Inland Seas of Alberta: A Geological History

Throughout Earth's history, the landscape we now know as Alberta has been repeatedly submerged beneath vast shallow seas. These ancient marine environments have left behind rich geological records, influenced Alberta's topography, and created many of its valuable natural resources. This report examines the major inland seas that once covered Alberta, their characteristics, and their lasting impact on the province.

The Evolution of Alberta's Ancient Seas

Alberta's geological history includes several major marine transgressions (advances of sea water over land) and regressions (retreats), creating a complex series of sedimentary deposits. These seas were part of larger geological sequences that affected much of North America over hundreds of millions of years.

Paleozoic Seas: The Early Foundations

Sauk and Tippecanoe Sequences

North America's first major cratonic sequence, the Sauk sequence, occurred from the late Proterozoic through the early Ordovician periods. During this time, a marine transgression spread across the continent, eventually covering most of North America with a shallow sea^[1]. This was followed by the Tippecanoe sequence (Middle Ordovician to Early Devonian), which may have created the deepest seas of the Paleozoic era. At one point during the Silurian period, the Taconic highlands in the east were the only part of North America that remained above water^[2][3].

The Elk Point Sea (Middle Devonian, ~375-400 million years ago)

One of Alberta's most significant ancient marine environments was the Elk Point Sea, which existed during the Middle Devonian period approximately 375-400 million years ago. This shallow inland sea extended southeastward from what is now the Northwest Territories to the Williston Basin, covering parts of North Dakota, southwestern Manitoba, southeastern Saskatchewan, and much of Alberta^[4].

The Elk Point Sea was characterized by its restricted circulation from the open ocean, as organic reefs from Great Slave Lake to the Yukon limited water exchange. Combined with the hot, arid climate of the time, these conditions created an environment perfect for the formation of massive evaporite deposits^[4]. Up to 215 meters of evaporites were precipitated, including halite (sodium chloride), anhydrite (calcium sulfate), and potash (primarily potassium chloride).

The areal extent of the Elk Point Basin within western Canada is estimated at 1.2 million square kilometers, with at least 500,000 square kilometers containing halite-bearing strata. In Alberta alone, the halite volume is calculated at an impressive 33,200 cubic kilometers^[5]. These vast salt deposits have significant economic importance today, with over 150 active salt caverns now used for hydrocarbon storage and waste disposal in Alberta^[5].

Ocean waters again flooded present-day Alberta during the late Devonian period (which ended about 360 million years ago), resulting in expansive coral reef growth across the province. These structures acted as natural reservoirs, absorbing and trapping oil created by decayed plant and animal remains, forming many of Alberta's important oil deposits that are exploited today^[6].

Mesozoic Seas: The Age of Dinosaurs

The Sundance Sea (Middle to Late Jurassic)

During the Jurassic Period, Alberta was partially covered by the Sundance Sea, an epeiric sea extending from what is now the Arctic Ocean through western Canada into the central western United States. This was not a single, continuous marine event but rather a series of five successive marine transgressions (sea level rises), each separated by periods of erosion^[7].

The Sundance Sea was remarkably rich in marine life. Fossils from this sea include Gryphaea (an extinct oyster), shark teeth, belemnites (squid-like animals), ammonites, crinoids, and bivalves. The sea was also home to large marine reptiles, including the 20-foot ichthyosaur Ophthalmosaurus, the plesiosaur Pantosaurus, and the massive pliosaur Megalneusaurus, which could reach lengths of 25 feet^[7].

The sea eventually receded when highlands to the west began to rise, marking another major transition in Alberta's geological history.

The Western Interior Seaway (Late Cretaceous to Early Paleocene, ~100-66 million years ago)

Perhaps the most famous of Alberta's ancient seas was the Western Interior Seaway (also known as the Cretaceous Seaway, Niobraran Sea, or North American Inland Sea), which existed from about 100 to 66 million years ago. This enormous inland sea effectively split the North American continent into two separate landmasses: Laramidia to the west and Appalachia to the east^[8][9][10].

At its maximum extent, the Western Interior Seaway was truly massive, stretching 5,000 kilometers in length and connecting the Arctic Ocean with the Gulf of Mexico. It measured about 970 kilometers (600 miles) in width and reached depths of approximately 760 meters (2,500 feet)^[10]. Despite its size, it was relatively shallow compared to modern oceans, with constantly shifting shorelines and fluctuating dimensions throughout its existence^[8].

The Western Interior Seaway formed through a combination of tectonic processes. Its origin can be traced to the early phases of the Laramide orogeny (mountain-building event) that eventually formed the Rocky Mountains. Geologists believe the seaway resulted from a downwarping of the continental crust ahead of the growing mountain chain^[10]. The seaway began with the Mowry Sea, formed when an arm of the Arctic Ocean transgressed southward over western North America, eventually merging with a northern extension of the Gulf of Mexico^[10].

The warm, shallow waters of the Western Interior Seaway hosted an incredibly diverse ecosystem. Marine life included various types of algae, microscopic radiolarians, invertebrates such as ammonites, belemnites, squid, crabs, snails, clams, and shrimp. Vertebrates included numerous species of fish, sharks, rays, skates, seabirds, turtles, crocodiles, and extinct marine reptiles like mosasaurs and plesiosaurs^[9].

One remarkable fossil found in Alberta from this period is the extremely well-preserved nodosaur Borealopelta markmitchelli, discovered in rock that was once the bottom of this expansive sea. This specimen, housed in the Royal Tyrrell Museum, represents one of the most incredible fossils of prehistoric life and provides tangible evidence of Alberta's marine past^[8].

Geological Impact and Resource Legacy

The repeated advance and retreat of these inland seas over millions of years left Alberta with a wealth of geological resources. Many of the natural resources mined in Alberta today, including coal, petroleum products, and bentonite clay, are direct results of the environments created by these ancient seas^[9].

The black shales of the Exshaw Formation, deposited in a marine setting during the Hangenberg event (an oceanic anoxic event associated with the Late Devonian extinction), are particularly significant. Rich in organic matter, these shales represent one of the most important petroleum source rocks in the Western Canada Sedimentary Basin^[11].

The evaporite deposits from the Elk Point Sea have also proven valuable. Saskatchewan's potash mines, which account for approximately one-third of the world's commercial production, extract potassium chloride from zones near the top of these ancient evaporite units^[4].

Conclusion

Alberta's landscape has been dramatically transformed multiple times throughout Earth's history by vast inland seas. From the Paleozoic Elk Point Sea to the Mesozoic Western Interior Seaway, these ancient marine environments have shaped the province's geology, created its natural resources, and left behind fascinating fossil evidence of prehistoric life.

The cyclical nature of these marine transgressions and regressions demonstrates the dynamic nature of Earth's systems over geological time. What we see today as stable land was, for much of Earth's history, the bottom of shallow, warm, biologically rich seas. This geological history not only helps us understand Alberta's past but also explains the province's present wealth of natural resources that have been so important to its economic development.

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• https://en.wikipedia.org/wiki/Sauk_sequence 
• https://en.wikipedia.org/wiki/Tippecanoe_sequence 
• https://ags.aer.ca/publications/atlas-western-canada-sedimentary-basin/chapters/chapter-9-middle-ordovician-lower 
• https://esask.uregina.ca/entry/elk_point_sea.html   
• https://geoconvention.com/wp-content/uploads/abstracts/2021/67413-salty-stratigraphy_-salt-caverns-in-the-elk-point.pdf  
• http://www.ramp-alberta.org/river/geography/geological+prehistory/paleozoic.aspx 
• https://en.wikipedia.org/wiki/Sundance_Sea  
• https://www.cbc.ca/natureofthings/features/100-million-years-ago-alberta-was-a-giant-sea-surrounded-by-tropical-forest   
• https://www.thecanadianencyclopedia.ca/en/article/western-interior-seaway   
• https://en.wikipedia.org/wiki/Western_Interior_Seaway    
• https://en.wikipedia.org/wiki/Exshaw_Formation