Application du Fischer plot et de l'analyse spectrale a la cyclostratigraphie d'une serie peritidale discontinue; exemple du Lias carbonate, bordure sud du Causse du Larzac, France

Universite Montpellier-II, ISTEEM, Montpellier, France

We provide a cyclostratigraphic partition for the southern "Causse du Larzac" Liassic carbonates, that lack stratigraphic fossils. This interval is subdivided into four lithologic units, and biostratigraphic data only concern the first unit and the top of the fourth unit. The second unit, 200 m thick, shows a vertical stacking of 170 shallowing up, peritidal cycles capped by emersive boundaries. Spectral analysis of the peritidal cycle thicknesses shows two bundling ratios: bundles of 9.3 cycles include bundles of 2.6 cycles. We conclude to a climato-eustatic control of cycle bundling, turned on eccentricity and precession of Earth orbital periods. We propose a 7.5 m.y. duration for the second unit and we locate the Hettangian/Sinemurian boundary in the middle, not at the top of this unit. Cyclostratigraphy allows to extend time lines biostratigraphically well defined in outer shelf towards the peritidal formations of the inner domains lacking biostratigraphic control. Moreover that allows global quantification of the amalgamated or missed eustatic cycles in a peritidal record.

This record provided courtesy of AGI/GeoRef.