Process-form Relationships Research Articles

Weathering in cold regions: some thoughts and perspectives

Weathering in cold regions has primarily focused on the notion of ‘cold’, such that process and landform theory have generally used this both as the developmental criterion and as the outcome of palaeoenvironmental reconstructions based on landforms or sediments. As a result of this approach, the process focus in terms of weathering has been that mechanical processes predominate, with freeze-thaw weathering as the prime agent, and that chemical processes are temperature-inhibited, often to the point of nonoccurrence. Here a reconsideration of the whole conceptual framework of weathering in cold environments is undertaken. It is shown that, contrary to popular presentations, weathering, including chemical weathering, is not temperature-limited but rather is limited by moisture availability. Indeed, summer, and oft-times even winter, rock temperatures are more than adequate to support mechanical and chemical weathering if water is present. Where water is available it is clearly shown that chemical weathering can be a major component of the weathering regime. The argument is made that there is no zonality to cold environment weathering as none of the processes or process associations are unique to cold regions; indeed, many cold regions show similar weathering assemblages to those in hot arid regions. Process-form relationships are also questioned. The assumption of angularity with weathering in cold regions is questioned, all the more so as hot arid studies identify exactly the same angularity of debris form. Further, that all forms have to be angular is shown by field examples to be no more than an artefact of original unquestioning and oft-repeated assumptions, now over a century or more old. The argument is made that there is a strong need for the reconsideration of the nature of weathering in cold environments, that current theory should be questioned and challenged, and field observation undertaken within this revised frame of reference.

Recent research on the nature, origin and climatic relations of blocky and stratified slope deposits

A selection of subaerial slope processes is discussed together with the morphological and sedimentological traces that are left by the processes. Emphasis is on mass transfers related to (coarse) blocky slopes and to scree accumulations, either on steep or on gentle slopes. New developments in the interpretation of more or less clearly stratified slope deposits are discussed in the light of the findings of research focusing on present-day process-form (process-material) relationships. The question of the climatic (and, more specifically, the periglacial) significance of the different processes and their morpho-sedimentary expression is a returning theme in this paper. It is concluded that many deposits are formed by azonal processes, although their activity (in terms of magnitude-frequency combinations) is often relatively high under periglacial conditions. Some of the deposits point to (cold-climate) extreme events. This especially is the case for frost-coated clast flows, aeolian transport of large platy clasts, and to a lesser extent debris flows.

The active temperate glacial landsystem: a model based on Breiðamerkurjökull and Fjallsjökull, Iceland

Accurate interpretations of ancient glaciated terrains rely heavily on our knowledge of process–form relationships in contemporary glacierized basins. A landsystems model for temperate, actively receding glaciers is presented based upon Breiðamerkurjökull and Fjallsjökull, Iceland. Historical documentation, maps and/or aerial photography documenting recession since 1903 provide a unique series of “snapshots” of the evolving glacial geomorphology at these snouts. This documentation is employed in association with sedimentological investigations to assess the evolution of sediment–landform assemblages at active temperate glacier margins, using the wealth of geomorphological and sedimentological information produced during the recent recession of Breiðamerkurjökull and Fjallsjökull. Three depositional domains are recognized: (1) areas of extensive, low amplitude marginal dump, push and squeeze moraines derived largely from material on the glacier foreland and often recording annual recession of active ice; (2) incised and terraced glacifluvial forms, such as recessional ice-contact fans and hochsandur fans, and simple and complex, anabranched eskers and small areas of pitted outwash; (3) subglacial landform assemblages of flutes, drumlins and overridden push moraines located between ice-marginal glacifluvial depo-centres. The lack of supraglacial sediment in active temperate glaciers like Breiðamerkurjökull and Fjallsjökull generally precludes the widespread development of chaotic hummocky moraine. The hummocky terrain previously termed “kame and kettle topography” has mostly evolved by melt-out into a complex network of anabranched eskers over the period 1945–1998 or actually comprises pitted or kettled outwash (sandar). The tills across the foreland were emplaced by subglacial deformation and lodgement, and comprise materials derived from pre-existing stratified sediments in addition to localized abrasion of rock surfaces and patches of lake sediments. Till sequences thicker than 2 m have been constructed by the sequential plastering of till layers onto stratified sediments and bedrock. Because this stacking is a sub-marginal process, it is suggested that complex till sequences similar to those observed at Breiðamerkurjökull/Fjallsjökull may be employed in the reconstruction of ancient glacier margins. Additionally, the geomorphology of the active, temperate landsystem at east Breiðamerkurjökull may contain subtle surge signatures, verifying the historical record of small surges by this part of the glacier. This illustrates the danger of employing landform–sediment associations from restricted study areas (e.g. parts of landsystems) as representative process–form models for glaciated terrains.

Process form relationships on USA east coast barrier islands

Process form relationships on USA east coast barrier islands

Physical Processes and Fine-grained Sediment Dynamics, Coast of Surinam, South America

ABSTRACT The prograding Holocene mud wedge between the Amazon and Orinoco Rivers in the trade wind belt of northeastern South America provides a modern-day example of muds accumulating under moderate wave-energy conditions. Gigantic shore-attached mudbanks (10 km 20 km), composed partly of thixotropic fluid-mud gel, front this coast every 30-60 km to form a buffer to wave attack and a temporary storage for fine-grained sediments. This mesotidal coast (tide range 2.0 m) with gentle offshore slope (0.0006) allows the exposure twice a day of extensive tidal flat deposits, which are backed by mangrove swamps on a well-developed chenier-plain complex. Field experiments were conducted in Surinam du ing 1975 and 1977 to provide new information on process-form relationships in this interesting but unusual muddy environment. Simultaneous measurements of waves, currents, tide elevation, suspended-sediment concentration, and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency. Suspended-sediment concentrations typically exceed 1,000 mg/l at the surface as incoming solitary-like waves partially disperse fluid mud into overlying water on a falling or rising tide. Redeposition of mud may occur near time of high tide. The strong attenuation of shallow-water waves by these muds provides conditions that are favorable for further sedimentation. High concentrations of suspended fluid mud, together with solitary-like waves from the northeast throughout the year, can lead to extraordinarily high net sediment transport rates in the nearshore zone. Calculations based on solitary-wave theory and on data obtained from this study indicate that 15-65 106 m3 of mud can move along shore each year without involving breaking waves, the concept of radiation stress and a nearshore circulation cell, or bedload transport. Farther offshore, outside the zone of wave dominance, wind-driven currents and the Guiana Current combine to transport muds to the northwest, consistent with the observed direction of mudflat migration.

Structure and Sedimentological History of the Overbank Deposits of a Mississippi River Point Bar

ABSTRACT Six structure morphotypes were recognized from the overbank deposits of Plaquemine Point, a point bar on the Mississippi River. They included ripple drift laminations, trough cross-stratification, massive structures, horizontal laminations, parallel laminations, and distorted laminations. The probable hydrodynamic conditions responsible for producing these morphotypes were established from existing literature. Utilizing sediment properties, process-form relationships, probable hydrodynamic conditions and vertical changes in structure suites, the sequence of structures was correlated with a flood cycle of the river. The sequence comprised (from bottom to top): 1) initial overbank flooding in lower flow regime and formation of ripple drift lamination, 2) high turbulence during rising fl od and formation of deformational structures, 3) upper flow regime during maximum flood and formation of horizontal laminations (erosion occurred at this stage), 4) lower flow regime in the falling stage of flood and formation of small scale trough cross-stratification, 5) formation of clay drapes. Each vertical sequence could be recognized by either complete or partial preservation of the sedimentary structures. An attempted correlation of the preserved structure sequences (seven in all) and the flood hydrographs of the river showed that not all the floods were recorded in the portion of the deposit studied.