Tectonic, eustatic and climatic controls on marginal‐marine sedimentation across a flexural depocentre: Paddy Member of Peace River Formation (Late Albian), Western Canada Foreland Basin

Abstract

In north‐central Alberta and adjacent British Columbia, clastic strata of the middle to late Albian Peace River and Shaftesbury formations were deposited in alluvial to shallow‐marine environments across the foredeep of the Western Canada Foreland Basin. A high‐resolution, log and core‐based allostratigraphic framework for the Paddy Member of the Peace River Formation established nine allomembers, PA to PI, bounded by flooding surfaces and apparently equivalent non‐marine surfaces. Within the estimated 2 Myr. duration of the Paddy, allomembers allow the evolving palaeogeography and changing relationship between accommodation and sedimentation rates to be analysed on time‐steps on the order of 105 years. Paddy strata fill an arcuate depocentre ca 300 km wide, across which the rocks thin eastward from 125 m to ca 5 to 10 m. The northern part of the basin is occupied by muddy, offshore marine deposits that pass abruptly southward into a linear, WSW‐ENE‐trending body of sandstone deposited in a wave‐dominated barrier‐strandplain, at least 350 km long. Extending >200 km to the south of the strandplain was a region of shallow brackish to freshwater lagoons and lakes that graded to the SW into alluvial facies. Within the lagoon region, few‐m thick, elongate and patchy sandstones represent river‐dominated deltas. In allomembers PA to PG, these sandstones are concentrated in the west and south, implying supply from the western Cordillera. In allomembers PH and PI, sandstones are mainly in the east and have a distinctive, quartz‐rich composition. They can be correlated eastward into the coeval Pelican Formation, and were sourced probably from the Canadian Shield on the opposite side of the basin. In the western foredeep, alluvial rocks comprise aggradational, unconfined floodplain deposits with ribbon sandstones, dissected, on at least nine separate levels, by palaeovalleys that are confined to the proximal foredeep. Valleys are 10 to 30 m deep, few km wide, and filled with multi‐storey channel‐bars of pebbly coarse sandstone or conglomerate. Valleys cut down from well‐developed interfluve palaeosols that record a falling and then rising water table. Alternating aggradation and degradation, and advance and retreat of the alluvial gravel front is attributed to cycles of varying rainfall intensity, rather than tectonism or eustasy. Apparently, coeval transgressive‐regressive successions in the lagoon and marine regions are attributed to few‐m scale eustatic changes. On the NE margin of the basin, tidal sandstone fills a northward‐opening estuary cut on the basal Paddy unconformity. This sandstone contains the first well‐documented specimens of Gnesioceramus comancheanus (Cragin), proving contemporaneity with at least part of the marine Joli Fou Formation to the east. Paddy allomembers change shape upward from short blunt wedges, through more acutely tapered wedges, to sheets. This change reflects initially rapid flexural subsidence, attributed to active thickening of the adjacent orogenic wedge. A waning rate of def...

The impact of Aptian glacio‐eustasy on the stratigraphic architecture of the Athabasca Oil Sands, Alberta, Canada

“…() and Plint et al . () have speculated on their presence in younger North American Cretaceous strata.…”

“…18). Although glacio-eustatically controlled depositional cycles have not previously been identified within the Aptian strata of the Western Canadian Foreland Basin, Plint (1991), Plint & Kreitner (2007), Maynard et al, (2010), Zhu et al (2012) and Plint et al (2017) have speculated on their presence in younger North American Cretaceous strata.…”

Trans-Atlantic correlation of Late Cretaceous high-frequency sea-level cycles

“…This observation may provide independent evidence for a linkage between sea-level fall and higher river discharge that triggered a short-lived advance of the gravel front in rivers, allowing pebbles to reach the lowstand shoreline. Eustatic fall of the estimated c.10 -20 m would, alone, be inadequate to steepen river gradients across a coastal plain at least 100 km wide, sufficient to initiate gravel supply to the coast (Plint et al, 2018).…”

High‐frequency sequences within a retrogradational deltaic succession: Upper Cenomanian Dunvegan Formation, Western Canada Foreland Basin

“…The restriction of palaeovalleys to the top of sequences B3 and A3 suggests, however, that sea‐level falls of greater magnitude, or greater duration, may have terminated these two sequences. Alternatively, valley incision may have been due to a change in the ratio of discharge to sediment load of rivers, as predicted by numerical models (Armitage et al , 2018; Zhang et al , 2019), recognized in Quaternary river systems (Gibling et al , 2011; Blum et al , 2013), and inferred for Cretaceous valley systems in Dunvegan allomembers H to E (Plint and Wadsworth, 2003), and in Upper Albian coastal plain strata of the Paddy alloformation (Plint et al , 2018).…”