High-frequency sea-level fluctuations recorded on a shallow carbonate platform (Berriasian and Lower Valanginian of Mount Saleve, French Jura)

The Berriasian and Lower Valanginian strata of Mount Saleve represent shall ow-marine to peritidal sedimentary environments dominated by carbonates. De positional sequences can be identified by a deepening-shallowing facies evo lution. Periodic emersion is indicated by root horizons, calcrete, and blac k pebbles. Elementary sequences are the smallest units (usually beds) displ aying such evolutions. They compose small-scale composite sequences again s howing a transgressive-regressive facies trend. These stack into medium-sca le composite sequences, which in turn build up large-scale sequences. Super position of emersion features directly on subtidal facies and the good corr elation of individual sequences over long distances suggest that relative s ea-level fluctuations played an important role in sequence formation. A 193-metre long, almost continuously exposed section is analysed in terms of sequence stratigraphy and cyclostratigraphy. Ten large-scale sequences h ave been identified. Due to superposition of high-frequency sealevel change s over a long-term trend, maximum-flooding zones and sequence-boundary zone s commonly occur rather than well-expressed single surfaces. Biostratigraph ic control by charophyte-ostracod assemblages, benthic foraminifera, and ca lpionellids allows to calibrate most large-scale sequence boundaries and to compare them with 3rd-order sequence boundaries of regional or global impo rtance. A repeated stacking of 2-6 elementary sequences into one small-scale sequen ce, and of 4 small-scale sequences into a medium-scale sequence is observed . At least 72 small-scale sequences making up 19 medium-scale sequences hav e been counted or are inferred from lateral correlation in the interval bet ween 3rd-order sequence boundaries Be1 and Va1. Based on Gradstein et al. ( 1995), sequence boundary Be1 is dated at 144.2 (+/- 2.6) Ma by Hardenbol et al. (1998), sequence boundary Va1 at 136.5 (+/- 2.2) Ma. This general time framework and the hierarchical stacking pattern suggest that the formation of the depositional sequences was, at least partly, related to processes c ontrolled by orbital cycles. The small-scale sequences may correspond to th e 100-ky eccentricity cycle of the Earth's orbit, and the medium-scale sequ ences to the 400-ky eccentricity cycle. Thus, a time span of 7.2 to 7.6 my for the studied interval is implied by cyclostratigraphy (compared to 7.7 /- 4.8 my proposed by Hardenbol et al. 1998). The elementary sequences are interpreted to have formed in tune with the 20-ky cycle of precession of th e equinoxes. However, because of low accommodation around large-scale seque nce boundaries, not all elementary sequences have been recorded, and small- scale sequences may be condensed. During longer-term sea-level rises, the h igh-frequency sealevel drops were in many cases too attenuated to create el ementary sequences. Late Cimmerian tectonic activity induced a major hiatus , but was not strong enough to mask the record of eustatic sea-level change s.