Rend. online Soc. Geol. It., 3 (2008)
Polyphase deformation in sedimentary basins: the case of the eastern sector of the Tertiary Piedmont Basin Andrea Bernardeschi(*), Rita Catanzariti(**), Michele Marroni(*)(**), Giuseppe Ottria(**), Luca Pandolfi(*)(**) to the tectonic event that generated the unit’s basal unconformity surface. Furthermore, by dating the rocks above the unconformity surface we can individuate the timing of the tectonic event that generated the unconformity. The Tertiary Piedmont Basin (TPB) considered as a part of a wider episutural basin known as epi-Mesoalpine Basin (Mutti et alii, 1995), represents an episutural basin developed from Late Eocene to Miocene, filled by a thick succession of predominantly terrigenous deposits (Fig. 1a), The study area is located at the eastern margin of the TPB where the succession, unconformably overlying the External Liguride Antola Tectonic Unit, is limited to the North by the Villalvernia-Varzi Line (Elter et Pertusati, 1973; Laubscher et alii, 1992) (Fig. 1b). The sedimentary succession, characterized by strong lithofacies variations and marked compositional changes, displays intense synsedimentary tectonic activity through the widespread occurrence of unconformity surfaces (Di Giulio et Galbiati, 1995; Mutti et alii, 1995; Di Biase, 1998). Nevertheless the TPB is regarded to be an almost undeformed succession, the eastern TPB represents a good example of polyphase-deformed basin (Mutti et alii, 2002; Marroni et alii, 2002). stratigraphic and tectonic evolution Based on the recognition of unconformity surfaces, 11 depositional were individuated and dated the use of nannofossil biostratigraphy. The succession started with hemipelagic marls (Monte Piano unit) and by the development of silicoclastic turbidite systems progressively shallower (Pizzo d’Oca, Grue and Rio Trebbio units) evolved to fan-delta deposits (Rivalta, Val Borbera, Monteggio and Persi units). The succession, after an abrupt depositional change, continued with turbiditic sedimentation (Monastero, Gremiasco and Castagnola units). Three main unconformity surfaces of regional tectonic meaning were individuated. These unconformities can be regarded as the results of the three main deformation phases (D1, D2, D3). The D1 structures, recognized only in the lowermost part of the succession, are sealed by the angular and erosional unconformity at the base of the Monastero unit, Middle Rupelian
Riassunto Deformazioni polifasiche nei bacini sedimentari: il caso del settore orientale del Bacino Terziario Piemontese Il Bacino Terziario Piemontese (BTP) rappresenta un bacino di tipo episuturale, impostato, a partire dall’Eocene, superiore in discordanza al di sopra delle unità alpine strutturata nel corso della Fase Mesoalpina. Dall’Oligocene il BTP, ed il suo substrato, vengono implicati nella formazione del prisma orogenico appenninico. Trovandoci in un bacino episuturale, le principali superfici di unconformity riconosciute all’interno della successione corrispondono quindi a tappe significative dell’evoluzione tettonica del settore studiato. Utilizzando un approccio integrato, (geologia strutturale unita alla stratigrafia e alla biostratigrafia a nannofossili), è stato possibile ricostruire l’evoluzione tettono-sedimentaria dell’area in esame. In base a queste discontinuità è stato possibile distinguere tre principali fasi deformative (D1, D2, D3). La fase D1, caratterizzata dallo sviluppo di strutture trastensive che mostrano chiari indizi di attività fin dal Priaboniano superiore (MNP 20), risulta sigillata da depositi del Rupeliano medio (MNP 23). La successiva fase D2, sigillata da depositi del Chattiano inferiore (MNP 24), mostra uno sviluppo di pieghe e thrust nord-vergenti, mentre la successiva fase D3, sigillata da depositi del Burdigaliano inferiore (MNN 2a), è caratterizzata dallo sviluppo di pieghe a piano assiale sub-verticale.
Key words: Northern Apennines, Polyphase Deformation, Tertiary Piedmont Basin, Unconformities
INTRODUction
The use of a multidisciplinary approach, (structural geology coupled with stratigraphy and nannofossil biostratigraphy), allows the reconstruction of the tectono-sedimentary evolution of a polyphase-deformed basin evolved at shallow structural level. In the case of basins affected by strong synsedimentary tectonics, the tectonic pulse that determines the basin’s margin variation is also responsible for the formation of unconformity surfaces that represent the response to tectonics acting in the basin. By this way, the rocks enclosed between two unconformity surfaces, of local and/or regional significance, constitute a depositional unit, representing the sedimentary response
Michele Marroni:
[email protected] (*) Dipartimento di Scienze della Terra, via S. Maria 53, 56126; Pisa Italy (**) Istituto di Geoscienze e Georisorse – CNR Lavoro eseguito nell’ambito del progetto CARG, FOGLIO 196 “CABELLA LIGURE” 97
Fig. 1 – Tectonic sketch map of the Northern Apennnines and Western Alps (1a), and close-up view of the study area (1b).
(MNP 23). The D1 phase displays sub-vertical transtensive faults, striking between NW-SE and N-S. The early activity of these faults seems to be responsible for the formation of pull-a-part basins since the Late Priabonian (MNP 20) where Late Priabonian-Early Rupelian units were deposited. Open to tight asymmetric folds with A1 axes ranging from N-S to E-W are also related to this phase. The D2 phase is characterized by the development of open to tight overturned asymmetric folds, associated with NE-verging thrusts that affect also the Antola tectonic unit. The A2 axes trend is scattered from E-W to N-S, with, respectively, a northward to eastward facing of the D2 folds. The D2 phase is sealed by the erosional unconformity at the top of the Monastero unit dated at the Rupelian-Chattian boundary (MNP 24). Open folds with sub-vertical axial planes and A3 axes showing a main strike between NE-SW and NWSE characterize the D3 phase. This phase is also responsible for the development of small NE-verging thrust. Early Burdigalian (MNN 2a) deposits seal the D3 structures.
Di Giulio A. & Galbiati B. (1995) – Interaction between tectonics and deposition into an episutural basin in AlpsApennine knot. In: Polino R. e Sacchi R., (Eds.) - Atti del Convegno “Rapporti tra Alpi e Appennino”: Accademia Nazionale delle Scienze detta dei XL, Roma, 1-19. Elter P. & Pertusati P. (1973) – Considerazioni sul limite Alpi-Appennino e sulle sue relazioni con l’arco delle Alpi Occidentali. Mem. Soc. Geol. It., 12, 359-375. Laubscher H., Biella G. C., Cassinis R., Gelati R., Lozej A., Scarascia S. & Tabacco I. (1992) – The collisional knot in Liguria. Geol. Rundschau, 81 (2), 275-289. Marroni M., Cerrina Feroni A., di Biase D., Ottria G., Pandolfi L. & Taini A. (2002) – Polyphase folding at upper structural levels in the Borbera Valley (Northern Apennines, Italy): implications for the tectonic evolution of the linkage area between Alps and Apennines. C. R. Academie des sciences, 334, 565-572. Mutti E., Papani L., di Biase D., Davoli G., Mora S., Segadelle S. & Tinterri R. (1995) – Il Bacino Terziario Epimesoalpino e le sue implicazioni sui rapporti tra Alpi e Appennino. Mem. Sci. Geol., 47, 217-244. Mutti E., Ricci Lucchi F. & Roveri M. (2002) – Revisiting turbidites of the Marnoso-Arenacea Fomation and their basin-margin equivalents: problems with classic models. Excursion Guidebook, 64 EAGE, Florence (Italy), p. 120.
References Di Biase D. (1998) – Stratigrafia fisica, facies e significato dei Conglomerati della Val Borbera (Bacino Terziario Piemontese). Ph. D. Thesis, Università di Parma.
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