FISCHER PLOT AND SPECTRAL-ANALYSIS APPLIED TO CYCLOSTRATIGRAPHY OF LIASSIC CARBONATES OF THE SOUTHERN CAUSSE DU LARZAC, SOUTH FRANCE

We provide a cyclostratigraphic partition for the southern <<Causse du Larzac>> Liassic carbonates, that lack stratigraphic fossils. This in terval is subdivided into four lithologic units, and biostratigraphic data only concern the first unit and the top of the fourth unit. The s econd unit, 200 m thick, shows a vertical stacking of 170 shallowing u p, peritidal cycles capped by emersive boundaries. Spectral analysis o f 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 precess ion of Earth orbital periods. We propose a 7.5 m.y. duration for the s econd unit and we locate the Hettangian/Sinemurian boundary in the mid dle, not at the top of this unit. Cyclostratigraphy allows to extend t ime lines biostratigraphically well defined in outer shelf towards the peritidal formations of the inner domains lacking biostratigraphic co ntrol. Moreover that allows global quantification of the amalgamated o r missed eustatic cycles in a peritidal record.