glacial facies models

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5.

GLACIAL DEPOSITS

Carolyn H. Eyles, School of Geography and Earth Sciences, McMaster University, Hamilton, ON, L8S 4L8, Canada Nick Eyles, Department of Geology, University of Toronto, Toronto, ON, M5S 3B1, Canada

INTRODUCTION The cryosphere is the name given to the approximately one fifth of the Earths surface affected by the freezing of water. The cryosphere includes snowfields, valley glaciers, ice caps, ice sheets, floating ice such as icebergs, ice that forms on the surfaces of lakes, rivers and seas, and ground ice that forms beneath landscapes frozen year-round (permafrost). The cryosphere has repeatedly expanded to cover one third of the global land area during the Pleistocene ice ages of the last two million years. During major glaciations, floating ice shelves and icebergs reached far onto continental shelves, where they influenced deep-marine environments, and changed ocean circulation by the release of huge volumes of meltwater (Benn and Evans, 1998; Dowdeswell and OCofaigh, 2002). Global sea level rose and fell as ice sheets waxed and waned, and influenced coastal evolution worldwide. In the more remote past, the Earth experienced six major intervals of glaciation when ice was present for tens of millions of years (glacio-epochs; Eyles, 2008). The earliest known glaciers formed about 2.8 billion years ago and some glaciations (those between roughly 750 and 600 million years ago) may have been so severe that they affected the entire planet (Fairchild and Kennedy, 2007; Hoffman, 2008)! Understanding the formation and characteristics of glacial sediments has important and practical applications in northern regions such as Canada. These sediments underlie many large urban centers and contain aquifers that supply drinking water to millions of people. Groundwater exploration and management programs, investigations for waste-disposal sites, aggregate-resource mapping, and the cleanup of contaminated sites all require knowledge of the

subsurface geology of glaciated terrains (Meriano and Eyles, 2009). The mineral-rich Precambrian shields of the northern landmasses are covered by extensive sheets of glacial sediment, and knowledge of ice dynamics and sedimentology is needed to locate economically valuable mineral resources, such as gold and diamonds, which lie, buried, beneath the cover of glacial deposits. The search for shallow gas, trapped in Pleistocene glacial sediments in Alberta, and for oil, coal and gas in older Paleozoic glacial strata in Brazil, Australia and India, has emphasized the importance of glacial sedimentology to energy exploration. Glacial sedimentology plays an important role in many other applications of environmental geology, such as in urban areas, in seismic-risk assessment and geological engineering, and is increasingly integrated with other disciplines such as geophysics. Recent Developments A major shift in focus has occurred since Facies Models was last published in 1992. Then, glacial facies modeling and knowledge of glacial processes was dominated by studies at modern glaciers flowing on hard rock (Fig. 1). In contrast, large Pleistocene ice sheets (and parts of todays Antarctic Ice Sheet) flowed across soft beds of wet sediment. Deformation and mixing of this sediment is now known to be an important process resulting in the formation of poorly sorted till (Boulton et al., 2001; Evans et al., 2006). Also, there have been major advances in quantifying rates of glacial erosion (and thus landscape modification) as a consequence of analyses of cosmogenic isotopes and thermochronometry. The flux of glacial sediment from glaciated basins is understood better, and it is now known that significant chemical

weathering can also take place in cold environments. Offshore, much has been learned about how ice sheets deposit sediment underwater on continental shelves and slopes (Boulton et al., 1996; Domack et al., 1999; Eyles et al., 2001; Dowdeswell and OCofaigh, 2002; Heroy and Anderson, 2005). This knowledge has arisen as a consequence of oil and gas exploration, ocean drilling of deep-sea sediments to obtain climate records, and geophysical mapping of northern seafloors. GLACIAL SEDIMENTARY ENVIRONMENTS The glacial environment is one of the more difficult to summarize because glaciers can affect depositional processes both on land (glacioterrestrial) and offshore (glaciomarine) and there are many sub-environments within each of these settings (Figs. 2, 3). In addition, the growth and decay of ice sheets gives rise to rapid timetransgressive deposition, commonly complicated by later reworking of deposits by marine and fluvial action. A broad periglacial zone surrounds ice sheets where it is too dry or slightly too warm for glaciers to grow. In this zone, freezethaw cycles and the deep-freezing of groundwater to form ground ice dominate sedimentary processes; there is also considerable potential for eolian processes to transport and deposit sediment such as loess (see Chapter 7). Also, ice sheets expanded onto continental shelves when the sea level was lowered during glaciation. Much glacial sediment is ultimately preserved in deep-water continentalslope successions as debrites, muddy contourite drifts, and turbidites (Hooke and Elverhoi, 1996; Sejrup et al., 2005; Tripsanas and Piper, 2008; see Chapter 12), and as horizons of ice-rafted debris in the deep ocean (Andrews, 1998). New information

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EYLES AND EYLES

Figure 1. A. Canadas most familiar glacier, the Athabasca Glacier, on the Icefields Parkway in Alberta. Most early glacial facies models were derived from study of easily accessible glaciers such as this one, which flows over bedrock. Pleistocene continental ice sheets behaved differently because they flowed over thick sediment. B. A geologist lying on bedrock looking up at the dirty base of a glacier flowing left to right (Glacier du Bosson, French Alps). The glacier is carrying debris within the basal ice (as englacial load). Observations at numerous glaciers show they transport very little englacial debris. The most effective means of moving sediment is where glaciers rest on soft beds composed of sediment that can be deformed and moved as the glacier flows (see Fig.5). within a few years in temperate areas, but takes many hundreds of years in the much colder and dryer Antarctic (Benn and Evans, 1998). The formation of glacier ice takes place in the accumulation zone of a glacier or ice sheet (Fig. 4B). There, the mass of ice gained each year is greater than that lost by melting. At lower elevations and under warmer temperatures, glacier ice melts at greater rates than it is formed and the glacier loses mass. This area is called the ablation zone. The point on a glacier where there is neither gain nor loss of mass is termed the equilibrium line and its position can be approximated by the position of the snow line visible on a glacier at the end of the summer melt season. Transfer of ice between the accumulation zone of a glacier and the ablation zone occurs through the process of creep or deformation. Glacier ice moves essentially under the influence of gravity in response to both vertical (compressive) and shear stresses. The rate of glacier movement is mostly dependent on the surface slope of the glacier, the thickness of the ice (shear stresses and rates of ice movement increase as ice thickness increases), and ice temperature (warm ice close to the melting point can deform and move much more rapidly than cold ice). The thermal regime of a glacier is a description of the temperature of the ice, which

Figure 2. Much sediment in glaciated areas, such as the Copper River Valley, Alaska, shown here, is moved and deposited not just by glacial processes per se. A braided meltwater river leaving the ice front has reworked almost all primary glacial sediment such as tills and glacial landforms. Small lakes add further variability. In the lower part of the image braided and anastomosed rivers co-exist (see Chapter 6). Close to the ice front, debris is reworked by gravity and slumping off steep mountain slopes. Eolian activity is significant and small dunes are forming. Most of the sediment load is transported to the ocean downstream. regarding glacially influenced deposition along continental margins is the key to understanding pre-Pleistocene glaciations, for which the sedimentary record is mainly preserved in marine basins (Eyles, 1993). THE GLACIER SYSTEM Glacial ice forms when snow accumulates and, at depth undergoes repeated cycles of partial melting, refreezing and recrystallization. Firn is the material that forms at an intermediate stage between snow and ice, and has a density greater than 0.5 g/cm3 (Fig. 4A). Glacial ice is formed with a density of 0.9 g/cm3 with further burial and recrystallization. This process occurs

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Figure 3. The figure illustrates a typical glaciated continental margin showing the principal glaciomarine environments and representative vertical profiles through sediment accumulating in these environments. Pleistocene glaciations have left a prominent glacial record on land but most sediment (perhaps as much as 90%) is deposited offshore on continental slopes, especially on trough-mouth fans. Glaciomarine deposits dominate the record of older glaciations in Earth history because the terrestrial record is easily eroded. affects not only the rate of movement but also the capacity of the ice to erode, transport and deposit sediment. Cold-based glaciers are typical of cold, high-latitude regions (e.g., Antarctica), where the temperature at the base of the ice is well below the pressure melting point (i.e., the temperature at which melting occurs, at the pressure present at the base of the glacier) and there is no water present. These glaciers typically move very slowly by

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