SYNERESIS CRACKS - SUBAQUEOUS SHRINKAGE IN ARGILLACEOUS SEDIMENTS CAUSED BY EARTHQUAKE-INDUCED DEWATERING

Syneresis cracks, often confused with subaerial desiccation phenomena, are traditionally ascribed to subaqueous shrinkage whereby salinity c hanges caused deflocculation of clay. This and other previously propos ed mechanisms fail to account for their occurrence in low-energy, typi cally non-evaporitic facies, stratigraphically sporadic distribution, intrastratal formation under shallow burial depths, variation in morph ology, degree of contraction, generation of sedimentary dikelets as cr ack fills, and deformation of dikelets and enclosing layers. Instead, it is suggested that ground motion from strong synsedimentary earthqua kes caused argillaceous sediments to dewater, interbedded sands and si lts to be almost simultaneously liquefied and injected into the result ing fissures, and then these dikelets to be distorted. Comparative rar ity of syneresis cracks in Phanerozoic versus Precambrian marine strat a is considered to be primarily an evolutionary consequence of rheolog ical changes caused by increased organic binding of clay flocs in the water column, greater input of organic matter into the sediment, and t he diversification of sediment-dwelling bacteria and meiofauna in Phan erozoic deposits. (C) 1998 Elsevier Science B.V. All rights reserved.