Allostratigraphy and paleogeography of the Harmon and Cadotte members of the Peace River Formation (Middle Albian), west-central Alberta and adjacent British Columbia

Abstract The mudstone-dominated Harmon and sandstone-dominated Cadotte members of the Peace River Formation were deposited during the middle Albian in the southern end of a Boreal marine embayment that occupied the foredeep of the Western Canada Foreland Basin. Sandstone of the Cadotte Member represents a shoreface environment and has traditionally been interpreted as the updip equivalent of offshore Harmon mudstone. Allostratigraphic correlation, based on marine flooding surfaces and unconformities shows that the Harmon alloformation can be divided into five regionally-mappable allomembers. Similar erosion surfaces allow the Cadotte alloformation to be divided into three allomembers. The new allostratigraphy shows that the informal Harmon and Cadotte alloformations do not have an interfingering relationship and hence are not genetically related. Harmon and Cadotte allomembers have been mapped over about 100 000 km2, but show no evidence of stratal lap-out or depositional topography; it is concluded that deposition took place on a shallow marine ramp of very low relief. The vertical stacking of Harmon and Cadotte allomembers across the foredeep indicates that rates of accommodation and sediment supply remained in approximate equilibrium throughout deposition of these units. In the far southern and eastern parts of the study area, changing gamma ray log response suggests that marine deposits of both the Harmon and Cadotte alloformations grade laterally into coeval coastal plain deposits that are mapped in outcrop as Gates Formation whereas in subsurface wireline log picks, these rocks are usually included in the Upper Mannville Group. The lateral boundary between offshore mudstone and coeval nearshore muddy sandstone can be mapped within individual Harmon allomembers; the mudstone to sandstone boundary has a lobate to cuspate pattern in plan view, suggesting that, at the southern extremity of the ‘Harmon Sea’ rivers built elongate deltas in a very shallow-water, low wave-energy setting. Lenticular, sharp-based sandstone bodies 17–42 m thick hang from allomember bounding surfaces in both the Harmon and Cadotte alloformations; these sandbodies probably represent paleovalley fills. Valley incision may have been linked to relative sea-level changes that also gave rise to the marine transgressive surfaces that are traceable over the entire study area. In the east, both the Cadotte and Harmon alloformations are completely truncated by a bevelling unconformity at the base of the Paddy and the laterally-contiguous Joli Fou alloformation.