PALEOECOLOGY OF MARINE BEDS IN THE MIDDLE PENNSYLVANIAN LOWER KITTANNING CYCLOTHEM IN NORTH AMERICA

Abstract

This paper evaluates the paleoceanography of marine strata within a Middle Pennsylvanian (Desmoinesian) cyclothem. Paleoecological data are used to interpret spatial and temporal changes in energy, salinity, and turbidity across the North American craton. Using mid-continent terminology, the marine part of the study interval commenced at the top of the Croweburg coal and continues up through the Verdigris Limestone. Cores from four boreholes, in Kansas, and exposures of the interval at 21 outcrops in 18 states were measured, described, and evaluated. Body fossils of marine invertebrates, some vertebrates, plant debris, and trace fossils were identified and evaluated in terms of their environmental and paleoecological significance. These biotic data are presented for eight geographic areas, as follows: (1) Appalachian Basin (Ohio, Pennsylvania, West Virginia, and eastern Kentucky); (2) Eastern Interior Basin (Illinois, Indiana, and western Kentucky); (3) Western Interior Basin (Missouri, Iowa, Kansas, outcrops and cores, and Oklahoma); (4) subsurface of northwestern Kansas; (5) Hartville Uplift (Wyoming) and Black Hills (South Dakota); (6) Fort Worth Basin (Texas); (7) Pedregosa Basin (Arizona); and (8) Paradox Basin (Utah and Colorado), Great Basin (Arrow Canyon, Nevada), and Death Valley (California). In the eastern and central United States the biota occurs in gray mudrocks and black, platy to fissile shales. Fossiliferous limestones occur at the top of the cycle in the central area and become better developed westward into Nevada and California, where the entire interval is dominated by limestone. Comparison of overall diversity and number of taxa versus lithology, mode of life, and feeding type in these eight areas indicates that the greatest diversity is in the Western Interior Basin. In this central region, calcareous skeletons in dark gray shales are commonly replaced by pyrite. Pyritization occurs in such environments because environments of dark gray mud are slightly more reducing than the more oxygenated depositional environments of lighter gray mudrocks and limestones. For ease of further comparison, the eight study areas are grouped into three regions, as follows: East, Central, and West. Comparison of these three regions indicates the following: (1) fragmentation of skeletons is greatest in the East, (2) epifaunal suspension feeders (typical of Paleozoic sedimentary sequences) dominate all three regions, and (3) there is some similarity between the general biotic diversity and total number of taxa in the black mudrocks and limestones in the Central region. Eustatic or tectonic sea-level change has traditionally been invoked to explain the differences documented in this stratigraphic interval and thus along our transcontinental transect. These two processes were active and, no doubt, may have been contemporaneous at some scale. Our data indicate, however, that temporal changes in water depths were similar in most basins across the North American continent. The biotic and lithologic changes, therefore, appear to be principally a function of differences in energy and salinity in response to temporal and spatial changes in climate and tidal conditions across a broad, shallow, island-studded epicontinental sea. Eustasy and tectonics controlled accommodation space, but temporal and spatial climate change was the primary control on sediment supply, wind-driven energy, and seawater chemistry.