256 C. SCHUCHERT MORRISON AND TENDAGURU rORMATlONS 



tion. Twenty miles to the southeast, near Centennial, the Sundance is 

 reduced to 156 feet and has Belemnites densus throughout the upper 50 

 feet. Five miles farther in the same direction, near J elm Mountain, the 

 Sundance is down to 92 feet thick and here all the higher beds with 

 marine fossils are absent. The top of the section here consists of 50 

 feet of buff to cream-colored, soft, massive sandstone devoid of fossils — 

 the same sandstone at Centennial is 85 feet, thick and is beneath the fos- 

 siliferous marine Sundance — and it is this member that makes up the 

 top of all the other sections down to the Colorado State line (north and 

 soutli face of Eed Mountain and at Bull Mountain) . This member at Bull 

 Mountain is 36 feet thick aaid shoAvs cross-bedding of the eolian type. In 

 other words, these sections demonstrate that the Sundance thins south- 

 ward from 179 feet to 50 feet and even to 20 feet. The "thinning takes 

 place by cutting out progressively the upper members, and apparently 

 not by overla]) l)iit ratliei' tlirough erosion.'^ This then appears to be clear 

 evidence that an erosion interval existed after Sundance time and before 

 the deposition of the younger transgressing Morrison. Knight also recites 

 other evidence showing that the Morrison clearly is a transgressing forma- 

 tion not only over the Sundance but across the Triassic as well. 



Correlation of Sundance and Morrison Formations 

 opinions and conclvi^ion as to age of the sundance 



It is now widely held that the Sundance sea, with a cool- water fauna, 

 began in the later part of the Jurassic to transgress widely over Alaska 

 and British Columbia and into the states of Montana, Idaho, AVyoming, 

 Colorado, New Mexico, and Utah. In the Great Plains region the deposits 

 have an average thickness varying between 200 and 400 feet, but increas- 

 ing to the west to upward of 1,000 feet, and in southwestern Wyoming, 

 it is said, to 3,500 feet. The nature of the sediments and the universal 

 presence of oysters indicate that the sea was a shallow one, and, further, 

 that it flowed over a land eroded to a low reHef . 



Stanton^" states that the marine Sundance fauna ' 



"belongs in the loivei' part of the Upper Jurassic. It is characterized by 

 Gardioceras cor di forme and other invertebrates, which indicate approximate 

 correlation with the Oxfordian of the European Jurassic." 



Last December Stanton informed the writer that the above correlation 

 of the 



"Sundance with European formations has been made through Alaska and other 

 boreal Jurassic areas in this way: The Chinitna formation in Alaska con- 



10 T. W. Stanton : Bull. Geol. Soc. Am., vol. 26, 1915, pp. 347-348. 



