Allostratigraphy of the Lower Colorado Group (Cretaceous) in south-west Alberta
Abstract
Clastic, upper Albian-lower Cenomanian strata were deposited in a low-accommodation backbulge depozone of the Western Canada Foreland Basin in SW Alberta. These strata are lithologically very heterogeneous and encompass a spectrum of depositional environments along an alluvial to offshore transect. These rocks are assigned, in subsurface, to the Lower Colorado Group, and in outcrop to the upper Blairmore Group. Lithological heterogeneity, as a result of rapid lateral facies changes, resulted in diverse nomenclature that obscured genetic relationships between time-equivalent strata. The present study integrates wireline log, core, and outcrop data to establish a high-resolution allostratigraphic framework which allowed recognition of the Basal Colorado, Joli Fou, Viking and Westgate alloformations, in ascending order. Each alloformation comprises several allomembers and parasequences. The Basal Colorado alloformation is a NE thinning wedge that progressively onlaps lower Albian Mannville Group strata. Deposition took place in a brackish-water embayment open to the south. Basal Colorado strata correlate with the Lynx Creek Member of the Mill Creek Formation in outcrop. The Joli Fou and Viking alloformations have a broadly sheet-like geometry. Both alloformations comprise three allomembers, composed of a complex array of river-dominated and mixed-influence deltaic successions. The location of successive deltas was strongly influenced by differential compaction of underlying deltas. Sandstone-rich Joli Fou rocks pass northward into offshore mudstone. To the south and west, marine Joli Fou and Viking deposits grade laterally into alluvial facies. Only the upper part of the Westgate alloformation extends into SW Alberta. Tectonic tilting is indicated by the lowermost (alluvial) and uppermost (marine) Westgate parasequences, which are SW and NE thickening wedges, respectively. Paleo-valleys, typically ~20 m deep, are incised into the top surface of most allomembers. Different examples of valley incision and fill can be attributed to both sea-level changes, and also to climatically-controlled changes in the sediment load to discharge ratio. Two pulses of thrust-induced loading and a long term (>1 myr) eustatic cycle provided the overall control on stratal architecture, whereas climate-controlled cycles in the ~400 and 100 kyr band exerted an important control on the development of allomembers and parasequences. Eustatic changes are estimated to have been ~20 – 30 m.