Grain-size sorting within river bars in relation to downstream fining along a wandering channel

The textural variability of river bed gravels at bar scales is poorly understood, as are the relations between variability at this scale and at reach and river scales. Surface and subsurface grain‐size distributions were therefore examined at reach, bar and bedform scales along lower Fraser River, British Columbia, Canada. Grain‐size variations within compound bars are conditioned by longitudinal position, elevation and morphological setting. Surface and subsurface sediments tend to decrease in median size from bar head to bar tail by 33% and 17%, respectively. Sediment size is constrained at some upper limit that is inversely related to bar surface elevation and which is consistent with competence considerations. The surface sediments on unit bars are finer and better sorted than the bed materials in bar‐top channels and along the main bar edges. Secondary unit bars tend to have a lower sand content than other features, a consequence of sediment resorting. Individual unit bars and gravel sheets exhibit streamwise grain‐size fining and lee‐side sand deposition. Over time, significant amounts of cut and fill do not ipso facto cause changes in surface grain sizes; yet, sediment characteristics can change without any significant morphological adjustment taking place. At the reach scale there is a clear downstream fining trend, but local variability is consistently high due to within‐bar variations. The surface median grain‐size range on individual bars is, on average, 25% of that along the entire 50 km reach but is 68% on one bar. While the overall fining trend yields a downstream change in surface median size of 0·76 mm km^?1, the average value for ‘head‐to‐tail’ size reduction on individual bars is 6·3 mm km^?1, an order of magnitude difference that highlights the effectiveness of bar‐scale sorting processes in gravel‐bed rivers. Possibilities for modelling bar‐scale variability and the interaction of the different controls that are identified are discussed.     

Sun, sand and sea urchins

Jamaica, land of holiday dreams, is also fascinating to the geologist. In particular, the fossil record is diverse, the ancient fauna containing many rarities. The fossil ochinoid fauna is especially large and varied for such a small island, with about 130 nominal species ranging from the late Cretaceous to the Pleistocene.     

Late-glacial ice advances in the western Italian Alps

The extent of ice on the south side of the Mont Blanc massif during two late-glacial readvances is known primarily from the distribution of crystalline erratics on sedimentary and metasedimentary terrain, and from the distribution of end moraines and outwash remants. Similar dual moraine complexes are found elsewhere in upper Val d'Aosta and indicate that the pattern is regional in character. During the earlier readvance equilibrium-line altitudes were ca. 450 ± 50m lower than at present, whereas during the later readvance they were ca. 350 ± 50 m lower. The moraines, not yet closely dated, were deposited sometime after 14,200 ¹⁴C Years ago by which time Lago'd Alice near the margin of the würm moraine system at Ivrea had been deglaciated, and before 8400 years ago, at which time Rutor Glacier in upper Val d'Aosta was no larger than at present. The younger moraines are believed to correlate with Egesen moraines on the north side of the Alps, whereas the older set may either correlate with an early phase of the Egesen or with Daun advance.     

Origin and terminal architecture of a submarine slide: a case study from the Permian Vischkuil Formation,Karoo Basin,South Africa

Submarine mass movement deposits exposed in the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, provide a rare opportunity to analyse and interpret their emplacement history and deformation processes at a scale comparable to seismic examples. An up to 80 m thick slide deposit, continuously exposed in two 2 km long sub‐parallel sections, passes from extensionally deformed material (clastic dykes and down‐dip facing low‐angle shear surfaces) down‐dip into a compressional toe zone with large (tens of metres amplitude) folds dissected by steep, up‐dip facing thrust planes. The compressional shear planes sole out onto a highly sheared décollement and cross‐cutting relationships indicate an up‐depositional dip younging in the timing of fold dissection. Lithofacies characteristics and detailed correlation of volcanic ash and other marker beds over more than 500 km² in the bounding undeformed stratigraphy indicate a low‐gradient (<0·1°) basin floor setting. The slide is abruptly overlain by an up to 50 m thick debrite with sandy clasts supported by an argillaceous matrix. Shear loading of the debris flow is interpreted to have driven large‐scale deformation of the substrate through the generation of high shear stresses at a rheological interface due to: (i) the abrupt contact between the slide and the debrite; (ii) the coincident thickness distributions of the debrite and slide; (iii) the distribution of the most intense folding and thrusting under the thickest parts of the debrite; (iv) the preservation of fold crests with only minor erosion along fold limbs; (v) the presence of the debrite under overturned folds; (vi) the presence of laterally extensive marker beds directly above deformation units indicating minimal depositional topography; and (vii) the demonstrably local derivation of the slide as individual folded beds are mapped into undeformed strata outside the areas of deformation. The debrite is directly overlain by fine‐grained turbidite sandstone beds that show widespread vertical foundering into the debrite. This case study demonstrates that intensely deformed strata can be generated by negligible amounts of down‐dip movement in a low‐gradient, fine‐grained basin floor setting with the driver for movement and deformation being the mass imbalance resulting from emplacement of episodic debris flows. Simple interpretation of an unstable slope setting based on the presence of such deformed strata should be treated with caution.     

The Dunedin Mayoral Election

In an electoral upset in 1995 Dunedin elected its first woman and first Sikh mayor. This paper argues that the symbolic discourse of the mayoral election confronted the Dunedin electorate with the question of ‘fairness’ or equity in the city. Turner's victory signalled the symbolic rejection by a plurality of voters of the ruling neo-liberal political economy of Dunedin. In the same election the incumbent council, in a political reproduction of the ruling order, was returned. The paper explores this contradiction. It concludes that Dunedin's struggle for a new ruling political alliance necessary to combat a stagnant city economy has just begun.     

Negotiating Technology — (Re)considering the Use of GIS by Indigenous Peoples

Geographic Information Systems (GIS) have been increasingly touted for their ability to help empower indigenous peoples. The opportunity to discuss the development of an iwi‐based GIS with Te Ruunanga O Raukawa provides a platform for reflecting critically on issues that impact the effective use of GIS by indigenous groups. The dialectical context of GIS is explored by (re)considering the opportunities and challenges of applying GIS as a cross‐cultural communication device. A position is negotiated which interprets GIS as conditionally empowering provided that indigenous peoples consider their symbolic representation in, and physical access to GIS before making an initial investment in the technology.