In the 1960's, Hsu and Maxwell imported two aspects of the geology of the
Northern Apennines and Sicily to the Franciscan: argille
scagliose--literally scaly clays--and olistostromes. Although the term
argille scagliose was coined in 1840 to describe rocks with a certain
mesoscopic fabric, by the 1960's the term had been broadened In Italy to
include many types of rock units with a chaotic aspect imparted by blocks
of diverse sizes disposed in a clay matrix. These chaotic complexes were
popularly inferred to have originated as submarine landslides. the term
olistostrome was used for sedimentary bodies, consisting partly of argille
brecciate, that were emplaced into normally bedded sections by slides or
debris flows. Hence, when these terms were first used for Franciscan
rocks, they strongly connoted an origin by sedimentary or mass-flow
processes.
During the past 30 years, the origin of much of the chaotic and disrupted
Franciscan has remained controversial, while our understanding of N.
Apennine geology has evolved. Can two major re-interpretations of Apennine
geology now help to resolve Franciscan controversies? Recent work in the
N. Apennines has enabled the subdivision of the chaotic complexes,
consisting partly of argille scagliose and brecciate, into two kinds of
units, which each have different origins and ages. Tectonosomes are
disrupted Cretaceous and early Tertiary stratigraphic sequences that were
probably deformed in a pre-Oligocene accretionary wedge; they now reside
in the extensive Ligurian nappe. The nappe itself, which contains these
remnants of a B-type accretionary wedge, is an A-type wedge that is
advancing onto the continental Adria plate. The olistostromes are present
in strata deposited in late Eocene and younger Epiligurian basins, riding
piggyback on the nappe, and in foreland-basin deposits partly overridden
by the nappe.
