EIFELIAN-GIVETIAN SILICICLASTIC CARBONATE RAMP SYSTEMS .2. CYCLOSTRATIGRAPHY AND PALAEOSTRUCTURATION

The French-Belgian Eifelian-Givetian (E-G) transitional beds (100-250 metres thick) developed along a storm-dominated pericratonic ramp (PRE AT & KASIMI, 1995). The locus of this second parr is on the small-scal e cyclicity and its probable control by Milankovitch forced sea-level oscillations. These E-G layers consist of ten types (cycles 1 to 10) o f 5(th) order upward-shallowing cycles (1 to 15 metres thick), Cycle d urations calculated along the ramp profile are between 17 and 53 ky an d about 35 ky on average, The cycles are ;symmetric and can be grouped into three major types: deep subtidal (cycles 1 to 3), shallow subtid al (cycles 4 to 8) and peritidal cycles (cycles 9 and 3) which are gen etically related and constitute a continuous series along the ramp pro file. Deep subtidal cycles consist of basinal argillites deposited bel ow storm wave base, overlain by storm-deposited line-grained siltstone s and shelly limestones; shallow subtidal cycles consist of stacked sk eletal packstone/grainstone deposits and reefal floatstone/rudstone bi ostromes Peritidal cycles are composed oi very shallow subtidal to int ertidal semi-restricted algal/calcisphaerid/ostracod fenestral wackest ones and laminites. The latter oi these display millimetre-thick lamin ations formed by algal binding of thin storm layers. The 5(th) order c ycles display the same stacking pattern within the six studied profile s end record an overall regressive prograding depositional sequence. T hese 5(th) order cycles are stacked to form several 4(th) order parase quence sets allowing definition of a regional third-order sequence of type 2 (sensu VAIL et al,, 1991). The overall characteristics oi these 5(th) order cycles i.e., 1) large lateral extension; 2) pronounced as ymmetry; 3) average duration between 17 and 53 ky; 4) a 1:5 bundling w ithin the 4(th) order parasequence sets; 5) a simultaneous development of subtidal and peritidal cycles, suggest a Milankovitch type climati c-eustatic control. The 1:5 recurrence ratio of metre-scale cycles wit hin the megacycles could represent the precession signal modulated by the short eccentricity signal. These eustatic sea-level fluctuations w hich are in accordance with the Milankovitch frequencies during the no n-glacial E/G period may be associated with lake and/or underground aq uifer volume variations. Systematic changes in the stacking patterns o f the 5(th) order cycles are used in conjunction with Fischer plots to define long-term sea-level changes, i.e, to define the 3(rd) order de positional sequence associated to the E/G transitional series, The E/G shelf margin systems tract (SMST) followed a long-term sea-level high stand (HST, cycles 1 to 3) and developed during a rapid sea-level fall (type-2 sequence boundary), followed by a long-term lowstand systems tract (cycles 4 to 10), The absence of any subaerial erosion err karst ification indicates that the sea-level fall was not important and did not exceed the subsidence rate. The absence oi emersion features, part icularly within the crinoidal and reefal buildups suggests that the am plitude of the sea-level fall was less than 20-25 metres during the E/ G transition, Based on conodont zonation, the duration of the lowstand systems tract was approximately 700 ky, so the relative sea-level fal l can be estimated to have been between 3 and 3.5 cm/1000 y. This valu e is in the range of the tectono-eustatic sea-level fluctuations.