ESTUARINE CYCLOTHEMS IN LOWER PENNSYLVANIAN STRATA OF EASTERN IOWA

Pennsylvanian outliers in eastern Iowa provide evidence for seven estuarine cyclothems within strata assigned to the “Caseyville Formation,” an interval known to be partially, if not wholly, of Morrowan age. Except for the first cyclothem, which fills in valleys and karst systems on eroded Silurian and Devonian carbonates, each cyclothem is marked by coal or carbonaceous shale at the base (local leaf beds). Each is dominated at most sites by gray mudstones, part silt-laminated and with varying siderite and pyrite nodules {locally rich in plant debris or megaspores). Each cyclothem is capped by a paleosol (up to 5 m thick) variously displaying abundant slickensides, rooting, or sphaerosiderites; some locally show channeling or an erosional top. Although fluvial facies (including sandstone channels) occur within the succession, the bulk of the included facies show evidence of marine influence during deposition: 1) linguloid brachiopods locally common in dark silty mudstones (5 of 7 cyclothems); 2) scattered bivalves (nuculoids?) and conchostracans (3 of 7); 3) all contain horizontal to subvertical burrows; 4) all contain pyrite nodules (sulfate source); 5) four cyclothems show well developed tidally-modulated silt laminae (rhythmites). However, no open-marine facies are identified, and the restricted benthic fauna, organic mudrocks, and tidal rhythmites support deposition in estuaries. Located over 400 km from the nearest coeval open-marine shale facies in southern Illinois, the northward flooding of large-scale estuaries deep into the cratonic interior of eastern Iowa apparently accompanied sea-level rise during a succession of Morrowan cyclothemic sequences.