Arkell.1956.Jurassic..

282 NORTH AFRICA
to western Algeria (Arkell & Lucas, 1953). After temporary emergence
in the late Bajocian to Lower Bathonian, the barrier disappeared finally
with the Middle Callovian transgression which caused the widespread
'equalization' noted in Barbary by Lucas (1942) and brought the influx
of northern ammonites such as Erymnoceras into Algeria and Arabia.
Any barrier must have been narrow, for on the latitude of Gabes, close to
the southern edge of the Atlas trough, over 2000 m. of Jurassic has been
proved in borings (Domergue & others, 1952, p. 19).
Geological dating of the Jurassic rocks is difficult owing to scarcity of
ammonites. The 600 m. of gypsum and anhydrite is followed by thin
limestones and marls in which a meagre fauna of small pelecypods and
gastropods occurs, which Dubar (in Domergue & others, 1952, p. 15)
SIDI MOSBAH SI DI STOUT T 0 UJ A N £
| i ! NNW
GYPSUM
FIG. 39.—Horizontal section through the marginal Jurassic of southern Tunisia.
After Mathieu, 1949.
considers Toarcian or Lower Bajocian. A small Gervilliid from these
beds or a little below (Mathieu, 1949, pi. iii, figs. 11, 12) is totally different
from a French Bajocian Oxytoma figured with it for comparison, and
Dr L. R. Cox (in lit.) considers it probably of Lower Jurassic or even
Upper Triassic age.
The Bathonian beds, probably Upper or Middle Bathonian as suggested
by H. Douville (1908), consist of a semi-continental or 'estuarine' facies
with plants and vertebrates, with marine intercalations containing oyster
beds and such shells as Astarte angulata and Trigonia pullus abundantly
(Domergue & others, 1952, pp. 15-16; some figures in Mathieu, 1949, pi. iii).
As in Egypt, Himalaya, Cutch and Burma, these beds are transgressive,
and towards the north they overstep the gypsum and over an area of
nearly 40 sq. km. rest on folded Lower Trias. Mathieu (1949, pp. 31-2)
infers that the date of folding was late Triassic or early Jurassic, and he
names the phase 'Matmatien' after a local district. But from the evidence
presented there could be more than one phase superimposed, and the
last folding could be as late as Lower (or even Middle) Bathonian. If
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