--> ABSTRACT: Origin of Cyclic Sedimentation in Middle Miocene Siliceous Rocks of Northern Japan, by Tada Ryuji, Azuma Iijima; #91003 (1990).

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ABSTRACT: Origin of Cyclic Sedimentation in Middle Miocene Siliceous Rocks of Northern Japan

Tada Ryuji, Azuma Iijima

Middle Miocene diatomaceous siliceous rocks, best known as the Onnagawa Formation, are characterized by decimeter-scale rhythmical alternation of light-colored porcelanite and dark-colored porcelanite or siliceous shale, which were deposited in a silled basin with the estimated sill depth of 150-500 m and surrounded by small islands. A hundred-micrometer-scale parallel lamination well develops in the siliceous rocks of the central part of the basin. The comparison of the average laminae thickness with the average sedimentation rate deduced from biostratigraphic data suggest the laminations are varves. In the studied area, which is located in the center of the basin, nearly 50% of the siliceous rocks are laminated. Degree of laminae preservation, which is believed to refle t bottom-water oxygenation level, shows cyclic fluctuation from perfectly laminated to perfectly bioturbated with a cycle thickness of approximately 2 m. Frequency of dark- and light-colored alternation also fluctuates from 5 to 20 dark layers per meter, showing 2-m and 11-m cycles. Based on the laminae thickness, a 2-m cycle and an 11-m cycle represent approximately 15,000 and 80,000 yrs, respectively. Taking into account the uncertainty of the estimation for the average lamina thickness, these values are close enough to the orbital precessional and eccentricity cycles.

MARs of biogenic silica, detritus, and organic matter are calculated based on chemical composition, dry bulk density, and average laminae thickness for each samples. The detritus MAR clearly shows that 2-m cycles with low MAR coincide with the well-laminated and low dark- and light-colored alternating frequency intervals. The organic matter MAR shows similar changes as the detritus MAR, although 2-m cycles are less clear. On the other hand, the biogenic silica MAR shows decimeter-scale fluctuation with low MAR in dark layers. The effect of diagenetic redistribution of silica is negligible in the studied section.

We conclude that decimeter-scale dark and light alternation most likely reflects hundred-year-scale fluctuation in biogenic silica flux, whereas meter-scale cycles reflect longer time-scale fluctuations in bottom-water oxygenation level and detritus MAR, which are related to orbital cycles.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990