Smoky Hill Chalk: Spectacular Cretaceous
David J. Bottjer
CHALK, A ROCK MADE PRIMARILY OF THE FOSSILS OF microscopic marine algae (coccoliths), is typically deposited in the deep sea. But in the Cretaceous, it accumulated extensively in shallow epicontinental seas, which covered large portions of many of the continents during that time (Bottjer 1986). Even though it was deposited in the Cretaceous at relatively shallow depths, chalk as a sediment typically accumulates in marine environments with little wave or current action. The chalk cliffs along the northern part of the English Channel— on the English coast in Dover (White Cliffs of Dover) and on the French coast in Normandy—are arguably the most prominent and famous of Cretaceous chalk exposures worldwide. However, this typical Cretaceous rock (the time period takes its name from the Latin word for “chalk”: creta) is also found along the Gulf Coast (Texas, Arkansas, Mississippi, Alabama) and in the western interior (Kansas, Colorado) of North America (Bottjer 1986).
Chalks found in the western interior not only were deposited in marine environments with relatively little wave or current energy, but also typically formed on seafloors where dissolved oxygen concentrations were low. This led to conditions for the development of a conservation Lagerstätte that is a stagnation deposit. Indeed, the chalk beds of the western interior, particularly the Smoky Hill Chalk Member of the Niobrara Chalk (western Kansas) (Figures 19.1 and 19.2), are renowned for their preservation of articulated Cretaceous marine reptiles, such as mosasaurs, plesiosaurs, pliosaurs, and large turtles, as well as pterodactyls, enormous fish, and the flightless marine bird Hesperornis. While