CARBONATE RAMP DEPOSITIONAL SYSTEMS FROM A LATE JURASSIC EPEIRIC PLATFORM (IBERIAN BASIN, SPAIN) - A COMBINED COMPUTER MODELING AND OUTCROPANALYSIS

No counterparts to epeiric-sea carbonate ramps are known in present-da y environments. This hinders the interpretation of the factors control ling the growth and evolution of these depositional settings. In this study we analyse the facies and geometries of two Jurassic examples bo th from outcrop study and through computer modelling. This analysis is constrained by two important features of these Oxfordian and Kimmerid gian ramps: firstly, they are very well exposed, allowing accurate rec onstruction of a 200-km section from proximal to distal ramp environme nts, and, secondly, a time framework for correlation, section reconstr uction and modelling is provided by a well-defined ammonite biostratig raphy. The modelling results in a synthetic stratigraphy which closely matches the reconstructed cross-sections and, when integrated with th e field study, constrains and provides additional quantitative data on the following aspects of carbonate ramp systems. Resedimentation by s torms is an important process in maintaining the ramp profile through time. Down-ramp transport distances of between 25 and 40 km are indica ted from the distribution of storm beds and shallow-water allochems an d from model-matching known stratigraphic thicknesses and geometries. Modelling sediment production within the time constraints from the amm onite biozones indicates that shallow-water carbonate production was 1 -2 orders of magnitude less than that predicted for present-day open-m arine carbonate platforms. Deeper-water production rates were reduced by lesser amounts. These proportionally higher, outer-ramp production rates also help to maintain ramp geometries through time. The enigmati c slope crest of ramps is shown to result from a combination of higher , shallow-water production and erosion rates, together with loss of ac commodation during highstands and high-stillstands in the modelled sea -level curves. The most parsimonious modelling of the two ramp sequenc es comes from a relative sea-level curve composed of a linear subsiden ce component superposed by 20- and 100-kyr cycles on a third-order cyc le. The third-order cycles and their timing do not correspond to those of the Exxon curve.