Very low term (250 Myr) quantication of the eustasy during mesozoic-cenozoic time based on coastal onlap measurement at the Tethys on world-scale
Résumé
Eustasy is a reference level of primary importance in the Earth Sciences. That control part of the stratigraphic record, the area of continental surfaces exposed to erosional processes, and the base level of the fluvial systems. Several long term sealevel curves for the past 250 My were published. The most popular one is the one proposed by Haq et al. (1987). Most agree that sea level rise occurred from the Permian to the Upper Cretaceous, and was followed by a sea level fall that continues today. Controversy persists, however (1) on the amplitude of this variations, (2) on the kinematics of the sea level fall or rise and (3) about the required mechanism, which must be related to the plate movements, associated with the Wilson Cycle. Our first objective is to question and to quantify the sea level variations for long (250 My) term time durations. We will focus on the Cretaceous, which displayed the highest sea level during the last 250 My. The first part of our study will be based on the measurement of the marine flooding of continents through time. This method requires new style world-scale paleogeographic maps, quantified in term of paleotopography, to measure both the coastal onlap and the hypsometry of the Earth at each time-interval. The main challenge is the quantification of the topography of the Earth for Mesozoic times. Our technique is based on a global-scale measurement of the marine flooding of continents on Earth paleogeographic maps. Sea level is inferred, for a given time interval, from the intersection of this world-scale flooding with the distribution of the world elevation, or hypsometry. The continental flooding is the percentage of the continental domain flooded by the sea. This percentage is defined from a specific geographic reference level that can be the shelf break or the present-day shoreline. The hypsometric curve is the cumulative curve of areas of land between pairs of contour lines as a percentage of the total land area. We used a new paleogeographical dataset for the Meso-Cenozoic: one at a world-scale (Vrielynck & Bouysse, 2001), and one at a Tethys-scale (Dercourt et al., 2000). These first results are based on the present-day continental hypsometry. But does the present-day altitude distribution apply for the past? Present-day continents are mainly subjected to erosion, and few subsiding domains occur. The present-day continental topography is different from the past one, mainly Upper Jurassic - Lower Cretaceous time, where large intracontinental basins with low relief, occurred. The present-day altitude distribution, which could be the best analogue of this period, is the Amazon watershed. Using this last one and the world-scale paleogeographic map, the highest sea level occurs during Upper Cretaceous time and the amplitude would be +100 m above present-day sea level. Using Tethys-scale paleogeographic maps, the highest relative sea level occurs during Cenomanian time and the amplitude would be +250 m above present-day sea level.