Anticosti foreland basin offshore of western Newfoundland: Concealed record of northern Appalachian orogen development

The Anticosti Basin, largely hidden beneath the Gulf of St. Lawrence, includes foreland basin successions that record multiple tectonic events associated with the Ordovician to Devonian evolution of the northern Appalachian orogen. Due to the lack of well ties and minimal onshore exposure, geophysical data must be used in mapping the offshore stratigraphy. Outcropping geologic boundaries are tied to magnetic lineaments that parallel stratigraphy. These lineaments are correlated with reflections on seismic profiles in order to interpret the subsurface. Seismic isochron maps for successive basin development episodes display differences in geometry, implying that orogenic loading varied through time. The geometry and subsidence rates recorded by the Middle Ordovician Goose Tickle Group imply that it formed in a pro-arc setting associated with loading during arc-continent collision that was most intense in the northern Newfoundland Appalachians. The geometry and subsidence recorded by the overlying Long Point Group imply pro-arc loading by Taconian allochthons in the Québec segment of the orogen. Diachronous subduction polarity reversal along the margin placed the Long Point Group in a combined retro-arc and pro-arc setting, comparable to that experienced by parts of the north Australian margin at the present day. The uppermost Silurian to Lower Devonian Clam Bank Formation and Lower Devonian Red Island Road Formation represent foreland basin successions associated with the later Salinian and Acadian orogenies. Their consistent thickness implies a broad, shallow basin, suggesting that the lithosphere was cooler and stronger than during earlier subsidence, and are consistent with a retro-arc setting.     

Electrical Resistivity Imaging and the Saline Water Interface in High-Quality Coastal Aquifers

Population growth and changing climate continue to impact on the availability of natural resources. Urbanization of vulnerable coastal margins can place serious demands on shallow groundwater. Here, groundwater management requires definition of coastal hydrogeology, particularly the seawater interface. Electrical resistivity imaging (ERI) appears to be ideally suited for this purpose. We investigate challenges and drivers for successful electrical resistivity imaging with field and synthetic experiments. Two decades of seawater intrusion monitoring provide a basis for creating a geo-electrical model suitable for demonstrating the significance of acquisition and inversion parameters on resistivity imaging outcomes. A key observation is that resistivity imaging with combinations of electrode arrays that include dipole–dipole quadrupoles can be configured to illuminate consequential elements of coastal hydrogeology. We extend our analysis of ERI to include a diverse set of hydrogeological settings along more than 100 km of the coastal margin passing the city of Perth, Western Australia. Of particular importance are settings with: (1) a classic seawater wedge in an unconfined aquifer, (2) a shallow unconfined aquifer over an impermeable substrate, and (3) a shallow multi-tiered aquifer system over a conductive impermeable substrate. We also demonstrate a systematic increase in the landward extent of the seawater wedge at sites located progressively closer to the highly urbanized center of Perth. Based on field and synthetic ERI experiments from a broad range of hydrogeological settings, we tabulate current challenges and future directions for this technology. Our research contributes to resolving the globally significant challenge of managing seawater intrusion at vulnerable coastal margins.     

The micromorphology of paraglacial and periglacial slope deposits: a case study from Morfa Bychan,West Wales,UK

Micromorphological analysis of the crudely stratified diamictons exposed in coastal sections at Morfa Bychan, west Wales, is presented and discussed. Three major units, Yellow Head, Blue Head and Brown Head, have been recognised, and their crudely stratified nature, plus clast orientations, indicate that they were all subject to mass movement down the flanks of the adjacent coastal hills. Earlier workers have suggested a periglacial environment for this mass movement, but recently it has been proposed that the Blue Head, at least, may have accumulated in a paraglacial environment through gravitational redistribution of recently deposited unstable till. It is shown here that the Yellow Head contains occasional dense silt-rich coatings on the upper surfaces of sand and gravel grains, although other cryogenic micromorphological phenomena were not observed. The Blue Head, however, contains no clear cryogenic micromorphological features, although there is abundant evidence for sediment shearing and grain rotation within the matrix. The Brown Head contains diffuse grain coatings and lenticular aggregates very similar to those observed elsewhere in periglacial solifluction deposits. Thus it is concluded that accumulation of the Blue Head was rapid, and took place in a paraglacial environment as a result of mudflow. The Yellow Head may represent periglacial solifluction immediately prior to glaciation, whereas the Brown Head accumulated above the Blue Head as a result of periglacial weathering of the adjacent exposed hill tops, and subsequent downslope solifluction of the weathering product. © 1998 John Wiley & Sons, Ltd.     

Ripple cross-laminated sediments on the East Antarctic Shelf: evidence for episodic bottom water production during the Holocene?

Biosiliceous sediments sampled from a submarine valley system on the continental shelf of East Antarctica contain intervals of ripple cross-lamination interspersed with massively bedded units. Based on radiocarbon dates from one core collected on the Mac.Robertson Shelf, the most intensely cross-laminated sediments were deposited between 6 and 3.5 kyr BP, with isolated cross-laminae deposited at other times in the Holocene. The cross-laminated sediments are interpreted here as a signal of episodic density currents flowing across the outer shelf, which result from the formation of high salinity shelf water (HSSW). This HSSW is formed in winter by brine rejection during sea ice formation and by the exchange and cooling of upwelled saline slope water, and it contributes to the bottom water produced along the continental margin of Antarctica. If this interpretation of the cross-laminae is correct, then bottom water formation and export from the East Antarctic shelf has exhibited temporal, and probably also spatial, variability throughout the Holocene. Such variability would have implications for oceanographers attempting to quantify Antarctic bottom water production rates based only on present day observations.     

The glacigenic deposits of Western Lleyn,North Wales: Terrestrial or marine?

This paper describes a complex sequence of glacigenic sediments occupying a faultbounded depression at Aberdaron Bay, western Lleyn. The sequence offers an insight into sedimentary environments during deglaciation of the Irish Sea Basin. A lower stratified diamict association (LDA) containing contorted units of fine sand/silt and displaying strong, consistent clast fabrics, is overlain by an upper diamict association (UDA) with weaker fabrics and extensive sand and gravel layers and channel fills. Certain characteristics of the sequence can be explained by a glacimarine depositional model, but there are several problems. In particular, the geometry of the sediments is difficult to explain without recourse to the melting of buried ice. An alternative model that overcomes these problems involves the decay of a terrestrial glacier containing reworked marine sediments. Supraglacial sediment flows released during decay of thinner ice covering the surrounding hills (UDA) would have rapidly buried a thick stagnant ice mass in the Aberdaron depression, facilitating slow melting and release of basal meltout till (LDA). A model is presented that accounts for the stagnation and in situ decay of a debris-rich Irish Sea glacier, and which could explain many of the deposits and landforms surrounding the Irish Sea Basin without recourse to high relative sea-levels.