Organic facies of the Oligocene lacustrine system in the Cenozoic Taubate basin, Southern Brazil

摘要

A quantitative study of palynofacies and organic facieswas carried out in the Cenozoic sedimentary succession of the Taubate basin. The palaeoenvironmental distributions of particulate organic components were examined in relation to various potential controlling factors (lithology, depositional environment, and stratigraphic sequence). About 30 sampleswere analyzed froma single borehole drilled in Sao Paulo State. This involved rigorous kerogen counts supported by 30 TOC and sulphur, 20Rock-Eval pyrolysis and 20 biochemistry analyses. In the studied area the Oligocene deposits are approximately 600 m thick. Data was obtained by counting 300 to 500 kerogen particles per sample, and then recalculating the percentages of the different constituent groups (phytoclast, palynomorph, and amorphous organic matter — AOM). The kerogen assemblage is palynomorph-dominated (N70), consistingmainly of freshwater lacustrine algae (Pediastrum and Botryococcus). Non-opaque phytoclasts predominate in most of the kerogen samples. However, significant variations in the proportion of biostructured and non-biostructured brown phytoclasts occur in some intervals. The fluorescent AOM content occurs in some levels and is composed of two main components: an intensively fluorescing structureless organic matter, which also has high lipid and protein content indicating a probable bacterial origin for the organic matter (microbial mats); and terrestrially-derived amorphous organic matter, which has been reworked to a different extent exhibiting a high content of carbohydrates. Comparison of the mean kerogen assemblages from different lithologies revealed that the finer-grained lithologies generally have higher palynomorph percentages, whereas coarser-grained lithologies present higher percentages of phytoclasts, emphasizing the important role of grain size in organic-matter distribution. The TOC content is generally less than 10, but reaches as high as 44 in some intervals, duemainly to the presence of coaly seams. The T_(max), Spore Coloration Index, and spectral fluorescence measurement values obtained from these samples indicate a low maturity stage. The integration of these results, together with the organic facies parameters, indicate that the analyzed sedimentary section represents a variation from a freshwater lacustrine environment with an oxidant regime to a restricted saline, lacustrine environment with a dysoxic–anoxic regime. There was an excellent correlation between the optical and geochemical parameters.