Pre-Holocene sediment dispersal systems and effects of structural controls and Holocene sea-level rise from acoustic facies analysis: SW Japan forearc

Abstract

Late Quaternary shelf, slope and intraslope basin sediment dispersal systems of Tosa and Muroto basins in the southwest Japan forearc are analyzed from a grid of 3.5 kHz acoustic profiles and sediment samples to investigate physiographic and sea-level controls on turbidite deposition. Large parts of the basins show a 2–10 m thick Holocene unit of transparent acoustic facies, draping a generally higher reflective lower unit of last glacial age, when parts of the shelves were coastal plains, and sediment supplied by coastal rivers was fed directly onto basin slopes and into canyon heads. Based on sedimentary facies distribution as interpreted from pre-Holocene seafloor morphologic and acoustic facies distributional patterns, five slope to base-of-slope clastic dispersal systems are identified: slope-apron, canyon-fan, hybrid, gullied-slope and canyon systems. Slope-apron systems are progradational turbidite systems, including the sediment-supplied, progradational slope presently dissected by slope valleys, and a base-of-slope apron if the necessary sediment supply and physiographic conditions exist. (Semi-) transparent reflections beneath the upper slope-apron system indicate mass-wasting, and prolonged reflectors beneath the central to lower slope-apron system indicate increased deposition of intercalated sand layers downslope. Canyon-fan systems evolved where major canyons are incised into the slope. Canyons funnel sediment either through leveed fan valleys, which show “patchy” subbottom reflections and sediment waves, or directly from the canyon mouth to depositional fans on the basin floor characterized by prolonged reflectors. Hybrid systems show some characteristics of both slope-apron and canyon-fan systems. Gullied slopes and canyons are sediment-starved dispersal systems on slopes and ridge flanks disconnected from the terrigenous sediment source. A ternary facies classification diagram reveals lateral genetic relationships of acoustic (and interpreted sedimentary) facies within and among the five systems. End-members of the (TMP) diagram are: T, transparent subbottom (“homogenized” sediment); M, multiple parallel reflectors (fine-grained, well-stratified sediment); and P, prolonged reflector (abundant coarse-grained sediment layers). This classification framework emphasizes that not only bulk content of coarse-grained sediment, but also degree of stratification/homogenization is critical in producing certain reflection configuration. With increasing homogenization, acoustic facies decreasingly indicates abundance of coarse-grained sediment.