Post-eruptive sedimentary processes in volcanic crater lakes: implications for deciphering the Samoan sedimentary record
The two largest islands of the Samoan chain, Savai'i and Upolu, possess almost 400 volcanic cones. Their craters form enclosed, internally drained basins that potentially retain long, detailed and uninterrupted sedimentary sequences. Because of the sparsity, fragmentary nature and low temporal resolution of records of environmental change from the tropical Pacific, these deposits have the potential to fill an important gap in our knowledge of global climatic and environmental change. To interpret such records we must understand the depositional processes that operate in these basins. Unfortunately, although the post-eruptive sedimentology of volcanic calderas and maars is relatively well-established, that of crater lakes remains poorly understood. The volcanic edifice of Mount Lanotō in southeast Upolu was selected for investigation. The form of the volcano and its crater are typical of those observed across the island. The sediments retained in the crater are composed largely of plant-organic-rich muds that display little visible evidence of stratigraphic variation. Mineral magnetic and chemical methods were therefore employed to document the types and distribution of sedimentary facies represented in the post-eruptive crater fill and, by inference, the processes of sedimentation that had operated in the crater. The earliest post-eruptive deposits are the result of the failure of the crater's oversteepened internal slopes. The crater floor subsequently collapsed to form a pit crater. The basal deposits in the pit crater are likely to be the product of the collapse of its walls and roof. However, the bulk of the material in the feature was laid down under lacustrine conditions. These deposits accumulated in a relatively deep-water environment. Across the rest of the accumulation zone, by contrast, water levels appear to have been shallow. Sedimentation during this phase was dominated by autochthonous plant-organic-rich deposits, with minor fine-grained clastic input. Deposition was intermittently interrupted by localised episodes of mass movement that reworked the regolith mantling the steep internal slopes of the crater into the accumulation zone in the form of low-angle fans. At the broad scale, the sedimentology of Lake Lanotō displays similarities with that of volcanic calderas and maar lakes. However, the morphological simplicity of the basin, the general absence of contemporaneous volcanic activity, the timing of the onset of lacustrine conditions, the derivation of the clastic deposits in the volcanic crater almost solely from the by-products of the volcanic eruption, and the high biological activity in the lake waters mean that there are important differences between the types and distribution of sedimentary facies identified in Lake Lanotō and those represented in models of deposition in maars and volcanic calderas.