How and When was the Niobrara deposited
The Niobrara Formation, commonly referred to as the Niobrara Shale, but also known as the Niobrara Chalk, was deposited in the Western Interior Seaway in the middle of North America during late Cretaceous Age. (145.5 – 65.5 million years ago) This inland sea divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada.
The Niobrara Formation and laterally equivalent rocks were deposited during a period of high eustatic sea level and crustal subsidence in the Western Interior Seaway, resulting in a major marine transgression and conditions favorable for carbonate deposition. In the eastern part of the seaway where clastic input was minimal, chalks and limestone are the principal lithologies of the Niobrara in the Denver Basin of eastern Colorado (Pollastro and Scholle, 1986).
Formal members of the Niobrara include the Fort Hays Limestone (~10 m thick) and the overlying Smoky Hill Chalk (?80 m thick). (Locklear&Sageman,2008) These members are separated by inter-bedded shales.
However a more detailed view of the Niobrara shows that their are three chalk (reservoir) zones, A, B, C along with the Fort Hays Limestone below, with inter-bedded marls and shale that serve as the the source rock for the reservoir zones.