New Unified Classification for Fine Grained Sedimentary Siliciclastic Material Based on Atterberg Limits

Abstract Fine-grained (argillaceous) sedimentary material comprising unconsolidated (i.e., argillaceous sediment and soil) and consolidated (argillaceous rock) is the most abundant sedimentary material. Argillaceous rock is further divided into argillaceous stone and shale based on structure, and into siltrock and clayrock based on grain size. However, the nomenclature of these materials is frequently inconsistent, and there is no unified classification system for these materials. This is due to the difficulty and expense of particle size analysis (PSA), particularly in silt and clay fractions. AIPEA and CMS have proposed using plasticity parameters, i.e., plasticity index (PI) and liquid limits (LL), to differentiate between silt and clay. In engineering geology, a quantitative limitation on unconsolidated and consolidated argillaceous materials is necessary. This is defined as the slake durability index after two cycles (I d2 ) value, which is argillaceous rock with a I d2 ≥ 2%. Argillaceous stone and shale can be distinguished based on the ease with which a fissility plane structure can be formed with a maximum thickness of 1 cm. The term shale is defined as a fine-grained sedimentary rock that is easily fissile, whereas argillaceous stone is defined as a rock that is not fissile (massive). This article presents a new unified classification for fine-grained sedimentary materials, including loose sediments, soils, and sedimentary rocks. This new unified classification is expected to facilitate the nomenclature of argillaceous sedimentary materials and the prediction of clay and silt fractions based on their plasticity, as an alternative to using PSA. To illustrate the utilisation of the novel unified classification, a series of samples of unconsolidated sediments, soils, and sedimentary rocks were employed, sourced from Lower Kutai Basin, East Kalimantan, Indonesia.