Hominin-bearing caves and landscape dynamics in the Cradle of Humankind, South Africa

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• 作者：Paul H.G.M. Dirks^a ; ^b ; ^c ; ^{paul.dirks@jcu.edu.au} ; Lee R. Berger^b ; ^c
• 关键词：Hominins ; Malapa ; Cradle of Humankind ; Geomorphology ; Tectonics ; Landscape
• 刊名：Journal of African Earth Sciences
• 出版年：2013
• 出版时间：February, 2013
• 年：2013
• 卷：78
• 期：Complete
• 页码：109-131
• 全文大小：4862 K

文摘

This paper provides constraints on the evolution of the landscape in the Cradle of Humankind (CoH), UNESCO World Heritage Site, South Africa, since the Pliocene. The aim is to better understand the distribution of hominin fossils in the CoH, and determine links between tectonic processes controlling the landscape and the evolution and distribution of hominins occupying that landscape. The paper is focused on a detailed reconstruction of the landscape through time in the Grootvleispruit catchment, which contains the highly significant fossil site of Malapa and the remains of the hominin species Australopithicus sediba.

In the past 4 My the landscape in the CoH has undergone major changes in its physical appearance as a result of river incision, which degraded older African planation surfaces, and accommodated denudation of cover rocks (including Karoo sediments and various sil- and ferricretes) to expose dolomite with caves in which fossils collected. Differentially weathered chert breccia dykes, calibrated with ¹⁰Be exposure ages, are used to estimate erosion patterns of the landscape across the CoH. In this manner it is shown that 2 My ago Malapa cave was ¡«50 m deep, and Gladysvale cave was first exposed; i.e. landscape reconstructions can provide estimates for the time of opening of cave systems that trapped hominin and other fossils.

Within the region, cave formation was influenced by lithological, layer-parallel controls interacting with cross-cutting fracture systems of Paleoproterozoic origin, and a NW-SE directed extensional far-field stress at a time when the African erosion surface was still intact, and elevations were probably lower. Cave geometries vary in a systematic manner across the landscape, with deep caves on the plateau and cave erosion remnants in valleys. Most caves formed to similar depths of 1400-1420 mamsl across much of the CoH, indicating that caves no longer deepened once Pliocene uplift and incision occurred, but acted as passive sediment traps on the landscape.

Caves in the CoH are distributed along lithological boundaries and NNE and ESE fractures. Fossil-bearing caves have a distinct distribution pattern, with different directional controls, a high degree of clustering, a characteristic spacing of 1700 m or 3400 m, and a characteristic bi-model fractal distribution best explained by a combination of geological and biological controls. It is suggested that clustering of fossil-bearing caves reflects a L¨¦vy flight patterns typical for foraging behavior in animals. The controlling element in this behavior could have been availability of water in or near groups of caves, resulting in preferential occupation of these caves with accumulation of diverse faunal fossil assemblages.

The tectonic drivers shaping the dynamic landscape of the CoH did not involve large, seismically active fault lines, but complex interactions between multiple smaller fractures and joints activated in a far field stress controlled by uplift. The landscape of the CoH, with its caves and water sources and dissected landscape provided a setting favored by many animals including hominins. A modern day analog for what the CoH would have looked like 2 My ago is found 50 km east of Johannesburg, near the SE margin of the Johannesburg Dome.