6(a). ‘Wrinkle structures’ – a critical review

In this section, we critically review a variety of so-called ‘wrinkle structures’ in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces, and true microbially induced ‘wrinkle structures’. The latter, however, are not easily differentiated from the numerous small-scale, crinkly irregularities on ancient bedding surfaces that originate from physical processes without participation of microbial communities, for instance: types of rill marks (e.g., Shrock, 1948; Reineck and Singh, 1980); swash marks (e.g., Reineck, 1956); foam and moving foam impressions (Häntzschel, 1935; Allen, 1966; Wunderlich, 1970; Reineck and Singh, 1980); adhesion warts and adhesion ripples (‘antiripplets’ after van Straaten, 1953; ‘eolian microridges’ after Hunter, 1969; see, e.g., Reineck, 1955; Kocurek and Fielder, 1982); rain impact ripples (e.g., Clifton, 1977; Reineck, 1984; Robb, 1992); and small-scale load structures (e.g., Allen, 1985). All these may occur, like microbially induced ‘wrinkle structures’, in the intertidal to lower supratidal zones. Furthermore, some current marks and fleur-de-lys patterns known from turbiditic successions (e.g., Dzulynski and Walton, 1965) as well as tectonically induced crenulations may develop similar appearances in response to physical processes.