CONDENSED SEDIMENTATION AND CONODONT BIOEVENTS IN THE ST. PETER SANDSTONE (ORDOVICIAN), CENTRAL MIDCONTINENT

The St. Peter Sandstone overlies the Sauk-Tippecanoe unconformity over a vast area of central North America, typified across much of its extent by remarkably clean quartzarenite lithologies. St. Peter depositional interpretations have been hampered by the apparent paucity of clear sedimentary structures and fossils in the sandstone succession, although many previous workers have stressed nearshore, littoral, and eolian processes for the deposition of a transgressive sheet sand. Very different depositional processes are interpreted for the western facies of the St. Peter in the subsurface of Iowa, Nebraska, and northeast Kansas, where the sandstone-dominated succession includes thin conodont-rich brown shales, phosphorites, pyritic/phosphatic hardgrounds, and oolitic ironstones. Extreme condensation of the thin organic shales is indicated by exceptionally high conodont abundances ranging as high as 66,000 conodonts per kilogram. Samples with the highest conodont yields also include high proportions of small immature and juvenile conodont elements (especially Phragmodus flexuosus). At least three such condensed shales are recognized in the St. Peter succession in western Iowa, each overlying a pyritic hardground and each capped by a thicker interval of quartzarenite. The condensed shales are apparently represented eastward by hardground surfaces in the sandstone succession. The extreme condensation identified within the western facies indicates that sediment accumulation became highly attenuated at times across areas of the cratonic shelf, probably during transgressive sea-level events. Organic, phosphatic, and pyritic deposition is interpreted to have occurred during deepening episodes within a stratified dysoxic or anoxic water body, and the absence of carbonate material suggests calcite undersaturation within the water mass. Fluctuations and episodic overturn of the pycnocline may have triggered bioevents within the seaway resulting in high juvenile and adult mortality of the pelagic conodont faunas. The St. Peter Sandstone cannot simply be a broadly diachronous shoreface and nearshore transgressive sheet sand as some have suggested, but it is interpreted to be a complex sandstone body deposited during several major eustatic sea-level cycles.