Published online by Cambridge University Press: 10 November 2010
COMPLETE PENNSYLVANIAN CRINOIDS
As one drives across the seemingly featureless till plains of central Illinois, geology and fossils typically do not come to mind. This is corn and soybean country. The ground is a rich, black soil; rocks are not part of the picture. Remarkably, however, rich Palaeozoic fossiliferous beds lie beneath these glacial deposits. The most famous is the Mazon Creek soft-bodied Lagerstätte, but these Pennsylvanian strata have also yielded an exceptional crinoid fauna from the LaSalle Limestone. By the standards of the faunas described in this book, this locality may not seem exceptional, but it is an exceedingly important fauna because it was one of the first Pennsylvanian crinoid faunas described principally from complete crowns (Fig. 165). Most crinoid faunas younger than the Middle Mississippian are dominated by cladid inadunates; and especially in the Pennsylvanian, crinoids are rarely preserved with arms. The majority of Pennsylvanian crinoids are found only as bowl- and saucer-shaped isolated aboral cups, so LaSalle crinoids offer an important glimpse of Late Palaeozoic cups with articulated arms.
CYCLICAL SEDIMENTATION
The Pennsylvanian of the midcontinental United States was represented by cyclical sedimentation and is well known for extensive and economically important coal deposits. Individual cycles are called cyclothems (in England, the Coal Measures), and each cyclothem has alternating marine and non-marine facies. In Illinois, cyclothems are composed of substantial portions of both marine and non-marine facies, whereas to the east non-marine facies dominated and to the west marine facies dominated.
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