ABSTRACT: Base-Level Controls on Nonmarine Deposits, Lower Cretaceous Mannville Group of Alberta

CANT, D. J., Geological Survey of Canada, Calgary, Alberta, Canada

The Mannville Group consists of a lower transgressive depositional systems tract overlain by a highstand systems tract. Within each of these, smaller scale, higher order sequences also occur.

In western Alberta, basal nonmarine conglomerates (Cadomin Formation) lie directly on an unconformity, beneath the transgressive systems tract. The alluvial fan to braided river conglomerate cannot be traced to a marine equivalent and may be interpreted as a nonequilibrium deposit, part of which was laid down before the basin began to subside.

The extensive nonmarine deposits in the remainder of the Mannville are generally fine-grained with minor sandy intervals. Most of the dominant finer-grained deposits lie directly behind transgressing shorelines, truncated at their seaward edges by ravinement surfaces under the shoreline sandstones. These transgressive nonmarine deposits consist in one case of brackish-water limestones and shales, but mainly are shales with several thick (to 3 m) coal seams. The other major type of nonmarine deposits are coarse-grained valley-fills. These can be distinguished from large contemporaneous channels (with some difficulty) because of the correlatability across them of the finer, enclosing sediments. At their seaward ends, valley-fills are estuarine as shown by their microfossil and trace fos il assemblages. Landward, many consist of up to 30 m fluvial sandstones (possibly braided), with fining-upward successions (possibly meandering) in their upper parts. These valley-fills were also deposited during base-level rises.

The grain-sizes, depositional styles, and lateral extent of most nonmarine deposits in the Mannville Group are therefore controlled largely by base-level changes.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)