Photographic evidence of variable bottom-current activity in the Suruga and Sagami Bays, central Japan

Abstract

Complex patterns of bottom-current behaviour were clarified by studies of sedimentary features and orientations of benthic animals in the deep Suruga and Sagami Bays on the Pacific side of central Honshu, Japan. Both the Suruga and Sagami Bay measure about 60 km in length as well as in width at their mouths and are about 1500 m deep in their central portions. The size of each bay is comparable to that of ancient forearc basins. The bottom sediments are characterized by turbidites and slump deposits. At 32 stations on the bottom of the bays, deep-sea photographs were taken, most of which clearly indicate bottom-current activity. Current-induced bottom features are current-lineations, moat-like scours around resistant objects, crag-and-tail structures behind obstacles, ripple marks, sand ridges and deformed biogenic structures such as burrows, mounds, tracks and faeces. These features are produced by bottom currents with rather high velocities. Other important current indicators are some benthic organisms, which in general show a sensitive response to currents and adopt particular orientations. Typical examples of megabenthos identified in the bottom photographs as effective current indicators are the small deimatid holothurian Peniagone japonica, the benthic shrimp Glyphocrangon hastacauda, sea anemones, and sea pens. Among them, the orientation of Peniagone japonica shows abrupt changes of current direction with time, for example, from N (0°) to SW (240°) during 15 min and from N to S and back to N, a complete rotation during 40 min. The results of these observations indicate that the bottom currents in deep bays tend to fluctuate rapidly in velocity and direction, probably owing to strong internal tidal waves in the very steep embayments. Upslope currents appear to be present at the mouths of submarine canyons. Thus, it should be borne in mind that palaeocurrent analysis of ancient bottom-current deposits or contourites is limited in application.