The geological layers that preserved a dinosaur world
In short: Deep beneath the Alberta landscape lies a 40-meter thick record of the Late Cretaceous. These light-colored sandstones aren't just rocks; they are the preserved remnants of ancient river systems and floodplains that once served as a massive graveyard for some of Earth's most iconic prehistoric inhabitants.
The Oldman Formation is a stratigraphic unit dating to the middle Campanian stage, roughly between 77.5 and 76.5 million years ago. Located primarily in southern Alberta, Canada, it is characterized by light-colored, fine-grained sandstones. These sediments were deposited in fluvial channel and floodplain environments, creating a landscape of river systems where dinosaur remains were eventually buried and preserved. The formation is situated entirely within magnetic polarity Chron 33n. It is recognized primarily for its rich concentration of dinosaur fossils and other prehistoric remains.
Geologically, the formation is composed of upward-fining, lenticular to sheet-like bodies that appear yellowish and blocky in outcrop. While sandstones dominate the fluvial channels, the formation also contains siltstones and mudstones, which were deposited in overbank and floodplain environments. The sediments originated primarily from northwestern Montana and thickened toward the southwest. At Dinosaur Provincial Park, the formation reaches a thickness of about 40 metres (130 ft). The formation was named based on exposures found along the Oldman River. These specific outcrops are located between the city of Lethbridge and the river's confluence with the St. Mary River.
Distinguishing the Oldman Formation from its neighbors requires precise geological analysis. Resting atop the Foremost Formation in a conformable manner, it sits below a regional disconformity that marks the Dinosaur Park Formation. Petrographic and sedimentologic variations differentiate these two units, despite their outward resemblance. The Oldman Formation corresponds to the Judith River Formation's Woodhawk and Coal Ridge Members, plus the Mclelland Ferry Member's upper section, within Central Montana. The unit is a member of the Belly River Group, which is also known as the Judith River Group. Before the discovery of the regional disconformity, the Dinosaur Park Formation was actually included as part of the Oldman Formation.
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- Summary by Intellect · source: Oldman Formation
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