マッドクラスト礫岩の運搬・堆積機構 : ニュージーランド南島ケープルス帯トリアス-ジュラ系中の例

概要

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Mudclast conglomerates in the Chrystalls Beach Complex of the Caples Terrane, New Zealand occur as beds of up to 4m thick. Individual conglomerate beds are composed of a lower sandy unit containing abundant angular rip-up mudclasts (division A) and an upper unit consisting of subrounded to rounded slump-generated mudclasts set in a muddy matrix with admixed sand (division B). The mudclast conglomerates resulted from deposition of sediment gravity flows generated by slumping and other processes of sediment failure on a steep slope. During the sediment failure and subsequent evolving stage into the flow, the cohesionless sandy sediments disaggregated into sand grains reflecting its friability, while the cohesive muds broke up into a lot of fragments as slump-generated mudclasts. Marked segregation of these mudclasts from sand, strongly suggests sorting and re-formation of the failed materials due to longitudinal and vertical density and or grain size gradient mechanisms in the highly turbulent flow. Sands were concentrated in the head and the lower part of the body of the flow while slump-generated mudclasts were swept into the upper part of the body. The flows were so turbulent that the head of the flow strongly eroded the substrates, resulting in abundant rip-up mudclasts in division A. The bipartitions of the conglomerate beds suggest rapid mass emplacement of such flows as a result of slope reduction.
地学団体研究会の論文
1992-03-25