Southern Alpine (Trento Plateau) and Northern Apennine cherts: Ages, Distribution and Petrography

The sedimentary successions on the south-eastern side of the Alps (Trento plateau, Southern Alps) and on the northern Apennines side are described and compared, with particular focus on the distribution of flints. The aim was to identify the potential they represented in prehistoric times for the people who settled in or passed by these two areas separated by the Po plain. These two areas present similar but also distinctive features, substantially deriving from their respective palaeogeographic positions along the margins of the Adria microplate and from the structural evolution of the Alps and the Apennine Mountains. Flints and radiolarites are rocks that are typical of deep marine environments. The studies therefore focused on such formations. The Trento plateau is the northernmost margin of the Adria plate. The Permian-Triassic sedimentary successions, overlying the crystalline basement, are essentially composed of continental, alluvial, evaporitic and shallow water marine carbonate facies. During the Ladinian stage there was an initial differentiation of the platform into basins in which the Buchenstein/Livinnallongo cherty limestone deposits were intercalated between vulcanite and tuff deposits. This formation mainly outcrops in the Dolomites, where these quite characteristic flints have been collected and used. During the Jurassic stage a drastic environmental change followed the breakup of the Pangaea leading to the opening of the Piemont-Liguria Ocean. The Southern Alps and part of the northern Apennine series (Umbria-Marches; Tuscany) belonged to the Adria passive margin that faulted and were subjected to subsidence into pelagic deposits from the Liassic stage on. Pelagic limestones were deposited in the whole area during the Dogger-Upper Cretaceous interval: flint-bearing radiolarites and limestones were deposited on continental slopes as well as in oceanic domains. The Adria margin was articulated in platforms, submarine plateaus and basins. Theseries that outcrop above a large raised submarine block (Trento plateau, Southern Alps) are described. The pelagic deposition above the Trento plateau was affected by erosions (sea bottom currents) and condensation compared to adjacent deeper areas (Lombard and Belluno basins). On the other hand the rifted block protected the sea bottoms from terrigenous inputs and made possible the deposit of undisturbed cherty limestones. The westernmost margin of the Apennine-Adriatic paleomargin has been overthrust by the Ligurids that were deposited originally farther west, near the continental slope and in the Piemont-Ligurian oceanic domain. Subduction of most of the oceanic crust and covers occurred during the eoalpine (Late Cretaceous) and mesoalpine (Eocene) tectonic phases. The rotation of the Corsica-Sardinia Block started during the Oligocene/Miocene and played an important role for the general structural setting of the Apennine Range. The Ligurids occupy a large part of the northern Apennines slopes; the easternmost area is dominated by the flintless Umbria-Marche siliciclastic foredeep successions. The Ligurids have been traditionally subdivided into internal (mainly outcropping on the Ligurian side) and external Ligurids (mainly outcropping on the Emilian side). The internal Ligurids are remnants of the oceanic crust (Piemont-Ligurian Ocean) and the connected covers. The external Ligurids (northern Apennines side) were deposited between the internal Ligurids and the Adria paleomargin. Here thick, monotonous siliciclastic and almost flintless (except for the Mount Sporno flints) successions were deposited during the Upper Cretaceous-Eocene interval. The basal complexes of the series are comprised of olistoliths /olistostromes showing analogies with both the internal Ligurids (western external Ligurids, WEL) and the Adriatic slope/marginal continental successions (eastern external Ligurids, EEL). These olistoliths/olistostromes have sometimes an impressive size (up to several kilometres), and contain well preserved sedimentary flint-bearing successions that were exploited locally. The ‘internal Ligurids-like’ successions have been known for a long time and they are very peculiar on the Apennine slope, often being associated with ophiolites, whereas the ‘Adria margin/slope-like’ successions are less well known and less frequent. A few large outcrops are distributed on the medium Apennine side (eastern external Ligurids, EEL) and they contain sedimentary cherty successions comparable to more internal portions of the Adria margin (mainly in Tuscany and Southern Alps successions). In particular the Triassic-Jurassic cherty limestones, the Aphtycus limestone and the Maiolica series contain high-quality flints exhibiting particular features, unknown for the internal Ligurids and the western external Ligurids (WEL). The quality and the dimensions of the flints are comparable to the outcrops identified in the Southern Alps. These outcrops certainly represented important procurement sources in prehistoric times and should be considered before taking into account more distant flint provenances for the Apennine sites. The Epiligurids were deposited above the tectonised Ligurids during the Eocene-Messinian period, in satellite basins trapped over the growing chain. Only the Antognola ormation (Miocene) contains flints that are abundant locally (Bologna, Modena). These flints were exploited in historic times and are known as ‘phtanite’. In the Apennine foothills the Imola Sands were deposited during the Early Pleistocene in a marginal marine (littoral) environment containing horizons of well-rounded marine conglomerates including mainly flint. These conglomerates, widely exploited in the area since the Lower Palaeolithic, were transported by the sea shore currents from the Romagna and Marches areas and include flints cropping out in these areas. This work mainly focuses on the description of the geologic framework of the cherty formations as well as on flint distributions and occurrences. Detailed petrographic descriptions (microfacies) combined with an assessment of the knapping suitabilities of the flints are ongoing and will be subjected to further studies in cooperation with the Ferrara University.