«CHROMATIC MODIFICATION AFFECTING CRETACEOUS SANDSTONES IN NORTHERN ITALY AND SOUTHERN SWITZERLAND G. Cavallo,1* G. Corredig1 and G. Vola2 Institute ...»
12th International Congress on the Deterioration and Conservation of Stone
Columbia University, New York, 2012
CHROMATIC MODIFICATION AFFECTING CRETACEOUS SANDSTONES
IN NORTHERN ITALY AND SOUTHERN SWITZERLAND
G. Cavallo,1* G. Corredig1 and G. Vola2
Institute of Materials and Constructions DACD-SUPSI, Switzerland
*corresponding author email@example.com
The Cretaceous sandstones investigated in this study belong to the Lombard Flysch Group (Pontida Formation, Sarnico Formation, Piano di Sirone, Bergamo Flysch) and are mainly exposed in Brianza and around Bergamo (Lombardy, Northern Italy), subordinately in Southern Switzerland around Mendrisio. These rocks have been used in the past and still in the present time as building materials.
Both ashlars in historical buildings and samples collected from quarries or outcrops FT exhibit a characteristic chromatic modification of the bulk colour of the rock ranging from red-orange to brown which is not limited to the surface.
Polarizing Light Microscopy (PLM) integrated with Scanning Electron Microscopy coupled with microanalysis and elemental X-Ray mapping (SEM/EDS) allowed to infer that the Fe oxides and/or oxi-hydroxides are related to alteration A processes affecting Fe-rich micas such as biotite forming part of the detrital fraction; a few samples exhibit alteration of K-rich micas such as glauconite. Furthermore, Febased compounds are associated with Mg,Ca-carbonates of the detrital fraction. Pyrite is also present as individual crystals sometimes associated with organic matter. The FeR oxides are also concentrated around the intergranular sparry calcite cement.
In situ observations both in quarries and monuments combined with analytical data indicate that the chromatic modification is mainly related to the diagenetic environment D and can be considered intrinsic to the rocks mainly due to biotite oxidation. The reddishbrownish colour is not an alteration process but the result of geochemical modifications of the original detritus occurring during the eogenetic phase of diagenesis.
Keywords: Lombard Flysch Group, Cretaceous sandstone, Chromatic modification, Biotite alteration, Diagenetic processes.
1. Introduction A research developed in the frame of the Interreg Project between Italy and Switzerland entitled The stone and the history. Safeguard of the landscape and architecture between Lecco (Northern Italy) and Canton Ticino (Southern Switzerland) (http://www.pietraestoria.eu), co-funded by the EC, was aimed at a detailed study of the Cretaceous sandstones extensively used as building material in this area. Observations carried out on historical building surfaces revealed a chromatic modification of the original greyish-greenish rock color into reddish-brownish, initially attributed to chromatic alteration. Visits to the quarries, especially those close to the city of Bergamo (Italy), showed the presence of grey sandstone with nodules and planar surfaces reddish 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 in color. These preliminary observations suggested the causes of color change were intrinsic to the rock.
The ICOMOS-ISCS illustrated glossary (Vergès-Belmin V., 2008) reports the term iron-rich patina for describing the chromatic modification even if it refers to thin layers, also as a result of longer exposure to open air. Discoloration (chromatic alteration) is more general and is reported as a change of the rock color. Both the patterns are reported as alteration phenomena.
The subject was not extensively treated until now; a few authors advocated biological causes as responsible for superficial alteration (Krumbein, 1992; Valls del Barrio et al., 2002). Our research was intended as an attempt to clarify the role of Febearing minerals in causing the reddish-brownish coloration, the significance of the diagenetic environment and the role of syn and post-depositional processes.
2. Geological and stratigraphic setting The discontinuous outcrops of the so-called “Lombard Flysch Group”, which includes Pontida Formation, Sarnico Formation, Sirone Conglomerate and Bergamo Flysch, is approximately E-W oriented, 85 km wide, extending along the Southalpine FT foothills between Brianza and Iseo Lake, northern Italy (Bersezio et al., 1990). The area covers almost five different Italian provinces, Varese, Como, Lecco, Bergamo and Brescia, including the southern part of Canton Ticino, southern Switzerland. The most important outcrops and quarries, most no longer active, are located in the Bergamo area, between Mapello and Sarnico, and subordinately in Brianza, between Oggiono and A Viganò (Bigioggero et al., 2000; Bugini et al., 2004; Cavallo and Corredig, 2011).
Lombard Flysch, Upper Cretaceous in age, was described in details by Venzo (1954) and reported in the geological map of Italy at the scale of 1:100.000. Subsequently, the stratigraphic information was improved by various authors (Fernandez, 1963; Aubouin R et al., 1970; Bichsel and Häring, 1981; Gelati et al., 1982; Bersezio et al., 1990).
Recently, the same formations were illustrated on various Italian geological maps at scale 1:50.000 (Bersezio et al., 1990; Jadoul et al., 2002; Bersezio et al., 2010; Bini et D al., 2010; Gaetani et al., 2010). The Pontida Formation (Middle Turonian - Turonian) is composed of hybrid lithic sandstones, mostly light brown to brown in colour, in beds of highly variable thickness organized in Bouma sequences, alternating with marls. The Sarnico Sandstone (Coniacian) presents alternating thin to medium layered grey sandstones and shales, sometimes organized in coarsening-upward cycles, with structures typical of the Bouma Sequence. The Sirone Conglomerate (Santonian) presents massive conglomerates with clasts of centimetric to decimetric size and less common conglomeratic sandstones in lenticular bodies. The Bergamo Flysch (Campanian - Middle Maastrichtian) is mostly composed of alternating sandstones and shales, thin to thick-bedded. These deposits of Late Cretaceous turbidite systems are involved in the neo-Alpine south-vergent fold and thrust belt.
2.1 Quarries location, exploitation and use The formations of the Lombard Flysch Group were widely quarried all over the Southalpine foothills for many centuries. The Sarnico Sandstone, also called “Sarnico Stone” in the Province of Bergamo and “Oggiono Stone” in the Province of Lecco, was already used as building material since the 11th-12th centuries (Figus, 2011); evidence of 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 early uses dates back even to the Lombard period (Bigioggero et al., 2000). Today only three quarries are active, two in Paratico (Brescia) and, a new one, in Gandosso, Bergamo (Regione Lombardia, 2008). Almost the same lithological unit, called “Molera Stone” in Brianza, and “Cote Stone” in the Seriana Valley, was quarried and used for the production of millstones, which were traded for many centuries all over the world (Valoti, 2010). The Sirone Conglomerate was used as millstone, as well (Bergamaschi et al., 2003). The Bergamo Flysch, under the name of “Credaro Stone”, is exploited in Credaro and Castelli Calepio (Bergamo) and used as building material, generally for rustic ashlars (Regione Lombardia, 2008). Recently two different lithofacies are available on the market, called respectively “Medolo” and “Berrettino” types (Bettoni et al., 1997). The sandstone called “Molera di Viganò” which is also part of Bergamo Flysch was also quarried and used in the Brianza area near the villages of Missaglia, Viganò and Montevecchia (Province of Lecco) as building material (Bugini et al., 2004).
In southern Switzerland the use of sandstones referable to the Pontida Flysch Formation is limited in and around Mendrisio (Zala, 2010).
3. Materials and methods FT Samples were collected from monuments and quarries; the locations are reported in the Table 1. A large number of samples comes from monuments and historical buildings located in Mendrisio (Canton Ticino, Switzerland) whilst the quarry samples come mainly from the Italian side as the outcrops in Canton Ticino are limited per se, hardly accessible or completely hidden by the structures and infrastructures built in the last A centuries.
Microscopic techniques were used, also according to the suggestions by VazquezCalvo et al. (2007).
Polarizing light microscopy (PLM) was carried out on polished thin sections in order to study the mineralogical paragenesis, the texture and the structure of the samples.
Petrographic examinations were combined with elemental analysis and X-ray mapping by means of an electron microscope coupled with microanalysis (SEM/EDS). A Jeol JSM-5910LV was used for BSE images and X-rays maps, operating at the following conditions: 20 kV, vacuum mode HV, working distance 9 mm, spot size 43.
12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012
4.1 Petrography All of the analyzed samples exhibit a similar mineralogical composition (Table 2) including, in order of decreasing abundance, quartz and fragments of quartzs-feldspatic rocks (magmatic and metamorphic rocks); variable amounts of chert fragments, lithic fragments of micritic-biomicritic-microsparitic and sparitic-microsparitic limestone (the presence of dolostone is not excluded); intraclasts, rare peloids, ooids and fossil fragments; K-feldspar, mica (mainly muscovite and chlorite, biotite is often altered to chlorite and associated with Fe-oxides) and opaque minerals (Fe-oxides and traces of Fe-sulfides recognizable by their cubic crystal habit). Traces of plagioclase, glauconite, zircon and garnet are present in some samples.
Samples referable to Pontida Flysch Formation (Bernoulli and Winkler, 1990;
Bersezio et al., 2010; Gaetani et al., 2010) labelled CSM, SMB and FCL (Table 1) exhibit a low amount of chert fragments, K-feldspar is orthoclase and the fossils content FT is represented by foraminifera, sometimes gastropoda, and fragments of bivalvia and brachiopoda. Micas exhibit both a preferred orientation sub-parallel to the original stratification and a random distribution. Layers showing black material as in the sample CSM 01 are most probably rich in organic matter.
Samples from the Sarnico Sandstone Formation (Bersezio et al., 2010; Bini et al., A 2010; Gaetani et al., 2010) labelled CE, VS, GAN, SA (Table 1) are characterized by a high presence of generally fibrous chert and chalcedony fragments, often opaque and are rich in organic matter; in addition to orthoclase, microcline was detected in subordinate amounts. The fossil content is represented by foraminifera and brachiopoda. Micas, R mainly chlorite, subordinate muscovite and altered biotite, are rare or occur only in traces. Micas are slightly more abundant in the samples from Oggiono (VS, CE). Traces of tourmaline, glauconite, zircon and garnet are also present, especially in the samples D from Gandosso (GAN) and Sarnico (SA).
Samples from the Bergamo Flysch Formation (Bersezio et al., 2010; Bini et al., 2010, Bugini et al., 2004) labelled ME and VIG (Table 1) exhibit a low content of micritic limestone; in addition to orthoclase, microcline is abundant. The fossil content is represented by fragments of brachiopoda. Micas are very abundant (mainly muscovite, biotite often altered to chlorite and associated with Fe-oxides). Micas exhibit both a preferred orientation sub-parallel to the stratification of the rock and a random distribution.
The texture of all the samples is grain supported; the frame consists of cement and granules; the cement is mainly sparitic calcite with variable amounts of micritic calcite;
a siliceous fraction cannot be excluded. Particle size distribution expressed in the Udden grade scale modified by Wentworth (1922) ranges between silt and very coarse sand, sometimes passing to granules. Fe-oxides are often arranged around the edges of the sparry calcite crystals of the cement suggesting the presence of ferroan calcite.
Crystals of biotite which exhibit their peculiar optical features are present in all of the samples in low or trace amounts but are generally altered to chlorite or, in the form of altered crystals, associated with Fe-oxides of black or dark-brownish color.
12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012
Legend QR: Qtz-Fs rock fragments of igneous, metamorphic or sedimentary origin (microcrystalline silica such as chert), CR: carbonate rock fragments (micritic limestones, sparitic limestones, sometimes dolostones, intraclasts, peloids and rare fossil fragments), OM: opaque minerals.
D +++ main component; ++ minor component; + accessory component; tr traces; - absent or not observed.
* Generally altered.
4.2 Microanalysis Microanalysis carried out on significant areas and micro-areas selected after optical microscopy observations allowed to determine the chemical composition of opaque phases otherwise undetectable, the composition of mineral defined on the basis of crystal morphology and habit when the optical properties were masked by alteration processes, the distribution of iron bearing compounds and heavy minerals.
Opaque phases can be classified as organic and inorganic compounds. The former are C-rich compounds exhibiting variable morphologies ranging from rounded to elongate (Fig. 1, 1a). The elongate morphologies often exhibit a layered micro-structure as consequence of the depositional conditions. The C-rich compounds are in some cases associated with Fe,S-bearing compounds, i.e. pyrite.
Ti and Fe-oxides, Zircon occur in traces; REEs were also detected in a few samples.
Micas often exhibit a deformed morphology but their recognition was possible on the basis of typical relict structure when the optical characteristics were masked by the alteration of bivalent Fe (Fig. 2). The chemical analysis carried out on a selected area 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 (Fig. 2a) indicates that such micas are Mg,Fe-rich micas referable to biotite (see also the Mg and Fe X-Ray mapping in Fig. 2b).