ABSTRACT: The geology of sand and gravel deposits in British Columbia

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Abstract

Almost 75% of British Columbia is covered by unconsolidated surficial sediments; deposits that vary from less than a metre toseveral hundred metres in thickness. Most of the surficial sediments owe their origin to a number of processes which have been active during the last few million years (Quaternary), but a few isolated deposits of Tertiary age are also known. All of the processes are still active today, but their relative importance has changed with time, especially during the last 10,000 years(Holocene). For instance, since the last glaciation (Fraser Glaciation) the pre-eminent role of glaciers to actively erode, transport and deposit sand and gravel in British Columbia has been superseded by fluvial and mass-wasting processes. During the Late Wisconsinan (25,000-10,000 years ago), much of the province was covered by a network of coalescing ice caps, valley, trunk, piedmont and cirque glaciers collectively termed the Cordilleran Ice Sheet. Isostatic depression of the land surface by the weight of the ice and a concomitant eustatic lowering of the sea level affected the configuration of the province’s terrain. At this time, changes in base level resulting from isostasy and eustasy promoted sediment erosion and deposition. Subsequent climatic warming witnessed the decay of the ice sheet through active retreat and in situ melting. Sediment trapped in the ice consequently underwent active deposition beneath and adjacent to the melting glaciers; hence, deposits associated with deglaciation tend to reflect rapid and episodic events. In many areas of British Columbia, thick accumulations of sand and gravel were deposited in front of advancing glaciers during the early stages of the Fraser Glaciation (e.g. Quadra Sand). Gravely facies correlative to this period provide significant sources of aggregate in many areas of southwestern B.C. (e.g. Saanichton gravel). At the end of the Fraser Glaciation, a number of deltas and river terraces developed in many of the isostatically depressed valleys. In the Fraser Lowland area, sand and gravel accumulations associated with the Fraser Glaciation include select facies of the Quadra Sand, Vashon Drift, Fort Langley Formation, Sumas Drift and Capilano sediments. Economically viable deposits of sand and gravel from this period are widespread and often exploited for aggregate purposes. The character and distribution of unconsolidated sediments can be generalized by examining modern geological processes and the landforms resulting from such processes. Ground moraine, kame terraces and eskers which are common to the glacialenvironment consist of “predictable” assemblages of sediment that conform to a documented range of texture, sorting and internal structure. Similarly, the sedimentological and stratigraphic composition of channels, terraces and fans that occur in the fluvial environment, also contain deposits with predictable attributes. These and other geological environments (e.g. marine, lacustrine, aeolian) are the target of considerable research by earth scientists who specialize in surficial studies. This research includes the collective examination and analysis of the external morphology of landforms and internal properties of the deposits. Thus, theidentification of unique landforms through air photographic interpretation provides the first level of analysis in the study of natural aggregate deposits, whereas the field description of subsurface exposures provides the second level of study. Although many ideal landforms such as fan-deltas and glaciofluvial terraces often contain high quality materials for aggregate production, within deposit variations are common. The influence of local bedrock, topography and complex composite geological histories limit generalizations. Such parameters as sediment thickness, sorting and composition, although often predictable, still display site specific characteristics for each deposit which, therefore, require individual evaluation. Landforms suitable for sand and gravel production occur in discrete areas of the province. In the mountainous regions of the province, natural aggregate occurs in the mid to lower valley environments. In the upper elevations, delta, fan and kame deposits hold the greatest promise for sand and gravel. Proximal ends of alluvial fans may support coarse rock particles with limited fine content. Raised glaciofluvial fan-deltas, although rare, mark the limits of tributary valley lake impoundment and often consist of coarse debris. Similarly, kame terraces, which represent the former margins of valley glaciers also contain poorly sorted mixtures of sand and gravel. In the lower valley environment, deltas, kames and eskers are secondary in abundance to dissected fluvial and glaciofluvial terraces. Terrace deposits, which represent former flood plains, contain sediments which are usually better sorted and graded than deltas, eskers and kames. Fluvial and glaciofluvial deposits are prominent landscape features in most valleys and at all elevations, but the sediments are always coarsest in their proximal reaches. In the coastal environments, glaciofluvial fan-deltas can occur up to 200 metres above sea level, depending on local isostatic depression, but generally are present within a few tens ofmetres above sea level. Productive glaciofluvial and fluvial deposits, show considerable range in elevation, but tend to be confined topographically to valley environments. In contrast, modern and raised ancient beach deposits show little vertical range but considerable lateral extent. The sediment in beach environments is often moderately well sorted and frequently coarser near the top and finer in the lower parts.

Bibliography of Canadian Geomorphology