Palynology and palaeoenvironment of the Bathonian-Oxfordian of the northern Switzerland sedimentary basin.

摘要

The northern Switzerland sedimentary basin of the Mesozoic era has been divided into three different parts: a shallow water area, a transitional region and a basinal region. In order to examine the Oxfordian marine palynomorphs of these three different regions, ten different localities were sampled.;Attention was concentrated on the four major groups of marine palynomorphs (acritarchs, dinoflagellates, foraminiferal test linings and tasmanitids). Their relative proportions were calculated and plotted for each section in the three different parts of the basin. Ratios of chorate cysts to proximochorate, proximate and cavate forms (C: PPC ratio) were calculated for each sample and compared graphically with the changes in proportions of the major palynomorph groups (acritarchs, dinoflagellates, foraminiferal test linings and were considered important in paleoenvironmental interpretations. Furthermore, the species diversity for each sample was ascertained and compared with changes in the proportions of the marine palynomorphs and with the changes in the C: PPC ratios.;A dinoflagellate zonation has been established; it is related to the standard zonations of Woollam & Riding (1983) for the English Jurassic and of Riley & Fenton (1982) for the Jurassic of Northwest Europe and has been correlated with the ammonite zonation established for the northern Switzerland by Gygi & Persoz (1986). All the dinoflagellate zones and subzones established but the mentioned authors have been recognized in northern Switzerland, though minor changes in the ranges of their characteristic species are apparent.;Changes in the proportions of different groups of palynomorphs have also led to some palaeoclimatic conclusions. In particular, foraminiferal test linings are more abundant in ultrahaline waters and an increase in salinity was most probably a result of a hot and dry climatic period. This is supported by the lower proportions or absence of terrestrial palynomorphs (spore and pollen grains) in such sediments.;The following were the principal paleoenvironmental conclusions: Increases in numbers of dinoflagellates were coincidental with decreases in acritarchs (mainly Micrhystridium species). Decreases in the C: PPC ratios coincided with increases in acritarchs (and also with increases in tasmanitids, where present). Water level fluctuations had more effect on the C: PPC ratios, and on the proportions of dinoflagellates, acritarchs and foraminiferal test linings, in the shallow water region than in the transitional and basinal regions. Species diversity in dinoflagellates typically exhibits an inverse relation with proportions of foraminiferal test linings. Finally, increase and decrease in the C: PPC ratios can potentially reflect rise and fall of water level in this marine basin and can amplify the deductions made from lithological changes and marine invertebrate distribution. (Abstract shortened by UMI.)