Abstract

The Tonganoxie Sandstone Member of the Stranger Formation (Douglas Group, Upper Pennsylvanian, Kansas) was deposited in a funnel-shaped, northeast-southwest-trending paleovalley that was incised during the uppermost Missourian sealevel lowstand and backfilled during the subsequent transgression. Quarry exposures of the Tonganoxie near Ottawa, Kansas, include nearly equal 5 m of sheetlike, vertically accreted siltstones and sandy siltstones, bounded above and below by thin coals with upright plant fossils and paleosols. Strata range from submillimeter-thick, 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 interval; 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). Synsedimentary convolutions at the tops of many climbing-ripple sequences and a variety of water-escape structures indicate rapid deposition. The vertical sequence of sedimentary structures indicates each bedset was deposited by a waning current with significant suspended load. The Tonganoxie succession has many similarities to fluvial overbank/floodplain deposits: sheetlike geometry, upright plant fossils, lack of bioturbation and body fossils, dominance of silt, and a punctuated style of rapid sedimentation from suspension-laden waning currents. Missing, however, are thick clay drapes or evidence of prolonged exposure and desiccation, which generally characterize a floodplain sequence with seasonal overbanking. Physical and biogenic sedimentary structures--including tetrapod trackways, surface grazing traces, abundant raindrop impressions, wind ripples, runnel marks, runnel marks, and runoff washouts--indicate that subaerial exposure was periodic and brief, and may have followed each sedimentation event. Analysis of stratum-thickness variations through the succession suggests that tides significantly influenced sediment deposition. Strata ranging through three orders of magnitude systematically thicken and thin, recording the influence of an ebb-dominated, 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 approximately 3.8 m/yr. These unusually high rates appear to have prevailed for only a short time and were probably spatially restricted within the basin. A fluvial-to-estuarine transitional depositional setting is interpreted for the Tonganoxie by analogy with modern depositional settings that show similar physical and biogenic sedimentary structures, vertical sequences of sedimentary structures, and aggradation rates.

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