Subaqueous debris flows of early Pleistocene age at Motunau, North Canterbury, New Zealand

Abstract

Abstract Subaqueous debris flow deposits of early Pleistocene age fill a series of shallow (1–6 m deep), wide (up to more than 200 m) channels within a relatively deep-water marine mudstone sequence at Motunau, North Canterbury. Four lithotypes are distinguished; each reflects a different mechanism of debris flow. Three lithotypes (A, B, C) have in common a well-sorted fine to very fine sand with a tail of silt and clay that comprises less than 20% by weight. These reflect an interrelated spectrum of sandy debris flows, and include the first recognised example (Lithotype C) of fine-grained submarine debris flows that have been predicted by M. A. Hampton on theoretical and experimental grounds. Lithotype D is a muddy debris flow similar to others previously described in the literature. Lithotype A consists of a mixture of lithoclasts and perigenic clasts of up to boulder size, plus abundant disoriented shell fragments, all floating in the sandy matrix; stratification and grading are absent. The coarse clas...