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

AbstractIn 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 deformation permitted wider dispersal of sediment across the basin, driving broad isostatic subsidence beneath increasingly sheet‐like rock bodies. A major hiatal surface, VE3, records non‐deposition or subtle erosion attributed to erosional unloading and uplift of the adjacent orogen. A subsequent marine transgression is attributed to renewed thickening of the tectonic wedge that triggered deposition of marine mudstone that thickens westward from 0 to >110 m over 300 km. A postulated Milankovitch‐band climatic control on both local gravel supply (via fluctuating rainfall), and shoreline movement (via ?Antarctic glacio‐eustasy or groundwater storage), might account for cycles of alternating incision and aggradation in the alluvial realm. The same mechanism may also explain why shallow‐marine units such as the Cretaceous Viking and Cardium formations contain abundant conglomerate in lowstand shoreface deposits (higher river discharge), yet have highstand shorelines dominated by sandstone (lower river discharge).