Paul Edwin Potter

Dispersal Centers of Paleozoic and Later Clastics of the Upper Mississippi Valley and Adjacent Areas

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.

Trace elements in marine and fresh-water argillaceous sediments

Abstract The trace elements B, Co, Cr, Cu, Ga, Ni, Pb, V and Zn were studied in 66 samples of both modern and ancient marine and fresh-water argillaceous sediments to determine their value as environmental discriminators. Samples of each group were widely scattered geographically and represent widely varying source areas, climates, tectonic conditions and rates of sedimentation. B, Cr, Cu, Ga, Ni and V are significantly more abundant in marine than in fresh-water argillaceous sediments. A discriminant function based on B and V alone and derived from 33 modern sediments decisively separated the modern marine and modern fresh-water sediments. When this discriminant function was applied to ancient sediments whose environment of deposition had been independently established by geologic evidence, 28 out of 33 sediments were correctly classified. Similar results were obtained using six elements with graphical methods.

Variance Components of Cross-Bedding Direction in Some Basal Pennsylvanian Sandstones of the Eastern Interior Basin: Geological Application

Dip direction of torrential cross-bedding is a vectorial property providing a rapid measure of the local direction of sediment transport. Over 500 measurements in the basal Pennsylvanian Caseyville and Mansfield sandstones of southern Illinois and Indiana were taken, to explore its possibilities and limitations as a measure of direction of regional sediment transport. The analysis of variance helped interpret the pattern of variability, indicate the reliability of averages, and improve the efficiency of sampling effort. The host topography of the Mississippian-Pennsylvanian unconformity exercised a strong control on the basal cross-bedding direction. Averages of many measurements along 260 miles of outcrop, however, accurately indicate a southwesterly direction of sediment transport for the Caseyville and a southerly direction for the Mansfield. The importance of a northerly source is suggested but would have to be evaluated by petrologic study.

Breccia and Small-Scale Lower Pennsylvanian Overthrusting in Southern Illinois

Two unusual exposures of Pennsylvanian sediments occur adjacent to a tunnel along the Illinois Central Railroad in northwestern Pope County, Illinois. At the north end of the tunnel is an 1,800-foot exposure of a breccia probably Caseyville in age. At the south end, are two small imbricate overthrusts of Caseyville age. Midway between the pene-contemporaneous breccia and overthrusts is the McCormick fault zone-anticlinal trend. Both breccia and imbricate overthrusts are believed to be free gravity slides related to Caseyville activity of the McCormick fault zone-anticlinal trend. These exposures show that slide structures of appreciable magnitude are not limited to argillaceous sandstone sequences of major geosynclines but can also occur locally in the relatively clean sandstone sequences of intracratonic basins.

Team Programs in Sedimentology

Sedimentology can be taught most effectively with a program that emphasizes principles and processes; plenty of field work in modern and ancient sediments; sedimentary petrology; and exposure to supplemental, quantitative geological and outside disciplines. At any level, an integrated team program maximizes teaching, training and research efficiency. Commonly three sedimentologists can effectively provide much, but not all, of the necessary instruction.

A Comparative Study of Upper Chester and Lower Pennsylvanian Stratigraphic Variability

The most general property of any sedimentary sequence is its areal variation of gross lithology. This variation, here termed stratigraphic variability, is investigated in the Upper Chester and Lower Pennsylvanian sediments of a portion of the Eastern Interior Basin with the hierarchical case of the analysis of variance. Using electric log data, the hierarchical case of the analysis of variance segregates the total variability of sandstone and limestone proportion into components associated with increasing increments of area. Variability of Upper Chester and Lower Pennsylvanian sandstones is broadly similar on all but the lowest sampling level. Tectonics appears to be the dominant control on the higher sampling levels, whereas length of section and hydrodynamic factors are most important within a square mile. Because limestone is predominantly an autochthonous rather than allochthonous sediment, its variability is much less. This contrast is also reflected in markedly differing distributions of sandstone and limestone thickness. The practical applications of this methodology are important for determination of confidence limits for one or more wells as samples of areas of variable size and for allocation of sampling in comparative studies.