Unit Name: Midale Beds
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Osagian (348 - 340 ma)
Age Justification: Brindle (1960) identifiers a fairly extensive coral-brachiopod fauna from the Midale Beds. However it does not appear to be very diagnostic and may be closely related to that found in the underlying Frobisher-Alida Beds.
Originator: Saskatchewan Geological Society, 1956.
This unit took its name from the Midale oil field, Saskatchewan, where, in April, 1953 the discovery well Shell Midale A-7-18, in Lsd. 7, Sec. 18, Twp. 6, Rge. 10W2M was completed in this part of the succession, between 1,393.5 and 1,416.7 m (4,572 and 4,648 ft).
The northern and western limit of the Midale is marked by its zero erosional edge that forms an arcuate trace at the sub-Mesozoic subcrop extending from Twp. 1, Rge. 30WPM to Twp. 1, Rge. 16W3M, reaching a maximum northern limit in Twp. 12, Rge. 20W2M. Its southern limit is controlled by the recognizability of its lower boundary, which for the most part is at the base of the Frobisher Evaporite. The evaporite is underlain by an argillaceous interval at the top of the Frobisher Beds that has been recognized as far west as Rge. 16W3M; its recognition farther west being limited by erosion of the marker bed. The thickness of the Midale Beds varies markedly. It is thinnest in extreme southeastern Saskatchewan, where it is about 15 m (49 ft) and thickest beyond the solution edge of the Middle Devonian Prairie Evaporite, where it reaches 45 m (148 ft).
The Midale Beds consists of two lithologic intervals, the lower part is defined as the Frobisher Evaporite and the upper is commonly described as the Midale carbonate. Haidl (1978) showed that the Frobisher Evaporite includes a variety of subaqueously deposited and supratidal anhydrites that may reach 9 m (30 ft) in thickness. The nature of the Midale carbonate varies markedly along the Midale trend of production in southeastern Saskatchewan. It includes oolitic-pisolitic and skeletal grainstones and packstones that in places have well-developed vuggy porosity, as well as dolomites and dolomitized burrow-mottled limestones and wackestones that have intercrystalline porosity. The vuggy grainstones and packstones are commonly known as the "Midale Vuggy" and the lime mudstones and wackestones are described as the "Midale Marly".
The top of the Midale Beds is placed at the top of an argillaceous carbonate interval that underlies the Midale Evaporite in the Ratcliffe Beds, and the lower limit is placed at the top of second argillaceous interval with the Frobisher Beds and immediately beneath the Frobisher Evaporite. Recognition of both of these argillaceous marker beds is not contingent on the presence of the evaporites, as they have been identified well beyond the depositional limits of both evaporite intervals (Hutt, 1963 and Kent, 1974). The Midale has been correlated with the upper Strathallen Beds of western Saskatchewan (Kent, 1974) and with the Shunda Formation of the Alberta Plains (Brindle, 1960).
Brindle, 1960, Fuzesy, 1960; Haidl, 1978; Hutt 1963; Kent, 1974; Saskatchewan Geological Society, 1956; Smith, 1980; Walters, 1983.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: D.M. Kent
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Apr 2003