Organic-facies determination: a key for understanding facies distribution in the basal peat layer of the Holocene Rhine-Meuse Delta, the Netherlands

Abstract

Peat and gyttja (organic lake sediment) are important indicators for past environmental conditions; they form in areas where the supply of clastic sediment is insufficient to fill the accommodation space. Previous studies of delta sediments, however, have focused mainly on clastic deposits, whereas organics have received only minimal attention. Identification of organic facies can be of value for understanding delta formation as it provides insight into, for example, palaeoenvironmental conditions and compaction susceptibility of these layers. Currently, however, field characteristics of organics are not welldocumented for the full range of facies present in delta plains. In this study, a field method – a classification key – for identifying organic facies in delta plains was developed based on concisely described organic facies in selected cores from the Rhine-Meuse delta, The Netherlands. This method was tested on archived borehole descriptions of the diachronous basal peat layer in the distal Holocene Rhine-Meuse delta; this was done by means of a semi-automated procedure to select and classify samples from a database. The resulting data set was used for mapping organic facies in the basal peat layer by means of indicator kriging and inverse distance weighting. The resulting facies distribution map shows consistent and geologically meaningful patterns, which were interpreted in terms of hydrological regimes. The formation of the basal peat layer, which marks the onset of Holocene aggradation, has been controlled by marinedominated, fluvial-dominated and seepage-dominated environments. Before 9000 cal yr bp, marine processes influenced the initial stage of organic accumulation in the western part of The Netherlands. After 9000 cal yr bp, on the flanks of topographical higher regions, seepage-dominated mesotrophic organic facies characterized the onset of accumulation. Simultaneously, nutrient-rich organic facies could develop in the Rhine-Meuse valley: gyttja formed in the sediment-limited Meuse realm, whereas reed peat accumulated in the Rhine realm, indicating lower water depths and thus a higher elevated surface level. It is concluded that the classification key for identifying organic facies is a strong tool to better recognize peat and gyttja types on the basis of macroscopic field characteristics. Application of the key on archived boreholes also proved to be feasible. Identification of organic facies and determination of their spatial distribution at delta scale, as is shown in this study, contributes to understanding delta evolution.