| Budget Amount *help |
¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 2006: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2005: ¥600,000 (Direct Cost: ¥600,000)
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| Research Abstract |
Rock-boring trace fossils are characterized by boring organisms such as bivalves, gastropods, brachiopods, barnacles, polychaete annelids, sponges, bryozoans, etc. In general, these trace fossils are used as indicators of ancient rocky shores, and are useful for reconstruction of pal eoenvironments.
The shallow-marine facies of Hokkaido rich in rock-boring bivalves was deposited during the Miocene. I studied rock-boring bivalves and their clavate borings at two localities in northern Hokkaido. In locality Akebono, basal part of the Sankebetsu Formation unconformably overlies the basement and is composed of fine-grained sandstone. Rock-boring bivalves such as Penitella kotakae and Platyodon nipponica are recognized, and the clavate boring of Penitella, Gastrochaenolites turbinatus, dominated in the basements. This occurrence represent autochthonous, and is presumed to wave-cut bench of ancient rocky shore. In locality Yamato, basal part of the Yamato Formation unconformably overlies the basement and is composed of basal conglomerate. Boring bivalves dominated in clasts of conglomerates, and no occurrence on the basements. Rock-boring bivalves, P sp., and their clavate borings, G. turbinatus, dominated in the clasts. This occurrence represent semi-autochthonous, and is presumed to boulder beach of ancient rocky shore.
The paleoecological characteristics of the molluscan and ichnofossil assembleges in these localities reflect changes in the sedimentary environment during a sea-level transgression. The rocky shores were formed during the transgression events in the Miocene time. Bioerosion on marine rocky surfaces demonstrates to be a very good tool to reveal geological episodes such as relative oscillations of the sea-level, erosion/sedimentation rates and intensity of physical disturbance.
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