Ichnology of Cyclothem Deposits in the Lower Permian Council Grove Group (Kansas, USA)
John W. Counts
University of Kansas, Department of Geology
The Lower Permian Council Grove Group in Kansas consists of alternating marine and continental deposits that record fourth-order glacioeustatic sea-level fluctuations. These sea-level changes are also recorded in discrete trace-fossil assemblages found throughout the upper part of the Council Grove Group. This study examines both spatial and temporal changes in trace-fossil assemblages to understand better the response of burrowing organisms to changing paleoenvironmental parameters and sea-level dynamics. Continental strata are characterized by weakly to extensively rooted paleosols with monospecific assemblages of adhesive meniscate burrows. Burrow characteristics—tortuous axes, variable orientations, and discrete packets of meniscate backfill—are consistent with those produced by such modern soil-dwelling insects as bugs (Hemiptera). Early Permian tracemakers had similar burrowing habits and anatomy, and were likely produced by Hemiptera. Crosscutting relationships with rhizoliths and rhizohaloes are inconsistent with suppositions that such traces were originally produced subaqueously and later overprinted by soil-forming processes. Marine units represent a variety of shallow water environments and are characterized by more diverse nearshore communities typical of marginal marine systems. Traces are absent or rare in some marine intervals. Vertical successions of trace-fossil assemblages are a direct response to changing sedimentary facies, relative sea-level position, or paleoclimate. Ongoing observations of core in western Kansas and equivalent outcrops in eastern Kansas show that assemblages are similar, but not identical, over hundreds of miles laterally. Data will ultimately be used to produce a model relating ichnocoenoses to environmental conditions, depositional setting, and relative sea-level position of cyclothem deposits.
AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid