TIDAL SEDIMENTATION FROM A FLUVIAL TO ESTUARINE TRANSITION, DOUGLAS GROUP, MISSOURIAN VIRGILIAN, KANSAS

The Tonganoxie Sandstone Member of the Stranger Formation (Douglas Gro up, Upper Pennsylvanian, Kansas) was deposited in a funnel-shaped, nor theast-southwest-trending paleovalley that was incised during the uppe rmost Missourian sealevel lowstand and backfilled during the subsequen t transgression. Quarry exposures of the Tonganoxie near Ottawa, Kansa s, include approximately 5 m of sheetlike, vertically accreted siltsto nes and sandy siltstones, bounded above and below by thin coals with u pright plant fossils and paleosols. Strata range from submillimeter-th ick, normally graded rhythmites to graded bedsets up to 12.5 cm thick with a vertical sedimentary structure sequence (VSS) consisting of the following intervals: (A) a basal massive to normally graded interval; (B) a parallel-laminated interval; (C) a ripple-cross-laminated inter val; and (D) an interval of draped lamination. The VSS-C intervals of thicker bedsets are characterized by climbing ripples that evolve from Type A (erosional-stoss) to Type B (depositional-stoss). Synsedimenta ry convolutions at the tops of many climbing-ripple sequences and a va riety of water-escape structures indicate rapid deposition. The vertic al sequence of sedimentary structures indicates each bedset was deposi ted by a waning current with significant suspended load. The Tonganoxi e succession has many similarities to fluvial overbank/floodplain depo sits: sheetlike geometry, upright plant fossils, lack of bioturbation and body fossils, dominance of silt, and a punctuated style of rapid s edimentation from suspension-laden waning currents. Missing, however, are thick clay drapes or evidence of prolonged exposure and desiccatio n, which generally characterize a floodplain sequence with seasonal ov erbanking. Physical and biogenic sedimentary structures-including tetr apod trackways, surface grazing traces, abundant raindrop impressions, wind ripples, runzel marks, runnel marks, and runoff washouts-indicat e that subaerial exposure was periodic and brief, and may have followe d each sedimentation event. Analysis of stratum-thickness variations t hrough the succession suggests that tides significantly influenced sed iment deposition. Strata ranging through three orders of magnitude sys tematically thicken and thin, recording the influence of an ebb-domina ted, diurnal tidal system with a well developed semimonthly inequality . By conservative estimate of sedimentation rates based on neap-spring tidal cycles, the sequence aggraded at an average rate of approximate ly 3.8 m/yr. These unusually high rates appear to have prevailed for o nly a short time and were probably spatially restricted within the bas in. A fluvial-to-estuarine transitional depositional setting is interp reted for the Tonganoxie by analogy with modern depositional settings that show similar physical and biogenic sedimentary structures, vertic al sequences of sedimentary structures, and aggradation rates.