MIDDLE-SHELF FACIES, DEPOSITION, AND STRATIGRAPHY OF THE CEDAR VALLEY GROUP, DEVONIAN OF EASTERN IOWA

The upper Givetian-lower Frasnian Cedar Valley Group shows significant southeastward facies changes across Iowa into the northern portion of the Illinois Basin. Inner-shelf facies of central and northern Iowa, which include peritidal depofacies and evaporites, are abruptly replaced in an offshore direction by a succession of fully subtidal marine facies across the middle-shelf region of southeastern Iowa. Stratigraphic sequences across the inner-shelf are bounded by subaerial erosional surfaces, whereas the same sequences are bounded by submarine hardground disconformity surfaces across the middle shelf. Middle-shelf strata of the Little Cedar Formation, lower Cedar Valley Group, include a condensed interval of Solon Member packstones and a thicker argillaceous Rapid Member. Although the Solon shows general offshore thinning and condensation, a thicker coral-rich biohermal or biostromal facies is locally identified in southern Iowa. The Rapid Member becomes dominated by sparsely fossiliferous to unfossiliferous mudstones in an offshore direction, apparently reflecting deposition in a stratified seaway with oxygen-deficient bottom waters. The overlying Coralville and Lithograph City formations show dramatic offshore condensation and thinning across the middle shelf area. In strong contrast to equivalent inner-shelf facies, these strata display an offshore loss of coralline facies, an increase in phosphatic enrichment in some beds, and the development of numerous submarine hardground surfaces. The highest strata of the Cedar Valley Group, the Shell Rock Formation, are entirely absent across the middle shelf, apparently due to widespread offshore sediment starvation and/or submarine planation.