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Journal of the Geological Society

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Discussion on large sea, small tides: the Late Carboniferous seaway of NW Europe

^{1 1}Geoclastica Ltd, 9 Jefferies Close, Marlborough SN8 1UB, UK (e-mail: rogerhiggs@geoclastica.com)
^{2 2}Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK (e-mail: p.a.allison@imperial. ac.uk)
^{3 3}The George Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, USA

The first 20% of the full text of this article appears below.

Roger Higgs writes: Wells et al. (2005a,b) produced an innovative and thought-provoking numerical model of tides in the Late Carboniferous seaway of NW Europe, predicting very low tidal ranges (<1 m), consistent with the scarcity of sedimentary evidence for (weak) tides reported in the corresponding rocks. Based on the model, Wells et al. (2005a) argued intuitively that tidal range was minimum during (glacioeustatic) highstands, and maximum during lowstands (and transgressions). I would argue the converse.

During highstands, not only was the water volume (mass) greater, leading potentially to larger tides, but also any tide in the adjacent ocean would necessarily be passed into the seaway, adding to the locally generated tide. This is a weakness of the model: the oceanic tidal influence was ‘not incorporated for simplicity’ (Wells et al. 2005a, p. 418).

In contrast, during lowstands, the seaway became a freshwater lake (or chain of lakes), perched at the level of the outflow sill, and bordered by incised highstand prograded deltas and alluvial plains (‘Lake Bude’ of Higgs 1991, 2004). Disconnected from the global ocean, the lake would have had an extremely low tidal range (<5 cm), as in all lakes, because of the relatively small water volume (Talbot & Allen 1996). Indeed, no evidence for tides has been reported in Lake Bude deposits (Ross, Crackington and Bude Formations of Ireland and England; Higgs 1991, 2004, and references therein). However, given the (scarce) evidence reported elsewhere in the seaway, sedimentary structures indicative of weak tidal currents (e.g. ripples with foreset slack-water mud drapes; rhythmic lamination; opposed ripples) may eventually be discovered in the Lake Bude formations, probably associated with the volumetrically minor shale bands with brackish and marine fossils (Higgs 1991, 2004. . . [Full Text of this Article]