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

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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 clasts were supported by water-sand-clay/silt-shell mixtures during transport. Lithotype B consists of abundant coarse shells plus sparse gravel to cobble size clasts floating in the sandy matrix; smoothly wavy discontinuous stratification is apparent. Gravel and shells were supported by water-sand-clay/silt mixtures during transport. Lithotype C consists of the fine-skewed sand without larger clasts or shells. The sand was supported by water-clay/ silt mixtures during transport. All three lithotypes could feasibly be deposited from a single debris flow—A from the concentrated frontal segment, and B and C from succeeding segments wherein concentration diminished and successively finer sizes of grains were removed from the water-sediment transport medium. In addition, depending on the sediments initially involved, individual debris flows could feasibly have deposited only one or two of the lithotypes. The general paleoenvironmental setting was like that of the present outer continental shelf. Seaward, differential relief gave rise to retrogressive slumping, which initiated and extended channels towards the shoreline by a process of headward sapping. Shorewards, liquefaction of sandy deposits fed sandy debris flows into the channels. Slump structures are seen, but the channel morphology suggests erosional modifications prior to infilling; erosion was accomplished in part by the debris flows, but possibly also by water currents that have left little or no record. Tectonism need not be invoked to account for the sequence described, but could have been involved at least as a trigger mechanism for some mass movements.