放散虫層状チャートを形成した底層流の速度を推定するための実験的考察

Biostratinomy of macrofossils such as conical shaped moluscus，bivalves and ammonites has been studied minutely in shallow marine environments．On the other hand microfossils have been hardly studied as much．Conical or elongate shaped microfossils for example，radiolarians and spongy spicules are important components of siliceous rocks，which are regarded to have deposited under CCD．Orientation of these fossils and their relatlon to current velocity are one of the interesting object for understanding pelagic environments．On behalf of conical radiolarian shells Early Jurassic Parahsuum ssp. are adopted in a flume experiment to clarify relationship between their arrangement and current velocity．
The result compiled in rose-diagrame indicates correlation between radiolarian shell arrangement and current velocity．1）When current velocity is fast（Re=1121，where Re is Reynolds number），concentration rate of shell apex directed upstream is clearly observed．This position could be hydrodynamically stable in the current for conical shaped radiolarians．2）When current velocity is under Re-971，shell arrangement vertical to the current occupied higher rate．But in less current velocity，concentration of this style is gradually diversified to increase
ratios of radiolarians arranged perpendicular to the current，which were previously facing to upstream．3）As for relation between rate of shell apex facing to downstream and current velocity，this correlation can not be accorded in the case of current velocity excessing over Re=1121 and under Re=336．The slower current velocity is，the higher rate of shell apex directed downstream is．These lines of observation will help us to interpret paleoenvironmental condition and current deduced from field data．
When current velocity excesses over Re=971 in the flume，ripple marks appeard．This limits possible range of current velocity of bedded cherts under Re=971，for pelagic bedded cherts have not hitherto produced ripple marks．