Abstract

Tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated. The Pleistocene fans of the Dead Sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin. Exposures are excellent. The rivers are currently incising in response to the low lake levels of the Holocene, producing cliffs >50m high. These reveal an interdigitation of lake deposits (dominantly fine grained carbonates) and the coarse river gravels which characterize the fan deposits. The bounds of each facies are sharp: river sediments are superimposed on lake sediments with little evidence of either reworking or loading; in turn, aragonite crusts are draped over alluvial gravels indicating the sudden return of still-water conditions. Neither can be explained by gradual adjustment of the hinterland river system after tectonic shock. Instead, it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment. In interpreting basin development in ancient settings, it is always tempting to assume that coarse deposits are indicative of tectonism. However, the Dead Sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence.

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