An allostratigraphic framework for the Late Albian to Early Cenomanian upper Fort St. John Group across the foredeep of the Western Canada Foreland Basin, NE British Columbia

Abstract

Abstract The Upper Albian to Lower Cenomanian Hasler and Cruiser shales and intervening Goodrich Sandstone are up to about 800 m thick and constitute most of the upper Fort St. John Group in the foredeep of NE British Columbia. These rocks exhibit pronounced lateral changes in lithology and thickness from west to east, making their lithostratigraphic boundaries highly diachronous. An allostratigraphic approach to correlation, based primarily on discontinuity surfaces represented in wireline well logs, allows subdivision of these broad lithostratigraphic units, and permits correlation of genetic stratal packages across facies transitions, and from subsurface to outcrop. Upper Fort St. John strata spanning the foredeep in British Columbia are shown to be correlative with Viking allomember VD, and with the overlying Westgate and Fish Scales alloformations, previously defined in Alberta on the basis of outcrop, core and wireline log data. Bounding surfaces have also been correlated northward to connect with successions described from the Sikanni and Liard river areas of British Columbia. In the British Columbia study area, pyritic, largely unbioturbated mudstone of allomember VD thickens from 20 to 200 m, indicating syn-depositional flexural subsidence, greatest in the north-western part of the study area. The Westgate alloformation is also dominated by weakly bioturbated mudstone, but includes more common, thin, fine-grained sandstone beds, many with wave- and combined-flow ripples, organized in parasequences up to approximately 20 m thick. Westgate allomembers WA through WD take on a progressively more tabular geometry upward, reflecting diminishing flexural subsidence with time. Allomember WD includes shoreface/delta front sandstones (‘Goodrich Formation’) that extend eastward from the Foothills outcrop for up to 70 km. The Fish Scales alloformation, the base of which is defined by surface FE1, marks an abrupt introduction of sand across a muddy sea floor in response to sea-level fall. The upper boundary of the Fish Scales is the condensed section/downlap surface ‘Fish Scales Upper’ (FSU) beneath the Dunvegan alloformation. The Fish Scales alloformation is a pronounced wedge, thickening westward from approximately 100 to approximately 400 m. It consists mainly of pyritic, unbioturbated mudstone to claystone with a variable content of fish debris, indicative of dysaerobic to anaerobic bottom-water conditions. Ammonites do, however, indicate oxic surface waters. The Fish Scales alloformation comprises allomembers FA and FB, separated by an erosion surface termed the Base Fish Scales Marker (BFSM) which, throughout the Peace River region, has a veneer of chert pebbles that mark a major sea-level lowstand; pebbles were subsequently reworked by marine transgression. A 15 m thick conglomerate-filled channel (or paleovalley?), exposed in Hasler Creek, directly underlies the BFSM surface and is interpreted to be the deposit of a large river that supplied sediment to the lowstand shoreline. In the far west, close to the Foothills, the Fish Scales alloformation becomes sandy and bioturbated, and includes stacked ‘Goodrich’ shoreface sandstone bodies that prograded a few tens of km eastward. Mapped to the SE, allomember FA pinches out close to the Alberta-British Columbia border, whereas allomember FB persists as a thin and highly radioactive phosphatic mudstone, the base of which comprises the merged FE1 and BFSM erosion surfaces. Despite up to about 800 m of subsidence, sedimentary structures throughout the succession show that the sea floor, which formed a homoclinal ramp, remained above storm wave base, attesting to a very high sedimentation rate and effective offshore dispersal of sediment, primarily by storm-driven combined flows. Bivalves of the genus Posidonioceramus have recently been proposed as a marker for the base of the Cenomanian Stage. These fossils are found in the study area as low as the base of Viking allomember VD, which implies that the Albian-Cenomanian boundary may lie at a level significantly lower than the long-accepted ‘Base of Fish Scales’ erosion surface.