ABSTRACT

The Mannville Group of the Lloydminster area at present contains nine informal formations. From core study it appears that there exist only three, separate, genetic intervals which are referred to as the lower, middle and upper Mannville. The lower Mannville is dominantly a tabular-shaped, quartzose, fine- to coarse-grained sandstone which is found in the topographic lows between northwest-trending ridges formed of Paleozoic carbonates. The unit is over 60 m thick and the dominant sedimentary structures are high-angle (30°) cross beds. The middle Mannville consists mainly of upward-coarsening, very fine to fine grained, quartzose sheet sandstones which are generally 6 to 9 m thick and contain a restricted marine microflora and microfauna. A typical sequence begins with a basal bioturbated shaly siltstone and proceeds upward through wavy and lenticular sandstones in silty shales, wave-rippled sandstones and siltstones with fine silt drapes, wave-rippled sandstones, and low-angle (0-10°), cross-laminated sandstones. This sequence is commonly capped by a coal or grey carbonaceous shale. Although sheet-like in appearance, the lateral continuity of middle Mannville sandstones is commonly broken up by ribbon-shaped deposits of shale or sandstone. The upper Mannville is formed of thick (up to 35 m), lenticular, ribbon-shaped deposits of cross-bedded sandstone which laterally grade into interbedded deposits of current-rippled sheet sandstones, siltstones, shales and coals.

The dominant controls on Mannville Group deposition were: i) the configuration of the Paleozoic surface, ii) sea-level fluctuations, iii) salt dissolution within underlying Paleozoic strata, and iv) the Columbian Orogeny.

In the Lloydminster area the lower and middle Mannville strata are dominantly quartzose whereas the upper Mannville contains both quartzose and lithofeldspathic sandstones. To the north of the study area all rocks above the quartzose lower Mannville are lithofeldspathic. Lloydminster area sandstones were derived from sedimentary, metamorphic and igneous terrains located to the south, east and west, whereas post - lower Mannville sandstones found to the north originated from dominantly metamorphic and igneous terrains located to the west.

The depositional sequence of the Mannville Group is proposed to be: i) post-Paleozoic incision followed by valley-fill deposition by an aggrading north-flowing fluvial system (lower Mannville), ii) marine transgression followed by dominantly regressive sedimentation in wave-dominated paralic environments (middle Mannville), iii) continued regressive deposition within an extensive north-flowing fluvival network (upper Mannville). Mannville deposition in the Lloydminster area was terminated by a major transgression which deposited the widespread marine units of the Colorado Group.

Stratigraphic and structural hydrocarbon traps are present within Mannville Group strata, with the former being the more numerous. Some common stratigraphic traps are created by: i) sandstone onlap against Paleozoic ridges, ii) up-dip margins of shale-filled channels, iii) sandstone pinchouts, and iv) differential compaction over thick intra-Mannville sandstones. Structural traps are commonly found in areas that have undergone salt dissolution.