Discover Canada and Explore the Fossils and Geodiversity of the Manitoba Escarpment
- Wayne Munday
- Oct 9
- 5 min read
Sip back and discover Canada and explore the fossils and geodiversity of the Manitoba Escarpment or Pembina Escarpment rising above the Red River Valley. Stretching nearly 675 Km from north east North Dakota into southern Manitoba near Morden, Miami and Thornhill its fossil record spans from the Devonian to Late Cretaceous periods. Fossil collecting in the region dates back to the nineteenth century with early discoveries including plesiosaur teeth described by J. Leidy and sediment mapping by Joseph B. Tyrrell, while bentonite mining in the twentieth century has occasionally uncovered marine fossils. The escarpment’s Pierre Shale and underlying Carlile Formation formed in the Western Interior Seaway a vast inland ocean influenced by the Sevier orogeny which created a retroarc foreland basin ideal for sediment accumulation and fossil preservation. Fine muds, clays, volcanic ash, and bentonite layers accumulated in low-oxygen waters, preserving a rich marine fossil assemblage of mosasaurs such as Tylosaurus pembinensis known locally as Bruce, plesiosaurs like Trinacromerum and Styxosaurus, predatory fishes, sharks, rays, flightless diving seabirds and giant cephalopods such as Tusoteuthis. Beneath these Cretaceous deposits lie Devonian limestones and dolostones of the Winnipegosis Formation, Prairie Evaporite Formation and Manitoba Group that was once a tropical reef ecosystem and today act as porous carbonate reservoirs for hydrocarbon exploration. Together, these rocks and fossils document a dynamic history of marine ecosystems, tectonics, sedimentation and fossil preservation along Manitoba’s ancient continental margin and much of it is on display at the Canadian Fossil Discovery Centre in Morden.
The Manitoba Escarpment is located where the Red River and Lake Agassiz plains give way to the uplands of the Pembina Hills near the towns of Morden, Miami and Thornhill. Fossil collecting in the Manitoba Escarpment goes back to the nineteenth century when railway construction through shale near Miami and Altamont uncovered fossil-bearing beds. In 1865 J. Leidy described a plesiosaur tooth from south of Selkirk. In the 1890s Joseph B. Tyrrell mapped the escarpment’s sediments and noted its potential for vertebrate fossils. Bentonite mining between the 1930s – 1990s occasionally unearthed fossils but many were unfortunately lost to spoil dumps. By the 1970s, local residents Henry Isaak and Don Bell who were the first to seriously collect and preserve all the fossils coming out of the Pembina Escarpment rallied fossil rescue and founded the Canadian Fossil Discovery Centre in Morden in 1971.
The Manitoba Escarpment stretches for nearly nearly 675 Km across the south west of the Province of Manitoba. Its most prominent rocks were laid down during the Late Cretaceous Period when this region was submerged beneath the Western Interior Seaway a vast inland ocean connecting the Arctic to the Gulf of Mexico. In these calm, low-oxygen waters, fine silts, muds, and volcanic ash accumulated in the Western Canadian Sedimentary Basin to form the fossiliferous Carlile Formation, dating to the Late Cretaceous Turonian Age between 93.9 – 89.8 million years ago and underlies the Pierre Shale which was deposited later in the Western Interior Seaway during the Late Cretaceous Campanian to Maastrichtian epochs.
Because the Manitoba Escarpment was submerged beneath a seaway and not a terrestrial habitat there are no fossils of dinosaurs but instead the fossil record is dominated by marine reptiles, fish, molluscs and abundant microfossils. Among the most famous fossils discovered is "Bruce the Mosasaur" a species of Tylosaurus pembinensis measuring over 13 meters in length and named after the 'Bruces' sketch by Monty Python. This mosasaur was discovered near Thornhill north of Morden in 1974 and is now on display at the Canadian Fossil Discovery Centre.
Beneath the Pierre Shale and Carlile Formation in southern Manitoba’s Red River Valley and Interlake Region are Devonian limestones and dolostones deposited between 419 - 359 million years ago in a warm, shallow tropical sea. Part of the Western Canada Sedimentary Basin these rocks include the Winnipegosis Formation of reefal carbonates, the Prairie Evaporite Formation of salt and gypsum and the Manitoba Group of open-shelf carbonates. They tell a story of fluctuating sea levels and different salinities in environments ranging from tidal flats and lagoons to open marine settings. Historically, petroleum exploration has targeted these Devonian carbonates in southern Manitoba as the porous carbonate rock has created permeable reservoirs with effective traps and seals for hydrocarbons. Rich fossil assemblages include corals, stromatoporoids, brachiopods and crinoids that reveal the presence of thriving Devonian reef ecosystem.
The Pierre Shale exposures of southern Manitoba dates to the Campanian Stage the fifth of six ages of the Late Cretaceous spanning from 83.6 - 72.1 million years ago. The Sevier orogeny or mountain building event that occurred from the Late Jurassic to early Paleogene profoundly influenced the formation of the Pierre Shale in southern Manitoba. As the Farallon Plate subducted beneath the continent, compressional forces uplifted the Sevier Highlands and caused the crust to flex downward to the east forming a a large sediment-filled depression or retroarc foreland basin on the overriding continental plate.
This depression extended into Manitoba and was later flooded by the Western Interior Seaway creating a deep marine environment ideal for sediment accumulation. Continuous subsidence provided space for the deposition of fine muds, clays and volcanic ash in low-oxygen waters producing the dark shales typical of the Pierre Formation.
Intermittent volcanic activity added bentonite layers where the marine sediments were subsequently altered through diagenetic processes primarily transforming volcanic glass into montmorillonite the main mineral in sodium bentonite. While long-term tectonic stability preserved the sediment record.
The Sevier orogeny connected regional mountain building with the marine deposition and fossil preservation recorded in the Pierre Shale, which was later uplifted, eroded, and reshaped by proglacial floods gouging gullies through shale layers during Pleistocene glacial activity when the retreating Laurentide Ice Sheet carved steep valleys and exposed the Cretaceous bedrock seen today along the Pembina Escarpment and Pembina Hills.
The fossil assemblage of the Pierre Shale includes abundant invertebrates such as ammonites, baculites and bivalves, alongside Tusoteuthis longa a giant squid-like and the northern most known occurrence in the ancient seas. The vertebrate fauna features apex predators such as mosasaurs and plesiosaurs including Trinacromerum kirki and the long-necked Styxosaurus. Large predatory fish such as Xiphactinus, Cimolichthys and Catatinus also inhabited these waters swimming alongside sharks and rays.
The seaway also supported flightless diving birds such as Hesperornis regalis, part of the hesperornithiform group, which hunted fish beneath the surface. Another notable reptile is the crocodyliform Terminonaris robusta (meaning "“enlarged snout”). These reptiles reflect an ecological differentiation where mosasaurs patrolled open waters, plesiosaurs prowled the near shores and crocodyliforms waited near to the foreshore of the coastal margin to ambush their unsuspecting prey.
