Volume n° 3 - from D01 to P13 32nd INTERNATIONAL GEOLOGICAL CONGRESS GEOLOGICAL FEATURES AND THE HISTORICAL AND ARTISTIC HERITAGE OF VITERBO, CITY OF POPES, EMPERORS AND THERMOMINERAL WATER Leader: U. Chiocchini Field Trip Guide Book - D06 Field Trip Associate Leaders: A. Lanconelli, S. Madonna Florence - Italy August 20-28, 2004 During-Congress D06 D06_copertina_R_OK C 24-05-2004, 9:21:02 The scientific content of this guide is under the total responsibility of the Authors Published by: APAT – Italian Agency for the Environmental Protection and Technical Services - Via Vitaliano Brancati, 48 - 00144 Roma - Italy Series Editors: Luca Guerrieri, Irene Rischia and Leonello Serva (APAT, Roma) English Desk-copy Editors: Paul Mazza (Università di Firenze), Jessica Ann Thonn (Università di Firenze), Nathalie Marléne Adams (Università di Firenze), Miriam Friedman (Università di Firenze), Kate Eadie (Freelance indipendent professional) Field Trip Committee: Leonello Serva (APAT, Roma), Alessandro Michetti (Università dell’Insubria, Como), Giulio Pavia (Università di Torino), Raffaele Pignone (Servizio Geologico Regione Emilia-Romagna, Bologna) and Riccardo Polino (CNR, Torino) Acknowledgments: The 32nd IGC Organizing Committee is grateful to Roberto Pompili and Elisa Brustia (APAT, Roma) for their collaboration in editing. Graphic project: Full snc - Firenze Layout and press: Lito Terrazzi srl - Firenze D06_copertina_R_OK D 24-05-2004, 9:19:35 Volume n° 3 - from D01 to P13 32nd INTERNATIONAL GEOLOGICAL CONGRESS GEOLOGICAL FEATURES AND THE HISTORICAL AND ARTISTIC HERITAGE OF VITERBO, CITY OF POPES, EMPERORS AND THERMOMINERAL WATER AUTHORS: U. Chiocchini1, A. Lanconelli2, S. Madonna1 1Dipartimento di Geologia, Ingegneria Meccanica, Idraulica e Naturalistica per il Territorio (GEMINI), Università della Tuscia, Viterbo - Italy 2Archivio di Stato di Roma - Italy Florence - Italy August 20-28, 2004 During-Congress D06 D06_R_OK A 24-05-2004, 9:24:14 Front Cover: The old walls at Porta Faul, western side of Viterbo D06_R_OK B 24-05-2004, 9:24:16 GEOLOGICAL FEATURES AND THE HISTORICAL AND ARTISTIC HERITAGE OF VITERBO, CITY OF POPES, EMPERORS AND THERMOMINERAL WATER D06 Leader: U. Chiocchini Associate Leaders: A. Lanconelli, S. Madonna Introduction characterised by two series: the KS and the HKS , The city of Viterbo is situated on the northwestern typical of the Vicano District. slopes of the Cimini Mountains (the Region of Rocks of felsic volcanism are considered hybrid Latium) and its gently dipping landscape was shaped because they are connected to a process of crustal during the huge, mainly ignimbritic explosive eruption melting of magmas rising from the mantle combined of the Pleistocene Cimini - Vicano Volcanic District with the felsic magmas produced by anatexis (Figure 1). The fi eld trip will allow participants to processes. Potassic magmas rise from the mantle observe inside the inner parts of the main products following a process of assimilation and differentiation. of the volcanic activity. The surrounding areas are The formation of felsic magmas, including the also characterised either by springs of thermomineral geodynamic environment where the volcanic territory water, utilized since Roman times for therapeutic of the Cimini Mountains lays, are very complex and purpose, or by some quarries of a particular lithifi ed not well defi ned until now. gray tuff locally known as “peperino”. Viterbo has an The Cimino Volcanic District important historical and artistic heritage, since it is The activity of this district developed between 1.35 one of the best-preserved medieval cities in Latium. and 0.8 Ma in three different stages. Today we can still see the formidable walls and Some endogenous lavic domes were fi rst set in place buildings constructed between XII and XIII centuries. through some NW–SE trending fractures. Then a The city is situated along the main medieval road that particularly energetic eruption followed, caused by connected the north and Rome – the Francigena Way riodacitic magma rising from the same fractures traveled by pilgrims and German emperors – and was from which the domes had previously originated. The disputed for a long time between popes and emperors, product of this eruption was a lithoid ignimbrite with who all left their marks on the city. a typical fl ame structure (Typical Peperino Auct.). The fl ow direction and distribution suggest that the Regional geologic setting magma expanded mainly north-east (Lardini and The urban area of Viterbo rises on a Pleistocene Nappi, 1987). volcanic territory, shaped during two different The second stage of the eruption occurred through magmatic cycles: the felsic cycle of the Tuscan some transversal fractures, with a quartz-latitic - Latium Province which gave birth to the Volcanic magma which determined the formation of some District of the Cimini Mountains (Marinelli, 1967); lavic domes. Some eruptions followed along the same and the potassic cycle of the Comagmatic Latium fractures: the fi rst were hydromagmatic eruptions Province represented by the Vicano Volcanic District. producing pyroclastic surges, the second eruptions There are also more recent deposits consisting of were characterised by a gas-rich magma producing some alluviums and eluvial-colluvial deposits. The another pyroclastic fl ow. volcanic substratum is made of either Tolfa Flysch or The third stage gave birth to a central volcano units of the Pliocene – Pleistocene neoautochthonous corresponding to the present-day Mt. Cimino. The cycles outcropping essentially along the northern and activity started building the apparatus that consists of eastern edges of the volcanic units (Sollevanti, 1983; the superposition of quartz-latitic magmas, followed Borghetti et alii, 1981, 1983). The Tolfa Flysh has by latitic and olivine-latitic magmas rising from some been classifi ed in different ways: by Boccaletti et alii, lateral mouths. 1980 as an internal Austroalpine unit; as a part of the The distribution of the volcanic rocks in the Cimini Tolfa Formation according to Abbate and Sagri, 1970; area is quite limited, and to the west it does not D01 to P13 n° 3 - from Volume as Tolfetani Flysch according to Fazzini et alii, 1972, include the Vicano District, as is attested to by the tectonically capping the Meso-Cenozoic units of the lack of products of this area in the pyroclastic rocks of Tuscan - Umbria - Marche Domain (western sector; the Vicano Volcanic District. Figure 1B). The Vicano Volcanic District The Cimino and the Vicano Volcanic Districts are There are lots of controversies regarding the evolution completely different from each other, both temporally of this district. According to Nicoletti (1969) it and for the magmas they produced. formed between 0.82 and 0.52 Ma; the latter refers to The magmatism of the Latium Province is the Vicano Tufo Rosso a Scorie Nere Unit. According 3 - D06 D06_R_OK 3 24-05-2004, 9:26:54 Leader: U. Chiocchini D06 Volume n° 3 - from D01 to P13 n° 3 - from Volume Figure 1 - Location of the Viterbo area and its main structural features. a) normal fault; b) main positive structural axis and c) main negative structural axis, obtained by gravimetric survey; d) normal fault obtained by seismic and geoelectric survey; e) normal fault detected on the base of fi eld evidence; f) drilling by the Terny Company (1, 2 , 3, 4, 5), INPS (U) and wells of ENEL and ENEL-AGIP joint venture (F1, C1, V1, V2, VT1); g) location of the stratigraphic – structural sections A – A’, B- B’, C – C’ (cfr. fi g. 4); h) main thermomineral springs (Ba, Bagnaccio; Ca, S. Caterina; Bu, Bullicame). D06 - D06_R_OK 4 24-05-2004, 9:26:56 GEOLOGICAL FEATURES AND THE HISTORICAL AND ARTISTIC HERITAGE OF VITERBO, CITY OF POPES, EMPERORS AND THERMOMINERAL WATER D06 Figure 2 - Geologic map in the Viterbo urban area. to Borghetti et alii (1981) this area developed between The stratigraphic data of a geological survey (Figure 0.4 and 1.144 Ma while according to Boccaletti et alii 2) show that the reconstruction of the pyroclastic (1987) it developed between 0.7 and 0.25 Ma. The units’ succession in the Cimino and Vicano Districts Consiglio Nazionale delle Ricerche (1987) estimates is quite clear. The Vicano District units, indeed, (the it rose between 0.7 and 0.14 Ma, while Bertagnini and Campo Farnese Unit, the Fosso di Piedilupo Unit, the Sbrana (1986) identify four different stages according Ignimbrite A Unit and the Vicano Tufo Rosso a Scorie to different kinds of eruptions. Nere Unit) lay on the older Bagnaia Unit (Typical The fi rst stage was characterised mainly by an Peperino Auct.) of the Cimino District. explosive Plinian eruption which deposited pumices In the Viterbo urban area the substratum of pyroclastic on the northern side of the district. The second, units varies from east to west. The data obtained from mainly characterised by a lava eruption, led to the the drillings in the urban area and the excavations formation of the stratovolcano. The third was again carried out for the building of houses along the road an explosive Plinian eruption with the emission of leading to San Martino point out that the substratum the main pyroclastic formations (ignimbrite A, B, C, is made of Lower Pliocene marine pelites, lying D; Locardi, 1965). The study of ignimbrite A and B immediately below the Bagnaia Unit, up to the showed that the deposits were the products of Plinian Bullicame-Riello area; to the west, instead, it is eruptions. In the case of ignimbrite C, also known characterised by the Tolfa Flysch. The same subsoil as Vicano Tufo Rosso a Scorie Nere, the eruption data show that the Ligurian and Austroalpine units D01 to P13 n° 3 - from Volume is much more complicated: after a Plinian eruption form the substratum of the Pliocene deposits of the some fractures led to the emission of the wide plateau Vulsino, Cimino, Vicano and Sabatino districts (Baldi of ignimbrite C. Then followed a collapse which et alii, 1974).