Morrison Formation

[The formation was named by Eldridge in this Monograph (Chapter II, Mesozoic geology, p. 51-151), within the area of which is its type locality; but Pikes Peak folio, by Whitman Cross, in which the formation was also described was published in 1894, before the publication of the Monograph.] Morrison formation. Throughout Denver region and for much of distance along east base of Rocky Mountains in Colorado, the Jura is essentially a formation of fresh-water marls, of average thickness of 200+/- feet. Its upper limit is sharply defined by Dakota sandstone, while the brown and pink sandstone closing the Trias as clearly marks its lower limit. To this formation has been assigned the name Morrison, from the town [near Denver] near which it is typically developed. The marls are green, drab, or gray, and carry in lower two-thirds numerous lenticular bodies of limestone of characteristic drab color and a texture compact and even throughout. About 20 feet above base occurs a persistent band of alternating limestones and sandstones, or all sandstones, 10 to 15 feet thick. At Morrison the top member of underlying Wyoming formation, consists of 15 to 25 feet of persistent sandstone, fine-grained, often massive, pink and brown. The clays of lower two-thirds are remarkable for their reptilian remains and from the dominating form have been designated "ATLANTOSAURUS clays." The upper third is generally a succession of sandstones and marls, of which the sandstones predominate. The most important sandstone occurs just above the "ATLANTOSAURUS CLAYS," is very persistent, and from contained Saurian remains has been called Saurian sandstone. It varies in thickness between 5 and 35 feet, and in its distance below the Dakota from 10 to 125 feet, although usually from 50 to 80 feet. The Morrison appears to be unconformable with the underlying Wyoming formation [now divided into (descending) Lykins formation, Lyons sandstone, Ingleside formation, and Fountain formation.] Pg. 183-188. Morrison formation. Type section of Morrison redefined. Strata originally assigned to Morrison formation include some that are equivalent to an older formation (Sundance of Jurassic) and others at top which contain fossil plants of Upper Cretaceous type. Section at Morrison comprises 10 units (numbered in descending order 1-10). Units 1-5 are Dakota group, 265 feet thick. Units 6 and 7 are Morrison formation, 160 feet thick. Units 8-10 are Sundance formation, 17 feet thick. According to G.L. Cannon, who accompanied writer [Lee] and who assisted in work on Denver Monograph [27], Nos. 1-3 of section were regarded as the two sandstones of the Dakota and the shale as the "Dakota fire clay." Other geologists have followed this usage, and when the "lower Dakota" and shale above it a few miles farther south were correlated with Purgatoire, No. 3 of Morrison section was regarded as Lower Cretaceous. No. 4 (100 feet thick) of section is part of Morrison as originally defined yet it contains fossil plants which Knowlton described as belonging to "Dakota flora." Unit 5 (10 feet thick) is conglomeratic sandstone (Saurian conglomerate) containing dinosaur bones and pebbles of quartz and jasper; sharply separated from underlying shale. Lee, W.T., 1927, Correlation of geologic formations between east-central Colorado, central Wyoming, and southern Montana: U.S. Geological Survey Professional Paper, 149, 80 p. Pg. 171-173. Morrison formation. Described in Ouray district, Colorado. Comprises the middle limestone and upper sandstone members of the "La Plata sandstone" of Cross and entire thickness of "McElmo" formation as these two formations are defined by Cross and Howe in Ouray folio. Thickness 700 to 750 feet. Comprises a basal limestone, sandstone, and shale member, herein named Wanakah, a middle sandstone member, and an upper shale member. Underlies Dakota(?) sandstone; overlies Upper(?) Jurassic sandstone. Age is [Early] Cretaceous(?). Pg. 9-10, 32-44, 58-63, charts. Morrison fo