Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota
Published:January 01, 1983
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1983. "Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota", Late Eocene and Oligocene Paleosols from Badlands National Park, South Dakota, Greg J. Retallack
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In the Pinnacles area of Badlands National Park, the Late Eocene and Oligocene White River and lower Arikaree Groups are largely superimposed fossil soils (87 of them in 143 m of stratigraphic section). The fossil soils can be recognized from irregular vertical tubular structures (fossil root traces and burrows), massive bioturbated layers with gradational contacts (soil horizons), complex cracking and veining (peds and cutans), and distinctive petrographic textures (sepic plasmic fabrics). Other features of these deposits, such as patterns of fossil bone accumulation, horizons of calcareous nodules and local variations in mineralogy, and trace and major chemical elements, are also evidence for fossil soils. The few late diagenetic alterations (those after burial of the soils) found in Badlands National Park have not obscured the abudnant early diagenetic alterations (those of ancient soil formation).
Ten different kinds of fossil soils recognized in the Pinnacles area of Badlands National Park have been classified into paleosol series, which are standard mapping units used by the U.S. Department of Agriculture. Each series represents a particular ancient environment and is evidence of past soil-forming factors, such as climate, organisms, topographic relief, parent material, and time of formation. Fossil soils of the Pinnacles area are evidence for progressively drier climate, from humid during Late Eocene time to semiarid during Late Oligocene time. There was also a concomitant change in vegetation from Late Eocene forest, to Early Oligocene open woodland, to early Late Oligocene savanna with stream-side gallery woodland, to mid-Late Oligocene savanna, and finally to latest Late Oligocene open grassland with scattered stream-side trees. Late Eocene mammalian faunas of forests and woodlands were supplanted by an invasion from elsewhere of savanna-adapted mammals, which persisted in recognizable evolutionary lineages throughout Oligocene time. Topographic relief was generally low and uniform, but for periods of marked stream incision during Late Eocene time, at about the Eocene-Oligocene boundary and during the late part of Late Oligocene time. Parent materials of most of the paleosols (except for those on the basal unconformity with Late Cretaceous marine rocks) consisted of volcanic ash fall, re-sorted soil material, and far-transported alluvium. Re-sorted soil material was most common in sediments of the stable wooded landscape during Early Oligocene time, but volcanic ash became increasingly more prominent in parent materials of less developed, sparsely vegetated, arid-land soils from early to latest Late Oligocene time. Time of formation of the soils became progressively less with time, from strongly developed soils during the Late Eocene and Early Oligocene to moderately developed soils during Late Oligocene time. Maximum rates of sediment accumulation calculated from estimates of the time of formation of the paleosols are progressively higher from the Late Eocene to the Late Oligocene. Comparisons between rates for sequences of different paleosols are an indication that changing vegetation, and perhaps to a lesser extent local tectonic movements, were the main factors controlling rates of sediment accumulation.