Eustatic Curve for the Middle Jurassic--Cretaceous Based on Russian Platform and Siberian Stratigraphy: Zonal Resolution

Abstract

We have used the stratigraphy of the central part of the Russian platform and surrounding regions to construct a calibrated eustatic curve for the Bajocian through the Santonian. The study area is centrally located in the large Eurasian continental craton, and was covered by shallow seas during much of the Jurassic and Cretaceous. The geographic setting was a very low-gradient ramp that was repeatedly flooded and exposed. Analysis of stratal geometry of the region suggests tectonic stability throughout most of Mesozoic marine deposition. The paleogeography of the region led to extremely low rates of sediment influx. As a result, accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations. The central part of the Russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the Jurassic and Cretaceous. The biostratigraphy of the Russian platform provides the basis for reliably determining age and eustatic events. The Mesozoic section of the central part of the Russian platform is characterized by numerous hiatuses. In this study, we filled the sediment gaps left by unconformities in the central part of the Russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions, such as northern Siberia. Although these sections reflect subsidence, the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled, so the subsidence rate can be calculated and filtered from the stratigraphic data. We thus have compiled a more complete eustatic curve than would be p ssible on the basis of Russian platform stratigraphy alone. Relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the Russian platform. For determining paleowater depth, we developed a model specifically designed for this region based on paleoecology, sedimentology, geochemistry, and paleogeography. The curve describes a series of high-frequency eustatic events superimposed on longer term trends. Many of the events identified from our study can be correlated to those found by Haq et al. (1988) and other sea level studies from other parts of the world, but there are significant differences in the relative magnitudes of events. Because the eustatic curve resulting from this study is based on a stable reference frame, the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins, basins, and other active regions.