Allostratigraphy of the Peace River Formation (Albian) in north-western Alberta and adjacent British Columbia

Abstract

Abstract The middle to upper Albian Peace River Formation was deposited at the southern end of the ‘Hulcross Sea’, an embayment of the Boreal Ocean that occupied the foredeep of the Western Canada foreland basin. The Peace River Formation can be allostratigraphically divided by numerous, regionally-mappable erosion surfaces, recognizable both in wireline logs, core, and outcrop. We propose recognition of the informal Harmon, Cadotte and Paddy alloformations, bounded by four regionally-mappable marine transgressive surfaces, HE0, CE0, PE0 and VE3 in ascending order. The Harmon alloformation consists predominantly of marine mudstone, and is divided into five allomembers bounded by marine flooding surfaces and/or thick bentonites. The Harmon forms a wedge thinning from >145 m in the SW to 5 m in the NE over approximately 300 km. There is no geometric evidence of a sigmoidal geometry, or that allomembers downlap onto a single, distinct surface. Deposition took place on a very low-gradient and low-relief marine ramp, strongly influenced by storms. The Cadotte alloformation consists of well-sorted swaley cross-stratified sandstone and was deposited on a wave-dominated strandplain shoreface. Ravinement surfaces divide the Cadotte into three, sheet-like allomembers, each typically about 10 m thick. Allomembers CA and CB are mainly fine-grained sandstone whereas allomember CC is dominantly medium-grained sandstone with abundant pebble conglomerate. Towards the north, sandstone grades laterally into heterolithic facies indicative of an offshore setting. The Paddy alloformation lies unconformably on the Cadotte, and comprises an array of facies, with alluvial and lagoonal deposits predominant in the south. Further north, sandstone indicative of a wave-influenced strandplain shoreline forms a belt trending WSW-ENE across the middle of the area; heterolithic offshore marine deposits lie to the north. The Paddy alloformation comprises nine allomembers, PA to PI, that onlap progressively onto the basal unconformity PE0 from SW to NE. The pronounced wedge shape and muddy lithology of the Harmon implies rapid flexural subsidence and largely vertical aggradation whereas the sheet shape of the Cadotte suggests a lower rate of subsidence when high-frequency eustatic change may have driven rapid regressions and transgressions. The lower Paddy allomembers PA to PF are strongly wedge-shaped, suggestive of renewed tectonically-driven subsidence whereas upper allomembers PG to PI are more sheet-like, suggesting that a tectonic load exerted a less important influence and instead, more uniform regional isostatic subsidence took place, driven by the load of the eastward-encroaching sediment body. Sandstone-rich Paddy strata are abruptly overlain, at the VE3 unconformity, by a blanket of dark marine mudstone that forms the basal part of the lithostratigraphic Shaftesbury Formation; the mudstone between the VE3 and VE4 surfaces is of upper Viking age, and has a pronounced wedge geometry which implies a major new pulse of flexural subsidence in the west.