Chapter 9
Middle Ordovician to Lower Devonian Strata of the Western Canada Sedimentary Basin

Authors:
B.S. Norford - Geological Survey of Canada, Calgary
F.M. Haidl - Saskatchewan Geological Survey, Regina
R.K. Bezys - Manitoba Energy and Mines, Winnipeg
M.P. Cecile - Geological Survey of Canada, Calgary
H.R. McCabe - Consultant, Winnipeg
D.F. Paterson - Saskatchewan Geological Survey, Regina

Introduction

Although originally depositionally continuous, Middle Ordovician to Lower Devonian rocks are preserved in two distinct and separate areas within the Western Canada Sedimentary Basin: the Williston Basin and the mountain outcrops (Figs. 9.1 to 9.4). The stratigraphic package primarily corresponds to the Tippecanoe Sequence (Sloss, 1963) but includes older (uppermost Sauk Sequence) and younger (lowest Kaskaskia Sequence) rocks in the mountains. Its present distribution is severely constrained by the effects of several later episodes of erosion. The entire succession is missing from virtually all of the basin north of the Meadow Lake Escarpment, and Middle Devonian rocks rest directly on the Precambrian in much of this region. The base of the succession is a regional unconformity, except in the Rocky Mountains, where it is conformable within the basin facies and within the immediately adjacent platform facies. Several other unconformities are present within the Middle Ordovician to Lower Devonian succession.

The thick outcrop sequences (up to 2300 m) of British Columbia, Alberta and the District of Mackenzie disappear abruptly eastward and southward in the subsurface, below Devonian unconformities. Lower Silurian Nonda carbonates (up to 270 m) are the only part of these sequences with any significant extension into the subsurface. Preserved extensively in the subsurface of southern Saskatchewan and Manitoba and southeastern Alberta are strata that represent an early development of the Williston Basin. A basal clastic unit (up to 68 m) is restricted to Saskatchewan and Manitoba and a sequence of platform carbonates (up to 400 m) extends over the entire area of preservation. These rocks extend into outcrop in Manitoba and in east-central Saskatchewan. Their easternmost extent is marked by outcrops at Lake Winnipeg; the western feather-edge (basal Red River Formation, subsurface) is about 150 km west of the Alberta-Saskatchewan boundary.

Previous Work

By 1890, Silurian rocks had been recognized in outcrop in southern Manitoba and in the southern Rocky Mountains; these included strata subsequently designated as Ordovician. Reconnaissance studies later documented the presence of the stratigraphic interval in the northern Rocky Mountains, in the Yukon, in the District of Mackenzie and in northern Manitoba. Drilling led to its discovery in the subsurface of the southern plains. In the 1964 atlas (McCrossan and Glaister, 1964), Porter and Fuller presented an excellent synthesis of the subsurface development and Norford outlined the outcrop distribution in the Rocky Mountains as far north as the British Columbia-Yukon boundary. Lower Devonian rocks later were recognized in northern British Columbia. Since 1964, extensive drilling, stratigraphic testholes and sedimentological studies have added much detail to the knowledge of the subsurface development in the plains. Knowledge of the outcrop regions of the Rocky Mountains and of the Territories has grown substantially through systematic mapping and stratigraphic studies but virtually no sedimentological investigations have been attempted. East of the Rocky Mountains and south of the Mackenzie River, studies of the subsurface revealed that Middle Ordovician to Lower Devonian rocks extended only short distances into the subsurface from the outcrop regions. Recent syntheses of the stratigraphic interval include Cecile and Norford (in press), Osadetz and Haidl (1989), and Norford (1991) for the mountain regions. These, together with Norford (1964) and Porter and Fuller (1964), provide a comprehensive bibliography of the Middle Ordovician to Lower Devonian stratigraphic interval of the Western Canada Sedimentary Basin.

Geological Framework

During the early Paleozoic, a miogeocline lay next to the western margin of the North American craton. The transition between them (close to Q-Q' and to the boundary between southern Alberta and British Columbia in Fig. 9.1) was abrupt at most times and marked by slope environments that separated the shallow waters of the craton from the deep ocean. The western margin of the North American continent probably lay just beyond the Cassiar and Purcell platforms (Fig. 9.1). The North American continent straddled the equator and sediments accumulated in warm, tropical seas. The craton itself was low lying and sediments were mature in character with little argillaceous detritus, but there were some persistent highs, such as the Peace River Arch, which from time to time shed quartz sands into the marine environments.

The Ordovician to Lower Devonian rocks that are now preserved in the Western Canada Sedimentary Basin are merely remnants of very extensive sheets of sediment that were deposited over much of the craton and its westward ocean margin. Later periods of erosion removed the entire stratigraphic interval from most of the region (Figs. 9.5 - 9.12, 9.15 - 9.19), but outliers are preserved in many places. Generally, the strata consist of shallow-water carbonates and minor clastics and evaporites on the craton, bordered to the west by basinal shales and limestones (Figs. 9.1, 9.6). The change of facies occurred at a fairly abrupt slope from the platform into deeper water. Outboard of this facies change, two highs hosted carbonate platforms (Purcell Platform and Cassiar Platform, Fig. 9.1), which themselves were bordered westward by further developments of basin facies. Volcanic complexes were developed near the platform margin on at least two occasions but seem to have had very limited lateral extents.

In the central part of the craton, subsidence of the ancestral Williston Basin began in Middle Ordovician time. The axis of this gentle depression shifted several times within the Tippecanoe interval but the areas of greatest subsidence were in northwestern North Dakota (Fig. 9.2; Porter and Fuller, 1964; Kent and Christopher, this volume, Chapter 27). Depositional thickening southward mostly developed within the Winnipeg, Red River and Interlake units. Thickness variations and facies distributions within various stratigraphic units indicate that some intrabasinal structures affected depositional patterns during Tippecanoe time (Osadetz and Haidl, 1989). The area of deposition at times extended well beyond the limits of the present-day structural Williston Basin, and many of the units were depositionally continuous with those of the Bow, Kakwa, MacDonald, Mackenzie, and Hudson platforms (Fig. 9.1). In the Canadian part of the Williston Basin, the subsurface development of these strata is thickest (about 480 m) in southeastern Saskatchewan, adjacent to its maximum (more than 760 m, Gerhard and Anderson, 1988) in northwestern North Dakota. The preservation of these lower Paleozoic strata during Late Silurian and Early Devonian erosion was due to epeirogenic tectonics creating a broad depression, so that the Ordovician and Silurian rocks were preserved below the Middle Devonian rocks.

At the north margin of their area of preservation, the lower Paleozoic rocks form an erosional escarpment (Meadow Lake) that limited a Middle Devonian sea within which the lower Elk Point evaporites were deposited (Meijer Drees, 1986). At the northeast margin of the Williston Basin, erosion on the Severn Arch (a Devonian or later feature) removed Upper Ordovician and Silurian strata that apparently were continuous between the Williston Basin and the Hudson Bay Basin (Fig. 9.19). The elbow in the distribution pattern (Fig. 9.1), between the easterly trending feather-edge in the subsurface and the northerly trending Manitoba outcrops, coincides with the boundary between the Churchill and Superior provinces (Fig. 9.22) and may indicate the resurrected effect of this Precambrian structural trend.

Sea-level changes driven by eustatic and glacio-eustatic events are recorded by unconformities and depositional cycles in the Williston Basin and in the mountains. Successive transgressions that began in the Middle Ordovician resulted in inundation of most of the Canadian part of the North American craton by Late Ordovician time, when carbonate deposition seemingly extended from the Mackenzie, Kakwa and Bow platforms eastward to the Williston Basin and the Hudson Bay Basin and northward into the present Arctic Islands. Within the subsurface of the Western Canada Sedimentary Basin, the present preservation of segments of these widespread carbonates is a function of subsequent erosion and the effects of episodes of uplift of the Peace, Sweetgrass-North Battleford and Severn arches, respectively in the northwest, southwest and northeast, and of the Transcontinental Arch (not shown within Fig. 9.1) in the south.

 

 

Last modified: August 8, 2008