Silurian Carbonate Platforms of Gotland, Sweden - Archives of Local, Regional and Global Environmental Changes

Sammanfattning: This thesis treats the Silurian carbonate platforms of Gotland, Sweden, which formed close to the equator within the East Baltic Basin. Three different time intervals, all involving changes in facies composition and structure of the platforms, have been studied; Late Homerian, latest Homerian ? earliest Gorstian, and late Ludfordian, respectively. Two of the intervals, the Late Homerian and the late Ludfordian, are known globally as periods of substantial environmental change, including sea-level changes, faunal extinctions (the Mulde and Lau events, respectively) and perturbations in the global carbon cycle, whereas the latest Homerian-earliest Gorstian is not associated with such global environmental changes. The aim of the project has been to study and interpret causes and effects of the environmental changes as reflected by sedimentary facies from low latitude shallow shelf environments. Facies analysis of outcrops and drillcores has formed the basis of the study, but for the late Ludfordian time interval, geochemical data (d13C and d18O) have been used in addition. The results of this study can be categorized into five main groups; 1) Stratigraphical improvements ? local refinement of the stratigraphy has been made in several different areas (e.g. the Ytterholmen islet and the Burgen and Lau outliers), and the knowledge about subsurface facies and geometries of stratigraphical units has been improved by recovery and study of several drillcores, 2) Increased understanding of depositional and erosional processes ? strata have been put into a context of depositional environments, and related to depositional and erosional processes. For example, the Burgsvik Sandstone has been re-interpreted as comprising reworked delta deposits, 3) Documentation of sea-level changes ? the studied strata have been put into a sequence stratigraphical context, revealing that sequence boundaries may not correspond with the traditional topostratigraphical boundaries. Evidence for forced regressions have been identified within all the three time intervals, of which the Late Homerian and the late Ludfordian are interpreted as glacio-eustatic, whereas the latest Homerian ? earliest Gorstian is interpreted as a relative sea-level fall affecting at least parts of the East Baltic Basin, 4) Relationships between sedimentary changes and faunal reorganisations ? This thesis has clarified relationships between faunal changes and environmental changes. The Mulde and Lau events are associated with the most rapid and substantial changes in facies composition and platform evolution within the entire Gotland succession. The significant decrease in biodiversity within the (post-event) Burgsvik Sandstone indicates that major faunal reorganisations also occurred on local scales due to autocyclic processes, 5) Silurian global climate ? based on the combined use of sedimentary facies and stable oxygen isotope (d18O) data, a glaciation is inferred to have occurred during the late Ludfordian. This hence challenges the common view of a prolonged Silurian greenhouse climate following the Late Ordovician-Early Silurian icehouse-period. The globally recognized, positive, carbon isotope excursion within the late Ludfordian has been closely linked to the glacio-eustatic sea-level fall.