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Doctor of Philosophy




Dr. Guy Plint


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.

Summary for Lay Audience

Sedimentary strata of Cretaceous age (~105 to 100 million years ago) currently underlie much of Alberta. These sediments were deposited in a shallow sea that flooded the North America and occessionally extended from the Polar Ocean to the Gulf of Mexico. The sea was able to flood the continent because the rising Rocky Mountains exerted a load on the crust, forcing it to subside to form what is known as a foreland basin. About 105 million years ago, the continuing break up of the supercontinent Pangea resulted in about 100-200 m of global sea-level rise that helped to flood the Western Canada Foreland Basin. This long narrow seaway existed for tens of millions of years. Rivers draining the Rocky Mountains to the west deposited sand and mud on river floodplains and along the coast. In the study area (southeast Alberta), these sediments accumulated in deltas, and were swept offshore by storms. Sea-level repeatedly rose and fell. During rise, muddy marine sediments flooded back across the low-lying coastal plain, whereas during sea-level fall, sandy deltas built out into shallow water. These cycles of rise and fall are recorded by packages of sediment that repeatedly grade up from deeper-water mudstone to shallow-water sandstone. Detailed correlation of these sandier-upward packages suggests that sea-level changes took place on several time scales, with million-year cycles punctuated by shorter, c. 500 and c. 100 thousand-year cycles. These depositional cycles probably formed because cyclical changes in the Earth’s orbit around the sun caused changes in climate. Traditionally, it is considered that periods of global cooling cause ice to accumulate in polar regions, lowering sea-level, whereas warming melts the ice and sea-level rises. However, current evidence suggests that the Cretaceous was a very warm ‘greenhouse’ climate with little or no polar ice. An alternative hypothesis proposes that ocean water was stored in terrestrial aquifers during periods of wetter climate (lowering sea-level); water was slowly released back to the ocean during drier climate phases (raising sea-level). It is possible that both mechanisms contributed to the sea-level cycles preserved in the Cretaceous rock record of SE Alberta.

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Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

Available for download on Monday, February 09, 2026

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