Reconstructing the history of sediment deposition in caves: A case study from Wonderwerk Cave, South Africa

Abstract

We applied cosmogenic isotope burial dating, magnetostratigraphy, and grain-size distribution analysis to elucidate the history of the sedimentary sequence, composed of fine quartz sands and silts, of Wonderwerk Cave, located on the southern edge of the Kalahari Desert, South Africa. The source for the quartz sand is the Kalahari sand dunes, presently located ∼100 km to the north of the cave. Field observations and grain-size analysis suggest a sediment transport scenario that includes eolian transport of Kalahari sand, abraded to a size of 70–100 µm, to the Kuruman Hills, temporary storage on the hill slopes and valleys surrounding Wonderwerk Cave, and later transport and deposition inside the cave. Our results suggest simple burial ages for sediments from both the front and back of the cave that range between 2.63 ± 0.17 Ma and 1.56 ± 0.10 Ma following initial exposure of 310–620 k.y. However, 26 Al/ 10 Be ratios of 3.98 ± 0.24 and 4.08 ± 0.22 measured in a sand sample collected from the surface outside the cave may imply an initial burial signal equivalent to 0.78 ± 0.15 Ma, thus reducing the possible age range of the buried samples to between 1.85 ± 0.23 and 0.78 ± 0.18 Ma. The paleomagnetic results for the front of the cave gave a polarity sequence of N > R > N||N, where N indicates normal polarity, and R indicates reverse polarity. This sequence can be correlated with both the older and younger cosmogenic burial age ranges. The correlation suggests that in the cave front, cosmogenic burial ages and the acquisition of stable remanent magnetization were not significantly affected by chemical and physical processes and that postburial production of cosmogenic isotopes was insignificant. In contrast, at the back of the cave, the paleomagnetic polarity sequence of R > N cannot be correlated with the cosmogenic burial ages, since the temporal gap between the initial penetration of the sediment into the cave and the final acquisition of a stable remanent magnetization may have been long (∼10 5 yr), and the single polarity transition can be correlated to any reverse-normal transition that occurred during the Quaternary. This highlights the need for caution when cosmogenic burial ages and paleomagnetic sequences are compared. The buried sediments in Wonderwerk Cave show similar grain-size distributions to the fine sand sediment presently exposed at the surface in the vicinity of the cave. Furthermore, calculated preburial 10 Be concentrations for the buried sediment are similar to those measured in sediment outside the cave. These similarities suggest that the environmental conditions and rates of geomorphic processes that persisted during sand deposition in Wonderwerk Cave during the late Pliocene and early Pleistocene may have been similar to those currently experienced in the southern Kalahari, the Kuruman Hills, and the western Ghaap Plain. These conditions favor the transport of fine-grained quartz sand to the vicinity of the cave.