Upper Pennsylvanian Paleosol in Stranger Shale and Underlying Iatan Limestone, Southwestern Iowa

Abstract

ABSTRACT Cores of the Iatan Limestone and overlying Stranger Shale in southwestern Iowa display characteristics of paleosol development other than the weathered-clay or caliche horizons or underclay type of paleosol profiles from the Upper Pennsylvanian in the Midcontinent described by other authors. Paleosol features in the Iatan-Stranger sequence include clay-filled fractures and solution channels (cryptokarst) in the top of the Iatan Limestone which display some degree of secondary clay orientation along the edges (neostrians). The clay-filled fractures widen upward and grade into the gray to red mottled, non-calcareous, blocky mudstone of the Stranger Shale, which is characterized by an absence of fauna and presence of slickensides, clay coatings on grains (argillans), quartz silt-filled f actures (skeletans), and possible former rooted zones. The mudstone matrix locally displays the sepic plasmic fabrics that are characteristic of soil development. Pedogenesis in this unit resulted from subaerial exposure during retreat of the sea following the transgressive-regressive marine cycle that deposited the Iatan Limestone. Solution-channelling of the top of the Iatan and leaching of clays within its fractures indicates initial exposure to a humid climate with strong meteoric flux. The Stranger paleosol, however, displays features of Entisol, Inceptisol, or Vertisol development in terrestrial deposits under a somewhat drier climate following the partial solution of the upper Iatan. These features include a thick (1 to 2.4 m) paleosol profile with weak morphologic expression which contains crystalline illitic clays and oxidized iron minerals. Lack of fluvial-alluvial features in the thick, monotonous quartz-silty mudstone sequence of th lower Stranger Shale suggests that fine aeolian detrital influx (desert loess) may have contributed much of the material to this unit. The lack of caliche or weathered clays in the Stranger mudstone indicates the importance of using multiple lines of evidence for recognizing paleosols.