Type Locality:

Composite section, continuously exposed along the valley of the Red Deer River between Sec. 7, Twp. 28, Rge. 18W4M (near the mouth of Willow Creek) and Sec. 7, Twp. 34, Rge. 31W4M (near the mouth of Big Valley Creek), Alberta.

Distribution:

Approximately 227 m (45 ft) thick at the proposed type section. Surface exposures of strata of the Edmonton Croup extend in an arcuate band along the eastern margin of the Alberta Syncline approximately between 50 deg N and 60 deg N. The beds are bounded on the east by the underlying Bearpaw Formation and on the west by the overlying Paskapoo Formation. The southern limit of the group occurs in Twps. 13 and 14, where Edmonton strata below the Whitemud Formation interfinger with beds of the St. Mary River Formation. Strata of Edmonton age are known to be present in the foothills of Alberta, where they form part of the Brazeau Formation. In those regions the lithology of the Edmonton equivalents is such that they cannot be separated satisfactorily from overlying and underlying nonmarine strata. Furthermore, shales of the underlying Bearpaw Formation are not known to be present in the central and northern foothills, and neither the overlying Whitemud nor the Battle Formation has been recognized so far within the faulted and folded strata.

Lithology:

The formation consists of deltaic and fluvial deposits of interbedded and interlensed fresh and brackish water sandstone, siltstone and shale. Typical sediments consist of soft grey and greenish grey, grey and white weathering, fine-grained, bentonitic, feldspathic sandstones; silty, grey green and brown bentonitic shales, coal seams and beds of carbonaceous shale. Included with the above are less common types, including concretions and beds of hard, brown weathering calcareous sandstone; thin nodular beds of red-brown weathering ironstone; and thin beds of bentonite. Most beds contain some bentonite and some beds contain a large amount of it. Near the base of the formation are beds containing oyster shot forming coquinas in some places. About 183 m (600 ft) above the base of the formation is Drumheller Member (Drumheller marine tongue) of Allan and Sanderson (1945). This zone consists of a thin bed or beds of fossiliferous limestone or sandy limestone. The outstanding features of the Horseshoe Canyon Formation are: (1) the lensing and interfingering of the strata so that no two sections are identical; (2) the great amount of bentonite in the beds; and (3) the numerous coal seams or zones (Allan and Sanderson list 12 seams) which are the main markers for correlating different measured sections.

Relationship:

The lower contact of the Horseshoe Canyon Formation is gradational with the underlying Bearpaw shales (Shepheard and Hills, 1970). Where the transition is exposed near the mouth of the Willow Creek the contact is placed arbitrarily at the base of the first thick, light grey weathering sandstone unit above the chocolate-brown, sandy shales of the Bearpaw Formation. At some localities the upper contact is gradational, as on Bow River, where the uppermost bed is a light grey weathering, argillaceous sandstone or, as on Red Deer River (Sec. 9, Twp. 30, Rge. 21W4), where light grey weathering grey shale grades upward into white weathering, light green-grey shale which underlies typical white weathering clayey sandstone of the Whitemud Formation. The Horseshoe Formation equates with part of the Wapiti Formation of northern Alberta, part of the Brazeau Formation of the south-central foothills, and part of the St. Mary River Formation and Blood Reserve Formation of southern Alberta.

History:

The Horseshoe Canyon Formation was proposed by Irish (1970) for the lower part of the Edmonton Group, equivalent to the lower and middle Edmonton of Allan and Sanderson (1945), and members A, B and C of the Edmonton Formation of Ower (1960).

Other Citations:

Allan and Sanderson, 1945; Irish, 1970; Ower, 1960; Selwyn, 1874; Shepheard and Hills, 1970; Tyrell, 1887.