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

Abstract

After prograding for several hundred kilometres during Middle Cenomanian time, the Dunvegan delta complex in north-west Alberta and adjacent British Columbia experienced stepwise transgression, commencing at about the Middle to Late Cenomanian boundary. Progressive drowning of the delta complex is recorded by Dunvegan allomembers B and A, each comprised of three simple depositional sequences, bounded by composite subaerial unconformity/flooding surfaces. Each sequence represents an array of deltaic depositional environments. Delta-front sandstones preserve little evidence, such as hummocky cross-stratification, for powerful storm wave action, although wave and combined-flow ripples are common. Delta-front sandstone bodies tend to be smaller and lobate in the lower part of the studied interval, and larger and more linear near the top. This suggests increasingly effective wave-driven redistribution of sand as more open-marine conditions were gradually established. The top surfaces of allomembers B and A are locally incised by sandstone-filled palaeovalleys up to 19 m deep; river incision may have been a response to relative sea-level fall and/or a change in the ratio of discharge to sediment load. Overall, the shoreline described a broad arc, open to the south east, with the sense of shoreline migration north-west to south-east. For each sequence, the shoreline migrated an average of 80 km between transgressive and regressive limits. The transgressive limit shows a progressive landward offset of about 15 km per sequence, culminating in complete drowning of the delta system above sequence A3. Isopach maps show that syn-depositional tectonic subsidence rotated the basin down to the south-west; palaeogeographic maps show, however, that the sea floor sloped to the south-east, implying that sediment redistribution effectively filled all tectonically generated accommodation and maintained a south-east inclined depositional surface. Transgressions and regressions across this surface were therefore driven primarily by eustasy rather than pulses of tectonic subsidence. Simple calculations based on inferred alluvial gradients of 10–20 cm/km suggest that eustatic excursions of ca 8–16 m would have been sufficient to generate sequence thicknesses on the order of 10 m. Limited geochronologic and biostratigraphic control suggests that the six simple sequences that form Dunvegan allomembers B and A each represent an average of about 41 kyr, suggesting that the orbital obliquity cycle was the primary control on high-frequency sea-level cycles.