Reevaluation of Wolfcampian cyclothems in northeastern Kansas: Significance of subaerial exposure and flooding surfaces

Ten cyclothems from the Wolfcampian of northeastern Kansas, including parts of the Council Grove and Chase Groups, were examined in detail with particular attention to discontinuity surfaces and paleosol development. These cyclothems are shown to be bounded by major discontinuities, or sequence boundaries, where marine limestones abruptly overlie paleosol profiles. Occurring within these cyclothemic sequences are prominent meter-scale cycles that are bounded by flooding surfaces, many of which overlie facies exhibiting evidence of subaerial exposure. They are developed within both the marine carbonate and shale intervals and variegated mudstone intervals of the cyclothems. These meter-scale cycles show a consistent carbonate-to-clastic pattern regardless of their stratigraphic position or component facies. Climate fluctuations within a generally monsoonal environment are determined to be the most likely forcing mechanism for the meter-scale cycles, with wetter climate phases resulting in the increased influx of terrigenous clastic sediment and drier climate phases favoring carbonate precipitation. Evidence of climate change at the scale of the cyclothemic sequences is also recognized in the studied interval. Cycles at both scales indicate that relative sea-level rise was associated with increasingly arid conditions and that sea-level fall was associated with an intensification of seasonal rainfall. Repeated sedimentary cycles were first recognized in the Pennsylvanian of Illinois by Udden (1912) and were later generalized by Weller (1930) into an idealized cycle. Moore (1931) was the first to describe the more complex cyclic pattern of the Pennsylvanian (lower Virgilian) of Kansas and Nebraska. The term cyclothem was introduced by Wanless and Weller (1932) to describe the Pennsylvanian cyclicity of the Illinois basin and was subsequently applied to the first description of Permian cyclicity within the midcontinent by Jewett (1933). The glacio-eustatic model of cyclothem development was first proposed by Wanless and Shepard (1936) for the Pennsylvanian and was soon followed by the classic paper of Elias (1937) on Lower Permian sea-level cycles. Moore (1936) introduced the concept of megacyclothems, or cycles of cyclothems, based on his work on the Virgilian cycles of Kansas. This concept was similarly adopted by several researchers to describe cyclic sequences of the Lower Permian [e.g., Hattin (1957)], and eventually the entire sequence was divided into megacyclothems (Moore, 1964). The facies sequence of Permian cyclothems described by Jewett (1933) began with red and green variegated mudstones, followed by a thin limestone or calcareous zone, a gray to yellow fossiliferous shale, and, last, a thick interval of limestone beds. This description was modified and elaborated by Elias (1937), who placed all the major facies encountered in Permian cycles into an idealized depth-related sequence. All the facies of this idealized cyclothem were never encountered in any single cyclothem, and some predicted facies transitions rarely, if ever, occurred. The actual cycles identified by Elias in the Lower Permian were essentially the same as those recognized by Jewett. A number of detailed sedimentary and paleontologic studies of individual cyclothems and their member-scale lithologic units were completed following the initial description and interpretation of late Paleozoic cyclicity. The Grenola Limestone (Lane, 1958), Beattie Limestone (Imbrie, 1955; Laporte, 1962; Imbrie et al., 1964), Wreford Limestone (Hattin, 1957), and Red Eagle Limestone (McCrone, 1963) were all the focus of investigation. In all these cases, however, the variegated mudstones were largely ignored and depositional and paleoecologic interpretations were confined to the fossil-bearing marine units. At present, the most widely accepted cyclothem model is that of Heckel (1977), which, like the earlier model of Wanless and Shepard (1936), assumes primary eustatic control. Heckel’s typical “Kansas-type cyclothem” consists of (1) a sandy “outside shale” unit deposited under primarily terrestrial and shallow marine conditions and typically containing coal beds; (2) a thin “transgressive” limestone unit; (3) a black, often phosphatic “core shale” representing deepest marine conditions; (4) a thick, multistoried “regressive” limestone unit; and (5) a return to an “outside shale” at the top. Although based primarily on the Upper Pennsylvanian (Mis1. Department of Geology, Kansas State University, Manhattan,