Recognition of tidal-bore deposits in the rock record; towards a facies model

A facies model for the deposits of tidal bores (upstream-propagating hydraulic jumps associated with the flood tide in estuarine rivers) has not yet been developed, despite the publication of data from laboratory experiments and some modern estuaries. Moreover, only one example of tidal-bore deposits has hitherto been identified in the rock record. Herein, we document evidence for tidal bores in an Upper Pennsylvanian (Virgilian, Gzhelian) incised-valley fill in northeastern Kansas, USA. Across the flanks and crest of a bank-attached bar remnant in this fill are thin (< 10 cm) lenses of muddy, massive sandstone and massive, plant-debris-rich sandstone bounded by erosion surfaces exhibiting symmetrical wave ripples and irregular scouring. This interval is enclosed by sandstones with bidirectional cross-stratification, rhythmites, mud drapes, flaser bedding, and related sedimentary structures that together record appreciable tidal influence. The erosion surfaces bounding the massive sandstone lenses, and the massive sandstone lenses themselves, are evidence for scour and sediment suspension by upstream-propagating bores followed by deposition from suspension after their passage. Not only are these deposits the first sedimentary record of tidal bores to be documented in the Pennsylvanian of the USA, they are only the second example of such deposits to be documented in the global rock record. On the basis of our observations, we propose a provisional facies model for tidal-bore deposits that can be refined by future work.