High‐Frequency Relative Sea‐Level Oscillations in Upper Cretaceous Shelf Clastics of the Alberta Foreland Basin: Possible Evidence for a Glacio‐Eustatic Control?

The Muskiki (mid–late Coniacian) and Marshybank (late Coniacian to early Santonian) Formations vary from about 50 to 100 m thick and are present throughout much of the Alberta foreland basin south of the Peace River. Together they form a sequence comprising transgressive and highstand systems tracts (TST and HST) that span about 1.5 Ma. Sequence-bounding unconformities developed during times of sea-level fall that appear to be regionally, if not globally, recognizable. The Muskiki–Marshybank sequence consists of at least 15, regionally traceable parasequences, each of which apparently lasted about 100 000 years. In much of the TST, parasequences generally shoal upward with little evidence of intervening relative sea-level fall. Towards the top of the TST, however, chert pebble beds mantle two parasequences and chert pebbles and/or erosive-based shoreface sand bodies cap most parasequences in the HST. These features suggest deposition as a result of relative sea-level fall, perhaps of only a few metres. Various lines of evidence suggest that the parasequences are due to eustatic fluctuations rather than to changes in subsidence or sediment-supply rates. The approximate 100 000 year frequency of the eustatic cycles is suggestive of an orbitally forced glacio-eustatic mechanism. Parasequences fall into five quasi-periodic groups that may reflect either changes in subsidence rate or eustatic fluctuations over several hundred thousand years.