ABSTRACT The Early Mississippian Northview Formation is a siliciclastic deposit reaching 25 m in thickness along an east-west trend in southwest Missouri, USA. Along this trend, the formation coarsens upward from shale to coarse siltstone with shaley interbeds and channel fills. The siltstones have high concentrations of two trace fossils: Nereites missouriensis and Zoophycos . Thus, the ichnofauna is dominated by namesake genera of the two deepest marine ichnofacies. Nevertheless, the Northview was deposited on a shallow carbonate platform. The vertical succession of trace fossils is consistent with deltaic deposition. The lowest shale is dominated by common elements of the Phycosiphon prodelta ichnofacies, and the lowest overlying siltstone is dominated by elements of the Rosselia delta-front ichnofacies. N. missouriensis occurs abundantly and dominates the overlying siltstones, with Zoophycos covering bedding planes near the top, a pattern found within the lower plain of other Paleozoic deltas. Geochemical proxy measurements indicate paleosalinity decreasing upward through the formation and an approaching freshwater (fluvial) source, raising questions about Early Mississippian source areas and tectonics.

ABSTRACT A succession of Ordovician and Mississippian carbonates, separated unconformably, is exposed across the southern flank of the Ozark Dome in southwest Missouri. Deposits of both periods exemplify typical facies of the Midwestern United States: carbonate tidal-flat assemblages for the Early Ordovician and carbonate shelf environments for the Early–Middle Mississippian. The basic stratigraphic sequence of these deposits has been known for over a century, but interesting features remain to be addressed. Thin discontinuous sandstones are present within the Early Ordovician Cotter Dolomite, but the informal Swan Creek sandstone member seems anomalous. This sandstone can exceed 5 m in thickness and is fairly continuous across southwest Missouri. Most Ordovician sandstones in Missouri mark major transgressions above regional unconformities, but not the Swan Creek, and there is no obvious source of the sand. Therefore, we hypothesize that the Swan Creek represents reworked eolian dunes blown across the broad peritidal environment. Clastic sandstone dikes, apparently sourced from the Swan Creek, cut across beds of Cotter Dolomite near faults. We propose that these dikes are evidence of local faulting and seismicity during the Early Ordovician. Early and Middle Mississippian limestones comprise a sequence of shelf deposits, although mud mounds and other facies changes near the Missouri-Arkansas line mark the edge of the Mississippian shelf and the transition to a ramp setting. Early Mississippian carbonate deposition was interrupted by a short and localized influx of siliciclastic sediment comprising the Northview Formation. The Northview has additional characteristics consistent with a river-dominated deltaic deposit, which we suggest as its origin. If correct, this hypothesis implies that the history of tectonic features in the Midwest is more complicated than yet known. Finally, facies changes within and between the local Mississippian formations may record an early crustal response to the impending Ouachita orogeny farther to the south.