Characterisation of palaeosols in the Lower Cretaceous Helvetiafjellet Formation, Svalbard. Palaeo-climatic implications

Abstract

Although the Cretaceous period is known to have been dominated by greenhouse conditions, the Early Cretaceous climatic conditions in Svalbard have been under some debate. Both indicators of warm climate such as coal seams, Ornithopod tracks and warm-water dinoflagellates, and indicators of cold climate such as arctic belemnites, glendonites and ice rafted debris have been reported. This study characterises two palaeosols (Palaeosol 1 and 2) developed within the Lower Cretaceous Glitrefjellet Member of the Helvetiafjellet Formation, and investigates their validity as palaeo-climatic proxies. This analysis is based on observed features through the logging of two cores (DH-1 and DH-1A), petrographic analysis of thin sections and XRD-analysis. The petrographic analysis revealed a high percentage of quartz within both palaeosols. The combination of high quartz content and the observation of kaolinite through XRD-analysis suggests highly leached conditions. The formation of kaolinite is favoured by subhumid to perhumid climates. Both palaeosols were interpreted to be composed of two main horizons. An upper horizon A, recognized by the accumulation of organic content and a mineral fraction, and a lower horizon C, which was recognized by a pale colour, high quartz content and relict primary structures, indicating modest alteration due to soil forming processes. As a result, the two palaeosols were characterised as enitsols. Such palaeosols are regarded as immature and are thus not indicative of specific climatic conditions. The immaturity of the palaeosols was interpreted to be a consequence of several factors, where (1) high quartz content, (2) palaeosol development within crevasse splay and floodplain deposits and (3) unfavourable clastic sedimentary environment where time for plant growth and accumulation was limited were regarded as the main contributing factors. During thin section analysis, iron ooids were observed within the transgressive lag that marks the top of the Helvetiafjellet Formation. Their formation was interpreted to be a result of sediment starvation on a shallow-marine shelf that formed due to transgression and flooding of the proximal coastal plain of the Helvetiafjellet Formation. Such deposits are indicative of warm climates. Therefore, although palaeosols 1 and 2 could not be classified with regards to specific climatic conditions, other observations such as high degree of leaching, kaolinite content and iron ooids supports warm and humid climatic conditions.