Arkell.1956.Jurassic..

434 INDOCHINA AND INDONESIA
SUMATRA
In several parts of the island there are patches of phyllites and silky
slates with interbedded sandstones, quartzites and occasional limestone
lenses, which at first were taken to be Palaeozoic or earlier. Like the
Alpine Bundnerschiefer which yield stretched belemnites, however, they
have proved to contain remains of Mesozoic fossils. The best faunule,
from a lens of limestone in the Jambi district (see fig. 65), yielded Astarte,
Opis, Lucina and Cypricardia, pelecypods which seem to be of Middle
Jurassic age (Tobler, 1923; Freeh & Meyer, 1922). In several places also
lenses of reef limestone with only recrystallized fossils are attributed
doubtfully to the Upper Jurassic, while Lower Jurassic is suggested by an
oolite lens with Pentacrinus, corals resembling Montlivaltia, and belemnites
(Volz, 1913; Tobler, 1923; Brouwer, 1925, p. 29; Wanner, 1931, p. 597).
Clearly it is possible that the whole Jurassic system exists in Sumatra
in a partly metamorphosed state. According to the map by Rutten (1938),
however, there are only four small patches of Jurassic beds besides the
main Jambi area (see fig. 65).
The Neocomian ammonite genera Neocomites, Kilianella and Olcostephanus
are also recorded from Jambi.
BORNEO
A large part of this island is built of the Danau formation, a series of
radiolarites, hornstones, siliceous shales, quartz sandstones, silicified
tuffs and igneous rocks, intensely folded and compressed but not extensively
overthrust (Easton, 1904; Martin, 1907; Wanner, 1931, p. 596).
The radiolarites have, as always, caused differences of opinion as to date.
They were at first thought to be Jurassic, but more recently almost all
ages except Jurassic have been attributed to the Danau: ranging from
Devonian to Paleocene. In west Borneo shallow-water Jurassic beds,
which have been little disturbed but extensively denuded, and include in
places Lias, transgress over the folded Danau; here the Danau is considered
to be Palaeozoic to Lower Triassic, and, from the contained Radiolaria,
mainly Permo-Triassic (Easton, 1904, p. 28; Umbgrove, 1938, pp. 25-7;
Kobayashi & Kimura, 1944, p. 241). In north Borneo, however, where
no fossiliferous Jurassic is known, intensely folded Danau rocks contain
derived fragments of radiolarites and foraminifera of Cretaceous to Lower
Tertiary age. It seems, therefore, that in different parts of the island the
so-called Danau formation is of widely different ages, and received its
tectonic deformation at widely different periods. (For full discussion see
Reinhard & Wenk, 1951, pp. 91-7.)
From the Sambas district, in the extreme west of the island (see fig. 65),
an ammonite from a nodule has been figured as 'Aegoceras' borneensis
Krause (1911, pi. vii); it may be a Xipheroceras and in that case of middle
Sinemurian date. From shales at another locality in the same district
have been figured Upper Toarcian Grammoceratids, apparently for the
http://jurassic.ru/