Abstract

The basal Upper Devonian red bed in the area between Drumheller and the Rocky Mountains commonly is considered a part of the Middle Devonian Elk Point formation¹. This “correlation” is perpetuated from early considerations (McGehee, 1949; Imperial Oil, 1950) which assigned the red bed interval to the Elk Point on the questionable basis of alleged lithologic similarity and presumed analagous stratigraphic position. These considerations are untenable for the following reasons:

(a) The basal Upper Devonian red bed is not a part of the Elk Point but is naturally separated from it by a notable regional unconformity caused by pre-Upper Devonian erosion of 50 to 800 or more feet of uppermost Elk Point strata in the subsurface of Western Canada. This unconformity is interpreted from evidence in the published cross-sections of Crickmay (1954), Law (1955), and van Hees (1956). Although these authors themselves did not interpret the contact below the red bed interval as an unconformity, the obvious structure in the Elk Point strata and suggested differential erosion of these, can hardly be interpreted otherwise. This natural and significant break in sedimentation between Upper and Middle Devonian strata has long been known from the lower MacKenzie River area to Great Slave Lake (Cooper, et al, 1942), and has been recognized and agreed upon by many in the region of the Mississippi Valley (Illinois State Survey, 1944).

(b) Thus, the red bed is not the same age as the Elk Point, nor is it the same age everywhere as it is trangressive with successive conformable Upper Devonian (Beaverhill Lake) strata, which rest unconformably upon different Middle Devonian and older rocks. The red bed is, thus, itself an indicator of the unconformity which separates the rocks of the Upper and Middle Devonian Epochs.

(c) The red bed is not traceable into Elk Point strata below the unconformity, but rather in different areas it corresponds to the following Upper Devonian formations, — Ghost River formation of Walcott, (1923, 1928), Warren (1927) in the frontal Rocky Mountains west of Calgary; Unit C of Sloss and Laird (1947) in northwestern Montana; the Mafeking formation of Crickmay (1954) in outcrops in Manitoba; the Watt Mountain formation of Law (1955) in northern and central Alberta;the ‘first red bed’ of van Hees (1956) in the subsurface across Saskatchewan, as well as his interval from the top of ‘first red bed’ to the base of the ‘second red bed’ in western Saskatchewan.

The top of the Elk Point is revised herein from the top of the basal trangressive red bed of the Upper Devonian to the bottom of the interval. This boundary coincides with the regional unconformity which in every respect is a more significant horizon for both formational and stratigraphic subdivision and nomenclature. The following points will illustrate this argument:

(a) It is fundamentally unacceptable to consider a ‘regional’ hiatus within the rocks of one stage for example, the Givetian Stage. Rather it is basically sound, in theory and in practice, that such an unconformity occurs between the Frasnian and Givetian Stages. These mark the natural unconformable break in Europe between the Upper and Middle Devonian Series (Schindewolf, 1954). It is in fact misleading to include a major unconformity within one formation as it is generally understood! It is therefore unreasonable to use one formational name which hides the fact that both Upper and Middle Devonian strata are present but separated by a regional unconformity.

(b) The top of the red bed represents a transgressive facies boundary and therefore cannot represent a time-marker between strata of the Upper and Middle Devonian Epochs. This red bed cannot be considered to represent late Middle Devonian sediments, when in fact below the red bed are late and early Middle Devonian strata.

(c) Many notable stratigraphers deliberately include (Woodring, 1953), or unsuspectingly hide regional unconformities within type or reference sections. An example is the Cedar Valley formation of Iowa (Cooper, 1942) which has led to unceasing arguments that this formation is either Upper or Middle Devonian in age. In fact, the Cedar Valley is Upper and Middle Devonian and the “hidden” unconformity is well known beyond the area of the type section.

This same stratigraphic procedure has been followed in Western Canada, and has been commonly applied to the Devonian succession in the subsurface between Drumheller and the frontal Rocky Mountains. There is every reason to refer to the basal Upper Devonian red bed of this area as basal Beaverhill Lake and not uppermost Elk Point.