A MODEL FOR THE ESTIMATION OF SUBSIDENCE AND SEA-LEVEL VARIATIONS - AN EXAMPLE OF THE OXFORDIAN OF THE SWISS JURA

During deposition of the Natica member and the lower part of the Haupt mumienbank member (Vellerat Formation) and the corresponding members o f the Balsthal Formation (Gunsberg member and Steinibach beds), many f acies changes typical of a shallow-marine carbonate platform influence d by episodic terrigenous input are recorded. Despite a constantly sha llow bathymetry with numerous emersions, no major erosional hiatuses a ppear and obliterate sedimentary sequences. Elementary sequences can b e correlated between the different sections, and a sequence hierarchy is evidenced and related to the two orbital eccentricity cycles. The m odel presented here postulates a rate of subsidence similar to the rat e of sea-level fall caused by Milankovitch type oscillations. Thus, du ring emersion the sediment is maintained close to the water surface. W ith an almost complete conservation of the elementary sequences, it is possible to relate the sediment accumulation to the amplitude of sea- level oscillation and to the accomodation space created by subsidence. In comparing the different sections and using equations relating accu mulation, subsidence and sea-level variation, values of total subsiden ce as well as amplitudes of short-term and long-term variations of sea level may be estimated. An important subsidence rate in the order of 65 m/My during three 400,000-year cycles is suggested. The main events marking the sedimentary history, such as formation of sequence bounda ries, transgressive surfaces and maximum flooding surfaces, are explai ned by short-term sea-level oscillations with amplitudes of about 1 to 3 metres. Features related to long term trends (third-order regional variation rates of roughly 15 m/My) can be distinguished only if highl ighted by shorter term sea-level variations.