Arkell.1956.Jurassic..

424 JAPAN AND KOREA
1936). The stratigraphy of this area has undergone much revision
(Matsumoto, editor, 1954).
The shales of the Wakino Series contain a remarkable fauna of freshwater
mollusca, including peculiar genera of Unionids, Anodontas,
Corbiculids and Trigonioides, with the gastropod Brotiopsis (Kobayashi &
Suzuki, 1936; Suzuki, 1943). Similar faunules occur at various places
in eastern Asia, and despite its superficially Purbeckian appearance it is
now considered to be Lower Cretaceous (Suzuki, 1949).
KOREA
The Mesozoic basin of deposition in which these thick coarse clastic
sediments were laid down, called the basin of Tsushima after the straits
(fig. 63), extended also over at least the southern part of Korea. There
similar sandstones, shales and conglomerates cover a large area, resting
unconformably on the crystalline basement and on Palaeozoic rocks and
retaining in broad outline something of their original basin shape despite
subsequent diastrophism.
Locally the lower part (Lower Daido Formation) is Jurassic, but not
marine, so far as known. A record of Hildoceras inouyei Yok. if confirmed
would prove a continuation on to the mainland of the Toarcian of Nagato,
but I am informed by Professor Matsumoto that no reliance can be placed
on the record. A rich flora occurs (Kawasaki, 1926, p. 118). There are
also several higher floras, the most celebrated of which is the Naktong
flora, which characterizes a continuation of the Wakino Series of Kyushu,
with some of the peculiar freshwater shells. The beds are ripple-marked
and sun-cracked and contain some intercalated ash beds. This seems to
herald the volcanism recorded on a large scale in the overlying equivalents
of the Inkstone Series. Correlation is as follows:—
KOREA JAPAN AGE
Upper Daido, Upper Keisho, and
Shiragi formations
Middle Daido or Naktong formation
Lower Daido formation
Inkstone series and Shimonoseki
formation
Wakino formation
Nakayama (Middle Toyora)
| CRETACEOUS
TOARCIAN
In North Korea is found in places a rich freshwater molluscan faunule
believed to be early Jurassic. It differs from all the faunules in Korea
and Japan but has its counterpart in Manchuria (Suzuki, 1949, p. 94).
CENTRAL HONSHU
On the northern side of the supposed Eo-Nippon barrier in the central
part of the mainland of Honshu there was irregular subsidence during
the Jurassic and Lower Cretaceous, with deposition of great thicknesses of
shales, sandstones and conglomerates, of facies similar to that in the more
westerly parts of the geosyncline. But while the Lias is here represented
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