Allostratigraphy of the Cretaceous, Lower Colorado Group in SE Alberta: Tectonic and eustatic controls on depositional cyclicity
Abstract
The Lower Colorado Group is a Cretaceous, shallow marine, clastic succession, 150-200 m thick, deposited in the Western Canada Foreland Basin during the Late Albian and Early Cenomanian (c. 105-100 Ma). High-resolution allostratigraphy based on marine flooding surfaces allowed recognition of four alloformations: Basal Colorado, Joli Fou, Viking, and Westgate, in ascending order. Alloformations can be similarly divided into allomembers and sequences.
The Basal Colorado alloformation, comprising five allomembers, thickens southward and records active thrust loading to the south. The overlying Joli Fou and Viking alloformations are broadly sheet-like and record tectonic quiescence, and even uplift of the Cordillera, as suggested by the westward thinning of the Viking alloformation. At this time, depocentres were laterally offset from one another, and progressively shift seaward, to the NE, as a result of very limited accommodation and progressive filling of up-dip accommodation.
A brief pulse of tectonic loading is inferred from Westgate sequence WCa that is of alluvial facies, and thickens to the SW. Sheet-like, sandy deltaic facies of Westgate sequences WCb to WCe prograde to the NE and suggest tectonic quiescence. The overlying, mudstone-dominated sequences WCf to WCj onlap westward but thicken to the SE. This reflects eustatic rise and subsidence of the intracratonic Williston Basin.
Abrupt thickness and lateral facies changes in parts of the Joli Fou, Viking and Westgate alloformations define linear belts that suggest syn-depositional fault movement that localized the position of deltas. Faults may be related to deep-seated structures in the Precambrian basement, and may have been active at times of low in-plane stress.
The Basal Colorado plus Joli Fou record a long-term (> 1 Myr) tectono-eustatic sea-level rise when the basin was flooded from both north and south, whereas the Viking alloformation records eustatic fall and regression. The Westgate records a second long-term eustatic rise, with transgression from the north. Higher-frequency sea-level cycles of ?10-20 m are superimposed on these long-term cycles, with allomembers and their internal sequences apparently spanning c. 500 and c. 100 kyr, respectively, possibly reflecting long and short orbital eccentricity cycles that modulated climate, driving sea-level change through aquifer eustasy and/or glacio-eustasy.