Three major dispersal centers of the Paleozoic, Cretaceous, and Tertiary elastics of the upper Mississippi Valley and adjacent areas were located through integrated study of petrography, directional structures, facies maps, and regional stratigraphic relations.

In pre-Mississippian time, nearly all the elastics originally were derived from the Precambrian rocks centered around the Lake Superior region of the Canadian Shield. Relatively modest streams supplied sands and muds and recycling on a stable craton produced mature and supermature sandstones.

The second and third major dispersal centers resulted from Appalachian orogenic activity. Although some contributions from the Lake Superior region and Canadian Shield continued, the tectonic borderlands of the northern Appalachian mountains were the chief dispersal center of Mississippian and Pennsylvanian elastics. In post-Pennsylvanian time, this center shifted to the east of the southern Appalachian mountains. The post-Devonian clastic sediments were transported to the shallow marine shelves, coastal plains, and small deltas of the craton by a series of large, recurring drainage systems.

Despite major shifts in dispersal centers, the slope of the craton in the upper Mississippi Valley and adjacent areas has persisted to the south and southwest throughout Paleozoic, Mesozoic, and Tertiary time.

Inherent in the foregoing regional results are some problems of general interest. These include sedimentary differentiation, intrastratal solution, and the relationships between paleoslope, regional unconformities, and cross-bedding directions.

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