Arkell.1956.Jurassic..

CHAPTER l6
JAPAN AND KOREA
The three island festoons which stretch along the Pacific coast of Asia
from Formosa through the Ryukyu Islands, Japan proper and the Kurile
Islands to Kamchatka represent the tops of sunken mountain arcs. They
are not, however, all of one date. While the folding of the Ryukyu and
Kurile arcs is of Tertiary origin, the folding of the Japanese arc is essentially
Mesozoic. The north island of Hokkaido (Yezo) goes with the
Kuriles and has complex Tertiary nappe structures, but in the main island
of Honshu and the southern isles of Shikoku and Kyushu (fig. 63) Tertiary
folding has taken place mainly along echeloned axes oblique to the islands
(Otuka, 1937). Here the main orogenies were Middle Triassic (post-
Ladinian, pre-Carnian: the Akiyoshi orogeny) and Lower Cretaceous
(the Oga and Sakawa orogenies), though evidences of lesser phases in the
late Jurassic are not lacking (Matsumoto, 1949). Movement was outwards
from Asia and presupposes a sunken rigid foreland under the
Pacific deeps. The foreland seems to have contributed no sediments to
the Mesozoic formations of Japan and therefore is inferred to have been
already submerged (Kobayashi, 19356; 1937; 1938; 1941).
Professor T. Kobayashi, to whose tremendous industry over the past
quarter century we owe an entirely new picture of Japanese geology,
postulates that after the Middle Triassic orogeny and Carnian transgression,
pre-Liassic epeirogenic movements warped up a gentle arch of folded
and thrust Palaeozoic and Lower Triassic rocks along the central axis of
south-western Japan. He calls this ancient hypothetical land Eo-Nippon.
During the Jurassic and Cretaceous epochs there formed on either side,
and were gradually filled, two more or less separated geosynclinal troughs.
Deposition on the outer (Pacific) side was mainly marine, with coarse
clastic and plant-bearing intercalations; on the inner (Asiatic) side it was
mainly lacustrine and deltaic, probably in more or less separate basins,
with marine intercalations representing temporary incursions of the
Jurassic and Cretaceous sea through breaches in the intervening barrier.
During the Jurassic there occurred on both sides epeirogenic movements
which greatly reduced and perhaps altogether halted sedimentation
in the Middle Jurassic: no definite Middle or Upper Bajocian or Bathonian
faunas have yet been found in Japan. After active and prolonged Upper
Jurassic sedimentation, with marine influence strong in both troughs,
there occurred two Lower Cretaceous orogenies. The first (Oga) is
equated with the Jurassic-Cretaceous interval. Its sphere of influence
was the inner trough and it produced Germanotype tectonics with thrusting
and imbricate structures along the inner side of the Eo-Nippon barrier.
418
http://jurassic.ru/