Large transverse bedforms and the character of boundary-layers in shallow-water environments

Theoretical work, laboratory studies, and field observations indicate that the oscillatory boundary layers generated by the tidal wave differ fundamentally in dynamics and kinematic structure from the unidirectional boundary layers of rivers. Unique to the former are mass-transport currents attributable to: (1) the wave motion itself, and (2) bed curvature in the presence of the oscillatory flow. The implication of this difference for bed-material transport is that the larger flow-transverse bedforms of shallow-water environments are divisible hydraulically between two major classes: (A) those related to tidal conditions, under which the fluid reverses in direction of flow with each reversal of the tide, permitting the initiation and maintenance of bed features by the spatially reversing, curvature-related mass transport, and (B) those related strictly to rivers and river-like flows, in which the fluid motion is unidirectional, and therefore the only mechanisms available for bedform initiation and maintenance are those creating a finite spatial lag between the transport rate and the bed waviness. Forms of Class B are best called dunes and bars, and only those attributable to Class A should be termed sand waves. The latter, restricted to oscillatory boundary layers of tidal origin, apparently correspond to the very much smaller; but also commonly symmetrical, ripple marks produced in wind-wave oscillatory boundary layers.     

After horton

The divergent and yet related problems of post-Hortonian studies of drainage density and channel network geometry are viewed against the difficulties of defining first-order channels and basins. It is proposed that the junction of an unbranched perennial (or blue-line) channel with another perennial channel be taken as the starting point for definitions and that the entire contour-crenulation network tributary to that point be considered the first-order stream. It is shown that the concept of network diameter may be used to describe the networks so delimited and that it appears to provide a useful starting point for interregional comparisons. Finally, an analysis of Blyth and Rodda's (1973) data on channel lengths and discharge indicates that network diameter may be as closely related to discharge as is channel length itself.     

Kyanite eclogites

Prior experimental work has shown that in the laboratory the mineralogy of eclogites is sensitive to the ratio of CaO ∶ MgO ∶ FeO and that the reaction pyroxene + kyanite?garnet + quartz proceeds to the right at high pressures in rocks rich in magnesium and to the left in rocks rich in calcium and iron. Typical basalts crystallized at high pressure never contain kyanite. The chemistry and mineralogy of a large number of naturally occurring eclogites show they belong to three classes.

1. Kyanite-free magmatic eclogites, rich in magnesium, from:
2. kimberlites
3. dunites and serpentinites.
4. Kyanite-bearing eclogites and grosspydites rich in CaO and low in FeO with intermediate MgO from:
5. kimberlites
6. gneisses.
7. Kyanite-free eclogites of metamorphic origin rich in iron with low magnesium and intermediate amounts of calcium from:
8. glaucophane schists
9. gneisses.

     

ON BED FORMS AND PALAEOCURRENTS

Bed forms arise by interaction between a fluid flow and the sediment transported over or close to the bed. The geometry of bed forms is two-dimensional or three-dimensional. Two-dimensional bed forms generate two-dimensional internal sedimentary structures and are adjusted to two-dimensional flow-vector fields. Three-dimensional bed forms generate three-dimensional internal sedimentary structures and are adjusted to three-dimensional flow-vector fields. Many shallow-water flow systems observable today are each characterised by flow-vector fields of unequal rank, and hence are hierarchically structured. Hierarchies of bed forms and internal structures which parallel the hierarchies of flow-vector fields can also be established in the case of these flow systems. A single type of bed form or internal structure from an hierarchically structured flow system cannot specify that system fully with regard to either its directional or flow-dynamic characteristics. An approach to palaeocurrent analysis that is more flexible and comprehensive than previously attempted may therefore be desirable.     

A revision of the ammonite faunas of the type Cenomanian. i. Introduction,ancyloceratina

This paper forms the first part of a revision of the ammonite faunas of the Cenomanian stratotype in the environs of Le Mans, Sarthe, France. The history of research on ammonites in the area is briefly noted, as is the lithostratigraphy. The systematics of the heteromorph ammonites (Ancyloceratina) is described, and the following taxa documented: Hamites simplex d'Orbigny, Sciponoceras baculoides (Mantell), Sciponoceras gracile (Shumard), Anisoceras plicatile (J. Sowerby), A. aff. plicatile, Idiohamites alternatus vectensis Spath, I. ellipticus radiatus Spath, Hypoturrilites gravesianus (d'Orbigny), H. tuberculatus (Bosc), H. mantelli (Sharpe), H. sp., Neostlingoceras carcitanense (Matheron), N. aff. carcitanense, Turrilites costatus Lamarck, T. acutus Passy, T. scheuchzerianus (Bosc), T. boerssumensis Schlüter, Mariella dorsetensis (Spath), M. cenomanensis (Schlüter), M. group of cenomanensis (Schlüter)-lewesiensis (Spath), M. spp., M. (Plesioturrilites) sp., Scaphites equalis J. Sowerby, S. obliquus J. Sowerby, S.(?) sp., and S. sp. juv.     

Ammonites from the Pawpaw Shale (Upper Albian) in northeast Texas

The lower part of the Pawpaw Shale in northeast Texas yields an abundance of diminutive limonitic ammonites, the assemblage being: Tetragonites sp. juv., Engonoceras serpentinum (Cragin, 1900), Conlinites wrighti gen. et sp. nov., Cantabrigites wenoensis (Adkins, 1920), Neophlycticeras (Neophlycticeras) sp., Neophlycticeras (Paradolphia) occidentalis sp. nov., Stoliczkaia (Stoliczkaia) clavigera Neumayr, 1875, Stoliczkaia (Lamnayella) worthense (Adkins, 1920), Stoliczkaia (Shumarinaia) asiatica Matsumoto and Inoma, 1975, Enigmaticeras riceae gen. et sp. nov., Flickia simplex Pervinquière, 1907, Ficheuria pernoni Dubourdieu, 1953, Ficheuria americana sp. nov., Anisoceras armatum (J. Sowerby, 1817), Hamites venetzianus Pictet, 1847, Mariella (Mariella) worthensis (Adkins and Winton, 1920), Mariella (Mariella) asper sp. nov., Lechites (Lechites) comanchensis (Adkins, 1920), Worthoceras worthense (Adkins, 1920), and Scaphites hilli Adkins and Winton, 1920.This assemblage is correlated with the Mortoniceras (Subschloenbachia) rostratum Subzone of the European sequence on the basis of the occurrence of that species in the underlying Weno Limestone, in the passage beds between the Pawpaw Shale and the succeeding Main Street Limestone, and the lower part of the Main Street.Some previous authors have interpreted the diminutive limonitic ammonites of the Pawpaw as being “dwarf” or “stunted”. The overwhelming majority of individuals are nuclei of much larger ammonites, with smaller numbers of adults of small species that occur in other facies and geographic locations, together with a smaller number still of paedomorphic dwarfs. The fauna is interpreted as a preservational one, where diagenetic pyrite formed in relatively small voids, either the early phragmocone chambers of larger ammonites, or the phragmocone and body chambers of genuinely diminutive species which oxidised to limonite during the weathering process.     

The influence of rock resistance on coastal morphology around Lord Howe Island,southwest Pacific

An Erratum has been published for this article in Earth Surface Processes and Landforms 27(7) 2004, 931. Lord Howe Island, in the northern Tasman Sea, is a remnant of a much larger Late Miocene basaltic shield volcano. Much of the island's coastline is exposed to waves that have unlimited fetch, but a marked contrast is provided by a fringing coral reef and lagoon that very effectively attenuate wave energy along a portion of the western coastline. The geology of the island is varied, with hard and resistant basalt lavas, breccias and tuffs of intermediate resistance, and highly erodible eolianites. This variability provides an excellent opportunity to examine the in?uence of rock resistance on the development of the spectacular rock coast landforms that occur around the island. The hardness of rocks and the extent of weathering around the coastline were assessed using a Schmidt hammer, and statistical analysis was undertaken to remove outlying values. On all but one occasion, higher mean rebound values were returned from fresh surfaces than weathered surfaces, but only half of these differences were statistically signi?cant. Shore platforms with two distinct levels are juxtaposed along two stretches of coastline and Schmidt hammer results lend support to hypotheses that the raised surfaces may be inherited features. Relative rock resistance was assessed through a combination of Schmidt hammer data and measurements of joint density, and constrained on the basis of morphological data. This approach formed a basis for examining threshold conditions for sea‐cliff erosion at Lord Howe Island in the context of the distribution of resistant plunging cliffs and erosional shore platforms. Copyright © 2004 John Wiley & Sons, Ltd.