Estimation of current velocity and direction from orientation of conical radiolarians in Lower Jurassic bedded cherts from Southwest Japan: indications of eddy-driven sedimentation

Abstract

Paleo-current activity of deep marine environments was analyzed by studying the sedimentation characteristics of conical-shaped radiolarian tests from Lower Jurassic bedded cherts in the Inuyama area, Central Japan. As representatives of conical-shaped radiolarians, Parahsuum spp. were analyzed in a flume experiment. The results indicate that the preferred azimuth orientation of the shells is correlated to the current velocity. This result is used to evaluate current velocity and direction in Lower Jurassic bedded cherts. The bedded cherts examined and ranging from the Triassic-Jurassic boundary to Toarcian (Lower Jurassic) are divided in ascending order into red, red-green, green and black to gray bedded chert units. Supplementary sections of the Lower Jurassic are also examined. Estimated current velocities within the red bedded chert sequences are usually higher than in the black bedded chert sequences, although current velocity varies in each horizon. Current direction within the examined sections varies between chert beds. Within single beds of the red-green and black cherts, current velocity has also been examined in intervals of less than 1 mm thickness. Examination of red-green bedded cherts reveals that current velocity altered rhythmically even during the deposition of a single bed. Within black bedded cherts, one bed shows slower current with varied current directions, while another shows rather rapid current with steady orientation. Faster currents during the deposition of red and red-green bedded cherts are concordant to inferred stagnant conditions in black bedded cherts with associated weak bottom-water circulation. The measured current direction generally varies both between beds and within single beds, which means bottom currents were not uniform direction but had a variety of directions in the sedimentary basin. Bedded cherts are representative of high productivity by silica-precipitating biogenic plankton and may be formed on topographic highs such as seamounts, where nutrients are supplied at a high rate by upwelling. The variation of current velocity and direction indicates a depositional basin with an irregular topography where current eddies may have formed.