Abstract

S. Q. Sun writes: The recent description and discussion by Allen & Underhill (1989) of the swaley cross-stratification of the Bencliff Grit (Upper Jurassic), Dorset coast, is an interesting attempt to explain the observed sedimentary structures and lithofacies association. They provide a very thorough description of the Bencliff Grit and conclude that the sedimentary structures in the Bencliff Grit example are a variety of hybrid bedforms, containing elements of both large (metre) scale trough cross-stratification and the decimetre-scale hummocky cross-stratification, produced by essentially unidirectional flows. However, from my own work (Sun 1989), I disagree with their identification of bedforms and mechanism producing them in this specific example.

First, they clearly demonstrate that swaley cross-stratification can be distinguished from trough cross-stratification in a number of ways: (1) swale margins have lower inclinations than do those of troughs, (2) swales are wider and shallower than troughs, (3) laminae drape swale margin concordantly rather than exhibiting discordant relationship with respect to the lower bounding surface, and (4) swaley cross-stratification looks the same in all vertical sections regardless of orientation, as opposed to the heterogeneous geometry of trough cross-stratification. I fully agree with their suggestion that the bedforms in the Bencliff Grit are extremely good candidates for interpretation as swaley cross-stratification on the basis of the first two criteria. Their widely-dispersed palaeocurrent pattern and notice of the conspicuous absence of angle of repose cross-strata are also consistent with my field observations. With respect to the last two criteria, it is highly unlikely to test

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