The Laramie Problem in the American West

The sedimentary sequence of the Western Interior of North America was first mapped by the Hayden Surveys of the 1860s and 1870s as they worked their way up the Missouri River through South Dakota and into North Dakota and Montana. In this major transect, they recognized the broad outlines of the stratigraphy of the Western Interior seaway of the Cretaceous: a basal transgressive Dakota Sandstone overlain by a thick and fossiliferous sequence of marine shale and limestone (Pierre Shale and equivalents), overlain by a regressive Fox Hills Sandstone, overlain by a thick, lignite-bearing unit. The so-called "Great Lignite'' contains units that we now know to be both Cretaceous and Paleogene in age, but at the time that was not clear. Superficially similar coal-bearing deposits were soon discovered in several stratigraphic units in additional areas in Wyoming, Colorado, Utah, and New Mexico. Eventually the Great Lignite came to be known as the Laramie Formation. The problem was that these coal-bearing units included rocks now known to be Late Cretaceous, Paleocene, and Eocene in age.

Discoveries of dinosaur fossils in the Great Lignite led to the realization that the K-T boundary was above the top of the Fox Hills Sandstone, but this realization took nearly 50 years to be resolved. The uncertainty over the age of the plant megafossils and the rocks that contained them continued until the middle of the twentieth century because of confused stratigraphy and flawed paleobotanical taxonomy. The stratigraphic confusion of the "Laramie

4.2 The Laramie Problem in the American West 37

Age

Formations

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