Geologia dell'area di Varzi (Appennino vogherese): note preliminari.

The geological survey of the Ligurids and the overlying epi-Ligurian Sequence outcropping in the Varzi zone (Oltrepo Pavese, Voghera Apennines) has brought to the acquisition of new stratigraphic and structural data in an area whose geological cartography goes back to about thirty years. The studied area is set behind an important tectonic line (Villavernia-Varzi Line) dividing the northern Apennines from the deposits of the Tertiary Piedmont Basin. The outcropping Ligurids are referable to the Cassio tectonic unit and belonging both to the Cretaceous “basal complexes” or pre-Flysch formations, and to the stratigraphically overlying upper Cretaceous-Paleogene turbiditic succession (Cassio-Viano or Val Tresinaro Sequence). Particularly, some outcrops, in the past included inside the “Chaotic pluriformational Complex” or attributed to the Pagliaro Shales, are correlable to the Viano Shales overlying the Mt. Cassio Flysch with a conformable and transitional boundary in the Emilia Apennines.The stratigraphic features of the epi-Ligurian Sequence show very close analogies with those present in many areas of the Emilia Apennines. The lower Eocene-Oligocene and lowermost Miocene part of the succession shows, besides the Monte Piano Marls, the Ranzano Fm. and the Antognola Fm., also a thick body of matrix-supported argillaceous polygenic breccias derived from mud and debris flows of Ligurian mainly shaly rocks. This body is correlable to the similar deposits that in the Emilia Apennines have been joined under the name of Baiso argillaceous Breccias, middle-late Eocene in age, approximately corresponding to the sedimentation span of time of the Monte Piano Marls. The uppermost part of the epi-Ligurian Sequence (Miocene in age) is represented by the Contignaco and Pantano Formations widely outcropping also in the Emilia Apennines and made of relatively fine-grained clastic deposits respectively with a siliceous and carbonatic dominant feature. Finally, both on Contignaco and Pantano Fms. prevailing clastic shelf deposits awarded to the Mt. Vallassa Sandstones outcrop. Their badly defined age (middle?-upper? Miocene) and their stratigraphic position suggest a questionable correlation either with the upper Miocene Termina Formation or with the middle Miocene inner shelf deposits belonging to the Bismantova Group of the Emilia Apennines.On the basis of their attitude, a strong structural difference between the lower (pre-Miocene) and the upper part of the epi-Ligurian Sequence is clear. The first one is partly involved in an overturned synclinal and it generally shows tectonic contacts among the formations and meaningful stratigraphical differences among neighboring zones. The second shows, on the whole, a more simple attitude (monoclinalic), usually plunging to N-NW and preserved stratigraphic boundaries. This suggests that also this sector of the Voghera Apennines has been interested, as in other areas of the Oltrepo Pavese and of the Emilia Apennines, by Oligocene and/or late Oligocene-early Miocene deforming phase. This late Oligocene-early Miocene tectonic phase has been connected to the widespread deposition of thick chaotic deposits of Ligurian materials (Val Tiepido-Canossa argillaceous Breccias) within the epi-Ligurian basin. A second meaningful Miocene (late Serravallian - early Tortonian?) deforming phase (mainly characterized by the development of a moderate brittle tectonics) is documented by the stratigraphic discontinuity at the base of the Mt. Vallassa Sandstones.