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1.
We present 10Be exposure ages from moraines in the Delta River Valley, a reference locality for Pleistocene glaciation in the northern Alaska Range. The ages are from material deposited during the Delta and Donnelly glaciations, which have been correlated with MIS 6 and 2, respectively. 10Be chronology indicates that at least part of the Delta moraine stabilized during MIS 4/3, and that the Donnelly moraine stabilized ∼ 17 ka. These ages correlate with other dates from the Alaska Range and other regions in Alaska, suggesting synchronicity across Beringia during pulses of late Pleistocene glaciation. Several sample types were collected: boulders, single clasts, and gravel samples (amalgamated small clasts) from around boulders as well as from surfaces devoid of boulders. Comparing 10Be ages of these sample types reveals the influence of pre/post-depositional processes, including boulder erosion, boulder exhumation, and moraine surface lowering. These processes occur continuously but seem to accelerate during and immediately after successive glacial episodes. The result is a multi-peak age distribution indicating that once a moraine persists through subsequent glaciations the chronological significance of cosmogenic ages derived from samples collected on that moraine diminishes significantly. The absence of Holocene ages implies relatively minor exhumation and/or weathering since 12 ka. 相似文献
2.
KEVIN KIERNAN 《The Australian geographer》1999,30(1):5-33
The easternmost extremity of the ice cap that developed in the Tasmanian Central Highlands during the time of most extensive Late Cainozoic glaciation lay on the doleritecapped Central Plateau east and north-east of Lake St Clair. During the Last Glacial Maximum (LGM), the more restricted ice cover included a small discrete ice cap (probably less than 250-300 m thick) that formed on the Central Plateau. The LGM ice limits on the southern part of the Central Plateau, including all five southern outlet valleys, are reported here. Earlier ice limits have been identified in two of these valleys, but on the plateau proper earlier glacial deposits have been generally extensively reworked beyond the LGM limit, such that confirmation of a glacial origin for diamictons on slopes is difficult. South of the plateau, the oldest deposits flooring lower reaches of two outlet valleys indicate that ice flowed southwards directly from the plateau, but later deposits indicate diffluent flow from the Derwent Glacier. 相似文献
3.
Three glacial stages (Deshkit 1, Deshkit 2 and Dishkit 3 glacial stages) are identified in the Nubra and Shyok valleys in northernmost Ladakh, northwest India, on the basis of geomorphic field mapping, remote sensing, and 10Be terrestrial cosmogenic nuclide surface exposure dating. The glacial stages date to ∼ 45 ka (Deshkit 1 glacial stage), ∼ 81 ka (Deshkit 2 glacial stage) and ∼ 144 ka (Deshkit 3 glacial stage). A mean equilibrium line altitude depression of ∼ 290 m for the Deshkit 1 glacial stage was calculated using the area accumulation ratio, toe-to-headwall ratio, area-altitude, and area-altitude balance ratio methods. Comparison of glaciation in the Nubra and Shyok valleys with glaciations in the adjacent Central Karakoram of northern Pakistan and northern side of the Ladakh Range of northern India indicates that glaciation was synchronous on Milankovitch timescales across the region during MIS-6, but differed greatly in extent, with more extensive glaciation in the Karakoram than the morphostratigraphically equivalent glaciation on the northern slopes of the Ladakh Range. This highlights the strong contrast in the extent of glaciation across ranges in the Himalaya-Tibetan orogen, necessitating caution when correlating glacial successions within and between mountain ranges. 相似文献
4.
Dating moraines by lichenometry enabled us to reconstruct glacier recession in the Bolivian Andes since the Little Ice Age maximum. On the 15 proglacial margins studied, we identified a system of ten principal moraines that marks the successive positions of glaciers over the last four centuries. Moraines were dated by performing statistical analysis of lichen measurements based on the extreme values theory. Like glaciers in many mid-latitude mountain areas, Bolivian glaciers reached their maximal extent during the second half of the 17th century. This glacier maximum coincides with the Maunder minimum of solar irradiance. By reconstructing the equilibrium-line altitude and changes in mass-balance, we think the glacier maximum may be due to a 20 to 30% increase in precipitation and a 1.1 to 1.2 °C decrease in temperature compared with present conditions. In the early 18th century, glaciers started to retreat at varying rates until the late 19th to early 20th century; this trend was generally associated with decreasing accumulation rates. By contrast, glacier recession in the 20th century was mainly the consequence of an increase in temperature and humidity. These results are consistent with observations made in the study region based on other proxies. 相似文献
5.
Two glaciers at Eyjafjallajökull, south Iceland, provide a record of multiple episodes of glacier advance since the Sub-Atlantic period, ca. 2000 yr ago. A combination of tephrochronology and lichenometry was applied to date ice-marginal moraines, tills and meltwater deposits. Two glacier advances occurred before the 3rd century AD, others in the 9th and 12th centuries bracketing the Medieval Warm Period, and five groups of advances occurred between AD 1700 and 1930, within the Little Ice Age. The advances of Eyjafjallajökull before the Norse settlement (ca. AD 870) were synchronous with other glacier advances identified in Iceland. In contrast, medieval glacier advances between the 9th and 13th centuries are firmly identified for the first time in Iceland. This challenges the view of a prolonged Medieval Warm Period and supports fragmentary historical data that indicate significant medieval episodes of cooler and wetter conditions in Iceland. An extended and more detailed glacier chronology of the mid- and late Little Ice Age is established, which demonstrates that some small outlet glaciers achieved their Little Ice Age maxima around AD 1700. While Little Ice Age advances across Iceland appear to synchronous, the timing of the maximum differs between glacier type and region. 相似文献
6.
Ridges that resemble terrestrial moraines are commonly visible at the foot of many mid-latitude crater walls in Mars Global Surveyor Mars Orbiter Camera images. These moraine-like ridges are often associated with hillside gullies, mantling material, and glacier-like flows, and are usually in contact with crater fill, suggesting possible interrelationships. We consider terrestrial glacier systems that may be analogs of martian moraine-like ridges and glacier-like flows and suggest that the formation of some gullies and crater fill is intimately tied to ice deposition, ice flow, and rock-glacier processes. Upper limits on age suggest the possibility that many of these features formed during the last, or last few, high obliquity cycles. 相似文献
7.
Grain size distribution of soils within the Cordillera Blanca,Peru: An indicator of basic mechanical properties for slope stability evaluation 总被引：1，自引：0，他引：1
This paper presents results of a study on the mechanical properties of sandy and gravely soils within the Cordillera Blanca, Peru. The soils were divided into groups according to their origin （glacial, fluvial, or debris flow）. The grain size distribution of forty three soil samples was used to classify the soils according to the scheme of the Unified Soil Classification System （USCS）. These distributions have then been used to estimate shear strength and hydraulic properties of the soils. There are clear differences between the soils which reflect their divergent origins. The glacial soils normally fit within one of two distinctive groups according to the proportion of fines （Group A, 7%-21.5%; Group B, 21%-65%）. The estimation of shear strength at constant volume friction angle and peak shear strength of the glacial sediments with low content of fines was made using published data relating to the measured shear strength characteristics of soils with similar origins and grain size distributions. The estimated values were supported by measurements of the angle of repose taken from fourteen samples from two moraines and by shear tests on samples from one locality. The results of the grain size distribution werealso used to estimate the average hydraulic conductivity using the empirical Hazen formula which results were verified by field infiltration tests at two localities. 相似文献
8.
The extent, basal conditions and retreat history of a Loch Lomond Stadial glacier are reconstructed based on detailed geomorphological and sedimentological assessment. We present new evidence from the vicinity of Coire Ardair that supports the former existence of a warm-based, locally-fed valley glacier, with probable cold-based ice on the surrounding plateau. This is broadly consistent with modelled creep-dominated flow in the upper catchment and sliding-dominated flow throughout much of the valley. A dense suite of moraines, primarily formed in ice-marginal environments, records a multi-phase recessional history: (1) active and oscillatory retreat; (2) a prolonged ice stillstand; (3) partial ice stagnation with occasional minor readvances; (4) increased oscillatory retreat with a substantial readvance event; and (5) rapid and uninterrupted retreat. We propose that a Coire Ardair glacier responded to sub-centennial scale climate fluctuations, possibly associated with the periodic delivery of warmer air masses to the region, rather than to a single, prominent shift in climate. 相似文献
9.
I.D. Zol&#x;nikov 《Russian Geology and Geophysics》2008,49(9):682-691
We revised geological data substantiating the unified 1983 Regional Stratigraphic Chart of Gorny Altai Quaternary deposits. Based on our own and literature data, we showed that Lower and Middle Quaternary glacial horizons are erroneously distinguished in the Yaloman-Katun’ zone of southeastern Altai. A new correlation is proposed, according to which the glacial complex of the maximum glaciation (MIS-6) corresponds to the Inya catafluvial series and the glacial complex of the first postmaximum glaciation (MIS-4 unit), to the Sal’dzhar catafluvial series. The lectostratotypes of both series are described. The event history of the second half of the Late Neopleistocene in Gorny Altai (MIS-3 and MIS-2) was less catastrophic for ancient biota and Paleolithic man than it was believed earlier. 相似文献
10.
J. M. Gray 《Geological Journal》1982,17(2):111-133
Evidence for 35 former cwm glaciers has been mapped in Snowdonia, N. Wales. Glacial limits are based on end moraines, boulder and drift limits, and the down-valley extent of hummocky moraine. Protalus ramparts indicate that at least 16 semi-permanent snow beds down to altitudes of 150 m also existed. Radiocarbon dates and pollen stratigraphy indicate that the glaciers and snow beds belong to the Loch Lomond (Younger Dryas) Stadial. The existence of a large number of ‘Older Series Moraines’ is questionned. Glacier reconstruction and contouring allows calculation of fan line altitudes, and trend surface analysis indicates that they rise from 450 m in the southwest to 700 m in the northeast. It is inferred that mean July temperature at sea-level was around 8·5°C. Two differences between north and south Britain are recognized. Firstly in Snowdonia, as in the Lake District and Southern Uplands, the dominance of southerly air streams appears less than in the Scottish Highlands. Secondly, whereas in the Scottish Highlands, the glaciers appear to have stagnated in situ following initial active retreat, some of those farther south were active during much of the time their margins retreated. 相似文献