Abstract: Mudflows and Associated Mass-Transport Deposits in the Geological Record

Abstract

Deposits ascribed to slope-dependent mass-transport mechanisms are not uncommon in the geological record. In terms of relative abundance, deposits of this type formed in subaerial environments are subordinate to those originating in subaqueous (and especially submarine ) conditions. This difference results partly from the lower preservation potential of subaerial deposits but there is evidence that mechanisms of this type are both common and more effective in subaqueous environments. Described fossil examples of subaerial mass-movement include mud and gravel flows associated with alluvial fans and the deposits of volcanic/pyroclastic origin known as lahars. Sediments of deformed or chaotic aspect, attributed to mass-movement under glacial or periglacial conditions, are widespread in the Pleistocene and early Holocene but are rarely recorded from older glacial episodes. The lithified products of subaqueous mass-transport encompass a wide range of sedimentary environments, from fluvial through lacustrine and deltaic to the deep ocean floor. Such sediments also span the entire spectrum of behavioural categories, from elastic through plastic to viscous or fluid conditions. A new classification of subaqueous mass-transport deposits which is essentially descriptive but carries important genetic implications will be discussed, with examples.