Low- and high-frequency sea-level changes control peritidal carbonate cycles, facies and dolomitization in the Rock of Gibraltar (Early Jurassic, Iberian Peninsula)

The Rock of Gibraltar comprises two tectonically separated limbs of an isol ated klippe of Liassic Gibraltar Limestone Formation. Both limbs have simil ar, c. 400 m thick sequences of inner carbonate platform facies arranged in high-frequency, metre-scale, shallowing-upward, peritidal cycles with emer gent, caliche caps. Four cycle types are recognized on the basis of vertica lly repeated successions of different sedimentary structures, lithologies, facies and biota. When compared with other Liassic cycles from fault-bound platforms of the western Mediterranean region all are found to be of simila r scale, facies and cycle type. Likely common origins are through Milankovi tch band allocyclicity, or autocyclic tidal flat progradation superimposed on regional subsidence. Within the Gibraltar Limestone high-frequency cycles are superimposed on a low-frequency (third order?) cyclicity that is revealed, through the use of Fischer plots, to control the occurrence of facies, biota, high-frequency cycle types and dolomitization. Falling sea-level and lowstand phases, with reduced accommodation space, are typified by restricted, inner platform fa cies and cycles and by early reflux dolomitization. Transgressive and highs tand phases, with more accommodation space, are characterized by the absenc e of early dolomites, the incoming of inner platform microfossils (i.e, for aminifera and calcareous algae) and by less restricted marine facies (i.e. oncoids, shelly rudstones, packstones and grainstones). Fischer plots have demonstrable value in the correlation and analysis of te ctonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas.