Integrated sequence stratigraphy: Facies, stable isotope and palynofacies analysis in a deeper epicontinental carbonate ramp (Late Jurassic, SW Germany)

摘要

Sequence stratigraphy in deeper water, epicontinental carbonates such as in the Upper Jurassic of southern Germany is difficult because the recognition of parasequences, sequences and sequence boundaries is impeded by the paucity in diagnostic sedimentological criteria or stratal surfaces. Using the "genetic stratigraphic" approach, and integrating facies, stable isotope (C,O) and palynofacies analysis two types of genetic depositional sequences can be discriminated: small-scale sequences are stacked into medium-scale sequences which may record a 400 kyr Milankovitch signal. The medium-scale sequences were correlated regionally using both gamma-ray logs and stable isotope records. Regional correlations show that the depocentres are controlled by underlying palaeotectonic elements (Late Palaeozoic troughs). The rise/fall turnarounds of medium-scale sequences are marked by negative δ~(18)O peaks (temperature maxima) and reduced absolute palynoclast contents. The fall/rise turnarounds are marked by positive δ~(18)O peaks (temperature minima) and high absolute palynoclast contents. The initiation and termination of sponge/microbial mounds show characteristic patterns: thrombolitic microbialites form during intervals of (1) reduced input of terrestrial palynomorphs interpreted as an increase in distality, (2) decreasing δ~(13)C trends interpreted to be related to decreasing nutrient supply and (3) decreasing δ~(18)O values interpreted as phases of warming and rising relative sea-level. In contrast, thrombolitic/stromatolitic microbialites were found to occur during phases of (1) increasing input of terrestrial palynomorphs interpreted as an increase in proximality, (2) increasing δ~(13)C values interpreted to reflect increasing terrestrial input and nutrient supply as well as increasing δ~(18)O values (interpreted as phases of cooling and relative sea-level falls). Isotopic and palynofacies evidence suggests that bioherms were terminated by sudden input of nutrients during relative sea-level falls. Sedimentological criteria were clearly not sufficient to delineate a robust sequence stratigraphic framework.