Palaeotidal speeds and ranges estimated from cross-bedding sets with mud drapes

Abstract

Tidally formed cross-bedding sets in which mud drapes are spaced out among sandy foresets occur in Cretaceous1–4, Eocene5 and Holocene6 sands deposited subtidally by strongly asymmetrical sand waves. A striking feature of some sets4,6 is that the horizontal drape spacing, measured in the direction of sand-wave advance, varies cyclically on a time scale related to the spring–neap tidal cycle as indicated by the numbers of drapes. Each drape forms part of a distinctive lithological couplet, consisting of a group of sandy foresets recording avalanching on the steeper side of the sand wave, followed by the drape itself, typically divided into upper and lower parts by a centrally placed silt‐sand parting. The drapes have been assigned to slack water, but the groups of sandy foresets to episodes of vigorous flow, the combination suggesting a quantitative model based on the tide. Here I show how the peak speeds and ranges of ancient tides can be estimated by combining an empirical bedload transport function with appropriate measurements from such cross-bedding sets.