The Hatton Bank continental margin—I. Shallow structure from two-ship expanding spread seismic profiles

Summary. The Hatton Bank passive continental margin exhibits thick seaward dipping reflector sequences which consist of basalts extruded during rifting between Greenland and Rockall Plateau. Multichannel seismic reflection profiling across the margin reveals three reflector wedges with a maximum thickness near 7 km, extending from beneath the upper continental slope to the deep ocean basin. We present results of the velocity structure within the dipping reflector sequences at eight locations across the margin, interpreted by synthetic seismogram modelling a set of multichannel expanding spread profiles parallel to the margin. At the top of some reflector sequences, we observe a series of 100 m thick high- and low-velocity zones, which are interpreted as basalt flows alternating with sediments or weathered and rubble layers. At the profile locations, the base of the dipping reflectors correlates with P -wave velocities near 6.5 km s⁻¹. However, elsewhere the reflectors appear to extend significantly deeper than the inferred 6.5 km s⁻¹ velocity contour, indicating that the velocity structure may not be controlled solely by lithological boundaries but also by metamorphic effects. Shear-waves were observed on two lines, permitting the calculation of Poisson's ratio. The decrease in Poisson's ratio from 0.28 to near 0.25 in the upper 5 km of crust may also indicate the effect of metamorphism on seismic properties, or alternatively may be explained by crack closure under load.     

Cretaceous continental margin evolution revealed using quantitative seismic geomorphology,offshore northwest Africa

The application of high‐resolution seismic geomorphology, integrated with lithological data from the continental margin offshore The Gambia, northwest Africa, documents a complex tectono‐stratigraphic history through the Cretaceous. This reveals the spatial‐temporal evolution of submarine canyons by quantifying the related basin depositional elements and providing an estimate of intra‐ versus extra‐basinal sediment budget. The margin developed from the Jurassic to Aptian as a carbonate escarpment. Followed by, an Albian‐aged wave‐dominated delta system that prograded to the palaeo‐shelf edge. This is the first major delivery of siliciclastic sediment into the basin during the evolution of the continental margin, with increased sediment input linked to exhumation events of the hinterland. Subaqueous channel systems (up to 320 m wide) meandered through the pro‐delta region reaching the palaeo‐shelf edge, where it is postulated they initiated early submarine canyonisation of the margin. The canyonisation was long‐lived (ca. 28 Myr) dissecting the inherited seascape topography. Thirteen submarine canyons can be mapped, associated with a Late Cretaceous‐aged regional composite unconformity (RCU), classified as shelf incised or slope confined. Major knickpoints within the canyons and the sharp inflection point along the margin are controlled by the lithological contrast between carbonate and siliciclastic subcrop lithologies. Analysis of the base‐of‐slope deposits at the terminus of the canyons identifies two end‐member lobe styles, debris‐rich and debris‐poor, reflecting the amount of carbonate detritus eroded and redeposited from the escarpment margin (blocks up to ca. 1 km³). The vast majority of canyon‐derived sediment (97%) in the base‐of‐slope is interpreted as locally derived intra‐basinal material. The average volume of sediment bypassed through shelf‐incised canyons is an order of magnitude higher than the slope‐confined systems. These results document a complex mixed‐margin evolution, with seascape evolution, sedimentation style and volume controlled by shelf‐margin collapse, far‐field tectonic activity and the effects of hinterland rejuvenation of the siliciclastic source.     

Crustal structure of the Newfoundland rifted continental margin from constrained 3-D gravity inversion

The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of the free air data over the Newfoundland margin allows us to generate a 3-D density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage. Results of the gravity inversion show good correspondence with Moho depths from seismic results. A shallowing of the Moho to 12 km depth is resolved on the shelf at the northern edge of the Grand Banks, in a region poorly sampled by other methods. Comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends. Such insights must act as constraints for future palaeoreconstructions of North Atlantic rifting.     

The Love wave scattering matrix for a continental margin (numerical)

Summary . This paper presents the numerical computation of the results previously obtained by the author through a scattering matrix formulation (together with plane wave and variational approximations) which describes the diffraction of plane, harmonic, monochromatic Love waves incident normally (from either side) upon the plane of discontinuity in a structure consisting of a half-space with a surface step — an idealized model of a continental margin. Magnitudes of reflection and transmission coefficients are computed numerically for different frequencies for a model which has been considered previously by Knopoff & Hudson and also by Alsop in their studies of the same problem. The results obtained under the plane wave approximation are compared with those obtained under the variational approximation in order to assess the effects of the body-wave contributions. Finally, the results of both approximations are compared with those obtained by previous authors.     

U.S. Bank Failures and Regional Economic Structure*

U.S. commercial bank failures approached historically high levels in the 1980s. The collapse of such institutions can have severe impacts on local economies, disrupting the supply of available investment credit. This paper situates bank failures within the broader transformation of global capitalism in the late 20th century. Next, it reviews the causes of bank insolvency, including mismanagement, deregulation, the glutted commercial real estate market, and depressed agricultural and petroleum-dependent local economies. It then illustrates multiple aspects of the spatial distribution of bank failures. A pooled time-series regression analysis, using primary causal variables as well as location quotients of state output in four industrial sectors, indicates that with the vital exception of petroleum, and to a lesser extent services, regional economic structure is not strongly related to bank failures. This finding likely reflects the structural and spatial transformation of the industry as it adapted to the highly competitive conditions of the last decade.     

Deep events in U.K. South Western Approaches

Summary. Apparently planar dipping events are observed in seismic data off south-west Britain within otherwise essentially transparent upper and middle crust. These are believed to represent Variscan thrusts, some of which were re-activated during the post-Carboniferous phase of extension that affected the southern U.K. They can be seen in two extensive commercial seismic surveys recorded to 6 s two-way-time (TWT) and, where laterally persistent, they have been mapped to reveal their essentially planar nature. Commonly these dipping events are associated with deeper, near-horizontal, or gently convex upwards events with which they often appear to converge. Where 'real', these are thought to indicate a complex fault system or possibly the top of the reflective lower crust. The thrusts are seen over the whole area except where granite is known to occur, and commonly exert a major control on the position and subsequent deformation of overlying sedimentary basins.     

Lithoprobe east: results from reflection profiling of the continental margin: Grand Banks Region.

Summary. The 1985 Lithoprobe East profiles resolve deep crustal structure of the Grand Banks off eastern Canada. Basins are 7 to 20 km deep, and bounded by major faults traceable to Mono depth. The lower crust is reflective along much of the survey, and the top of this reflective layer has a variable depth. Prominent and often surprisingly flat Mono reflections are observed. Puzzling rotated fault blocks are imaged at the continent-ocean transition.     

Coastal resilience and late Holocene tidal inlet history: The evolution of Dungeness Foreland and the Romney Marsh depositional complex (U.K.)

Dungeness Foreland is a large sand and gravel barrier located in the eastern English Channel that during the last 5000 years has demonstrated remarkable geomorphological resilience in accommodating changes in relative sea-level, storm magnitude and frequency, variations in sediment supply as well as significant changes in back-barrier sedimentation. In this paper we develop a new palaeogeographic model for this depositional complex using a large dataset of recently acquired litho-, bio- and chrono-stratigraphic data. Our analysis shows how, over the last 2000 years, three large tidal inlets have influenced the pattern of back-barrier inundation and sedimentation, and controlled the stability and evolution of the barrier by determining the location of cross-shore sediment and water exchange, thereby moderating sediment supply and its distribution. The sheer size of the foreland has contributed in part to its resilience, with an abundant supply of sediment always available for ready redistribution. A second reason for the landform's resilience is the repeated ability of the tidal inlets to narrow and then close, effectively healing successive breaches by back-barrier sedimentation and ebb- and/or flood-tidal delta development. Humans emerge as key agents of change, especially through the process of reclamation which from the Saxon period onwards has modified the back-barrier tidal prism and promoted repeated episodes of fine-grained sedimentation and channel/inlet infill and closure. Our palaeogeographic reconstructions show that large barriers such as Dungeness Foreland can survive repeated “catastrophic” breaches, especially where tidal inlets are able to assist the recovery process by raising the elevation of the back-barrier area by intertidal sedimentation. This research leads us to reflect on the concept of “coastal resilience” which, we conclude, means little without a clearly defined spatial and temporal framework. At a macro-scale, the structure as a whole entered a phase of recycling and rapid progradation in response to changing sediment budget and coastal dynamics about 2000 years ago. However, at smaller spatial and temporal scales, barrier inlet dynamics have been associated with the initiation, stabilisation and breakdown of individual beaches and complexes of beaches. We therefore envisage multiple scales of “resilience” operating simultaneously across the complex, responding to different forcing agents with particular magnitudes and frequencies.     

The PUMA experiment west of Lewis, U.K.

Summary. In August-September 1984 a 165 km long, reversed, wide-angle, seismic line was shot along part of the BIRPS WINCH deep seismic reflection profile west of Lewis in the Outer Hebrides. The experiment was recorded using a new type of sea-bottom receiver, the PUMA (Pull-Up Multichannel Array). This consists of an 1100 m long array of hydrophones connected to a sea-bottom recording package. Airgun and explosive shots recorded by the PUMA provide a densely sampled record section, allowing even low amplitude arrivals to be traced across the section. An initial, 1-D interpretation of the data reveals a crustal thickness of 27 km, and confirms that a band of reflectors seen at around 8.6 s TWT on the normal incidence data, and interpreted as the 'reflection Monol, coincide with the Moho determined by wide-angle reflections and refractions.     

Acidity of twelve northern New England (U.S.A.) lakes in recent centuries

Studies of sediment cores from 12 acidic lakes in granitic, forested and uninhabited catchments in northern New England, U.S.A. produced diatom-inferred pH (IpH) 5.2 to 5.8 and alkalinity (Ialk) –12 to 31 µeq l^–1, with slowly declining values at some lakes, for one to four centuries prior to logging. Increases of IpH (0.05 to 0.60) and Ialk (5 to 40 µeq l^–1) correlate with logging in the catchments in the early-1800s to early-1900s. Recovery to pre-logging IpH and Ialk correlates with forest succession toward conifers, and is completed in the late-1800s to mid-1900s. Beginning at 1915–1920 (4 lakes), 1930–1950 (4 lakes) and 1965–1970 (4 lakes), IpH and Ialk start decreasing below pre-logging values due to atmospheric acidic deposition, leading to respective total decreases of 0.10 to 0.45 (X=0.25) and 5 to 25 µeq l^–1 (X=15). Inputs of anthropogenic Pb, Zn, V, polycyclic aromatic hydrocarbons, and soot from the atmosphere are first detectable in early- to late-1800s sediment, and Cu in late-1800s sediment, increase rapidly in the late-1800s to mid-1900s, and level off or decrease since the 1960s — partly due to emission controls. Decreasing Ca, Mn, and possibly Zn relative to other metals and normalized to organic content, and increasing flux of Fe to the lakes, indicate soil and water acidification after 1900.This is the fourteenth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series.