ABSTRACT: Alberta preglacial sand and gravel deposits.

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Abstract

Tertiary and Pleistocene sand and gravel deposits predating Laurentide glacial deposits (hereafter collectively called preglacial deposits) have garnered geological interest in Alberta for over a century and are described in a number of publications. Preglacial deposits are economically valuable sand and gravel deposits and as such are included in the Alberta Geological Survey mineral aggregate resource inventory. Data on preglacial deposits were assembled from the AGS inventory, the published reports and supplemental field investigation. The resulting collection of information is used to describe the overall sedimentalogical and economic character of preglacial deposits and a theory of their origin and formation. All deposits classified as preglacial rest unconformably on bedrock. There is never any intervening tillbetween the bedrock and the sand and gravel. Deposits commonly are covered by till, glaciolacustrine or glaciofluvial materials or Recent gravels. The preglacial deposits are interpreted as predating Laurentide glacial deposition in Alberta. Cross-bedding and pebble imbrication is common in preglacial deposits and measurements indicate a southwest to northeast stream flow direction. The material deposited in the preglacial deposits commonly is coarse gravel. There is some decrease in the maximum size of clasts in deposits as distance increases from the mountains. This decrease in size agrees with the stream flow indicators and suggests preglacialsand and gravel deposits formed in rivers which flowed generally away from the Rocky Mountains. Pebbles and cobbles were collected and identified at ten, presumed, preglacial sites ranging from the United States border to north of Peace River. The deposits are at various elevations and distances from the mountains. No granitic or gneissic clasts from the Precambrian Shield of northeastern Alberta arefound in the preglacial deposits although the overlying tills or Recent gravels commonly contain clasts of this origin. Quartzites and sandstones are present in all deposits. Lithologies were described for all clasts and tentatively correlated with bedrock formations in the Rocky Mountains. Comparison of the lithologies from the deposits studied resulted in the identification of seven distinct regional lithologic groupings. It is believed that these groups reflect the source formations in the mountains and the extent of the river basinsin which deposition occurred. Analysis of the elevations of all deposits appears to define at least four levels. The highest deposits occur atop the highest hills on the Plains: the Cypress Hills, the Swan Hills, and Obed Mountain. Fossil evidence(Eocene to Miocene) [Storer, 1978] supports the view that these highest preglacial deposits are also the oldest. The Hand Hills (Pliocene to Pleistocene) [Russell, 1958 and Storer, 1978], the Wintering Hills, Whitecourt Mountain, the Pelican Mountains, and Halverson Ridge all belong to the second major, andnext lowest, level. These deposits are characterized by heavy gravel caps on prominent hills or ridges. The third erosion surface is at about Plains level and is represented by deposits at Grimshaw, Entwistle, Cluny, and Fort McLeod. The fourth, and lowest, level is incised into the Plains and includes deposits near the Simonette River, Watino, Villeneuve, and Kipp. These deposits are dated as Late Wisonsinan[Edwards and Scafe, 1996] and contain mammoth teeth and tusks which attest to a cold climate and the approaching Laurentide glacier. This sequence of deposit elevations is attributed to continental uplift which increased the elevation of the Plains during this preglacial period so that the rivers continued to erode through underlying bedrock, asmuch as a kilometre in some places. Topographic highlands covered by gravel deposits were produced when gravel armoured certain areas and protected them from erosion, eventually leaving the former river bed as a positive feature. The model developed in this study divides preglacial deposits into six regional groupings based on location and lithologic similarity. A group reflects the bedrock source area in the mountains for the cobbles and pebbles and the extent of the fluvial basin. The deposits also are divided into four stratigraphic units basedon age and elevation. These units are the Cypress Hills Formation and equivalents (about Early Oligocenedepositional age), the Hand Hills Formation and equivalents (about Pliocene), the Upland Gravels (Pleistocene), and the Saskatchewan Sands and Gravels (Late Wisconsinan). These units reflect the erosional history of the Plains. A total of 203 deposits and 170 sites of probable preglacial origin are identified. Preglacial sand and gravel deposits provide a geological record of events on the Alberta Plains from about 50 million years ago until about 25 000 years ago. The preglacial sands and gravels are not just interesting geological features, they are valuable economic resources. Hard, resistant cobbles and pebbles are common in all groups and produce mineral aggregate above average in quality and worth about $50 million annually. Preglacial deposits contain placer gold concentrations as high as 0.17 g/m 3 . These values represent viable by-product production of as much as $1 million annually from aggregate mining in large volume operations in the Villeneuve deposit. Diamond indicator minerals are present in preglacial deposits and will play a role in identifying diatremes on the Alberta Plains. The ability to distinguish preglacial deposits is crucial for the proper interpretation of sample results during mineral aggregate, placer gold, or diamond exploration.

Bibliography of Canadian Geomorphology