Abstract: Progressive Changes in Siliciclastic Depositional Styles, Middle and Upper Pennsylvanian Cyclothems, Midcontinent, USA

Robert L. Brenner

Middle through Upper Pennsylvanian strata in the Midcontinent record changes in sedimentary styles from terrigenous to marine process dominance. During the middle Pennsylvanian (e.g., Desmoinesian Cherokee Group) deltaic complexes were fluvially dominated with only minor effects of marine waves or tidal processes. Later in the Pennsylvanian, fluvial processes weakened in relation to marine processes, perhaps due to climatic changes affecting fluvial discharge. Upper Pennsylvanian cycles (e.g., Missourian Kansas City Group), contain intervals of laminated siltstone, shale, and flaser-bedded sandstones, suggesting that tidal processes became more dominant. Tidal influence continued to increase through the Late Pennsylvanian to the extent that estuarine deposits comprise much of silicicl stic deposits (e.g., upper Missourian--lower Virgillian Douglas Group).

Maximum regressive was generally marked by scoured surfaces formed as sea level dropped below regional base levels. Subsequent rise in sea levels, caused back-filing of scours with fluvial sands and conglomerates. Reduced flow rates within fluvial channels caused by sea level rise, resulted in lower sediment transportation rates, lower gradients, and abandonment of some channels. Coastal water tables rose as marine transgression progressed, allowing the formation of laterally extensive peat bogs represented by coals that cap siliciclastic intervals. In some intervals, such as the Chanute Shale (Missourian Kansas City Group), an argillacous, crinoidal limestone bed occurs at the coal horizon, with coal having developed on topographic highs over underlying fluvial-deltaic complexes, whi e limestone formed in marine waters that covered adjacent low-lying areas. Shallow marine muds were deposited over coals as marine waters inundated coastal peat bogs.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994