Late quaternary speleogenesis and landscape evolution in a tropical carbonate island: Pango la kuumbi (kuumbi cave), zanzibar

Abstract

Kuumbi Cave is one of a group of caves that underlie a flight of marine terraces in Pleistocene limestone in eastern Zanzibar (Indian Ocean). Drawing on the findings of geoarchaeological field survey and archaeological excavation, we discuss the formation and evolution of Kuumbi Cave and its wider littoral landscape. In the later part of the Quaternary (last ca. 250,000 years?), speleogenesis and terrace formation were driven by the interplay between glacioeustatic sea level change and crustal uplift at rates of ca. 0.10-0.20 mm/yr. Two units of backreef/reef limestone were deposited during ‘optimal’ (highest) highstands, tentatively correlated with MIS 7 and 5; (mainly) erosive marine terraces formed in these limestones in ‘suboptimal’ highstands. Kuumbi and other sub-terrace caves developed as flank margin caves, in the seaward portion of freshwater lenses during such ‘suboptimal’ highstands. Glacioeustacy-induced fluctuations of the groundwater table may have resulted in shifts from vadose (with deposition of well-developed speleothems) to phreatic/epiphreatic conditions in these caves. At Kuumbi, Late Pleistocene (pre-20,000 cal. BP) ceiling collapse initiated colluvial deposition near-entrance and opened the cave to large plants and animals, including humans. A phase of terminal Pleistocene human occupation ca. 18,500-17,000 cal. BP resulted in the deposition of a dense assemblage of Achatina spp. landsnails, alongside marine molluscs and mammal remains (including zebra, buffalo and other taxa now extinct on Zanzibar). The Holocene part of the cave stratigraphy near-entrance records phases of abandonment and intensified late Holocene human use.