Date of this Version
The Journal of Geology, Vol. 56, No. 4, Jul., 1948
The lithologic and paleontologic differences between the Mississippian sediments of the Mid-Continent region and those of the northern Rocky Mountain and Black Hills regions suggest either that the Mississippian seas were not directly continuous between these two regions or that sea connections were greatly restricted. The purpose of this paper is to present and analyze the available subsurface data in Nebraska and surrounding states in the light of this problem. The study is primarily lithologic. Mississippian sediments are known to be widely distributed in the subsurface of much of Iowa and Kansas, in southeastern and extreme southwestern Nebraska, and in southeastern Colorado. These sediments are lithologically similar to those of the Mid-Continent outcrop areas. Likewise, Mississippian sediments are known to occur widely in the subsurface of much of Wyoming, in western and northwestern South Dakota, in northwestern Colorado, and in extreme northwestern Nebraska. These sediments are lithologically similar to those of the northern Rocky Mountain outcrop areas. However, Mississippian rocks seem to be absent in the subsurface in large areas between these two regions, and the Mississippian sediments of these two regions are lithologically dissimilar. Therefore, it appears that there was either no direct sea connection between the Mid-Continent and northern Rocky Mountain regions during the Mississippian or that the sea connection was greatly restricted. Mississippian sediments could have been deposited between these two regions and removed by post-Mississippian erosion. It is unlikely, however, that great thicknesses were removed because of the apparent absence of good evidence suggesting facial changes. However, the pre-Pennsylvanian rocks are known to be deeply buried within much of the critical area, and the subsurface has not been thoroughly tested by drilling.