The Evolution of a Cenomanian Delta Complex in the Western Canada Foreland Basin: Paleogeographic and Stratigraphic Responses to Tectonic and Eustatic Forcing

Guy Plint
University of Western Ontario, London, Ontario, Canada

The Dunvegan Formation is of deltaic origin and records > 400 km of SE-directed deltaic progradation along the axis of the foredeep. Successive delta complexes are separated by regional transgressive mudstones that provide a means of dividing the succession, which is up to ~250 m thick, into genetic allomembers, bounded by transgressive surfaces. Allomembers can also be interpreted as depositional sequences, within which prodelta, delta front, and coastal plain environments can be recognized.

The delta plain was traversed by extensive paleovalleys that have been mapped using 4800 well logs. Paleovalley patterns change from rectilinear to dendritic over time. This change may be attributable to variable syn-depositional activity on faults extending up from Paleozoic strata. Paleovalleys are separated by extensive interfluve paleosols (subaerial unconformities) that are recognizable on the basis of distinctive pedogenic textures that probably represent tens of k.y. of non-deposition. Depositional sequences are interpreted to be the result of eustatic changes on the order to 10-20 m on time-scales of < 200 k.y. Geometric considerations suggest that depositional sequences cannot be attributed to changes in pattern or rate of tectonic subsidence.

Isopach maps show that, in the NW, Dunvegan deltas were initially deposited in an area of rapid subsidence, in water perhaps as much as 100 m deep in which tall muddy clinoforms were deposited. Clinoforms lap out ~80 km from the delta front, suggesting that offshore dispersal of mud was relatively inefficient. The deltas gradually prograded onto an area experiencing a lower subsidence rate where stratal surfaces are effectively parallel over > 400 km. A low subsidence rate allowed the sea floor to aggrade to < ~40 m depth, above which storm-driven wave re-suspension resulted in efficient off- and along-shore dispersal of mud.

The lateral change from steep, tall clinoforms, through less steep, less tall clinoforms, to no clinoforms over a 950 km dip transect suggests that offshore mud dispersal was strongly controlled by effective wave base, which appears to have lain at about 40 m. Clinoforms, which are rare in the Canadian portion of the basin, seem to form only when accommodation > supply, resulting in relatively deep water and consequent inefficient mud dispersal. The scarcity of clinoforms in >shelf= mudstone units implies that for most of the time, supply > accommodation, wave-driven dispersal was efficient, and water depth rarely exceeded ~40 m, even at distances of >300 km offshore.

AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010