Journal of Conference Abstracts

Low- and High-Frequency Sea-Level Changes Control Peritidal Carbonate Cycles, Facies and Dolomitization in the Rock of Gibraltar (Early Jurassic, Iberian Peninsula)

Dan Bosence (bosence@gl.rhbnc.ac.uk)¹, Jason Wood², Ted Rose¹ & Hairou Qing³

¹ Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK

² School of Earth and Environmental Sciences, University of Greenwich, Chatham Maritime, Kent, ME4 4AW, UK

³ Geology Department, University of Regina, Regina SK, Canada S43 OA2

The Rock of Gibraltar comprises two tectonically separated limbs of an isolated klippe of the Liassic, Gibraltar Limestone Formation. Both limbs have similar, circa 400 m thick sequences of inner carbonate platform facies arranged in high-frequency, metre-scale, shallowing-upward, peritidal cycles with emergent, caliche caps. Four cycle types are recognised on the basis of vertically repeated successions of different sedimentary structures, lithologies, facies and biota. These vary from those developed in more interior platform top environments at the base of the succession to cycles developed in more marine influenced platform-top environments higher in the succession. Strontium isotopes prove a Sinemurian age for the Gibraltar Limestone, and together with Carbon and Oxygen stable isotopes, indicate early dolomitization from refluxing pore waters with slightly increased salinities. When compared with other Liassic carbonate sections from fault-bound platforms from the western Tethys Ocean all are found to be of similar scale, facies and cycle type. Likely common origins are through Milankovitch band allocyclicity, or autocyclic tidal flat progradation superimposed on regional subsidence. Despite the occurrence of small-scale cycles on fault-bound platforms tectonic controls are discounted because of the facies successions within cycles and the regularity and similarity of cycles throughout individual sections and the Tethyan region. Within the Gibraltar Limestone high-frequency cycles are superimposed on a low-frequency (3rd 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 facies and cycles and by early reflux dolomitization. Transgressive and highstand phases, with more accommodation space, are characterized by the absence of early dolomites, the incoming of inner platform marine microfossils (i.e. foraminifera and calcareous algae) and by less restricted marine facies (i.e. oncoids, shelly rudstones, packstones and grainstones). Fischer plots are shown to have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas.

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