The formation history and palaeoenvironmental significance of sedimentary cone deposits in Smoke-Tortoise Cave, Naracoorte, South Australia

Abstract

Sediment cones represent the main sources of clastic infill for many of the Naracoorte Cave Complex (NCC) cavities. However, the formation history and palaeoenvironmental significance of these cone deposits remain largely unassessed. This study aims to provide improved insights into NCC sediment cone dynamics and tests the hypothesis that the timing of solution pipe openings has a distinctive climatic signature, which could create temporal biases within pitfall fossil deposits. A series of sediment sequences in Smoke-Tortoise Cave (SMT), Naracoorte, including two cone deposits, the sediment floor and a modern analogue deposit, are investigated using geochronological, geochemical, and palaeoenvironmental techniques. By evaluating modern analogue deposits, it is determined that their optically stimulated luminescence (OSL) signal is fully reset prior to entering the cave, providing a modern age of 0 ka and confirming the suitability of OSL dating in this cave setting. Eleven single-grain OSL samples constrain the SMT cone sediments to two distinct depositional events; an older phase at ~305–270 ka (Marine Isotope Stage (MIS) 9 and early MIS 8) and a younger phase at ~230–200 ka (MIS 7), with deposition ceasing for both cones at 200 ka. These results, combined with analyses of published chronological datasets for other NCC cone deposits, imply that there is a climatic control on the opening and closing of solution pipes, with cone deposits preferentially accumulating during warmer interglacial and interstadial climate cycles. The complex histories of solution pipes suggest short-lived, discontinuous opportunities for sediment and fossil accumulation inside the caves, creating potential accumulation biases or gaps in palaeontological records. Comparisons between geochronology and geochemical results reveal that the SMT sediment floor accumulated as the distal part of Cone 2. The absence of pollen in SMT Cave potentially reflects the low preservation potential of pollen from old sediments in closed cave systems.