Arkell.1956.Jurassic..

MADAGASCAR 343
deltaic sandstones and clays which persist up to the Bajocian inclusive
and appear to form a continuation of the Triassic Isalo Sandstones
(Besairie, 1946, p. 17; Basse, 1949, p. 60).
[TRIAS
The Isalo Sandstones begin with an unconformity and basal
conglomerate. The thickness is up to at least 500 m. The lowest beds
are Eo-Trias; the highest probably Upper Trias, consisting of sandstones
and shales with Myophoria vulgaris and Mytilus psilonoti, which correlate
with the Lugh Series and Adigrat Sandstone of Abyssinia (Barrabe,
1929, p. 77; Basse, 1949, p. 58). The correspondence with Jubaland is
unmistakable (see p. 319).]
THE AFRICAN SHIELD
A considerable excursion into the stratigraphy of the northern part of
the African shield was made in the chapter on North Africa (pp. 256, 286),
because various rocks there had been loosely referred to the Jurassic. It
was seen that all these alleged Jurassic sediments turned out to be
Cretaceous where not Triassic. The Jurassic appeared to be represented
by a lacuna, from which it is to be inferred that it was a period of erosion
on the northern part of the shield. The same applies to the southern
part also.
In South Africa the whole of the uppermost (Stormberg) division of
the Karroo is customarily referred to the Trias, excepting the Drakensberg
Volcanics (= Stormberg lavas) at the top, which are thought to be
Rhaetic-Lias (Du Toit, 1926, p. 277). These immense lava sheets are
presumed to have resulted from fissure eruptions on a land surface. The
next sediments laid down in South Africa are Neocomian, and by the time
they were formed the Karroo surface had been profoundly eroded.
In East Africa and Madagascar there is a much less definite top to the
Karroo, and from what has been mentioned in earlier parts of this chapter
it will be apparent that in those regions the sequence of events in the
Rhaetic and Lower Lias periods is by no means yet fully elucidated.
The earliest marine horizon in both Madagascar and East Africa is the
Bouleiceras zone, basal Toarcian, which represents a transgression across
Karroo-type sandstones. It is quite possible that along the eastern margin
of the continent Karroo-type sedimentation continued throughout the
Lower and Middle Lias, and in Tanganyika even up to the base of the
Bajocian. (For the latest information on the Karroo, see the symposium
on the Gondwana Series issued by the XlXth International Geological
Congress, Algiers, 1952, pp. 181-256.)
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