ABSTRACT

The following is a summary of a paper on the Elk Point group in Alberta south of Township 60. The detailed paper will be published by the Geological Survey of Canada.

The Elk Point was introduced as a formation by J. R. McGehee in 1949. It was described from the cores and samples of a number of wells drilled in the Elk Point area and was extended from that area to include homotaxial units in southern Alberta and Saskatchewan. It was given group status by Belyea in 1952 because it includes a number of separately mappable units which are not necessarily co-extensive.

The sediments of the Elk Point group were deposited in a northwesterly trending basin extending from southern Manitoba to the Northwest Territories. A belt in the central part of the basin contains from one to three large salt beds and was designated the 'evaporite basin' by Crickmay (1954). Its western limit in Alberta is shown.

Crickmay in 1954 designated nine informal members in the Elk Point area, where the Elk Point group is near its maximum development. From the top down this sequence is comprised as follows:--Member 1: green and red shales, dolomite, anhydrite; Member 2: the 'first salt'; Member 3: fossiliferous limestone and dolomite, containing an upper zone with reef-type lithology and a lower zone, consisting of argillaceous limestone (correlated by Crickmay with the Winnipegosis); Member 4: orange-red shale, siltstone and dolomite (correlated with the Ashern by van Hees); Member 5: the 'second salt'; Member 6: white anhydrite and limestone with ostracodes; Member 7: grey and red shale; Member 8: the 'third salt'; Member 9: orange-red shales, siltstone, anhydrite with coarse sand grains and glauconite towards the base. van Hees (1956) placed members 1 to 4 in the Upper Elk Point and members 5 to 9 in the Lower Elk Point.

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The Lower Elk Point seems to be present only in the northeastern part of the map-area. No positive evidence as to its age has been obtained. The Upper Elk Point, of known Middle Devonian age, is more extensive than the Lower Elk Point. The individual members are not recognizable beyond the limits of the 'evaporite basin', having changed facies southward and westward into anhydritic claystones, shaly limestones and dolomite, shale and interbedded siltstones and sandstones, the latter increasing in abundance westwards; red and green colors are common.

The Elk Point group rests on truncated Cambrian and Ordovician sediments. The subcrop of the Ordovician on the pre-Devonian surface is present in the southeast part of Alberta where it is overlain by three members of the Elk Point group. At California Standard Parkland 4-12 (Lsd. 4, Sec. 12, Twp. 15, Rge. 27, W4M) in southwestern Alberta a thin Elk Point sequence rests on limestones, shown by Raasch and Campau (1957) to be Middle Cambrian. Between these two areas and northward over central Alberta, the Elk Point rests on light grey, calcareous, glauconitic siltstones with interbedded maroon and green shales of the Upper Cambrian. In western Alberta the Elk Point overlies fine to coarse-grained, poorly sorted quartzose sandstones which may in part be correlative with sandy beds included in the Elk Point elsewhere, or sandstones of more than one age may occur in the area. Warren (personal communication) has found lower Ordovician faunal elements in sandstones below the Devonian in Altoba and Canyon Clearwater No. 1 (Lsd. 5, Sec. 31, Twp. 34, Rge. 9, W5M). In the Windfall area of Central Alberta, sandstones present below the Devonian carbonates may also be Devonian in age.

Some inference as to the development of the Elk Point basin of deposition can be made. The Lower Elk Point beds, present only in the northeast part of the map-area, terminate against truncated Ordovician carbonates (Buller, 1958, and van Hees, 1956), or else, they change facies eastward (Walker, 1957). The Upper Elk Point beds, which reach a thickness of about 700 feet within the 'evaporite basin', become thinner to the southwest by rapid pinch-out of the salt member and by wedging out of the marine limestone member, the uppermost member being the most extensive. This combination of events suggests gradual subsidence permitting progressive onlap of Elk Point deposits over a positive area in southern and southwestern Alberta. The southern part of the province may have remained relatively high into Upper Devonian time and some shelf clastics, here included on the basis of lithology with the Elk Point, may in fact be equivalent to the overlying Beaverhill Lake.

There is some evidence that the Elk Point sediments reflect to some extent the topography of the underlying surface. For example in the area northeast of Calgary the Elk Point is thin over Upper Cambrian, but thicker in the Calgary area where it overlies Middle Cambrian. Thick Lower Elk Point sediments are restricted to the area bounded to the southeast by Ordovician carbonates and to the west by dolomites dated as Upper Cambrian by de Mille (1958).

Both northeasterly and northwesterly structural trends seem to have influenced the configuration of the Elk Point basin. The margin of the Lower Elk Point in eastern Alberta has a northeast depositional strike and northeasterly trending isopach thin trends subdivide the positive area of southern Alberta. One thin extends from the latitude of the Bow Valley and one from the southwest corner of the province. On the other hand, a northwesterly trend is apparent in the west margin of the 'evaporite basin'. These trends seem to reflect pre-Elk Point features, in part erosional but possibly indicative of structural weakness or movements which controlled the depositional and erosional patterns of the overlying Devonian sediments. The southern Alberta arch may result from the crossing of these northeasterly and northwesterly trending elements.

Some correlation between the configuration of the Upper Devonian reef chains and the trends of the Elk Point isopachs is evident. For example, the shelf-margin reef complex follows the wedge-edge of the Ordovician carbonates, the basal Beaverhill Lake reefs seem to follow Elk Point isopach trends in southern Alberta; and the Duvernay embayment and flanking Cairn stromatoporoid reefs in southern Alberta seem to follow the Elk Point embayment north of the Princess area. (Belyea, 1957 and 1958).

Tovell (1958) pointed to northeasterly and northwesterly trending structural elements south of the 51st Parallel as having influenced Mesozoic sedimentation. The trends of the structure contours on Tovell's maps are similar to those of the Elk Point in this area. This suggests that early structures, possibly Precambrian, although modified by later epeirogenic movements, have influenced later erosional and depositional patterns.

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