Precambrian Basement Features in Northern Alberta

As additional information becomes available it is possible to amplify or to modify previous hypotheses and concepts concerning the eological history of northern Alberta. This paper discusses the topography and structure of the Precambrian basement rocks and their influence on Devonian sedimentation.

The present topography on the Precambrian rocks in northern Alberta has been illustrated by Darling and Wood (1955), Sproule (1956) and Sikabonyi (1957). The Precambrian basement rocks and their overlying arenaceous deposits have been discussed by Baillie (1956), Goodman (1956), Greenwalt (1956), Guthrie (1956), and Sproule (1956), and the Peace River region has been the subject of a symposium of the Alberta Society of Petroleum Geologists (DeMille, 1958; Lavoie, 1958; Williams, 1958).

On a map of the present topography on the Precambrian rocks it is difficult to visualize properly and to evaluate the significance of the various discernible features. Thus an attempt (fig. 2) has been made to reconstruct the topography of the basement rocks late in the Devonian period-at the time of deposition of the shales of the Exshaw formation-and to discuss the tectonic and paleogeographic significance of the major topographic features.

The region covered in the reconstruction is the northern portion of the Province of Alberta, the southern boundary of which is drawn at the southern boundary of township 66 (latitude 54� 40'' north). The northern, eastern and western boundaries are the provincial borders-latitude 60 degrees north, and longitudes 110 degrees and 120 degrees west, respectively. The method of topographic reconstruction is essentially that used by Greenwalt (1956); the data on which it is based were obtained mainly from the Schedules of Wells of the Oil and Gas Conservation Board, Calgary, Alberta. An isopach map was onstructed of the strata between the base of the Exshaw formation and the Precambrian basement. If the pre-Exshaw surface is considered to be a planet and is rotated into a horizontal position, the isopachs become in effect contours on the surface of the basement. These relative contours (fig. 2) are at 250 feet intervals and the lowest has been designated as zero feet. The contact of the Exshaw strata with earlier Devonian sediments (Storey, 1956; Warren, 1956) is a paraconformity* but as there is no significant angular discordance this surface is considered to be suitable for use in the econstruction, and the data utilized to be within an acceptable margin of error.

The top of the Elk Point group was considered for use as a datum, but was not used for several reasons: in northwestern Alberta the upper boundary of Elk Point strata is in some areas diachronous (Law, 1955); towards the southeast this boundary, the base of the Slave Point formation, becomes a paraconformity; in the McMurray area there is evidence of solution of Elk Point evaporites (Carrigy, 1958); in the Peace River area Elk Point strata are absent over the basement high.

East of the erosion edge of the Exshaw formation reconstruction has involved an extrapolation of the datum control plane. From faunal evidence it is generally considered that the Devonian strata of Western Canada thinned out against a low-lying Precambrian landmass o the east (Warren and Stelck, 1958) which may have been covered by a veneer of Lower Paleozoic sediments (Cooke, 1947). It is assumed that in the latitudes of northern Alberta the eastern limit of deposition of Devonian sediments was 200 miles east of the erosion edge of Exshaw strata, that is, approximately 50 miles east of the Alberta-Saskatchewan boundary. On the basis of this assumption thicknesses of pre-Exshaw Devonian sediments have been estimated for the area from the present erosion edge east to the Alberta-Saskatchewan boundary.

The method of topographic reconstruction has several limitations ncluding an increasing margin of error towards the northeast; nevertheless the map constructed (fig. 2) still yields information of considerable interest. In particular, the topography illustrated, that of the surface of the mPrecambrian basement at the time of deposition of the Exshaw formation, is an approximation to that of the pre-Devonian surface on which Middle and Upper Devonian sediments were deposited.

The data available on which to base reconstruction are subject to more than one interpretation. Comparison should be made of the Peace River region as illustrated on figure 2 and on figure 6 of DeMille (1958). The reconstruction presented here is based primarily on an erosional concept, whereas DeMille''s is a more complex structural hypothesis. * Dunbar and Rodgers (1957) divide unconformities into four types the term paraconformity describes the structural relation between unconformable units where the beds are parallel and the contact is a simple bedding plane.