HAL Id: tel-01152641
https://tel.archives-ouvertes.fr/tel-01152641
Submitted on 18 May 2015HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
the Argentera Massif: stratigraphic, sedimentologic and
diagenetic evidence of syndepositional tectonics
Luca Barale
To cite this version:
Luca Barale. The Meso-Cenozoic stratigraphic succession adjoining the Argentera Massif: strati-graphic, sedimentologic and diagenetic evidence of syndepositional tectonics. Stratigraphy. Universita degli Studi di Torino, 2014. English. �tel-01152641�
Scuola di Dottorato in Scienze della Natura e Tecnologie Innovative
Ciclo XXVI
The Meso-Cenozoic stratigraphic succession
adjoining the Argentera Massif: stratigraphic,
sedimentologic and diagenetic evidence of
syndepositional tectonics
Luca Barale
Scuola di Dottorato in Scienze della Natura e Tecnologie
Innovative
Tesi di Dottorato di Ricerca in Scienze della Natura e
Tecnologie Innovative
Indirizzo: Scienze della Terra
THE MESO-CENOZOIC STRATIGRAPHIC
SUCCESSIONS ADJOINING THE ARGENTERA
MASSIF: STRATIGRAPHIC, SEDIMENTOLOGIC
AND DIAGENETIC EVIDENCE OF
SYNDEPOSITIONAL TECTONICS
Luca Barale
Tutor: Prof. Luca Martire,
Co-tutors: Dott. Anna d’Atri, Dott. Fabrizio Piana
Luca Barale
e-mail: luca.barale@unito.it – luca.barale@hotmail.it Università degli Studi di Torino
Dipartimento di Scienze della Terra Via Valperga Caluso, 35
10125 Torino – Italy
Tutors: Luca Martire
Dipartimento di Scienze della Terra Università di Torino
Anna d’Atri
Dipartimento di Scienze della Terra Università di Torino
Fabrizio Piana CNR-IGG - Torino
Reviewer:
Mikel López-Horgue
Departamento de Estratigrafía y Paleontología Universidad del País Vasco
Examining Committee: Francesco Dela Pierre
Dipartimento di Scienze della Terra Università di Torino
Mikel López-Horgue
Departamento de Estratigrafía y Paleontología Universidad del País Vasco
Gianreto Manatschal IPGS-EOST
Université de Strasbourg
Date of final examen: March 13th, 2014
Università degli Studi di Torino
Scuola di Dottorato in Scienze della Natura e Tecnologie Innovative Indirizzo: Scienze della Terra – Ciclo XXVI
Coordinatore del Dottorato: Fernando Camara Artigas
Anni Accademici 2011–2013
AKNOWLEDGEMENTS
I would like to thank my tutor Luca Martire (Dipartimento di Scienze della Terra, Università di Torino), and my co-tutors Anna d’Atri (Dipartimento di Scienze della Terra, Università di Torino) and Fabrizio Piana (CNR, Istituto di Geoscienze e Georisorse) for the support, the suggestions and the help they gave me during all the phases of the research work and the writing of this Thesis.
I extend my thanks to Carlo Bertok (Dipartimento di Scienze della Terra, Università di Torino), for his help and suggestions during most part of the research work and the writing of the Thesis.
I also thank Piergiorgio Rossetti (Dipartimento di Scienze della Terra, Università di Torino) for the precious help in micro Raman spectroscopy and in petrographic analysis of the “Valdieri Marbles”; Simona Ferrando (Dipartimento di Scienze della Terra, Università di Torino) for guidance and assistance in fluid inclusion microthermometry; Anna Fusconi (Dipartimento di Biologia Vegetale, Università di Torino) for the help and assistance in epifluorescence microscopy.
Stefano Bernasconi and Maria Isabel Míllan (Geological Institute, ETH Zurich) are kindly acknowledged for performing stable isotope analyses on carbonate samples.
Carles Martín-Closas (Departament d'Estratigrafia, Paleontologia i Geociències Marines, Universitat de Barcelona) is thanked for the determination of the Charophytae samples.
I thank Mikel A. López-Horgue (Departamento de Estratigrafía y Paleontología, Universidad del País Vasco), who read the first version of this Thesis and provided helpful suggestions.
The Maritime Alps Natural Park, particularly in the person of Luca Giraudo, is kindly acknowledged for allowing fieldwork within the Park territory, and for providing hospitality in the Park structures.
The research was supported by the University of Torino (Doctoral School of Sciences and Innovative Technologies funds; ex 60% funds) and by the IGG (Istituto di Geoscienze e Georisorse) of the the Italian CNR (National Research Council). Funds related to the ProGeoPiemonte Project (financed by Compagnia di San Paolo and University of Torino) have been also utilized.
Financial contributions by GeoSed, Italian Association for Sedimentary Geology, (Contributo Giovani 2011) and IAS, International Association of Sedimentologists, (Postgraduate Grant Scheme, 2nd session 2012) are kindly acknowledged.
TABLE OF CONTENTS ABSTRACT ... 9 RIASSUNTO ... 11 1 INTRODUCTION ... 13 2 GEOLOGICAL SETTING ... 17 2.1 INTRODUCTION... 17 2.2 ARGENTERA MASSIF... 19 2.3 DAUPHINOIS DOMAIN... 20 2.4 PROVENÇAL DOMAIN... 22 2.5 SUBBRIANÇONNAIS DOMAIN... 23
2.6 THE SUBBRIANÇONNAIS-PROVENÇAL QUESTION... 24
2.7 ALPINE FORELAND BASIN... 26
2.8 ALPINE TECTONICS... 27
3 METHODS ... 29
3.1 ANALYTICAL TECHNIQUES... 29
3.2 EXTRACTION OF CHAROPHYTAE GYROGONITES... 30
4 STRATIGRAPHIC SUCCESSION ... 32
4.1 INTRODUCTION... 32
4.2 ARGENTERA BASEMENT... 33
4.3 PERMIAN-LOWER JURASSIC SUCCESSION... 33
4.3.1 Permian deposits (Middle?-Upper? Permian)... 33
4.3.2 Lower Triassic quartzarenites... 35
4.3.3 Middle Triassic carbonates... 36
4.3.4 Upper Triassic-Lower Jurassic deposits... 38
4.4 THE DAUPHINOIS-PROVENÇAL DIFFERENTIATION AND THE CAIRE PORCERA PALEOESCARPMENT... 39
4.5 JURASSIC-LOWER CRETACEOUS DAUPHINOIS SUCCESSION... 39
4.5.1 Introduction... 39
4.5.2 Entracque Marl (Middle? Jurassic-Berriasian?)... 41
4.5.3 Lausa Limestone (Valanginian?-early Aptian?)... 44
4.5.3.1 Caire Porcera Limestone Member (Valanginian?-Hauterivian p.p.?)... 50
4.6 JURASSIC-LOWER CRETACEOUS PROVENÇAL SUCCESSION... 53
4.6.1 Garbella Limestone (Middle? Jurassic-Berriasian?)... 53
4.6.2 Lower Cretaceous succession... 57
4.6.2.1 Testas Limestone (Hauterivian p.p.?-Barremian?) ... 57
4.7 APTIAN-UPPER CRETACEOUS SUCCESSION... 61
4.7.1 Marne Nere (Aptian-Cenomanian)... 61
4.7.2 Puriac Limestone (Turonian p.p.-Campanian?)... 65
4.7.2.1 General features... 65
4.7.2.2 Lithozone with siliciclastic input ... 66
4.7.2.4 Lithozone with platform-derived bioclasts ... 72
4.8 ALPINE FORELAND BASIN SUCCESSION... 73
4.8.1 Introduction... 73
4.8.2 Trucco formation (Lutetian ?–lower Bartonian)... 74
4.8.3 Nummulitic Limestone (Bartonian)... 75
4.8.4 Globigerina Marl (upper Bartonian–lower Priabonian)... 78
4.8.5 Annot Sandstone (Priabonian)... 79
4.9 CARNIEULES... 79
5 STRUCTURAL SETTING... 82
5.1 INTRODUCTION... 82
5.2 MAIN TECTONIC CONTACTS... 82
5.2.1 Stura Fault Zone (SFZ)... 82
5.2.2 Argentera boundary fault system (ABF)... 85
5.2.3 Serra Garb Thrust (SGT)... 85
5.2.4 Limone-Viozene Zone (LiVZ)... 86
5.3 MINOR FAULT SYSTEMS... 86
5.4 TECTONIC UNITS... 87
5.4.1 Entracque Unit... 87
5.4.2 Roaschia Unit... 88
5.4.3 Limone-Viozene Zone (LiVZ) and Refrey Unit... 88
6 HYDROTHERMAL DOLOMITIZATION ... 89
6.1 INTRODUCTION... 89
6.2 DOLOMITIZATION OF THE PROVENÇAL SUCCESSION... 90
6.2.1 Previous works... 90
6.2.2 Areal distribution... 91
6.3 HOST ROCKS... 93
6.4 FEATURES AND DISTRIBUTION OF THE DOLOMITIZED BODIES... 94
6.5 PETROGRAPHIC ANALYSIS... 98 6.5.1 Dolomite types... 98 6.5.2 Calcite types... 105 6.5.3 Fe-oxides... 106 6.5.4 Organic matter... 106 6.6 DOLOMITIZATION FACIES... 108 6.6.1 Partial dolomitization... 108 6.6.2 Complete dolomitization... 108 6.6.3 Breccias... 111 6.6.4 Cavities... 114
6.6.5 Punta Bussaia cavity... 118
6.7 CATHODOLUMINESCENCE (CL) OBSERVATIONS... 119
6.8 EPIFLUORESCENCE... 122
6.9 MICRO RAMAN SPECTROSCOPY... 126
6.10 SEM OBSERVATIONS... 128
6.11 STABLE ISOTOPE GEOCHEMISTRY... 131
6.11.1 O and C isotopes... 131
6.12 FLUID INCLUSION ANALYSIS... 131
6.13 REWORKED DOLOMITE... 136
6.14.1 Age of dolomitization... 137
6.14.2 Earlier dolomitization... 138
6.14.3 Burial conditions during dolomitization... 139
6.14.4 Hydrothermal minerals... 140 6.14.5 Rock fabrics... 147 6.14.5.1 Partial dolomitization ... 147 6.14.5.2 Complete dolomitization ... 149 6.14.5.3 Breccias ... 150 6.14.5.4 Cavities ... 151 6.14.7 Dolomitization process... 158 6.15 REGIONAL CONTEXT... 162
7 OTHER EVIDENCE OF HYDROTHERMAL FLUID FLOW... 164
7.1 INTRODUCTION... 164
7.2 VALDIERI MARBLES... 164
7.2.1 Introduction... 164
7.2.2 Areal and stratigraphic distribution... 165
7.2.3 Lausa Limestone... 167 7.2.4 Marne Nere... 170 7.2.5 Puriac Limestone... 171 7.2.6 Garbella Limestone... 172 7.2.7 Veins... 172 7.2.8 Discussion... 172 7.3 AUTHIGENIC ALBITE... 174 7.3.1 Introduction... 174 7.3.2 Albite features... 175 7.3.3 Discussion... 177
7.4 FE-CARBONATE VEINS OF THE SABBIONE VALLEY... 179
8 SYNSEDIMENTARY TECTONICS ... 183
8.1 INTRODUCTION... 183
8.2 STRATIGRAPHIC EVIDENCE... 184
8.2.1 Introduction... 184
8.2.2 Paleozoic succession... 185
8.2.3 Upper Triassic-Lower Jurassic succession... 185
8.2.4 Middle?-Upper Jurassic succession... 186
8.2.5 Cretaceous succession... 186
8.2.6 Alpine Foreland Basin succession... 188
8.3 SEDIMENTOLOGICAL EVIDENCE... 188 8.3.1 Introduction... 188 8.3.2 Entracque Marl... 189 8.3.3 Lausa Limestone... 190 8.3.4 Puriac Limestone... 191 8.4 DIAGENETIC EVIDENCE... 193 8.4.1 Introduction... 193 8.4.2 Hydrothermal dolomitization... 194 8.5 DISCUSSION... 195 8.5.1 Paleozoic-Triassic tectonics... 195
8.5.3 Late Jurassic-Early Cretaceous tectonics... 201
8.5.4 Late Cretaceous tectonics... 203
8.5.5 Latest Cretaceous-Paleogene tectonics... 204
8.6 SUMMARY... 204
9 - CONCLUSIONS... 207
9.1 STRATIGRAPHY AND SEDIMENTOLOGY... 207
9.2 HYDROTHERMAL DOLOMITIZATION... 209
9.3 SYNSEDIMENTARY TECTONICS... 210
9.4 PROBLEMS AND PERSPECTIVES... 211
REFERENCES... 213
ABSTRACT
The Mesozoic stratigraphic successions cropping out at the southeastern margin of the Argentera Massif (Maritime Alps, NW Italy) have been deposited on the European paleomargin of the Alpine Tethys, in a sector corresponding to the transition between the Provençal platform and the Dauphinois basin. Different kinds of stratigraphic, sedimentologic, and diagenetic evidence indicate that synsedimentary tectonics played a key-role in the evolution of this part of the European margin throughout the Mesozoic.
The stratigraphic successions of both Dauphinois and Provençal domains are characterized by rapid lateral variations, interpreted as the result of synsedimentary fault activity. Moreover, the essentially pelagic-hemipelagic Dauphinois succession is characterized at different stratigraphic levels by clastic inputs, both intrabasinal and extrabasinal, culminating in the abundant and varied clastic component of the Upper Cretaceous Puriac Limestone. These clastic inputs provide a twofold evidence of synsedimentary tectonics, as they both document tectonically-triggered gravitational processes, and suggest a tectonically-driven evolution of the source areas
Middle Triassic and Jurassic carbonates of the Provençal succession are affected by an intense hydrothermal dolomitization. The study of this dolomitization joined classic field survey and petrographic analysis to a series of analytical techniques, including optical methods (cathodoluminescence, epifluorescence, micro Raman spectroscopy), fluid inclusion microthermometry, SEM-EDS and stable isotope geochemistry. Dolomitization was related to the expulsion of overpressured fluids through faults and fractures during episodes of fault activity, and was associated with repeated hydrofracturing processes documented by common fracture frameworks and tabular breccia bodies. Indirect stratigraphic evidence documents that the dolomitization occurred in the early Early Cretaceous (Valanginian?-Hauterivian?), at a very shallow burial depth. The dolomitizing fluids were hot
(about 200°C) brines, which were involved in deep hydrothermal circuits probably affecting also the crystalline rocks of the Argentera Massif.
Three main phases of Mesozoic synsedimentary tectonics have been reconstructed. The first (Late Triassic-Middle Jurassic) resulted in the differentiation between Dauphinois and Provençal domains and has a long-lasting influence on the whole Mesozoic evolution of this area. It was followed by a Late Jurassic?-Early Cretaceous phase, that caused the hydrothermal dolomitization of the Provençal carbonate succession, and a Late Cretaceous tectonic phase, during which different episodes of tectonic activity promoted the deposition of various clastic inputs in the Puriac Limestone succession.
RIASSUNTO
Le successioni stratigrafiche mesozoiche che affiorano al margine sud-orientale del Massiccio dell’Argentera (Alpi Marittime, Piemonte) sono state depositate sul paleomargine europeo della Tetide Alpina, in un settore corrispondente alla transizione tra la Piattaforma Provenzale e il Bacino Delfinese. Diverse evidenze di tipo stratigrafico, sedimentologico a diagenetico indicano che la tettonica sinsedimentaria ha svolto un ruolo fondamentale nell’evoluzione mesozoica di questa parte del margine continentale europeo.
Le successioni stratigrafiche dei domini Delfinese e Provenzale sono caratterizzate da brusche variazioni laterali di facies e di spessore, interpretate come il risultato di attività tettonica sinsedimentaria. Inoltre la successione delfinese, essenzialmente pelagico-emipelagica, è caratterizzata a diversi livelli stratigrafici da apporti clastici, sia intra- che extra-bacinali, particolarmente abbondanti nei Calcari del Puriac (Cretacico superiore). Questi apporti detritici forniscono una doppia evidenza di tettonica sinsedimentaria, in quanto da un lato sono il risultato di processi gravitazionali a probabile innesco tettonico, e dall’altro testimoniano un’evoluzione delle aree sorgenti anch’essa legata verosimilmente ad attività tettonica.
La successione carbonatica provenzale di età Triassico medio-Giurassico è localmente interessata da un’intensa dolomitizzazione idrotermale. Lo studio di questo fenomeno è stato condotto attraverso rilevamenti di terreno ed analisi petrografica, affiancati da una serie di tecniche analitiche, comprendenti metodi ottici (catodoluminescenza, epifluorescenza, microspettroscopia Raman), microtermometria delle inclusioni fluide, SEM-EDS e geochimica degli isotopi stabili.
La dolomitizzazione era legata all’espulsione di fluidi in sovrapressione attraverso faglie e fratture durante episodi di attività tettonica, ed era associata a ripetuti processi di idrofratturazione, documentati dalla presenza di reticoli di fratture e di corpi tabulari di brecce. Vincoli stratigrafici indiretti indicano che la
dolomitizzazione è avvenuta all’inizio del Cretacico inferiore (Valanginiano?-Hauteriviano?), in condizioni di seppellimento molto superficiale. I fluidi responsabili della dolomitizzazione erano salamoie ad alta temperatura (intorno ai 200°C), ed erano legati ad una circolazione idrotermale profonda che con tutta probabilità interessava le rocce cristalline del Massiccio dell’Argentera.
In base ai dati ottenuti, sono state ricostruite tre fasi principali di tettonica sinsedimentaria mesozoica. La prima (Triassico superiore-Giurassico medio) ha portato alla differenziazione tra i domini Delfinese e Provenzale, ed ha quindi avuto un’influenza durevole sull’evoluzione mesozoica dell’area di studio. A questa sono seguite una seconda fase (Giurassico superiore?-Cretacico inferiore), legata alla dolomitizzazione idrotermale della successione carbonatica provenzale, e una terza fase (Cretacico superiore), durante la quale si sono depositati i diversi apporti clastici nei Calcari del Puriac.
1 INTRODUCTION
The Western Alps represent one of the best-known geological regions in the world. In the last decades, a large number of detailed studies on the structural and metamorphic evolution have been carried out nearly on the whole orogenic chain (e.g., Coward and Dietrich, 1989; Rosanbaum and Lister, 2005; Beltrando et al., 2010): in some sectors, however, stratigraphy and paleogeography of the sedimentary successions are still poorly known, as studies on this argument did not keep pace with researches on metamorphism and tectonics. The north-eastern side of the Argentera Massif (Maritime Alps) is one of these sectors. A-part from pioneering studies dating back as far as the XVIII century (see the historical review in Carraro et al., 1970), modern geological research on the Argentera Massif and the adjoining sedimentary successions started from the 1950s, mainly due to French authors (e.g., Fallot and Muret, 1954; Faure-Muret, 1955; Lanteaume, 1958). After this period, the study of the Argentera basement and the tectonic evolution of the area has regularly continued until today (e.g., Perello et al., 2001; Musumeci and Colombo, 2002; Corsini et al., 2004; Compagnoni et al., 2010; Rubatto et al., 2010; Lombardo et al., 2011; Sanchez et al., 2011). On the contrary, the study of the sedimentary successions adjacent to the Argentera Massif, especially those on the Italian side of the massif, has been rather neglected in the last decades, the work of Bersezio et al. (2002) on the Upper Cretaceous Puriac Limestone representing the unique exception. The present-day geological knowledge about these successions substantially derives from studies earlier than 1970, and is summarized in the Geological Map of the Argentera Massif at 1:50,000 (Malaroda, 1970, explanatory notes by Carraro et al., 1970) and in the Geological Map of Italy at 1:100,000, Foglio Demonte (Abiad et al., 1970, explanatory notes by Crema et al., 1971). The study area is located in the Maritime Alps, representing the southern termination of the Alpine chain, at the boundary between NW Italy and SE France. It extends to about 80 Km2 along the mountain ridge separating the Gesso and
the Vermenagna valleys, and is comprises in the municipalities of Entracque, Valdieri, Roaschia, Vernante and Limone Piemonte (Cuneo province, Piemonte). The southern sector of the study area (Sabbione Valley-Grande Valley) is part of the Maritime Alps Regional Park.
This work focuses on the Mesozoic evolution of this area, with particular reference to synsedimentary tectonics. The sedimentary successions cropping out at the north-eastern side of the Argentera Massif have been referred in the literature to the Dauphinois, Provençal, Subbriançonnais and Briançonnais paleogeographic domains. During the Mesozoic, these domains represented part of the European paleomargin of the Alpine Tethys. As indicated by different lines of evidence, synsedimentary tectonics played a key-role in the evolution of this part of the Tethyan European paleomargin throughout the Mesozoic. Detailed studies about the timing and modes of synsedimentary tectonics are missing for the studied sector. Only Ivaldi et al. (1998) invoke synsedimentary tectonics to explain the important thickness changes in the Meso-Cenozoic sedimentary successions at the southeastern termination of the Argentera Massif. On the other hand, in the adjacent External Ligurian Briançonnais units, multiple evidence of Mesozoic synsedimentary tectonics has been recently documented (Bertok et al., 2011; 2012).
Reconstruction of synsedimentary tectonics in stratigraphic successions involved in orogenic chains commonly depends on the recognition of indirect evidence. In the study area, the clues of synsedimentary tectonics are based on three main lines of evidence: stratigraphy, sedimentology and diagenesis. Stratigraphic evidence mainly concerns lateral variations in sedimentary successions, interpreted as the result of an active contribution of fault activity in modifying the morphology of the basin. Sedimentological evidence is represented by the various clastic inputs recorded in the basinal Dauphinois succession cropping out in the Gesso and Stura valleys. Diagenetic evidence is mainly represented by the intense hydrothermal dolomitization that affects the Provençal Jurassic limestones in the southern part of the study area. This dolomitization has an Early Cretaceous age,
and is related with the circulation of overpressured hot fluids through faults and fracture frameworks.
Moreover, the stratigraphic successions of the study area are affected by other interesting modifications, resulting in the development of very peculiar rock types. They are the so called "Valdieri marbles", derived from a localized intense recrystallization of the Mesozoic Dauphinois succession (Carraro et al., 1970); the abundant authigenic albite in the Cretaceous Marne Nere unit, near Entracque (Carraro et al., 1970); the decimeter-thick veins of Fe-carbonates in the Jurassic Provençal limestones of the Sabbione Valley, developed at the contact with the Argentera basement.
The present Thesis is articulated in four main parts.
- The first part (chapters 2-3) introduces the geological setting of the study area and the regional geology of the Provençal and Dauphinois domains and provides an outline of the utilized analytical methods.
- The second part (chapters 4-5) describes the stratigraphic successions cropping out in the study area, with particular reference to the clastic inputs characterizing the Mesozoic Dauphinois succession. An outline of the structural setting is also given, describing the geometric features of the tectonic units and the main tectonic boundaries.
- The third part (chapters 6-7) is mainly focused on the hydrothermal dolomitization of the Jurassic Provençal succession. An exhaustive description of field and petrographic features of dolomitized rocks is followed by the presentation of a wide spectrum of analytical data (fluid inclusion microthermometry, cathodoluminescence, micro Raman spectroscopy, isotope geochemistry, SEM-EDS). Genetic processes of the studied hydrothermal dolomitization are then discussed and compared with other models described in the literature.
Other phenomena probably related to hydrothermal fluid flows (Valdieri marbles, authigenic albite, Fe-carbonate veins of the Sabbione Valley) are only shortly
described here, as no detailed studies have been carried out yet, and most aspects of their genesis still have to be clarified, including their age.
- The fourth part (chapter 8) is dedicated to Mesozoic synsedimentary tectonics. Stratigraphic, sedimentological and diagenetic pieces of evidence are discussed. The main phases of tectonic activity are then reconstructed and inserted in the frame of the regional geodynamic context.
A geological map of the study area at the scale 1:20,000, realized on the basis of a 1:10,000 geological survey, is attached to this Thesis.
2 GEOLOGICAL SETTING
2.1 Introduction
The Western Alps are part of the Alpine chain, an orogenic belt originated from the collision between the European plate and Adria (Coward and Dietrich, 1989). The actual nature of Adria is still debated, as it has been considered by different authors as an independent micro-plate or as a promontory of the African plate (e.g., Handy et al., 2010). These plates were separated during the Mesozoic by the Alpine Tethys, a segment of the wide Tethys Ocean originated during early Mesozoic times from the breakup of the previous Pangaea supercontinent and the subsequent divergence of the two resulting margins. During the Cretaceous, the spreading phase of the Alpine Tethys ended, and the oceanic basin began to close. The oceanic closure was due to the relative translation of Africa toward the NE and the concomitant movement of the Iberian plate and the southern part of the European plate toward the E, caused by the opening of the North Atlantic Ocean (Coward and Dietrich, 1989). This led to the subduction of the Tethyan oceanic crust, followed by continental collision between Adriatic and European plates. The present-day chain is therefore a puzzle of tectonic units deriving from both Adriatic and European continental crust and from oceanic crust of the Alpine Tethys.
The southern part of the Western Alps (Maritime and Ligurian Alps) is composed of five main domains, i.e. groups of tectonic units sharing similar tectono-stratigraphic features. From the most external to the most internal, they are the Dauphinois (whose southern portion is known as Provençal), the Subbriançonnais, the Briançonnais, the Pre-Piedmont and the Piedmont-Ligurian domains (Fig. 2.1). In the past literature, these domains were considered to correspond to main Mesozoic paleogeographic domains. In particular, Dauphinois, Subbriançonnais and Briançonnais domains would have been part of the European continental margin, while Pre-Piedmont Domain would represent the transition between the
European continental margin and the adjacent oceanic basin, represented by the Piedmont-Ligurian Domain (e.g., Vanossi et al., 1984; Lemoine et al., 1986; Seno et al., 2005).
Dauphinois and Provençal domains represent the more internal part of the margin (European proximal margin, Mohn et al. 2010) developed on continental crust. The European continental crust crops out in the External Crystalline Massifs, represented in the study area by the Argentera Massif. Dauphinois and Provençal domains differentiated in the Early-Middle Jurassic, in response to the extensional tectonics related to the Alpine Tethys opening. The Dauphinois Domain evolved as a subsident basin, while Provençal Domain remained an area of shallow water sedimentation until the Cretaceous. The study area corresponds to the easternmost sector in which the transition between Dauphinois and Provençal domains is presently observable. The Subbriançonnais domain has been interpreted instead as the transition zone between the Dauphinois domain and the Briançonnais one (representing the more external part of the European paleomargin), characterized by strong lateral variations of the Mesozoic successions (Debelmas and Kerckhove 1980; Mohn et al. 2010). The Mesozoic successions of the different paleogeographic domains are truncated at the top by a regional discontinuity surface, corresponding to an important hiatus (uppermost Cretaceous-Lower Eocene). The Alpine Foreland Basin developed above this surface, and was filled up by a Middle Eocene-Lower Oligocene succession (Sinclair, 1997; Sinclair et al., 1998; Ford et al. 1999).
The study area consists of a set of tectonic units, referred to the Dauphinois, Provençal and Subbriançonnais domains (Carraro et al., 1970): they are the Entracque Unit, the Roaschia Unit, the Limone-Viozene Zone (LiVZ) and the Refrey Unit (see chapter 5). These tectonic units form a SE-NW trending narrow belt, comprised between the Argentera Massif to the SW and the more internal Briançonnais and Helminthoides Flysch units to the NE.
Fig 2.1 – Geological scheme showing the main units of the South-Western Alps and the position of the study area (red rectangle).
2.2 Argentera Massif
The Argentera Massif is one of the External Crystalline Massifs, i.e. the sectors of the Dauphinois-Helvetic Domain in which the European continental crust presently crops out. The Argentera Massif is largely composed of Variscan migmatites with abundant relicts of pre-anatectic rocks (Compagnoni et al., 2010; Lombardo et al., 2011). It consists of two major tectono-metamorphic units: the Gesso-Stura-Vésubie Terrane (GSV, the Malinvern-Argentera Complex of Carraro et al., 1970) to the NE, and the Tinée Terrane to the SW. These units have distinct lithological associations and underwent different metamorphic evolutions, but both contain relicts of high-pressure and/or high-temperature mineral assemblages (granulite/eclogite facies) (Compagnoni et al., 2010; Lombardo et al., 2011). The
two units are separated by the Ferriere–Mollières shear zone, a 100- to 1000-meters wide zone composed of mylonitic rocks, interpreted as both metasedimentary rocks and mylonites derived from high-grade metamorphic rocks of the Tinée and GSV terranes (Bogdanoff, 1986). The Argentera Massif is bounded on its northeastern side by an important fault system (Argentera boundary fault system, ABF), with main NW-SE striking fault segments connected by minor NE-SW and E-W segments.
2.3 Dauphinois Domain
The Dauphinois Domain is the most external of the Mesozoic Alpine Domains, representing the proximal part of the European paleomargin of the Alpine Tethys (Mohn et al., 2010). The stratigraphic succession commonly starts with Lower Triassic continental to littoral terrigenous sediments, followed by Middle Triassic carbonates and Upper Triassic evaporites. Upper Carboniferous and Permian continental deposits are only locally present (Debrand-Passard et al., 1984). Starting from the Early-Middle Jurassic, the Dauphinois Domain was affected by important extensional tectonics, as a consequence of the Alpine Tethys opening (e.g., Lemoine et al., 1986; De Graciansky and Lemoine, 1988). The extension resulted in the creation of a basin, formed by a series of tilted blocks that deepened eastward toward the open Tethyan Ocean (Lemoine, 1984a, b; 1985). This basin is known as Dauphinois Basin (or French Subalpine Basin), also mentioned as Vocontian Basin or Vocontian Trough for the Late Jurassic-Late Cretaceous interval (Debrand-Passard et al., 1984). In Jurassic and Cretaceous times the Dauphinois basin represented a gulf located along the northwestern margin of the Alpine Tethys. It was an area of relatively deep-water, hemipelagic sedimentation, surrounded by platform sectors to the south (Provence), to the west (Ardèche) and to the north (Vercors-Jura). The Jurassic-Cretaceous succession of the Dauphinois basin is characterized by marl-limestone alternations with rich ammonite faunas. In the Late Jurassic-Albian interval, hemipelagic
sedimentation was punctuated by inputs of resedimented material coming both from the surrounding shallow water platforms and from the basin itself (Debrand-Passard et al., 1984; Friés and Parize, 2003; Courjault et al., 2011). The Dauphinois basin was a highly subsident basin. In depocentral areas, the Jurassic-Cretaceous succession reaches several thousand meters of thickness (Debrand-Passard et al., 1984).
The stratigraphic succession cropping out on the Italian side of the Argentera Massif deposited in a marginal sector of the basin and is consequently reduced (Bersezio et al., 2002). The reference succession for this area is that of the Puriac Valley (Sturani, 1962). It starts with a thin interval of Lower Triassic coastal deposits, resting on the Argentera Massif basement. The rest of the stratigraphic succession is detached from the substrate in correspondence of a level of tectonic breccias (carnieules), mostly developed at the expenses of a Lower-Middle Triassic evaporitic interval (Carraro et al., 1970). The first term is represented by Middle Triassic peritidal carbonates, followed by Upper Triassic pelites and Raethian-Hettangian bioclastic limestones. The succession continues with a Sinemurian condensed interval of ammonite-rich limestones with phosphatic hardgrounds, followed by Toarcian marls and Aalenian-Bathonian p.p. limestone-marl couplets (Cancellophycus Limestone). Above is a thick (300-400 m) succession of black shales of Bathonian p.p.-Callovian age (Terre Nere). The top of the Jurassic succession is represented by a calcareous interval of Kimmeridgian-Tithonian age, with resedimented beds at the top (Tithonian Bar). Lower Cretaceous deposits are represented by cephalopod-bearing, cherty limestones of Berriasian-Barremian age and Aptian-Cenomanian hemipelagic sediments (Marne Nere). Above an erosional unconformity (early-middle Turonian; Bersezio et al., 2002) the top of the Mesozoic succession is represented by a thick upper Turonian-Campanian succession of limestones and marly limestones, characterized by important detrital inputs (Puriac Limestone; Sturani, 1962; Bersezio et al., 2002).
to that of the Puriac Valley. The absence of many characteristic intervals in the Jurassic-Lower Cretaceous succession did not allow distinction of the different units present in the Puriac Valley succession. For this reason, in the literature, only two units have been separated from a cartographic point of view. They are the Rhaetian-Oxfordian succession and the Tithonian Bar, the latter comprehending also the Lower Cretaceous deposits (Carraro et al., 1970; Malaroda 1970; see also par. 4.5). The Aptian-Cenomanian Marne Nere and the Upper Cretaceous Puriac Limestone complete the Mesozoic succession.
2.4 Provençal Domain
The Provençal Domain represented the southern prosecution of the Dauphinois Domain and corresponded, during the Mesozoic, to an area of shallow water sedimentation. The stratigraphic succession of the Provençal Domain starts with Upper Carboniferous-Permian continental deposits, followed by Lower Triassic continental to littoral terrigenous sediments, Middle Triassic peritidal carbonates and Upper Triassic evaporites (Debrand-Passard et al., 1984; Durand, 2008). Jurassic sedimentation basically occurred in a carbonate platform environment, commonly subject to emersion in the Early Jurassic. Starting from the Oxfordian, a regressive trend culminated in the Berriasian transition to peritidal environment (Lanteaume, 1968; Dardeau and Pascal, 1982; Debrand-Passard et al., 1984). Classic Provençal successions continue in the Cretaceous with shallow water or continental deposits (Debrand-Passard et al., 1984). The Maritime Alps sector, at the northeastern margin of the Provençal platform, had a different Cretaceous evolution. In this area, Valanginian tectonism resulted in drowning of the carbonate platform and transition to open marine shelfal conditions (Debelmas and Kerckhove, 1980). The Valanginian-lower Cenomanian p.p. succession is represented by condensed deposits characterized by discontinuity surfaces and abundant authigenic minerals as glaucony and phosphate (Pasquini et al., 2004; Decarlis and Lualdi, 2008; Barale et al., 2013). After early Cenomanian drowning
of the shelf, sedimentation continued with the deposition of hemipelagic sediments (Cenomanian p.p.-Campanian), the latter being the lateral equivalent of the Upper Cretaceous Puriac Limestone in the Dauphinois Domain.
The study area, extending in the northeastern part of the Maritime Alps, has a much reduced succession. It starts with some tens of meters of Permian-Lower Triassic continental to littoral deposits and Middle Triassic peritidal carbonates. This interval is bounded at the top by a Late Triassic to Middle Jurassic stratigraphic hiatus, followed by Middle?-Upper Jurassic platform carbonates, commonly dolomitized (Campanino Sturani, 1967). The Jurassic succession is truncated by a regional unconformity, overlain by the Alpine Foreland Basin succession. Only locally, between the Jurassic carbonates and the regional unconformity, are a few meters of hemipelagic Cretaceous deposits present (Fallot and Lanteaume, 1956; Carraro et al., 1970).
2.5 Subbriançonnais Domain
The Subbriançonnais Domain represents the transition between the Briançonnais and the Dauphinois domains. In the study area, a series of tectonic units of intermediate structural position between the Briançonnais units to NE and the Dauphinois and Provençal units to SW are attributed to the Subbriançonnais Domain (Carraro et al., 1970; Malaroda, 1970). These tectonic units extend from the Tenda Pass to the SE to the right side of the Stura Valley to the NW. As a whole, they have been indicated as Colle di Tenda Unit by Carraro et al. (1970) or Colle di Tenda Unit l.s. by Lanteaume (1958).
The crystalline basement of these units is not known. The oldest outcropping term of the stratigraphic succession is represented by Middle Triassic peritidal carbonates, followed by a thin stratigraphic interval of Late Triassic-Early Jurassic age, consisting of continental red and green shales and peritidal to open-platform carbonates. The succession continues with a thick shallow-water limestone succession (200-300 m) of Middle?-Late Jurassic age, frequently dolomitized
(Campanino Sturani, 1967). The Cretaceous succession is subject to important lateral variations. It is absent in SE (Tenda Pass area) as well as in NW (Valdieri) sectors, whereas it reaches thicknesses exceeding hundred meters in the central sector (upper Roaschia Valley), where it is formed by bioclastic limestones, probably of Early Cretaceous age, and by Upper Cretaceous deep water hemipelagic marly limestones and resedimented arenitic limestones (Zappi, 1960).
2.6 The Subbriançonnais-Provençal question
In classical interpretations, main structural domains of the Western Alps roughly correspond to main paleogeographic domains, and tectonic contacts between them set on pre-existing paleogeographic boundaries (Debelmas and Lemoine, 1970; Debelmas and Kerckhove, 1980; Lemoine et al., 1986; Stampfli et al., 1998).
The Subbriançonnais Domain is one of the structural domains the Western Alps are classically divided in, and extends from the Maritime Alps to the Swiss Prealps. It is composed of a set of tectonic units interposed between the more external Dauphinois units and the more internal Briançonnais units. It is characterized by strong lateral variations in the Mesozoic successions, commonly attributed to important synsedimentary tectonics (Debelmas and Kerckhove, 1980). Subbriançonnais successions, in general, do not register the Early Jurassic emersion characteristic of the Briançonnais Domain (Maury and Ricou, 1983; Mohn et al., 2010).
The Subbriançonnais would have represented, during the Mesozoic, a paleogeographic domain interposed between Dauphinois and Briançonnais domains. The reconstruction is complicated by the controversy about the existence and the extension of the Valais Ocean. This should have been a narrow oceanic basin, whose remnants are presently preserved in the Valais units in Savoy, which separated the Briançonnais-Subbriançonnais block from the Dauphinois and Helvetic domains during the Cretaceous (e.g. Stampfli et al.,
1998; Handy et al., 2010; Loprieno et al., 2010). The actual extension of the Valais Ocean have been questioned by different authors, as no evidence of an oceanic suture exists south from the Petit Saint-Bernard Pass (De Graciansky et al., 2011) and particularly in the Maritime and Ligurian Alps (Seno et al., 2005; Bonini et al., 2010). Masson et al. (2008) query the existence itself of the Cretaceous Valais Ocean, based on new age data of the Valais ophiolites.
In the last years, however, the systematic correspondence between Alpine structural domains and Mesozoic paleogeographic domains has been largely questioned, and, among the others, the problem of the paleogeographic position of Subbriançonnais units has been raised. Maury and Ricou (1983), claimed that the Subbriançonnais represents only an Alpine structural domain, whit no meaning as a Mesozoic paleogeographic domain. According to these authors, the Subbriançonnais corresponds to a wide Eocene strike-slip shear zone, composed of a puzzle of tectonic units of different paleogeographic pertinence, referable both to the Briançonnais and to the Dauphinois domains.
The interpretation of the Subbriançonnais as a merely structural domain perfectly applies to the situation of the study area. The Colle di Tenda Unit (sensu Carraro et al., 1970) has been attributed to the Subbriançonnais Domain on the base of its structural position, i.e. intermediate between more external Dauphinois and Provençal units and more internal Briançonnais units. This attribution is due to Carraro et al. (1970) and Malaroda (1970), and has been maintained in the Geological Map of Italy at 1:100,000 (Foglio Demonte: Abiad et al., 1970; Crema et al., 1971). The attribution of the Colle di Tenda Unit to the Subbriançonnais Domain would imply its appartenence to a distinct paleogeographic domain, according to the the classical interpretation that matches Alpine structural domains to Mesozoic paleogeographic domains.
However, the Colle di Tenda Unit shows a Mesozoic succession very similar to that of the adjacent Provençal units. Both stratigraphic successions are in fact characterized by the lack or extreme reduction of Early Jurassic deposits, and by the presence of Middle?-Upper Jurassic shallow-water carbonates with very similar
facies. This second point, in particular, has been already highlighted by previous authors (Lanteaume, 1958; Campanino Sturani, 1967; Crema et al., 1971). Finally, the Cretaceous successions of both northern Provençal units near to the Argentera Massif and Colle di Tenda Unit are strongly reduced or completely lacking. The same stratigraphic features characterize also part of the Subbriançonnais units north of the Stura Valley (Unità Rocce Forni-M.Salè-M. Savi; Carraro, 1962).
The Colle di Tenda Unit, the northern Provençal units and part of the Subbriançonnais units north of the Gesso Valley have similar Mesozoic successions and thus no paleogeographic distinctions among them can be justified on a stratigraphic point of view. They can be considered part of a single paleogeographic domain (Provençal), characterized by the development of Middle-Upper Jurassic shallow water carbonates and by the reduction of Early Jurassic and Cretaceous deposits. Lanteaume (1958, 1968) and Maury and Ricou (1983) came to similar conclusions, including the Colle di Tenda Unit in the Provençal paleogeographic domain.
2.7 Alpine Foreland Basin
In all the described domains, the top of the Mesozoic succession is truncated by a regional discontinuity surface, which corresponds to a hiatus spanning the latest Cretaceous-Middle Eocene interval. It resulted from a prolonged sub-aerial exposure related to a significant uplift of the Mesozoic European margin during the first stages of the Alpine continental collision (Crampton and Allen, 1995). The following lithospheric flexure, caused by the load of the growing orogenic belt, resulted in the progressive development of the Alpine Foreland Basin in the external part of the Alps (Ford et al., 1999). The Alpine Foreland Basin succession shows rather uniform features and it is considered as deposited in a single basin, developed in discordance above the Mesozoic successions of the underlying domains.
The succession starts with a discontinuous interval of continental to coastal deposits (Trucco Formation: Varrone and Clari, 2003), followed by the classic trilogy of lithostratigraphic units including Middle Eocene Nummulitic Limestone, Upper Eocene Globigerina Marl and Upper Eocene-Lower Oligocene Annot Sandstone (Campredon, 1972). This succession reflects the evolution of the Alpine Foreland Basin in response to the increasing flexural subsidence, from a mixed carbonate-siliciclastic ramp (Nummulitic Limestone) to a slope environment (Globigerina Marl) and then to a turbidite basin (Annot Sandstone) (Sinclair, 1997). In the internal sectors, the history of the Alpine Foreland Basin ends with the local deposition of an olistostrome interval (Kerckhove, 1969; Dumont et al., 2012; Perotti et al., 2012), followed by the overthrusting of Helminthoides Flysch units in the Oligocene-Middle Miocene (Seno et al., 2005; Dumont et al., 2012). In parallel, sediment deposition migrated toward the SW in shallow-marine and continental basins (Ford et al.,1999)
2.8 Alpine tectonics
The Alpine orogenic belt originated from the convergence of European and Adriatic plates since the Cretaceous (Coward and Dietrich, 1989). After a phase of oceanic subduction (Late Cretaceous-Eocene), continental subduction and underthrusting of the European paleomargin below the Adriatic plate resulted in the formation of an orogenic wedge during the Early Oligocene (e.g., Tricart, 1984; Simon-Labric et al., 2009). The progressive wedge accretion triggered the formation of the flexural Alpine Foreland Basin on the European plate, and its propagation toward NW, from Eocene to earliest Oligocene (Dumont et al., 2012). Following this stage of nappe stacking, the deformation of the European margin was mainly accommodated by large strike-slip fault systems (Sanchez et al., 2011).
In the southern sector of the Western Alps, a large strike-slip shear zone acted since Early Oligocene as the kinematic boundary between the Ligurian-Maritime
Alps and the rest of the Western Alps, with sinistral main movement in the Oligocene followed by prevalent dextral movements from Late Miocene-Pliocene up to present (Molli et al., 2010). This shear zone, with approximate E-W direction, comprehends the strike-slip faults of the “Stura couloir” (Ricou, 1981), the boundary faults of the NE side of the Argentera Massif, and the Limone-Viozene zone (Piana et al., 2009).
Strike-slip tectonics along NW-SE shear zones also controlled the exhumation of the Argentera Massif, which occurred through various steps since the Oligocene, with a transition from transpressional to transtensional regimes around 8–5 My ago (Bigot Cormier et al., 2000; Bogdanoff et al., 2000; Sanchez et al., 2011). Exhumation involved independent crustal blocks following different uplift patterns (Bigot Cormier et al., 2000; Bogdanoff et al., 2000). Recent and present-day tectonic activity in the Maritime and Ligurian Alps is mainly related to dextral movements along some of these NW-SE-trending structures (Sanchez et al., 2010, 2011).
3 METHODS
The following is a description of the utilized analytical methods. Unless otherwise specified, they have been carried out at the Department of Earth Sciences, University of Torino.
3.1 Analytical techniques
Petrographic studies were carried out on selected samples by optical microscopy and cathodoluminescence aimed to distinguish different dolomite generations. Cathodoluminescence observations were carried out on polished thin sections using a CITL 8200 mk3 equipment (operating conditions of about 17 kV and 400 µA).
Fluid inclusion petrography has been studied on thick (80 µm), bi-polished thin sections. Microthermometry of primary fluid inclusion assemblages on dolomite and calcite was performed using a Linkam THMSG600 heating-freezing stage coupled with an Olympus polarizing microscope (100x objective).
Micro Raman spectroscopy on polished thin sections was performed with a LabRAM-HR 800 (HORIBA - JOBIN-YVON) spectrometer using a HeNe laser (633 nm, 20 mW).
Microdrilled samples were measured for their carbon and oxygen isotope composition following the standard method of McCrea (1950) in which carbonate powder is reacted in vacuum conditions with 99% orthophosphoric acid at 25 °C (time of reaction: 4 h for calcite and 6–7 h for dolomite). Measurements were carried out partly at the MARUM Stable Isotope Laboratory, Bremen, Germany (using a Finnigan MAT 252 mass spectrometer) and partly at the Stable Isotope Laboratory of the ETH Geological Institute, Zurich, Switzerland (using a Finnigan MAT 253 mass spectrometer). The isotopic ratios are expressed as δ13C and δ18O per mil values relative to the VPDB (Vienna Pee Dee Belemnite) standard (precision ±0.05‰).
SEM observations (with a Cambridge S-360 instrument) were made on gold-coated rock fragments in order to investigate the microstructure of dolomite and calcite phases. Rock fragments have been obtained both by sawing and by mechanical split. Calcite samples and part of the dolomite samples have been etched with HCl (5% solution) at room temperature for variable times (1-8 minutes). The other dolomite samples have been etched with HCl (5% solution) at 50°C for 20-60 seconds.
In situ chemical analyses on carbon-coated thin sections were carried out to determine chemical compositions by means of an EDS microprobe Link System connected to a SEM (Cambridge S-360).
Epifluorescence observations of polished thin sections were also made to highlight the presence of organic matter in dolomite and calcite. Observations were performed using a Nikon Eclipse E400 epifluorescence microscope, equipped with the UV-2A, B-2A, and G-2A filter combinations (excitation wavelength of 330-380 nm, 450-490 nm, and 510-560 nm respectively, corresponding to UV, blue and green light), at the Department of Plant Biology, University of Torino.
3.2 Extraction of Charophytae gyrogonites
The extraction of Charophytae gyrogonites from micrite samples of Garbella Limestone resulted necessary for their specific determination. A certain number of partly-extracted gyrogonites has been obtained, which are still half embedded in the rock, but with the other half well exposed. All attempts to extract them more than so, both mechanically and with acid attack, inevitably ended with their breakage.
The partial extraction has been obtained with the acetic acid technique described by Kariminia (2004), slightly modified as follows: 1) sample crushing in fragments of 3-5 millimeters in size; 2) acid attack at room temperature with a solution of 72% acetic acid and 28% distilled water, for 15-20 hours; 3) rinsing with abundant distilled water and drying in oven; 4) observation under the binocular microscope in order to individuate semi-extracted fructifications on the surface of
the attacked micrite fragments; 5) mechanical isolation of the 1-2 millimeter-large rock portion containing the gyrogonite from the rest of the micrite fragment.
4 STRATIGRAPHIC SUCCESSION
4.1 Introduction
The stratigraphic successions cropping out in the study area spans the Permian-Paleogene interval and record different stages of the geodynamic evolution of this sector. The lower stratigraphic interval (Permian to Early Jurassic) is the result of a continental to platform sedimentation, and shows similar characters throughout the study area, even if important lateral thickness variations are observed. Starting from the Early-Middle Jurassic, the study area began to differentiate into two distinct sedimentation domains. A shallow domain (Provençal platform) rested in the southern sector, while in the northern sector the increasing subsidence created a deeper environment of sedimentation (Dauphinois basin). During the Cretaceous, the partial drowning of the Provençal platform led to a relative facies homogenization between the two sectors, although important thickness differences persisted, related to the paleotopography of the basin. After a generalized emersion, resulting in a stratigraphic gap spanning the uppermost Cretaceous-Middle Eocene interval, the whole sector returned under similar conditions with the development of the Alpine Foreland Basin (Fig. 4.1).
A description of the essential characters of the different stratigraphic successions present in the study area is given below. For the reason above mentioned, the Jurassic-Lower Cretaceous successions of the Dauphinois basin and of the Provençal platform will be described separately. If already existing in the literature, names of lithostratigraphic units have been maintained (e.g., Puriac Limestone). If not, Jurassic-Cretaceous lithostratigraphic units have been named after study area localities. A particular attention has been paid to the detrital inputs recorded in the stratigraphic succession, whose relation with synsedimentary tectonics will be analyzed in a dedicated chapter (8). The studied successions show in general a rather high degree of recrystallization and preserve scarce fossil remains; therefore, the chronostratigraphic attribution of most units
is difficult. In most cases, the age of lithostratigraphic units can be only inferred indirectly, based on correlations with adjoining stratigraphic successions.
4.2 Argentera basement
The Argentera crystalline basement directly adjacent to the study stratigraphic successions is mainly composed of migmatitic granitoid gneiss (anatexites of Carraro et al., 1970). Locally (lower Sabbione Valley, Monte Frisson) masses of granitoids and migmatitic amphibolites are present (Lombardo et al., 2011). However, a more detailed description of the crystalline basement is out of the scope of this work (for an exhaustive review, see Compagnoni et al., 2010). In the study area, the stratigraphic successions and the crystalline basement are separated by tectonic contacts, and in no case a stratigraphic contact is preserved.
4.3 Permian-Lower Jurassic succession
4.3.1 Permian deposits (Middle?-Upper? Permian)
Permian deposits are present only in the southern sector of the study area (Monte Frisson and upper Sabbione Valley), being absent in the northern sector (Entracque and Roaschia units). They consist of a several hundred meters-thick succession made up of arenites and conglomerates (Fig. 4.2a), with minor pelite intercalations. These sediments are commonly grey, greenish or pinkish, but locally they show a distinctive red-purple color. Conglomerates are generally matrix-supported, with arenitic or pelitic matrix. Clasts are rounded, ranging from a few millimeters up to a few decimeters in size. They are composed of volcanic (mainly rhyolites), metamorphic (migmatites, gneiss, quartzites) and intrusive rocks (granites), and white or pink polycrystalline quartz. Arenite main components are quartz and feldspar grains associated with lithic clasts, made up of the same lithotypes than the above described conglomerate clasts.
←
Fig 4.1 – Stratigraphic scheme of the Provençal and Dauphinois Mesozoic successions and of the Alpine Foreland Basin succession in the study area.
These grains are associated in variable proportions giving rise to arenites of different composition, from arkoses to lithoarenites and locally quartzarenites. At the Colle dell’Arpione, a tectonic slice of rhyolithes, attributed to the Permian by Malaroda (1970), is present.
Permian deposits resulted from sedimentation in a continental environment (alluvial fan–alluvial plain: Faure-Muret, 1955; Barrier et al., 2009). The absence of Permian deposits in the northern part of the study area and in general on the northeastern side of the Argentera Massif is probably related to an highly irregular paleotopography, as already suggested by previous authors (Faure-Muret, 1955; Vernet, 1963). The precise age of this succession is difficult to asses. Permian deposits cropping out on the southern side of the Argentera massif have been attributed to the whole Permian period (Malaroda, 1994, 1999). The complex stratigraphy, characterized by marked facies heteropies and thickness changes (Malaroda, 1994), and the almost complete lack of fossils did not allow a precise cronostratigraphic attribution of this succession. The deposits cropping out in the study area correspond to the Monte Bego Unit of Malaroda (1994, 1999), attributed to the Middle-Upper Permian and roughly corresponding to the “Série du Bego” of Faure-Muret (1955).
4.3.2 Lower Triassic quartzarenites
In the southern sector of the study area (Sabbione Valley, Monte Chiamossero) this stratigraphic interval consists of some meter decimeter-thick beds of quartzarenites and pebbly quartzarenites (Fig. 4.2b), locally showing a cross bedding. Arenite grains are rounded and are composed of quartz, rhyolites, crystalline rocks and minor feldspars. Conglomerate pebbles are mainly
represented by polycrystalline quartz (white but also pink and violet; Fig. 4.2c). Locally (Monte Chiamossero), at the top of the quartzarenites, a few decimeters of dark red pelites are preserved. In the northern sector (Alpetto Valley) this stratigraphic interval is represented by an alternation of greenish arenites and siltites, in centimetric levels. Arenites are locally cemented by a brown, Fe-bearing carbonate (probably ankerite). At the confluence between Alpetto and Lausetto valleys, these deposits host part of the barite-quartz-sulphides mineralization, exploited in the past centuries, which largely affects the adjoining crystalline rocks, (Lausetto Mine: Martina, 1967; Mari and Mari, 1982).
Lower Triassic quartzarenites were deposited in a high energy environment, as indicated by the textural and compositional maturity and by the observed sedimentary structures (cross bedding). They represent coastal deposits, likely deposited as barrier islands. The alternations of fine arenites and siltites present in the northern sector originated in a quieter environment, probably represented by coastal lagoons.
Clasts derived from the crystalline basement or from the Permian deposits, even though the origin of the abundant pink and violet quartz clasts is unknown.
4.3.3 Middle Triassic carbonates
They are represented by finely-crystalline dolostones, dolomitic limestones and limestones, with well-defined, decimeter-thick bedding. Microbial/algal lamination is commonly found. Collapse breccias and flat pebble breccias are frequent. The fossil content is represented by rare brachiopods (Fig. 4.2d) and tiny gastropods. In the study area, Middle Triassic carbonates crop out as tectonic slices of limited extent and thickness (Monte Chiamossero, Sabbione Valley, Gias d’Alpetto, Desertetto Valley), and thus a detailed description of their characters is difficult. They extensively crop out in the high Roya Valley (Mont Agnolet), where they are affected by important phenomena of hydrothermal dolomitization (see chapter 6).
Fig 4.2 – Permian-Lower Jurassic succession. a: Permian conglomerate. Alberghi Lake, Palanfré. b: Lower Triassic pebbly quartzarenites. La Valera, Sabbione Valley. c: pebble of polycrystalline pink quartz in Lower Triassic pebbly quartzarenites. La Valera, Sabbione Valley. d: dolomitized brachiopod shell in Middle Triassic dolostone. Monte Chiamossero. e: Upper Triassic red shales, showing a marked slaty cleavage. Costa di Raiet, Entracque. f: stellate crinoid ossicle (Pentacrinites? sp.) in Sinemurian bioclastic packstone. Costa Balmera, Entracque.
Middle Triassic carbonates are the result of peritidal sedimentation in arid, sabkha-like conditions.
4.3.4 Upper Triassic-Lower Jurassic deposits
In the study area, the Upper Triassic-Lower Jurassic deposits consist of a thin stratigraphic interval, commonly involved in tectonic slicing that makes it difficult to reconstruct in detail the stratigraphic succession. This interval has very similar characters in the Entracque and Roaschia units, whilst in the southern sector (Sabbione and Grande valleys) this part of the succession is almost completely lacking. In the Roaschia Unit, this stratigraphic interval crops out in subvertical tectonic slices at the core of strike-slip fault zones.Tectonic slices of minor extent are also present along the basal thrust of the Roaschia Unit (Serra Garb thrust). In the Entracque Unit, these deposits are present as tectonic slices intermixed within slices of Middle-Upper Jurassic succession in the Gias d’Alpetto sector, west to Entracque (in this sector, the whole set of tectonic slices has been mapped as a unique unit, named “Rhaetian-Oxfordian”, by Malaroda, 1970).
Gathering all the observations done in various sectors, the following stratigraphic succession can be reconstructed. It is substantially similar to those proposed by previous authors (Malaroda, 1957; Carraro et al., 1970), whose age attribution has been followed.
- Red and green shales (Fig. 4.2e), showing a marked slaty cleavage, not preserving any fossil nor internal structure (Upper Triassic).
- Fine-grained limestones and dolomitic limestones, locally with algal lamination, containing flat pebble breccias and bivalve coquina levels (Rhaetian-Hettangian). Dark shale levels are also present, with small bivalves attributed to Cardita munita by Malaroda (1957).
- Bioclastic packstones and wackestones with abundant stellate crinoid ossicles (Pentacrinites? sp.; Fig. 4.2f), bivalves, brachiopods, echinoid spines, belemnites and ammonite fragments (Sinemurian).
The above described stratigraphic intervals indicate a progressive shift from a continental sedimentation (red and green pelites), to inner platform-lagoonal (mudstones-dolomitic limestones) and then to frankly marine, outer platform environments (bioclastic packstones).
4.4 The Dauphinois-Provençal differentiation and the Caire Porcera paleoescarpment
On the eastern side of the Argentera massif, the transition between Dauphinois and Provençal successions is at the present day located near Caire Porcera, a few kilometers south of Entracque. This transition, considered as a tectonic contact by previous authors (Carraro et al., 1970) is here interpreted as an essentially preserved primary feature, even if the general setting of this sector is complicated by Alpine tectonics (Provençal Jurassic succession locally overthrusting the Cretaceous Dauphinois succession). According to this interpretation, the Provençal-Dauphinois transition preserved at Caire Porcera originated as an Early?-Middle Jurassic paleoescarpment, progressively covered by slope deposits connecting the Provençal platform to the Dauphinois basin (Fig. 4.1). The origin and the evolution of the Caire Porcera paleoescarpement are discussed in par. 8.5.2.
4.5 Jurassic-Lower Cretaceous Dauphinois succession
4.5.1 Introduction
The Dauphinois succession of the study area is quite different from the reference succession cropping out at the NW side of the Argentera Massif (Puriac Valley; Sturani, 1962; Carraro et al., 1970) due to the absence of many characteristic intervals, especially in the Jurassic succession. In the geological literature, the Jurassic-Lower Cretaceous succession of the study area was divided in two lithostratigraphic units (Carraro et al., 1970, Malaroda, 1970). The lower unit
comprehends a thick succession (300-400 m) dominated by marls, with breccia beds at the top. This unit has been basically correlated to the Middle-Upper Jurassic Terre Nere. However, the scarcity of outcrops and the tectonic complications, i.e. the interposition of numerous tectonic slices of Upper Triassic-Lower Jurassic sediments, led those authors to create a comprehensive cartographic unit, named “Rhaetian-Oxfordian”, for all these deposits. The second unit is formed by limestones, with breccia beds at the base, and has been reattached to the Upper Jurassic “Tithonian Bar” (Carraro et al., 1970; Malaroda, 1970). This unit comprehends also the Lower Cretaceous deposits that could not be separated in this area from the Jurassic ones, due to the absence of the key level represented by the resedimented “Tithonian bed” (Carraro et al., 1970). The distinction between a lower marly unit (here named Entracque Marl) and an upper calcareous unit (Lausa Limestone) has been maintained (Fig. 4.3a), though a new dating is proposed for this part of the succession. It is based on the analysis of clasts in the breccias, which revealed to be the unique way to obtain some indirect age information in absence of fossils. The discriminating factor is the presence of clasts of coarsely-crystalline dolostones, which have been interpreted as clasts of hydrothermal dolostones deriving from the dolomitized Provençal succession. Breccias, containing hydrothermal dolostone clasts, are certainly younger than the dolomitization event, which is quite well constrained within the Early Cretaceous (probably in the Valanginian-Hauterivian, see chapter 6). The Entracque Marl contains breccia beds in the upper interval, which contain limestone clasts and clasts of finely-crystalline Triassic dolostones, but do not contain coarsely-crystalline dolostones. Conversely, the Lausa Limestone starts with breccia beds containing clasts of coarsely-crystalline dolostones, interpreted as hydrothermal dolostones deriving from the Provençal succession.
The lowermost interval of the Lausa Limestone is thus younger than the dolomitization event, i.e. at least post-Valanginian. The upper boundary of this unit, that separates it from the overlying Marne Nere, is better constrained in terms of age. It corresponds in fact to a marked change in the sediment
