Stratigraphy and paleoenvironments of the early to middle Holocene Chipalamawamba Beds (Malawi Basin, Africa)

[摘要] We describe the Chipalamawamba Beds, early to middle Holocene deposits atthe southern margin of long-lived Lake Malawi. The beds are exposed becauseof downcutting of the upper Shire River. The Chipalamawamba sediments aremedium to coarse, yellow to brown sands deposited in lenses varying inhorizontal extent from a few meters to several hundreds of meters. Fourunits are recognized; the first three mainly contain lacustrine sedimentsdeposited during lake high stands about 10.6–9.7 cal ka BP (Unit 1),7.6–6.5 cal ka BP (Unit 2) and 5.9–5.3 cal ka BP (Unit 3). Sediments of Unit 4 overlay Units 1to 3, are coarser and display regular foresets and oblique-bedding,suggesting deposition in riverine environments after installation of theShire River (~ 5.5–5.0 ka BP). Freshwater mollusk assemblages andbioturbation regularly occur in the lacustrine sediments, but are largelyabsent from Unit 4. Diverse and often contradicting hypotheses on the lakelevels of Lake Malawi have been proposed for the early and middle Holocene.The Chipalamawamba Beds allow straightforward recognition of water levelsand provide strong evidence for oscillating lake levels during this period,rather than continuous high or low levels. Sedimentation rates have beenhigh and individual shell beds have typically been deposited during a fewdecades. Because the Chipalamawamba Beds contain a sequence of molluskassemblages with intervals between subsequent shell beds ranging from acentury to a few millennia, they enable paleontological analysis of thefauna with an unusually high temporal resolution. That some mollusk lineagesinhabiting Lake Malawi are in the early stages of diversification andradiation increases the paleobiological relevance of these beds.

[学科分类] 地球化学与岩石