Additional Contributor:
D.G. Smith - Canadian Hunter Exploration Ltd., Calgary
Mannville Group and equivalent strata comprise the oldest Cretaceous rocks over most of the Western Canada Sedimentary Basin (Figs. 19.1, 19.2) and represent a major episode of subsidence and sedimentation following a long period of uplift, exposure and erosion of older strata (Fig. 19.3). Throughout the text all strata within the generally unconformity-bounded interval are referred to as the Mannville. Precise age determinations are difficult to make because most fossiliferous marine strata are restricted to the northern part of the basin, and index fossils are rare in the continental to brackish sections farther south (Fig. 19.2). Mannville strata blanket the entire basin, ranging from less than 40 m thick in some areas of the plains to more than 700 m thick in the Rocky Mountain Foothills (Fig. 19.4). The section is erosionally truncated along its eastern and northeastern limits. Mannville strata are not mapped as a distinct unit in northwestern Alberta and northeastern British Columbia, where they grade into marine shales that cannot be readily distinguished from younger and older marine strata (Fig. 19.4).
Enormous reserves of coal, natural gas, and conventional and heavy oils are found in the Mannville (Smith et al., this volume, Chapter 33; Hay, this volume, Chapter 32). Coals deposited on extensive low-lying coastal plains are preserved primarily in the present-day foothills and adjacent western plains. Oil and gas are trapped in numerous fluvial and valley-fill reservoir sandstones in the south, and in more regionally extensive shoreline sandstone complexes throughout northern and central regions. The latter category includes the giant heavy oil and oil sands accumulations of Alberta and Saskatchewan (Fig. 19.4).
Geological control for the Mannville is extensive, including abundant petroleum borehole data, outcrop and open pit exposures in the northeastern Alberta oil sands area, and outcrop and coal mine exposures in the Rocky Mountain Foothills. Outcrop exposures are poor along most of the northeastern edge, however, and petroleum borehole data are insufficient for detailed mapping of the interval in the Liard Basin in the extreme northwest.
Because Mannville strata are extremely widespread and heterogeneous, few comprehensive syntheses of the entire group exist. Rudkin (1964) reviewed the Lower Cretaceous of the entire basin, but more recent overview papers have been more geographically limited (e.g., Jackson, 1984).
Some excellent original works of regional scope do exist, however. Stott (1968, 1973, 1982) examined Mannville strata in foothills outcrops and the adjacent subsurface throughout northwestern Alberta, northeastern British Columbia, and the southern Northwest Territories. Glaister (1959; southern plains), Williams (1963; central plains), and Mellon (1967; Rocky Mountain Foothills and plains) described and interpreted Mannville deposits over large areas of Alberta. Subsequent regional work in Alberta has focussed on more restricted stratigraphic intervals and/or areas: McLean (1977, 1982) on parts of the Mannville section in the foothills; Smith et al. (1984) on the "Deep Basin", adjacent to the northernmost foothills of Alberta; Macdonald et al. (1988) on the central foothills; McLean and Wall (1981) on deposits of the mid-Mannville Moosebar Sea across Alberta, and Rosenthal (1988) on related strata of west-central Alberta; Hayes (1986) on the lower Mannville of the southern Alberta plains; Gross (1980) and Putnam (1982) on the east-central plains; and Harrison et al. (1981), Flach (1984) and Keith et al. (1988) on the oil sands of the northeastern Alberta plains.
Christopher (1974) described the Mannville Group of the southwestern part of Saskatchewan, and later synthesized the Mannville depositional history of the entire province (Christopher, 1984). Little has been written on the poorly exposed Mannville section of Manitoba, which was reviewed by McNeil (1984).
Mannville strata make up the second major Cordilleran-derived clastic wedge of the foreland basin, and record the first basin-wide sedimentation (Smith, this volume, Chapter 17). During the Early Cretaceous, accretion of allochthonous terranes in the western Cordillera compressed older miogeoclinal rocks and thrust them onto the continental margin. Stacked thrust sheets loaded the craton edge and caused it to subside, providing accommodation in the foredeep for the huge volume of sediments shed from the upthrusted sheets (Smith, this volume, Chapter 17). Mannville deposition was initiated as these sediments were introduced to the foreland basin, and as base level began to rise in advance of an Early Albian transgression of the Boreal Sea.
Within the foreland basin, several tectonic elements influenced Mannville sedimentation (Fig. 19.1). Three elements - the foredeep, Peace River Arch and Liard Basin - were regional depocentres. Mannville strata thicken abruptly westward from an eastern hingeline toward the foredeep. The Peace River Arch is a block-faulted basement feature that was high during Devonian time, but was a marked low during the Early Cretaceous (O'Connell, this volume, Chapter 28). The thickest Mannville section is preserved where the Peace River Arch and the foredeep converge in northeastern British Columbia (Fig. 19.4). A thick Mannville succession is also found in the fault-bounded Liard Basin, but this area has not been extensively drilled and is documented only in reconnaissance studies (Leckie et al., 1991).
In contrast, the Williston Basin did not subside actively during Mannville deposition. To the west of Williston Basin, the Sweetgrass Arch, a basement feature similar to the Peace River Arch, was uplifted during the earliest Cretaceous and was part of a positive area that was not covered until late in Mannville deposition. The northeastern edge of the Williston Basin was defined by the Punnichy Arch, which formed as the Devonian Prairie Evaporite dissolved in flanking areas to the south and north (Christopher, 1984). Mannville strata thicken north of the Punnichy Arch, where salt solution provided accommodation and abundant clastics were supplied from the Precambrian Shield (Jackson, 1984; Fig. 19.4).
Mannville deposition took place over a profound unconformity surface that truncates strata ranging from lowermost Cretaceous in the foothills to lower Paleozoic at the eastern margin of the basin (Fig. 19.3). Huge valley systems that cut into the unconformity surface influenced sedimentation patterns of the entire Mannville, as discussed below. Although it has been suggested that relief on the unconformity was controlled by tectonic factors (e.g., Christopher, 1984), Cant and Stockmal (1989) considered that differential erosion of dipping Paleozoic carbonate units also may have influenced valley and highland formation across the central plains.
Last modified: August 15, 2008
