Recognition of Tidal Bore Deposits in the Stratigraphic Record

Abstract:

Tidal bores are upstream-propagating hydraulic jumps that episodically form the leading edge of flood tides in upstream-narrowing, gently sloping, coastal rivers which experience high tidal ranges (6 m+, typically). They attain 9 m in height and can penetrate more than 100 km inboard of a shoreline. The deposits of tidal bores, if they can be confidently diagnosed in ancient successions, constitute an unequivocal line of evidence in support of deposition within the tidal-fluvial zone of a lowland river. Deposits of modern tidal bores have been documented, and two recent studies have interpreted tidal bore deposits in ancient (Jurassic and Pennsylvanian) sedimentary rocks. The ancient tidal bore deposits recognized thus far comprise laterally discontinuous, erosionally-based beds of massive to faintly stratified sand, locally muddy and rich in detrital plant debris. Paleoflow structures, where preserved, indicate upstream-directed flow. The beds are enclosed by other, more pervasively stratified sandstones that were the product of ebb and flood-oriented currents in the ancient tidal rivers. The interpreted bore deposits are anomalous in the context of these normal current deposits, and indicate erosive scouring of the substrate followed by the en masse deposition of sand from suspension and upstream advection. Multiple horizons of putative tidal bore deposits are recognized in both the Jurassic and Pennsylvanian examples, suggesting that they were not the product of low-frequency, high-magnitude events such as tsunami or debris flows. It is anticipated that more examples of ancient tidal bore deposits will come to light now that diagnostic criteria are available, and that these will contribute to the fuller recognition of tidally-modulated fluvial deposits in the rock record.