The Franciscan-Apennines connection, 30 years later

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.