Earth Sciences Report 2002-03

Quaternary Geological Setting of the Athabasca Oil Sands (In Situ) Area, Northeast Alberta

Executive Summary

Earth Sciences Report 2002-03 summarizes baseline investigations by the Alberta Geological Survey (AGS) into the groundwater resources of the Athabasca Oil Sands (In Situ) Area of northeast Alberta. Specifically, this report describes the nature and distribution of thick Late Tertiary and Quaternary geological sediments that overlie the bedrock surface above the oil sands deposits in the region between the Cold Lake area and Fort McMurray.

The surface of the underlying bedrock in the study area represents one of the major unconformable surfaces of the Western Canada Sedimentary Basin-the pre-Quaternary unconformity, which spans the period of erosion from the Late Cretaceous-Early tertiary to the onset of glaciation in the Early Quaternary. The bedrock surface is an erosional landscape consisting of isolated upland remnants and deep, broad relicts of paleoriver channel systems that formed during the Late Tertiary, but which were later modified by glacial and present-day fluvial processes. Elements of the bedrock landscape have been grouped into four major physiographic units based on differences of elevation and relief. The geometry and physical attributes of the buried channels are discussed.

An understanding of the distribution and geometry of the paleoriver channel systems becomes important in the overall evaluation of groundwater resources as these are the loci for accumulations of thick, permeable sediment, which comprise the major drift aquifers of the region. A series of geological cross-sections provide slices through more than 300 m of Tertiary and Quaternary sediment that lie above the deepest of the bedrock channels, illustrating the major stratigraphic units of the drift. Eight Tertiary-Quaternary units have been defined primarily through the interpretation of oil and gas exploration logs. These log interpretations have been validated and calibrated by lithological and geochemical analyses of samples collected from seven coreholes drilled by AGS. An attempt has been made to correlate these stratigraphic units in accordance with the lithostratigraphic model developed for the Cold Lake area to the south.

The lowermost of the drift units, the Empress Formation, consists dominantly of coarse sediment deposited by preglacial and glacial fluvial systems. A series of structure contour and isopach maps illustrate that these basal fluvial sediments are present as thick sequences within the floor of the buried bedrock channels, as well as terrace caps on the bedrock interfluves that separate the channels. Collectively, these fluvial sediments of the Empress Formation constitute the largest drift aquifer in the region. The presence of weathered horizons on the surfaces of buried till units, as determined from core samples, prove the region was subjected to multiple glacial and interglacial intervals during the Quaternary. Glaciofluvial sediments that lien between these tills may also form local- to regional -scale aquifer systems. Sample analyses also highlight the natural occurrence of bitumen in tills from place to place, as well as increased levels of arsenic in buried weathered horizons of older tills. The current level of interpretation does not permit the subsurface mapping of the stratigraphic units other than the Empress Formation.

A terrain analysis map has been constructed primarily from the interpretation of aerial photographs to show the nature and distribution of landforms and surface geological materials deposited in the period from the last glaciation to the present. Although lacking the rigours of a surficial geology map , particularly with respect to a paucity of field ground truthing, the terrain analysis map provides useful information regarding the regional distribution of geological material, which affect land-use planning and construction of infrastructure related to the petroleum industry.

Place Keywords: 73m; 74d; 83p; 84a

Theme Keywords: athabasca oil sands; quaternary