Arkell.1956.Jurassic..

THE APENNINES 213
facies, and some elements are locally reduced or absent; but on the whole
the succession is widely recognizable in the autochthonous or 'limestone
Apennines'. In the north-west, however, intense tectonic complication
occurs. The autochthon is itself believed by some authorities to
be torn from its roots in the form of a vast thrust sheet (the Tuscan nappe,
or Tuscanids), and the lower parts at least are metamorphosed regionally
(especially in the Apuan Alps—see fig. 22, where the celebrated Carrara
marble has been produced from Lower Lias limestone). Above all this,
however, there rests with violent structural unconformity a heterogeneous
sheet of mixed incompetent rocks, containing exotic blocks of all sizes,
known as the Ligurian 'nappe' or Ligurids. The total thickness is up to
1000 m. The principal element in this sheet is a laminated clay, the
'Argilli scagliosi', of Cretaceous and Tertiary age, in which float exotic
masses of limestone, sandstone, granite and green igneous rocks, both
intrusive and extrusive (the so-called 'ophiolitic series' or 'ophiolites').
These are partly in place and partly transported. At the base of the
Ligurids in the north are radiolarian cherts and limestones with Calpionella
alpina, which in the Mediterranean area is a reliable index fossil of
the Tithonian and Lower Neocomian (Steinmann, 1907, 1913). These
rocks rest on all horizons up to Eocene. The Calpionella beds are
penetrated and altered by intrusive green igneous rocks, but also contain
conglomerates in which have been found pebbles of apparently the same
igneous rock. From this it has to be concluded that violent igneous
activity in the northern Apennines occurred during the Tithonian or
possibly Lower Neocomian (Teichmiiller & Schneider, 1935, p. 8).
The Ligurian 'nappe' or 'allochton' is continuous in the Ligurian
Apennines of the north, but becomes broken up southwards by erosion
of domes and anticlines through which the Tuscanid series protrudes as
tectonic windows (as in the Apuan dome), until in Umbria it is reduced
to shreds and klippes. (Tilmann, 1926; and a good account also in
Kober, 1928, Der Bau der Erde, pp. 195-201).
Steinmann called the radiolarian cherts 'abyssites' because they contain
Radiolaria and no macro-fossils, but more recent work in many countries
has shown that such rocks are more likely of shallow-water origin and
connected with the presence of submarine eruptives; moreover, they
contain algae, as Steinmann admitted.
The source of origin of the so-called Ligurian nappe, and the mode of
transportation, present capital problems. It may be many years yet
before final solutions can be found. Any theory must take into account
the widespread occurrence of similar phenomena farther down the
Italian peninsula—where, however, only post-Jurassic rocks are involved,
so that we are not here concerned with the details—and in Sicily. (For
a full discussion of the problems of dating the Argille scagliose see
Brueren, 1941. He favours the view that in the north (Etruscan Apennines)
it is Cretaceous and perhaps in part even Jurassic, and equivalent to the
Schistes lustres.)
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