Jurassic radiolarian fossils from siliceous mudstone in the Mino
terrane, in Gifu Prefecture, central Japan
Yusuke Kitagawa1 and Atsushi Matsuoka1
1Department
of Geology, Niigata University, 950-2181, Japan
Studies on the evolution of Mesozoic radiolarians have increased gradually since
1980s. As necessary conditions for this kind of study, slow deposition rate, good
preservation of fossils and densely spaced sampling are required. The Mino terrane
distributed in central Japan satisfies these conditions. The Inuyama area and the
Kamiaso area have been actively surveyed for Triassic-Jurassic radiolarian studies for a
long time. However, there is a problem that few sections meet these conditions and
radiolarian studies have been conducted only in limited areas. Therefore, this
undergraduate study aims to discover new sections suitable for examination on
radiolarian evolution.
The Kamiasao unit of the Mino terrane in the Mino district, Gifu Prefecture,
central Japan, is an accretionary complex consisting of stacked chert - clastic sequences.
It consists of basaltic rocks (lava, tuff), limestone, siliceous claystone, bedded chert,
siliceous mudstone, mudstone, sandstone, and conglomerate. Bedded chert and siliceous
mudstone generally yield abundant radiolarian fossils.
A siliceous mudstone sequence is exposed at Hogiwaki along the Nagara River in
Gifu Prefecture over a length of 10 - 15m. This sequence grades downward into bedded
chert and upward into mudstone. Seven rock samples were taken from the sequence
and were treated with a solution of 5% HF. Well-preserved radiolarian fossils were
obtained from all of these samples. As a result of observation using an optical
microscope, it was clarified that some of these samples contained characteristic taxa of
Jurassic radiolarians such as Striatojaponocapsa, Hsuum, Pantanellium, and others.
In the later half of the undergraduate work, radiolarian fossils from these samples
will be observed in detail and stratigraphical changes in morphology will be examined.
The evolution of Jurassic radiolarians will be clarified.