Abstract

Meter-scale, peritidal carbonate cycles are a common feature of the geological record but debate continues about what processes lead to their formation. Three conceptual models, or a combination thereof, are commonly invoked to explain cycle formation; eustasy, tectonics, or autocyclicity. These three models are tested with a large new dataset from different Early Jurassic plate margins from western Tethys. Study of seven logged sections from Spain, Italy, Greece, Tunisia, Morocco, and Gibraltar enables an analysis of the possible roles of local versus regional patterns and controls on cyclicity within a Sinemurian time slice. Cycle types are diverse and include shallowing-upward cycles (parasequences) but also deepening-upward and diagenetic cycles (high-frequency sequences) and subtidal cycles. Numbers of cycles per section and cycle stacking patterns within this time slice vary from section to section. Statistical tests (runs tests, time series, and bundling) all indicate random stacking of cycles within sections and an absence of any bundling of thicknesses or of facies trends. Assessment of cycle types by their occurrence and stacking patterns indicates little support for either eustasy or autocyclicity being the dominant cycle-forming mechanism. However, the variability in numbers of cycles per section, thickness variations of the sections, cycle type variability, and randomness of stacking patterns all favor a pulsed, tectonic control for the creation and filling of accommodation space. This conclusion is further supported by evidence that has largely arisen during the course of this study of syndepositional extensional tectonics in the Sinemurian on these rifted Tethyan margins. Although tectonics appears to be the dominant control, superimposed eustasy and/or autocyclic processes cannot be discounted.

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