Palynofacies patterns, acritarch diversity and stable isotope signatures in the Lower Muschelkalk (Middle Triassic) of N Switzerland: Evidence of third-order cyclicity

Abstract

The Lower Muschelkalk of N Switzerland is characterised by a relatively monotonous succession of marly and dolomitic limestones with siltstone layers and thin sandstone beds. The studied Leuggern, Weiach and Benken wells comprise completely cored sections of the Lower Muschelkalk (Anisian). Due to the lack of characteristic marker beds and vertical facies changes, cyclic interpretation based on sedimentary features proves to be difficult. Therefore, palynofacies patterns, acritarch diversity and stable isotope signatures are used to detect eustatic signals related to relative sea-level changes. The major flooding phase is recognised in the Middle Wellenmergel (Pelsonian). It is characterised by maximum abundance of marine plankton and the highest amount of refractory opaque phytoclasts. Transgressive deposits are identified by a significant increase of marine constituents and a relatively high number of translucent woody fragments of the phytoclast group. Highstand deposits show a high terrigenous input of blade-shaped, opaque plant debris. Palynofacies of late highstand deposits is dominated by bisaccate pollen grains. This sequence stratigraphic break down allows correlation of the Swiss wells and their correlation with Anisian deposits of other parts of the Germanic Basin. Acritarch diversity is analysed by a morphometric study on vesicle size, process length, process bifurcations, and the number of processes. These parameters correlate with the eustatic evolution during Anisian times as interpreted from palynofacies data. Vesicle size, process length and the percentage of specimens with bifurcated processes are seen to increase during transgression and intervals of maximum flooding. They are decreasing within highstand phases. The number of processes shows the opposite trend and is high during the late highstand. Stable isotope curves both of δ13C and δ18O point out two intervals of maximum flooding. These geochemical signatures correlate with the palynological data. The example from the Middle Triassic of N Switzerland highlights the value of using organic facies and plankton associations in an integrated approach for reconstructing the eustatic history of an epeiric sea characterised by monotonous depositional series lacking significant sedimentary features.