Shedding Light Into The Dark: Magnetic Fabrics of Cave Sediments

Abstract

Magnetic fabrics of marine and continental sedimentary rocks from a variety of depositional environments has been studied and documented in numerous studies. In contrast, caves and associated sedimentary deposits have received much less of attention in the literature. Such geological environments act like natural sediment traps, collecting colluvial, fluvial and percolated particles, and therefore provide invaluable records of environmental conditions of the past in generally protected contexts. Cave sedimentary records are of particular importance to understand recent Quaternary changes including hominid evolution and adaptation to the changing environment. Therefore, deciphering the origin and depositional history of archaeological and paleontological remains found in cave deposits are fundamental to evaluate artifact and fossil contextualization. We will show how rock magnetic data based on the anisotropy of magnetic susceptibility (AMS) allows fabric analysis and characterization of depositional modes in cave sediments. When caves are at or just below the water table, they are subjected to frequent flooding and consequent deposition of slackwater sediments by stream flows. As the local water table lowers through time, due to progressive river incision, cave passages are gradually abandoned and eventually intersected by the slope of the topographic surface due to stream incision, ceiling collapse, or fissuring, all leading to the formation of cave entrances. At the cave entrance, processes such as scree, slope wash, and sliding bed deposits will produce a variety of gravel accumulation, depending on water availability and particle size. Such variety of depositional processes has a direct impact on the sedimentary fabrics and hence on the AMS, which in turn we can use to better understand the origin of the deposits. We will report a number of examples on both fossiliferous and sterile cave deposits where AMS approach has allowed to determine paleoenvironmental conditions, as well as to establish whether a given sedimentary unit has preserved primary depositional fabrics or had experienced post-depositional perturbation.