Cyclical Sedimentation of the Pennsylvanian Period and Its Significance

Abstract

The Pennsylvanian strata of western Illinois exhibit a cyclical repetition of beds as follows: (i) sandstone and sandy shales, (2) underclay, (3) coal, and (4) marine limestones and shales. Each cycle was initiated by uplift which was reflected first by erosion and later by continental deposition of clastic materials. This was followed by a long period of stability during which weathering developed a typical poorly drained profile of weathering, now represented by underclay. Then peat, later transformed to coal, accumulated in more or less extensive swamps. These conditions were terminated by subsidence so that the area was inundated by a shallow sea in which marine strata were deposited until uplift brought the cycle to a close. The cyclical repetition of beds is regional rather than local, and analysis of the conditions necessary for the formation and preservation of such strata over broad areas furnishes the basis for a reconstruction of the, physiography of the Pennsylvanian period. The conception of widespread sedimentary cycles provides new criteria for correlation in Pennsylvanian areas, as a definite succession of strata not dependent upon certain key horizons is established. It also furnishes a new basis for paleontologic studies, since faunules restricted to different habitats may be grouped with regard to definite marine invasions and an approximation of the total marine fauna of each invasion may be obtained.