摘要:

ABSTRACTThresholds for channel change were investigated on two proglacial outwash fans, one silty, the other gravelly, over, different temporal and spatial scales. Short-term channel change was monitored in detail over a period of seven weeks from July to August 1988, whilst longer term changes were observed between summer 1986 and 1988. A model predicting channel change under varying threshold conditions was devised.Input controls showed that base level fluctuations, discharge variations, suspended sediment concentrations, grain size variations and sorting, sediment structures and stream gradients played different roles not only on an inter- and intra- fan basis but also over varying time scales. Both minor and major thresholds influenced channel change processes of sediment transport; channel scour, accretion, division and abandonment; bank collapse; bar formation; fan erosion and deposition.Model outputs were defined in terms of planimetric change, braiding intensity, fan incision and extension, and cross-sectional change. A special intransitive threshold condition causing abrupt base level increase during the course of a jökulhlaup (R.I. 2–3 years) triggered complete channel system metamorphosis on the gravel fan and significant changes on the alluvial fan.

ISSN:0435-3676    

DOI:10.1080/04353676.1992.11880344

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

ABSTRACTThe extensive occurrence of glacier ice cores to landforms and deposits of the Neoglacial and Little Ice Age periods in the St. Elias Mountains of the southwest Yukon is described. These glacier ice cores were produced during active glaciation and have been preserved, in locations such as the Donjek Glacier moraine, for over 500 years. Incorporation of ice in landforms and deposits occurs during glacier surges, glacier advance or glacier backwasting as well as during stagnation of the glacier. The primary terrain ranges from sequences of moraines, providing a detailed record of glacier fluctuations, to ice contact landforms. The degradation of the ice-cored terrain is controlled by hydrological and glaciological processes, which expose the ice core, rather than by melt beneath the surficial materials.

ISSN:0435-3676    

DOI:10.1080/04353676.1992.11880345

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

ABSTRACTThis paper investigates the origin of a series of ring structures developed on Myrdalssandur, south Iceland, following a major, volcanically - triggered jökulhlaup in 1918. The ring structures are characterized by boulder-rich rims up to 4 m high and 40 m in diameter. The rims were found to be composed of diamicton which dips steeply into the central hollow, which is infilled with laminated fines. The deposit generated by the flood comprises pre-surge, hyper-concentrated flood surge, and post-flood stage sediments. The diamicton deposits indicate that the ring structures were produced by the in situ melting of debris-rich ice blocks, transported on the surface of the hyperconcentrated flood surge, to form‘rimmed’kettles. Laboratory experiments confirmed that ring morphology is dependent on the sediment concentration and depth of submergence of the ice block into the adjacent sediment, and have allowed the interrelationships to be defined by empirical equations. A sequence of four types of kettle holes has been distinguished in response to increasing sediment concentration of the transported ice block: Type 1, a‘normal’kettle hole; Type 2, a‘rimmed’kettle; Type 3, a‘crater’kettle; and Type 4, a‘till-fill kettle’or‘kettle mound’. The results suggest that other recent and Pleistocene kettle sediments, and their host deposits, may also be of jökulhlaup origin.

ISSN:0435-3676    

DOI:10.1080/04353676.1992.11880346

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

ABSTRACTVentifacts, developed on a variety of lithologies and at a range of scales, are reported for the first time from the Falkland Islands, South Atlantic. Detailed observations of ventifact locations and site stratigraphies indicate most examples are associated with a major occasion of wind activity following a period of severely cold climate but before restoration of vegetation cover and peat formation.14C dates (c. 13.6-11.0 ka B.P.) from basal peats consign ventifact formation to the Late Glacial period. Abundant material susceptible to wind movement was available at that time. The erosion and movement of sand continues at many sites but at none can it be demonstrated that ventifact formation is entirely a recent or contemporary process. The14C dates also reveal that climatic amelioration following the last cold episode occurred significantly earlier than previously considered.

ISSN:0435-3676    

DOI:10.1080/04353676.1992.11880347

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

ABSTRACTBy means of studies of lake sediment cores retrieved from Nicholson Lake, Vestfold Hills (68°35' S, 78°10' E), Antarctica, a climate chronology could be established, stretching from 6350 BP to the present. Climatic fluctuations were deduced on the basis of variations in organic and water contents, diatomic assemblage and wet bulk density with sediment depth. The chronology was based upon radiocarbon datings of sediment samples. Cool periods in the longer time-scale lasted from 4950 BP to 3230 BP and from 1300 BP to 250 BP, approximately, which were consistent with long-term fluctuations found elsewhere. Brief but more pronounced cold episodes occurred about 3650–3550 BP, 2950–2650 BP, 2300–2000 BP, 1300–1100 BP and 400–250 BP. Some of the cold events were similar in age with cold events occurring elsewhere, e.g. the 2300–2000 BP event.

ISSN:0435-3676    

DOI:10.1080/04353676.1992.11880348

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor