The Niobrara Formation, also called the Niobrara Chalk is a geologic formation in North America that was laid down between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian ages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member overlying the Fort Hays Limestone Member. The chalk formed from the accumulation of coccoliths from microorganisms living in what was once the Western Interior Seaway, an inland sea that divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, and pterosaurs as well as several primitive aquatic birds. The type locality for the Niobrara Chalk is Knox County in northeastern Nebraska.
The Niobrara consists of chalky limestones (Fort Hays Limestone, Lower through Middle Coniacian, about 20 meters thick) and chalks (Smoky Hill Chalk, upper Coniacian through lower Campanian, about 180 meters thick) deposited in the middle of the seaway during the maximum transgression (advance) of the Niobrara cyclothem.
Shark teeth and other vertebrate fossils are rare in the Fort Hays Limestone. Shimada (1996) lists Scapanorhynchus sp., Cretalamna appendiculata, Paranomotodon sp., Squalicorax falcatus, Cretoxyrhina mantelli and Ptychodus mortoni.
The Smoky Hill chalk represents a completely different depositional environment and sharks' teeth are more commonly found. Stewart (1990) lists the following species from the Smoky Hill chalk: Squalicorax falcatus, Squalicorax sp. aff. kaupi, Scapanorhynchus raphiodon, Psuedocorax laevis, Cretoxyrhina mantelli, Cretalamna appendiculata, Rhinobatos incertus, Ptychodus mortoni, P. martini and P. anonymous. Stewart (1990) also suggests that small enterospirae (coprolites) found only in the lower chalk suggest that an unknown species of shark was also present. In 1998, the teeth of an undescribed species of paleoanacoracid shark were was discovered in the lower chalk (Mikael Siverson, personal communication) and will be published in the near future.
Individual shark teeth are a fairly common find in the Smoky Hill chalk. When they weather out of the softer chalk, most sharks teeth are in pristine condition and require little or no preparation besides washing in plain water. They do, however, deteriorate rapidly when exposed to weathering. Finding a large, nearly perfect sharks tooth with its shiny, brownish-grey enamel weathered to a pasty white, brittle layer is really disappointing.
Compete and articulated dentitions have been found for Squalicorax, Cretoxyrhinidae and three species of Ptychodus. Isolated vertebrae and calcified fin elements are also found occasionally in the Smoky Hill chalk. Evidence of predation / scavenging by sharks (bite marks, severed bones and embedded teeth) has been found on nearly all species of marine reptiles and the remains of most large fish.
History of explorationEdit
The Niobrara Chalk was first studied during an expedition led by Othniel Charles Marsh of Yale University in 1870. This and following expeditions to the area in 1871 and 1872 yielded the first of many fossil vertebrate remains commonly attributed with the formation. Excavations continued through the following years up to 1879 under the direction of professional fossil collectors such as B. F. Mudge and S. W. Williston appointed by Marsh.
The Niobrara Chalk has been continuously explored ever since, with specimens being found by H. T. Martin of the University of Kansas and George F. Sternberg, the son of the famous fossil collector Charles H. Sternberg. Much of the best material from the formation is on display at the Sternberg Museum of Natural History in Hays, Kansas.
The Smoky Hill Member of the Niobrara Chalk contains the majority of the fossils found in the formation, and is subdivided into 23 marker beds. Most vertebrates are present from the upper half of the member. Most of the vertebrate remains were collected and described before the stratigraphy of the Niobrara Chalk was fully understood. Specimens were described as being from layers referred to as being either of gray-blue shale or yellow chalk. This dichotomy is not indicative of different stratigraphic units as was previously thought, but rather is seen as a weathering phenomenon that can be found at varying points in the same outcrop.
The Niobrara Formation is overlain by the marine Pierre Shale.