Arkell.1956.Jurassic..

NORTHERN ASIA
of Asia. In the west the last orogeny occurred in Triassic times, when the
Urals received their last and probably main folding (Vialov, 1939). With
this episode the Angara and Scandinavian shields were welded together.
The Ural folds—which may be traced far to the east of the present mountains,
under the Ob basin—were planed down before deposition of the Jurassic
rocks, and there were no further major disturbances west of the Lena.
The plateau basalts and dolerites of Angara Land are also Triassic, like
the Karroo lavas and dolerites, and were eroded to remnants before the
Jurassic Angara Beds were deposited (Odintsov, 1937). (Some intrusive
dolerites in the Kuznetsk coal basin, which intrude Middle Jurassic coal
measures (Yavorsky, 1937), may be much later.)
This stable western region belongs tectonically with European Russia
and, like it, was not invaded by the sea until the Upper Jurassic, and then
by a tongue of the Arctic Ocean which spread south on both sides of the
Ural Mountains and left relatively thin deposits. In the east, on the
contrary, geosynclines were actively forming during the Jurassic around
and between the horsts. The deposits laid down in these troughs, partly
marine but mainly continental or brackish, reached thousands of metres
in thickness while those forming farther west reached a few score or at
most hundreds of metres. The greatest thickness of Jurassic recorded is
about 7500 m. in Mongolia, largely conglomerates and sandstones. On the
Kolyma River in extreme NE. Siberia 6000 m. is reported, and 2000 to
4000 m. is common in the Amur geosyncline and elsewhere towards the
Pacific coast.
Like Australia during the Jurassic, Asia was largely covered by freshwater
lakes, in which collected conglomerates, sandstones, shales and
coal measures. The important coal measures of Siberia and China belong
mainly to this period. The flora is not uniform all over the continent.
From the Ob to Transbaikalia conifers predominate over cycads and
ferns, whereas farther east and south-east ferns take first place and cycads
second, indicating a warmer and damper climate towards the Pacific
(Obrutschew, 1926, p. 330). Farther south red beds and lack of flora
indicate aridity in the interior (Kobayashi, 1942). The Angara flora
has been the subject of many studies, and the stratigraphy of the Angara
Beds and their plants is a vast subject outside the scope of this book.
Excellent summaries will be found for Siberia in Obrutschew (1926)
and for China in Lee (1939). (See also Bexell, 1935; Nagibina, 1946;
Leuchs, 1935). In the type sections along the Angara River, between
Irkutsk and Lake Baikal, the Jurassic conglomerates, sandstones and coal
measures are overthrust by pre-Cambrian gneiss and granite, mylonitized
along the thrust-plane, which is in turn folded (Tetiaev, 1937). Near
Lake Baikal the Jurassic is represented by polygenetic conglomerate
with lenses of arkose, resting on Lower Cambrian; in the north it is only
gently inclined, but towards the south end of the lake it becomes intensely
folded and cleaved, and is finally overfolded by pre-Cambrian granite
(Tetiaev, 1937a). This region, which from information then available
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