Mudclast conglomerates: an example from the Triassic-Jurassic in the Caples Terrane, South Island, New Zealand

Mudclast conglomerates (intraformational conglomerates including abundant mudstone intraclasts) in the Triassic-Jurassic Chrystalls Beach Complex at Quoin Point, southwest of Dunedin, New Zealand, form, together with upper thin-bedded turbidites, thinning- and fining-upward sequences on the order of 100 m thick. The mudclast conglomerates occur as beds up to 4 m thick, either amalgamated or separated by mudstones. Individual conglomerate beds show bipartitions, and are composed of a sandy lower unit containing abundant angular mudclasts of several centimeters across (division A) and an upper unit consisting of subrounded to rounded mudclasts up to 30 cm in diameter set in a muddy matrix with admixed sand (division B). The angular mudclasts in the division A are interpreted as fragments ripped up from the muddy substrate by turbulent flows. Larger and more rounded mudclasts in the division B are attributed to disruption and fragmentation of unconsolidated or semiconsolidated muds as a result of subaqueous sediment failure and to subsequent abrasion of them in the flow. The mudclast conglomerates were resulted from deposition from sediment gravity flows on a slope or base-of-slope. Slumping and other processes of sediment failure in the upper part of the slope were the main sediment source for the conglomerates. During sediment failure and the subsequent evolving stage into the flow, cohesionless sandy sediments disaggregated almost completely, while cohesive muds broke up into fragments of varying sizes. Sand and mudclasts were likely separated in the flow due to density difference, and deposited to produce the distinctive divisions A and B, respectively.