A regional subsurface study of 200 cores and several thousand well logs has led to an interpretation of the depositional history for the Lloydminster area of western Canada. Major geologic controls on deposition were tectonism in western North America, a series of northwest-trending ridges formed of Paleozoic carbonates found in the central plains, salt collapse within the depositional basin, and eustatic changes associated with Boreal and Gulfian seas. On the basis of genetic units, the Mannville Group has been divided into three units: lower, middle, and upper.
The lower unit is dominantly a siliceous, fine to medium-grained sandstone which is found in the lows between the northwest-trending ridges of Paleozoic carbonates. The unit is up to 60 m thick with the dominant sedimentary structures being high-angle (30°) cross-stratification.
The middle unit consists mainly of upward-coarsening, very fine to fine-grained, quartzose sheet sandstones, 6 to 10 m thick, associated with a restricted marine microflora and microfauna. A typical sequence begins with a basal bioturbated shaly siltstone and proceeds upward through wavy lenticular sandstones in silty shales, wave-rippled sandstones with mud drapes, wavy-bedded sandstones, and low-angle (0 to 10°) cross-laminated sandstones. This sequence is commonly capped by a coal or gray carbonaceous shale. Although sheetlike in appearance, the lateral continuity of the sandstones is commonly broken by shale or sand-filled channels.
The upper unit is comprised of thick (up to 35 m) lenticular channel fills of cross-bedded, lithofeldspathic or quartzose sandstones which grade laterally into interbedded deposits of current-rippled sheet sandstones, siltstones, mudstones, and coals.
The vertical sequence of environments is interpreted to be: post-Paleozoic incision followed by valley-fill deposition of an aggrading north-flowing fluvial system (lower unit), transgression followed by regressive sedimentation in wave-dominated paralic environments (middle unit), and continued regressive deposition within an areally extensive, north-flowing anastomosed fluvial network (upper unit).
Mannville deposition in the Lloydminster area was terminated by a major transgression which deposited the widespread marine shales of the Colorado Group.