction data from the Mochras borehole (NW Wales). Located near the presumed locus of exhumation, Mochras penetrated thick successions of Lower Jurassic and Oligo-Miocene sediments. Our results indicate that both these successions experienced considerably deeper burial prior to early Cretaceous and Neogene exhumation, respectively. Apatite fission-track analysis (AFTA) and vitrinite reflectance data from the borehole reveal evidence for exhumation-related cooling episodes beginning during the Cretaceous (150–80 Ma) and Cenozoic (50–0 Ma). Regional AFTA results and stratigraphic constraints refine the timing estimates to the early Cretaceous (120–115 Ma) and Neogene (20–0 Ma). Estimates of exhumation magnitudes during these episodes based on palaeothermal and compaction data are c . 2.5 km and c . 1.5 km, respectively. Palaeothermal data provide no evidence for elevated Palaeogene palaeotemperatures indicative of deeper burial, and thus question the basis of Palaeogene exhumation.

Depositional and diagenetic signatures of Late Eocene–Oligocene sediments, South Carolina

Abstract Surficial and near-surface soils of the South Carolina Coastal Plain reflect a variety of lithologies and depositional environments that are difficult to differentiate because of intense leaching and abrupt or laterally inconsistent facies changes. Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray (SEM/EDX) observations, and X-ray diffraction (XRD) analyses indicate that onshore Late Eocene to Late Oligocene Barnwell Group sediments are transitional facies ranging from high-energy fluvial deposits to offshore siliciclastic shelf sands. Interfingering of the units results in alternation of mineralogic signatures within a low-gradient fluvial/transitional/marine depositional system. Late Eocene and Early Oligocene offshore sediments were deposited in a mixed carbonate–siliciclastic, middle- to outer-shelf environment that was subjected to periods of erosion or non-deposition during transgressive events. Detrital and diagenetic characteristics of the onshore kaolinite-enriched, Late Oligocene Upland Unit sediments reflect deposition in a high- to low-energy fluvial system. Differentiation between these uppermost sediments and the underlying low-energy fluvial deposits of the Late Eocene Tobacco Road Sand is based on distinctive hydroxy-interlayered vermiculite (HIV) signatures. Intervals of HIV-enrichment are coincident with accumulations of carbonaceous material and identified as paleosols; these “soils” are used to infer offshore transgressive periods. Onshore sediments of the Late Eocene Dry Branch Formation contain high concentrations of smectite and flocculated, relatively poorly crystallized kaolinite flakes reflective of marine depositional conditions. At the base of this unit, authigenic Ca-minerals (Ca-zeolites and calcite) and quartz lepispheres (opal-CT) form coatings on and between sand grains. Late Eocene siliceous microfossils that contribute to opal-CT formation are identified in southwestern North Atlantic continental margin shelf, slope and rise deposits and are a valuable correlation tool for onshore sediments.

Origin of anomalous Tertiary subsidence adjacent to North Atlantic continental margins

At the beginning of the Tertiary, the northern North Atlantic continental margins were characterized by voluminous igneous activity caused by continental rifting in the presence of the Iceland hot-spot. Many of the Mesozoic sedimentary basins close to these margins have been permanently uplifted by igneous underplating and remain close to or above sea level at the present day. However, a significant number of other basins underwent rapid subsidence at the same time, sometimes by as much as 3 km of sediment-loaded subsidence in 15–20 Ma. Several hypotheses attempting to explain the cause of this Tertiary subsidence have been proposed. The observational data are summarized here and then each hypothesis is considered in turn. It is argued that many of these are implausible. One possible alternative explanation links Early Tertiary subsidence/uplift and magmatism. If basaltic melts are trapped within the lithosphere, it is possible to produce either uplift or subsidence depending on the density of the trapped melt. At depths greater than 50 km, melts will solidify rapidly in the garnet stability field to form primary eclogite, which is dense enough to cause subsidence, while satisfying the constraints of long-wavelength Airy isostasy. Unfortunately, such an hypothesis is difficult to test. The simplest explanation for Tertiary subsidence is still a minor episode of lithospheric stretching, although to date there is only limited evidence for sufficient Tertiary normal faulting. Further work must be carried out before this simple and testable hypothesis can be confidently rejected.

Palaeogene peraluminous magmatism, crustal melting and continental breakup: the Erlend complex, Faeroe-Shetland Basin, NE Atlantic

Well 209/3-1, located close to the centre of the Tertiary Erlend complex, north of the Shetland Isles, contains an unusual volcanic sequence in which thick basalts of N- to T-type MORB affinity overlie highly peraluminous, cordierite-bearing dacites. The dacites from the Erlend complex are almost identical in terms of their petrography and major-element, trace-element and isotope geochemistry to those of dacites from two previously-described localities in the North Atlantic, and indicate the formation of the dacites by anatexis of aluminous upper crustal sediments, which were probably pelitic in character. The discovery of graphite in the Erlend dacites suggests that carbonaceous shales may be implicated as parent material. The geological settings of the Erlend volcanic sequence, and the other examples in the Rockall Trough and on the Vøring Plateau, correspond to intra-continental, pre-break-up rift systems underlain by thinned continental crust, occurring close to the continental margin. A possible petrogenesis for the dacites involves the partial melting of upper crustal sediments as a result of the extensive contemporaneous basic magmatism documented to have occurred within a wide area of the North Atlantic continental margins during the initial stages of continental break-up. This mechanism suggests that the observed volcanic sequence may be more common than is obvious from the present, very limited, drilling evidence.

Crustal structure and magmatism of North Atlantic continental margins

Enormous variations exist along rifted margins of the North Atlantic in the magmatic activity that accompanied continental breakup. The North Atlantic margins exhibit a full range of magmatic behaviour. One end of the spectrum (commonly termed ‘volcanic’ margins) is found in the northern North Atlantic off Greenland, Norway and northwest Britain, where huge volumes of igneous rock were added to the continental crust as it rifted, and the adjacent oceanic crust is considerably thicker than normal. The other end of the spectrum (so-called ‘non-volcanic’ margins) occurs further south off France and Spain, where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust. Results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced. The volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle, and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere. Large quantities of igneous rock were produced as the continents broke up to form the North Atlantic where the newly initiated Iceland plume brought abnormally hot mantle beneath the rift. Much smaller quantities of melt were produced further south, where mantle temperatures were normal during rifting. The distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust. Where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years, as is common during the initial stages of rifting, then the rising asthenospheric mantle has time to cool by conduction. Heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region. As a consequence of the reduced mantle temperatures, considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts. This explains the abnormally thin oceanic crust found adjacent to the non-volcanic rifted continental margins in the North Atlantic. As the new ocean widens, the spreading centre becomes isolated from the continental margins on either side by newly formed, warm oceanic lithosphere, and the rate of rifting generally increases. The oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces, and all the melt formed by decompression of the mantle solidifies close to the spreading axis. Basaltic melts can intrude continental crust far more easily than oceanic crust. Basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts. This redistributes melt laterally towards the continent in both volcanic and, to a lesser extent, in non-volcanic margins and may be a contributory factor in leaving unusually thin crust in the first-formed ocean immediately adjacent to some rifted margins. It also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the Rockall area northwest of Britain.

Magmatic tectonic evolution of a volcanic rifted margin

Many North Atlantic continental margins are underlain by huge volcanic edifices near the continent-ocean boundary. A deep crustal hole drilled on the Vøring Plateau (ODP Leg 104) has provided important constraints on the initial breakup history and the subsequent margin evolution by penetrating more than 900 m of igneous rocks and interbedded sediments below a post-early Eocene cover. The rocks constitute two distinct, different volcanic series. The upper series, comprising a seaward dipping reflector wedge, consists of transitional mid-oceanic tholeiitic lava flows and thin volcaniclastic sediments. Dacitic flows, some dikes and thicker interbedded sediments constitute the lower series. The margin evolved by Paleocene crustal extension accompanied by uplift and pervasive intrusion of the rift zone. Just prior to breakup, magma from shallow crustal melts produced the lower series. The upper series was constructed during an intense, rapidly waning, subaerial volcanic surge following breakup near the Paleocene-Eocene transition. It covers both the new oceanic crust and large areas of the continental crust. The dipping wedge was formed by subsidence due to loading and thermal contraction, possibly amplified by tectonism. When the surge abated, the injection center rapidly subsided and normal oceanic crust was formed. There is a direct temporal and compositional relationship between the onshore North Atlantic Volcanic Province and the margin volcanism. Compared to the transverse part of the province including the Iceland Hotspot, which has been active over 60 m.y., the volcanic margins reflect a 2500 km long transient segment related to continental breakup, lasting only about 3 m.y. The volcanic surge and uplift at breakup are related to a modest regional increased temperature at the base of the lithosphere. This mode of margin formation has had important paleoenvironmental implications.

The Barremian-Aptian arid phase in western Europe

Evidence is presented from nine areas in western Europe (Speeton, Yorkshire; southern North Sea; Channel and Weald Basins, southern England; Jeanne d'Arc, Lusitanian and Aquitanian Basins of the North Atlantic continental margins; Lower Saxony Basin, north Germany; the Paris Basin and southern France) for a phase of aridity which began during the Hauterivian-middle Barremian and continued to dominate the climate of the region until the mid-Aptian, although conditions during this period were not uniformly dry throughout. Criteria for recognising arid environments include the occurrence of evaporites, abundances of carbonate-rich sediments, red beds, firmgrounds, carbonate nodules and pyrite concretions, depletion of kaolinite, spore assemblages of limited morphological diversity, and the presence of numerous coniferalean and cheirolepidiacean pollen grains. The change from generally humid to dry climates during the Barremian is thought to have been linked to a lowering of sea-level. The arid phase clearly falls within the time span of the first Cretaceous Oceanic Anoxic Event.

Cenozoic unconformities and depositional supersequences of North Atlantic continental margins: Testing the Vail model

Integrated outcrop, borehole, and seismic reflection stratigraphy from the U.S. and Irish margins of the North Atlantic basin reveals a framework of Cenozoic depositional supersequences and interregional unconformities that resembles the Vail depositional model. Paleo-bathymetric and paleoceanographic analyses of associated microfossil assemblages indicate a genetic link between the depositional framework and the relative position of sea level.

Early Cretaceous »events« in the evolution of the eastern and western North Atlantic continental margins

At many North Atlantic continental margins, the early Neocomian is characterized by a major stratigraphic turning point from Late Jurassic-Berriasian carbonate bank/pelagic carbonate deposition to Valanginian-Barremian hemipelagic sedimentation with thick Wealden-type deltaic to deep-sea fan sequences.

British paleomagnetism, Iapetus Ocean, and the Great Glen fault

New paleomagnetic data for Britain establish the width of the Iapetus Ocean as ∼1000 km in mid-Ordovician time but do not yet allow precise estimates of the rate of its subsequent closure. The new lower Paleozoic data from northern Scotland are similar to data from the rest of Britain but are not compatible with North America data on the Bullard et al. reassembly. If this discrepancy is correct, lines of major transcurrent dislocation have yet to be identified either in the North Atlantic continental shelves and margins or in eastern North America, and the Great Glen fault was not the main locus of offset between Britain and North America. The tectonic complexities associated with this configuration require further evaluation.

Preliminary Geologic Report on U.S. North Atlantic Continental Margin: ABSTRACT

The U.S. Geological Survey is conducting a geologic and geophysical study of the outer continental shelf (submerged coastal plain) and the adjacent slope from Georges Bank to Cape Hatteras. The study also includes the emerged coastal plain because it is a more accessible extension of the submerged part or shelf. The total study area is about 324,000 sq km, of which the shelf and slope constitute about 181,000 sq km and End_Page 2148------------------------------ the emerged part constitutes 143,000 sq km. The shelf width ranges from about 80 km at Cape Hatteras (Cape Hatteras is actually well out on the shelf) to about 195 km off Raritan Bay and on Georges Bank. Preliminary conclusions suggest a considerably thicker sediment section than has been described previously and, locally, fairly complex structures. Onshore horizons of unconformity such as the base of the Miocene, base of the Tertiary, and base of the Upper Cretaceous, appear to continue seaward as horizon markers. Samples and sample analyses made it possible to construct a preliminary geologic map of the shelf and slope to a depth of 2,000 m. The oldest beds cropping out in the submarine canyons and on the slope are of early Late Cretaceous age. Beds of Jurassic and Triassic age are nown in deep wells onshore and probably are present beneath the shelf in the area of this study. Such beds are reported beneath the Scotian Shelf where they include limestone, salt, and other evaporites. The region is large, the sediment wedge is thick, structures such as folds and faults are present, and the hydrocarbon potential may be considerable. End_of_Article - Last_Page 2149------------