Sequence Stratigraphic Analysis of Individual Depositional Successions: Effects of Marine/Nonmarine Sediment Partitioning and Longitudinal Sediment Transport, Mannville Group, Alberta Foreland Basin, Canada

Abstract

In the Falher Member of the Mannville Group (Aptian-Albian) of western Canada, two shoreline successions contain the reservoir conglomerates for the giant Elmworth gas field. The Falher B succession has a basal sheetlike shoreface unit of hummocky cross-stratified sandstone that thins seaward and terminates about 30 km north (seaward) of the landward limit of the transgression. Another 25 km farther basinward, the succession shows a 20-30-m-thick sandstone, unattached to the prograding shoreface, and an overlying coarsening-upward shoreface succession with thin muds and coals, interpreted as back-barrier deposits. These basinward facies are the results of a relative sea level fall and the early stage of the subsequent rise. In the upper (Falher A) succession, immediately andward (south) of the barriers, fluvial valleys were incised into nonmarine mudstones and coals during the base-level fall. As relative sea level subsequently rose, in nonmarine areas the valleys were filled by estuarine and fluvial sands, then a widespread sheet of fine-grained nonmarine sediment was deposited. At the same time, the shoreline migrated back across the shelf. As it reached the original shorezone (structurally controlled), reworking of underlying deposits successively generated three gravelly barrier islands superimposed on the sandy shoreface succession. The conglomeratic reservoirs all rest above the unconformities, in the transgressive depositional system. Because this sequence is essentially a set of linked facies and not a composite of stacked individual facies successions, it is affected by sediment partitioning between facies. During relative sea level rise, little marine sedimentation occurred because sediment was trapped mainly in nonmarine areas. Conversely, during sea level fall, most deposition occurred in marine areas because of the absence of nonmarine accommodation. Westward (alongshore) toward the thrust belt, no falling or lowstand sea level succession developed. Instead, a wide regressive shoreface sandstone with a transgressive cap occurs. Subsidence rates were higher in this area, and relative sea level appears always to have risen, but at varying rates. The surface under the transgressive facies changes from a type 1 unconformity in eastern, lower subsidence areas to a type 2 unconformity in western, higher subsidence areas. Any two-dimensional sequence stratigraphic model, therefore, is inadequate to describe the lateral variation of the sequence and distribution of shoreface sandstones, because the subsidence gradient was not parallel to the direction of shoreface progradation.