Ice-contact Sediments Research Articles

Towards an understanding of sediment deposition on glaciated continental shelves

A simple vertical stratigraphic sequence of Quaternary deposits is recognized on 20 glaciated shelves from around the world reflecting the transition from the most recent glacial to interglacial period. This sequence includes some or all of the following: (1) ice-contact (ice-deposited and/or ice-loaded) sediments; (2) ice-proximal sediments; (3) ice-distal sediments; (4) paraglacial coastal sediments; and (5) post-glacial sediments. Typically, the sequence unconformably overlies sedimentary rocks on the outer shelf and a mixed bedrock basement on the inner shelf. The relative volume of these deglacial units provide important clues for the reconstruction of ice sheet dynamics. Deposition of unit (1) occurred initially during the period of globally (eustatic) lowered sea-level, and later with units (2) and (3) during a period of high sea levels related to isostatic loading. Deposition of unit (4) occurred during the initial fall in sea level (isostatic recovery) when ice had retreated and was rapidly ablating. Deposition of unit (5) occurred during more complex and local sea level fluctuations, that include such effects as collapse of the crustal forebulge. Minor variations to this association of sea level and sedimentation pattern occur, particularly if the glacial cycle is out of phase with the global eustatic cycle. Outstanding seismostratigraphic problems include: distinguishing ice-loaded glacial-marine sediments from subglacially deposited till; distinguishing till from other ice-contact deposits such as grounding-line fans; and distinguishing till from non-glacial debris flow deposits. The incorrect identification of till provides poor data control on ice sheet reconstruction, ice sheet properties and dynamics, especially as related to sediment transport.

Sedimentology and Depositional Setting of Late Pleistocene Drumlins, Galway Bay, Western Ireland

Subaqueous sediments comprise the cores of a group of drumlins formed at about 17 ka YBP towards the margin of the Late Pleistocene ice sheet is Galway Bay, western Ireland. Four major lithofacies associations (L.F.A.s) are identified. Regional ice flow patterns and isostatic depression together with sedimentary characteristics indicate that the three lowermost L.F.A.s accumulated in shallow-water settings fed by meltwater venting under high hydrostatic pressure. These sediments are tabular, lensate or channelled and have not been deformed by glacially-influenced tectonics. L.F.A.1 is an apron of interbedded mud and muddy diamict deposited from sediment plumes and ice-rafting. L.F.A.2 infills channels cut in the underlying muddy diamict and consists of interbedded poorly-sorted pebbly gravels, sands and diamicts formed by sediment gravity flows. L.F.A.3 is tabular and comprises a massive diamict derived from powerful meltwater jets. L.F.A.4 is a massive diamict carapace which overlies stratified facies and was deposited by basal ice processes. The model presented envisages subaqueous deposition below floating ice followed by till deposition and drumlinization. Coarsening upwards facies transitions are interpreted in terms of progressive change from ice-distal to ice-contact sedimentation. The sequence preserved in the drumlin cores forms part of a regional apron of sediments probably of glaciomarine origin which normally occurs in front of the drumlin belt in Irish coastal areas. Deposition probably occurred in isostatically-depressed zones around the drumlin ice sheet when high relative sea levels occurred. Local streamlining of ice marginal sequences was probably a result of accelerated ice flow triggered by rapid calving, subglacial channel development, sea level change or soft bed conditions. It is concluded that high relative sea levels, calving and high longitudinal stretching rates provide suitable mechanisms which help to explain drumlinization and the rapid disintegration of the last ice sheet in western Ireland.

Coquitlam Drift: a pre-Vashon Fraser glacial formation in the Fraser Lowland, British Columbia

Coquitlam Drift is formally defined and stratotypes established for it in the Coquitlam – Port Moody area, B.C. It is a Pleistocene formation consisting of till, glaciofluvial, ice-contact, and glaciomarine sediments deposited between 21 700 and 18 700 years BP, during the Fraser Glaciation (late Wisconsin) and prior to the main Vashon glacial maximum at about 14 500 years BP. The drift was deposited in short pulses by valley and piedmont glaciers fluctuating into the Fraser Lowland from the Coast Mountains to the north and Cascade Mountains to the east.

Sediment Transport and Accumulation in a Fjord Basin, Glacier Bay, Alaska

Sediment transport and accumulation were studied over several years to define the sedimentary environment in a fan-terminated, active valley glacier and adjacent fjord at Queen Inlet, Glacier Bay National Monument, Alaska. Recently exposed ice-contact sediment at the glacier snout is entrained during summer and early fall by meltwater streams crossing a subaerial-intertidal outwash fan. Suspended sediment in the meltwater (sediment loads exceed 1 g/liter at the inlet head) subsequently yields a cold, surface sediment plume overlying warmer saline water in the fjord basin. Mixing of these layers produces flocculation and settling occurs in discrete layers; each layer is considered to represent one tidal cycle. No gravel is present in fjord floor sediment, although both colluvium and outwash fan sediments contain gravel. This gravel distribution eliminates sliding, slumping of the outwash fan, and ice rafting as major contributors to fjord basin sediment in Queen Inlet. Piston cores of muddy fjord basin sed...

A Comparison of a Fossil Pleistocene Molluscan Fauna from North Dakota with a Recent Molluscan Fauna from Minnesota

Geological and paleontological studies of the glacial deposits in the Missouri Coteau district in south-central North Dakota have produced collections of mollusks which lived in lakes and streams approximately 8,000 to 12,000 (radiocarbon) years before the present (late Wisconsinan Age, Pleistocene Epoch). The geology and present position of the glacial sediments suggests the presence of buried stagnant glacier ice in the sediments at the same time the molluscan fauna existed. The relatively complex species composition of the molluscan fauna suggests that these lakes were temperate and at least mesotropic. The great thicknesses of drift in the area, taken in conjunction with the fact that mollusks were able to occupy essentially superglacial lakes shortly after the emplacement of the sediments of the Missouri Coteau, further suggests that the local climate was temperate during the late part of the Wisconsinan Age during which active glaciation was still taking place in North Dakota. The fossil mollusks are compared with the molluscan fauna of a modern lake in Beltrami County, Minnesota. The differences in climate which now exist between the area of the Missouri Coteau and Beltrami County, Minnesota are suggested to be the increment of net climatic change in southern North Dakota from early Twocreekan time to present. Recent mapping of the glacial deposits of the Missouri Coteau of south-central North Dakota by the North Dakota Geological Survey and the University of North Dakota Department of Geology has shown that the present topography of the area was formed largely by stagnation of the margin of the Pleistocene (late Wisconsinan, see Fig. 1) continental glacier. Among the glacial features found in this area are numerous deposits of ice-contact lake sediments and icecontact outwash sand and gravel containing abundant invertebrate fossils. These late Wisconsinan fossils, including gastropods, pelecypods, and ostracods are modern in appearance. The fossil fauna is not compared here with the Recent molluscan fauna of the area as the mollusks of North Dakota are as yet only poorly known. However, few, if any, mussels occur in the lakes of the Coteau, and the snails are mainly pulmonates that are able to survive the fluctuations of the ephemeral lakes. The fossil fauna is typified by a wide distribution of the prosobranch genera Valvata and Amnicola. In this respect the fossil fauna more closely resembles those found in the more humid climate to the east. The fossil mollusks from south-central North Dakota therefore, will be compared with mollusks from a selected lake (Long Lake) in north-central Minnesota. Traditionally, Pleistocene time divisions have been erected on lithologic criteria which are assumed to reflect climatic conditions. Those climate conditions which caused continental glaciers to advance