The Origin of High-Frequency Peritidal Cycles: A Regional Comparative Project from Liassic Platforms of Western Tethys

Dan Bosence¹, Marc Aurell², Marcelle Boudagher-Fadel³, Francesca Casaglia⁴, Simonetta Cirilli⁵, Mohammed Mehdie⁶, Luis Nieto⁷, Emily Procter¹, Javier Rey⁷, Rudy Sherreiks⁸, Mohammed Soussi⁹, and Dave Waltham^1
1 Royal Holloway University of London, Egham, Surrey, England
² Universidad de Zaragoza
³ University College London, England
⁴ ENI – Exploration & Production
⁵ Piazza Universita, Perugia, Italy
⁶ Université Ibn Tofail, Maroc
⁷ Universidad de Jaén, Spain
⁸ Geologische Staatssammlung, Munich, Germany
⁹ Université de Tunis, Tunisia

High frequency, metre-scale, cycles occur throughout the Phanerozoic but the debate still rages about their recognition; whether strata have random or cyclic organisation, and if there is a single driving mechanism, is it autocyclic tidal flat/island progradation, allocyclic fault-related tectonism or climatically controlled eustacy?

This presentation introduces a collaborative project that assesses the nature and origin of high-frequency cycles originally formed in western Tethys. All are Sinemurian in age and this provides a unique opportunity to assess the relative importance of different possible mechanisms of cycle formation. Time-equivalent sections were logged bed-by-bed, from seven carbonate platforms from different basin margins (North and South Tethys) and different isolated microplates (Apulia and Pelagonia). New dating has been achieved through using a combination of micropalaeontology (benthic foraminifers and algae) and strontium isotopes. Markov chain analysis of bed transitions indicates that the strata are organised into metre-scale cycles. However, cycle structure is varied, and comprises a) asymmetric, shallowing-upward cycles with or without emergence or, b) emergence of subtidal facies that then deepen upward to form asymmetric cycles, or c) more symmetric deepening then shallowing cycles.

Different locations are characterised by a dominance of particular cycle types, particular emergent facies, cycle stacking patterns, or evidence for syn-depositional extensional tectonics. The range of preserved cycle types rules out autocyclicity as a control. The local specificity of cycle types and their stacking argues against a eustatic control and, together with evidence of syndepositional tectonics, favours a tectonic origin for these high-frequency cycles.