Sagging and collapse sinkholes over hypogenic hydrothermal karst in a carbonate terrain

Abstract

We show that clusters of karst sinkholes can occur on carbonate hypogene karst terrains. Unlike common doline karst of dissolution origin, the studied sinkholes form mainly by sagging and collapse. Thermal survey, OSL dating and morphologic analysis during quarrying and excavations are applied to study the sinkholes at the Ayyalon karst, Israel. The thermal survey shows the spatial pattern of rising warm water plumes, whose temperature is >2°C warmer than the surrounding aquifer water. These plumes dissolve the limestone, creating large voids and maze caves. Mass wasting forms surface sinkholes mainly by sagging and collapse. Both types of deformation often occur within the same depression. Lack of hydrologic connection between the surface and underground voids constrain drainage and promote rapid accumulation of colluvium, dust and pedogenic clays. These have filled the sinkholes up to their rim before the late Holocene. OSL dating constrains the rate of sediment accumulation within the sinkholes. The average filling rate (thickness divided by elapsed time) is ~47mmka−1 for the last 53±4ka in Sinkhole 1, while in Sinkhole 2 (“Nesher Ramla karst depression”), the rate is ~61mmka−1 from ~200 to 78ka, and ~173mmka−1 since ~78ka. Between ~170 and 78ka, Sinkhole 2 was intensively used by Middle Paleolithic hominins. The studied sinkholes may be considered as a type locality for hypogene sinkhole terrain on carbonate rocks.