A palynofacies study of past fluvio-deltaic and shelf environments, the Oligocene-Miocene succession, North Sea Basin: A reference data set for similar Cenozoic systems

Abstract

Abstract Correct interpretations of depositional environments are fundamental for evaluating the geological history of a sedimentary basin. Palynofacies analyses are a valuable supplement to sedimentological and seismic studies. In order to develop a palynofacies reference dataset for fluvio-deltaic and shelfal successions, a study of the assemblages of sedimentary organic particles from seven different well-defined depositional environments within the uppermost Oligocene – lower Miocene succession onshore Denmark (eastern North Sea Basin) has been performed. The study deals with the following environments; floodplain, lagoon, washover-fan flat, prodelta, shoreface, offshore transition and shelf. The sedimentary organic particles were grouped into four major categories; 1) Structured wood particles, 2) Amorphous organic matter (AOM, in the present study mainly consisting of partly degraded vitrinite), 3) Cuticle and membranes and 4) Palynomorphs. The palynomorphs were grouped into eight subcategories; 1) Microspores, 2) Non-saccate pollen, 3) Bisaccate pollen, 4) Botryococcus, 5) Other freshwater algae, 6) Fungal hyphae and –spores, 7) Acritarchs and 8) Organic-walled dinoflagellate cysts. A combination of a univariate box plots and a multivariate Principal Component Analysis (PCA) of the palynofacies data clear revealed the quantitative characteristics and variations within each discrete environment as well as their principal similarities and differences. In spite of some natural overlaps, for example between the lagoon and offshore transition environments, the data revealed distinct characteristics, e.g. a strong dominance of wood particles in the shoreface environment, a strong dominance of bisaccate pollen in the washover-fan flat environment and a near absence of dinocysts in the floodplain environment. An overall increase in relative abundances of dinocysts and a decrease in abundances of non-saccate pollen in the proximal-distal trend were also outlined. This study outlines a palynofacies reference dataset that can be used as a tool for interpreting depositional environments in equivalent settings, preferentially combined with other information such as seismic data, well logs, and/or lithology.