Ramp sedimentation across a middle Albian, Arctic embayment: Influence of subsidence, eustasy and sediment supply on stratal architecture and facies distribution, Lower Cretaceous, Western Canada Foreland Basin

Regional mapping of Middle Albian, shallow-marine clastic strata over ca 100 000 km(2) of the Western Canada Foreland Basin was undertaken to investigate the relationship between large-scale stratal architecture and lithology. Results suggest that, over ca 5 Myr, stratal geometry and facies were dynamically linked to tectonic activity in the adjacent Cordillera. Higher frequency modulation of accommodation is most reasonably ascribed to eustasy. The Harmon and Cadotte alloformations were deposited at the southern end of an embayment of the Arctic Ocean. The Harmon alloformation, forming the lower part of the succession, constitutes a wedge of marine mudstone that thickens westward over 400 km from <5 m near the forebulge to >150 m in the foredeep. Constituent allomembers are also wedge-shaped but lack distinct clinothems, a rollover point or downlapping geometry. Ubiquitous wave ripples indicate that the sea floor lay above storm wave base. Deposition took place on an extremely low-gradient ramp, where accommodation was limited by effective wave base. Lobate, river-dominated deltas fringed the southern margin of the basin. The largest deltas are stacked in the same area, suggesting protracted stability of the feeder river. A buried palaeo-valley on the underlying sub-Cretaceous unconformity may have influenced compaction and controlled river location for ca 3 Myr. Adjacent to the western Cordillera, a predominantly mudstone succession is interbedded with abundant storm beds of very fine-grained sandstone and siltstone that reflect supply from the adjacent orogen. Bioturbation indices in the Harmon alloformation range from zero to six which reflects the influence of stressors related to river-mouth proximity. Harmon alloformation mudstone grades abruptly upward into marine sandstone and conglomerate of the overlying Cadotte alloformation. The Cadotte is composed of three allomembers 'CA' to 'CC', that represent the deposits of prograding strand-plains 200 x 300 km in extent. Allomembers 'CA' and 'CB' are strongly sandstone-dominated, whereas allomember 'CC' contains abundant conglomerate in the west. The dominantly aggradational wedge of Harmon alloformation mudstone records flexural subsidence driven by active thickening in the adjacent orogen: the high accommodation rate trapped coarser clastic detritus close to the basin margin. In contrast, the tabular, highly progradational sandstone and conglomerate bodies of the Cadotte alloformation record a low subsidence rate, implying tectonic quiescence in the adjacent orogen. Erosional unloading of the orogen through Cadotte time steepened rivers to the extent that they delivered gravel to the shore. These observations support an 'anti-tectonic' model of gravel supply proposed previously for the United States portion of the Cretaceous foreland basin. Because Cadotte allomembers do not thicken appreciably into the foredeep, accommodation changes that controlled these transgressive-regressive successions were probably of eustatic origin.