Stratigraphy and Depositional History within Incised-Paleovalley Fills and Related Facies, Douglas Group (Missourian/Virgilian; Upper Carboniferous) of Kansas, U.S.A.
The Douglas Group (Stephanian) of eastern Kansas contains several paleovalleys that were eroded during falling sea level and filled during lowstands and subsequent transgressions. One paleovalley exhibits 34 m of incision, is approximately 32 km in width, and can be laterally traced along outcrop and into the subsurface to the south for approximately 140 km. A fluvial to estuarine to marine facies mosaic can be delineated both laterally, from north to south, as well as within individual vertical sections. Paleovalleys were filled with a fining upward succession; the lowest facies is cross-bedded conglomerate and sandstone. The conglomerate contains clasts and fossils eroded from older units exposed within the paleovalley. Sandstone beds exhibit large scale (up to 1 m thick) trough and tabular-planar cross beds. Paleocurrent directions are generally southwest and indicate deposition via large-scale fluvial systems that were constrained within the paleovalleys. Overlying the fluvial sandstone is a diverse suite of lithofacies including planar-bedded sandstones and siltstones, heterolithic facies, sheet-like sandstone, bioturbated sandstones, and marine facies. The planar-bedded sandstones and siltstones can exhibit neap-spring tidal cycles which were formed in high-intertidal settings. Heterolithic facies are typically laminated and contain pinstripe laminations, starved ripples, and well-developed tidal cycles (cyclical tidal rhythmites). Neap-spring tidal cycles are common and range from 1 cm in thickness in heterolithic facies to as much as 1 m in thickness in planar-bedded siltstones. An interpretation invoking very high localized depositional rates is substantiated by the presence of buried upright trees, some of which have attached foliage. Tidal rhythmites are well developed in siliciclastic facies immediately overlying coals. The heterolithic and silty rhythmites were apparently developed within the estuarine turbidity maximum where high turbidity and locally high depositional rates resulted from estuarine circulation patterns and tidal amplification. The sheet-like sandstone bodies are dominated by small-scale trough crossbedding and ripple- and planar laminations. Paleocurrents are bimodal to the southwest and northeast, reflecting ebb- and flood-tidal currents. Features such as flat-topped ripples, rain-drop imprints, and tetrapod trackways indicate deposition within the intertidal zone. Estuarine to marine sequences contain progressively higher diversities of biogenic structures. "Flaggy" bioturbated sandstones indicate significant marine influences. These sandstones are capped by widespread marine shales and limestones that extend far beyond the limits of paleovalleys. Shales can be extensively bioturbated, lack laminations, and locally contain marine body fossils. Limestones form widespread lithostratigraphic markers and contain abundant marine fossils such as bivalves, fusulinids, brachiopods, crinoids, and bryozoans. Some of the limestones consist of shelly lags which indicate the development of transgressive surfaces of erosion. There are two major sequences developed within the Douglas Group. The sequence boundaries can be placed at the contact between incised fluvial sandstones and eroded underlying, commonly marine, strata. The fluvial and estuarine facies were deposited during lowstand and subsequent sea-level rise. The highstand system includes marine shales and limestones which were erosionally incised during subsequent fall in sea level.