Anatomy of a Marine Transgression: Tectonically Controlled Sediment Partitioning and Eustatic Rise, Cenomanian Dunvegan Formation, Alberta and British Columbia

Michael J. Hay¹ and A. Guy Plint^2
1 University of Western Ontario, London, ON
² Department of Earth Sciences, University of Western Ontario, London, ON

The Dunvegan Fm. consists of ten allomembers bounded by marine transgressive surfaces in prodelta and delta-front successions, and well-developed interfluve paleosols in the coastal plain. A total of 15 cores, 25 outcrop sections, and ~1400 wireline logs provide the basis for an allostratigraphic analysis of the uppermost allomember, A. In the far NW, allomember A is entirely nonmarine. Further east, marginal marine facies include fluvio-estuarine valley fills, hydromorphic paleosols, and brackish water mudstones and coals. Marine units include bioturbated shoreface sandstones, heterolithic inner shelf deposits, and outer shelf mudstones. An isopach map for allomember A shows an overall wedge shaped geometry, thinning from >40 m in the NW to as little as 5 m in the E. At least three backstepping sequences are recognizable in the subsurface. The progressive north-westward backstepping of successive delta-front sandstones reflects a broad relative sea-level rise during deposition of this allomember. The lowest sequence only extends about 150 km from the orogen, whereas the middle sequence extends for at least 450 km. In contrast, the uppermost sequence can be traced only about 250 km from the orogen. The more wedge-shaped lower and upper sequences suggest that accommodation was generated primarily by flexural subsidence. In contrast, the sheet-like geometry of the middle sequence, which apparently extends beyond the forebulge, suggests significant eustatic control on accommodation. This interval provides a classic illustration of increasing accommodation rate whereby sediment was progressively partitioned into the rapidly-subsiding updip coastal plain, progressively starving the delta front, leading to more than 250 km of shoreline backstep.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005