The Parona chaotic complex: a puzzling record of the Messinian (Late Miocene) events in Monferrato (NW Italy)

Sedimentary Geology 152 (2002) 289 – 311 www.elsevier.com/locate/sedgeo

The Parona chaotic complex: a puzzling record of the Messinian (Late Miocene) events in Monferrato (NW Italy) F. Dela Pierre a,b,*, P. Clari a,b, S. Cavagna a, E. Bicchi b b

a Dipartimento di Scienze della Terra-Via Accademia delle Scienze, 5-10123 Turin, Italy CNR-c.s. geodinamica delle catene collisionali-Via Accademia delle Scienze, 5-10123 Turin, Italy

Accepted 20 December 2001

Abstract The Parona chaotic complex (PCC) from Monferrato (NW Italy) is a me´lange composed of carbonate-cemented blocks floating in a matrix of weakly consolidated mud breccias. It forms a minor (1 km2) chaotic sedimentary body enclosed in a larger Messinian me´lange that rests unconformably on older deposits and is followed, again unconformably, by Pliocene sediments. Three main facies have been recognized in the blocks. (a) Bioclastic rudstones and coquinoid grainstones of early Messinian age, composed of oligotypic assemblages of gastropods and the brackish-water bivalve Cerastoderma, mixed with shallow-water, marine biota (mollusks, corallinae algae, benthic foraminifers, echinoid fragments, rare ahermatypic corals). These sediments record shallow-water carbonate sedimentation with episodic brackish incursions and show a peculiar diagenetic overprint (selective dissolution of aragonitic grains and early dolomitic cements). (b) Monogenic breccias, composed of clasts of coquinoid grainstones floating in a dolomitic microcrystalline matrix. (c) Polygenic conglomerates, composed of rounded clasts of pre-Messinian pelagic sediments, of lower Messinian bioclastic carbonates and of Messinian evaporitic carbonates. Finally, a few blocks of strongly cemented micritic limestones, interpreted as methane-derived carbonates for the geochemical signature of the carbonate phases (strongly depleted in 13C), are present. The matrix enclosing the blocks is a mud breccia containing centimeter-sized clasts of marly sediments. Micropaleontological analyses have shown that lower Messinian and upper Tortonian planktic foraminifers are mixed with lower Miocene, Oligocene and Eocene forms. The PCC results from complex processes involving: (1) deposition of the peculiar sediments preserved in the blocks and (2) their dismemberment and mixing. As for the depositional history, composition of the blocks has allowed to sketch the following evolution. (a) Deposition of bioclastic rudstones and coquinoid grainstones that are interpreted as lower Messinian carbonate platform deposits for their strong compositional and diagenetical similarity with the lower Messinian carbonates cropping out in the Mediterranean region. The Monferrato bioclastic sediments are the northernmost example of Messinian shallow-water carbonates yet known. (b) Deposition of monogenic breccias. They formed during the evaporitic phase of the Messinian that was accompanied by a relative sea-level drop and by erosion of the previously deposited coquinas. Depositional and diagenetical processes during the evaporitic phase are also recorded by clasts of evaporitic carbonates in the polygenic conglomerates. (c) Deposition of polygenic conglomerates that are interpreted as upper Messinian post-evaporitic fan delta facies resulting from an important erosional phase. After deposition, this succession was dismembered in the blocks forming the present-day me´lange. Geometry, stratigraphic relationships, and internal characteristics of the PCC point to an origin related to gravity-driven phenomena, triggered by tectonics. However, the faunal mixing in the mud breccias and the occurrence of blocks of methane-derived

*

Corresponding author. Dipartimento di Scienze della Terra-Via Accademia delle Scienze, Universita´ Torino, 5-10123 Turin, Italy. Tel.: +39-11-5621179; fax: +39-11-541755. E-mail address: [email protected] (F. Dela Pierre). 0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 2 ) 0 0 0 9 7 - 0

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carbonates suggest that mud diapirism also played a role for the genesis of the PCC. This latter mechanism, which has been poorly considered until now, could also have been effective in the genesis of Messinian chaotic deposits that extensively crop out in NW Italy. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Me´langes; Messinian; Mud diapirism; Monferrato; NW Italy

1. Introduction The Messinian stage is one of the more intriguing and debated topics of Neogene stratigraphy because during this interval, the Mediterranean region underwent drastic paleogeographic, paleohydrologic, and paleobiologic changes as a result of the so-called ‘‘Messinian salinity crisis’’ (Hsu¨ et al., 1973, 1977; Clauzon et al., 1996; Krijgsman et al., 1999). Thick successions of evaporites were deposited in a mosaic of basins differing in size and in paleogeographic conditions, whereas extensive carbonate sedimentation occurred at the edges of same basins (e.g. Rouchy and Saint Martin, 1992). Together with ‘‘in situ’’ Messinian deposits, large volumes of chaotic sediments that are mainly made up of gypsum olistoliths, resedimented gypsum arenites and breccias are known from many Mediterranean areas (see Roveri et al., 2001) and crop out extensively also in the Tertiary Piemonte Basin (TPB) in NW Italy (Fig. 1). Here, they consist of blocks of selenitic gypsum and evaporitic vuggy carbonates and breccias floating in a fine-grained, poorly exposed matrix. Despite their large areal extent, these deposits have so far received little attention from geologists, and their meaning for the correct interpretation of the complex and still obscure events that has affected this area in the Messinian have never been assessed. In this paper, we provide a detailed description of an areally limited chaotic body that have been recently discovered in the Monferrato domain (Fig. 1). This particular association of sediments, informally termed as the Parona chaotic complex (PCC), is of great interest for the understanding of the Late Miocene evolution of NW Italy for the following reasons. (a) It contains blocks of bioclastic sediments that suggest shallow-water carbonate sedimentation during the Messinian, previously unknown in NW Italy. (b) The unusually good outcrop conditions of both the blocks and the enclosing matrix have allowed to

discuss the possible mechanisms leading to the dismemberment of a portion of the Monferrato sedimentary succession during the Messinian, and to suggest a hypothesis for the genesis of the coheval chaotic deposits of NW Italy. The study has been performed through a multidisciplinary approach including: (a) detailed geological mapping and sampling of the most representative blocks; (b) petrographic study in thin section of compositional and diagenetical features of selected samples; (c) SEM observations and EDS analyses, on polished, gently etched rock surfaces and thin sections, in order to recognize the different carbonate phases (calcite, aragonite, dolomite); (d) micropaleontological analyses of the foraminiferal assemblages of 10 samples collected in the matrix including the blocks; (e) C and O stable isotope analyses on carbonate cements and bulk rocks.

2. Regional geological setting The Tertiary Piemonte basin (TPB, Fig. 1) masks the subsurface junction between the Alpine and the Apenninic thrust belts (e.g. Biella et al., 1997) and is composed of different tectonosedimentary domains (Langhe, Alto Monferrato, Borbera-Grue, Monferrato, Torino Hill, Fig. 1) showing partially independent tectonosedimentary evolution and filled by Eocene to Neogene sediments. These successions were deposited on a complex wedge made up of Alpine and Apenninic (Ligurian) units coupled by the mesoalpine collisional tectonic event (Mutti et al., 1995; Piana, 2000). The TPB domains are overthrusted to the North onto the Padane foredeep along the Late Neogene to Plio-Quaternary Padane thrust front (Roure et

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Fig. 1. Structural sketch map of northwestern Italy. IL = Insubric line; SVZ = Sestri – Voltaggio zone; VVL = Villalvernia – Varzi line; PTF: Padane thrust front; TH = Torino Hill domain; AM = Alto Monferrato domain; BG: Borbera Grue domain; M1 = Messinian me´lange; M2 = ‘‘normal’’ Messinian succession. Plio-Quaternary deposits are left blank. Structural Model of Italy (1990), modified.

al., 1996), which is now buried under the alluvial Po Plain sediments. The northernmost part of the TPB is the Monferrato, which represents the NW termination of the Apenninic thrust belt (Fig. 1). Recent researches (Clari et al., 1995; Piana and Polino, 1995) have shown that it consists of two main tectonostratigraphic units (the Western Monferrato and the Eastern Monferrato) showing different pre-Langhian successions, composed of strongly deformed Eocene to Lower Miocene siliciclastic and carbonate sedi-

ments resting unconformably on deformed Apenninc Flysch of Mesozoic age. The sedimentation was mainly controlled by transpressive tectonics along the Rio Freddo deformation zone (Piana and Polino, 1995), a kilometer-wide shear zone that separates the Monferrato from the adjacent Torino Hill (Fig. 2). The Langhian to Messinian succession, on the contrary, shows similar characteristics, both in the Western and Eastern Monferrato (Fig. 2), and is represented by Langhian to Serravallian shelf calcar-

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Fig. 2. (a) Structural sketch map of the Torino Hill and Monferrato domains (modified after Piana, 2000). TH = Torino Hill; RFDZ = Rio Freddo deformation zone; WM = Western Monferrato; EM = Eastern Monferrato. (b) Schematic cross section of the Miocene to Pliocene succession of Monferrato. Not to scale. Ol – Aq = Oligocene to Aquitanian; Bu = Burdigalian; L – S = Langhian to Serravalian; TO = Tortonian; Ms = Messinian. TH = Torino Hill; WM = Western Monferrato; EM = Eastern Monferrato; RFDZ = Rio Freddo deformation zone. The chaotic arrangement of the Messinian deposits is shown.

enites and marls followed unconformably by Tortonian marls that are, in turn, overlain, through a pronounced angular unconformity, by sediments of Messinian age. In Monferrato, as well as in other sectors of the TPB, Messinian deposits consist of a complex me´lange (M1 in Fig. 1; Bonsignore et al., 1969; Boni and Casnedi, 1970; Ghibaudo et al., 1985), quite different from the ‘‘classical’’ succession described by Sturani (1973, 1976) at the NW edge of the Langhe domain (M2 in Fig. 1). This, in fact, consists of lagoonal evaporites (balatino gypsum) and lacustrine silts and clays (the so-called ‘‘Lago Mare’’ facies), whereas in Monferrato, only scattered blocks floating in a poorly exposed, fine-grained matrix are recognizable. Larger blocks are made up of selenitic gypsum (Fig. 3) and show a lateral extent of several hundreds of meters

and thickness of up to 100 m; smaller blocks (few meters to few tens of meters) consist of fine-grained massive to brecciated vuggy limestones and dolostones. Gyspum blocks are made up of alternating thick (up to several meters) beds of twinned selenitic gypsum and thinner beds (10 – 40 cm) of grey marls and are comparable to the evaporites of the Gessoso Solfifera Fm. cropping out all over the Apenninic thrust belt (Vai and Ricci Lucchi, 1977; Decima et al., 1988). Limestone and dolostone blocks commonly show voids or carbonate pseudomorphs after gyspum crystals and have been interpreted as evaporitic carbonates. Moreover, locally, isolated masses of Tortonian and pre-Tortonian pelagic sediments and of Lucina-bearing strongly cemented limestones of methane-related origin (Clari et al., 1994) are present in the me´lange. Finally, in the Parona area, the

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Fig. 3. Schematic geological map of the SW edge of the Eastern Monferrato. Dashed lines indicate the fine-grained, poorly exposed matrix of the Messinian chaotic succession; abbreviations refer to composition of larger blocks. PT = pre-Tortonian marls; TO = Tortonian marls; Gs = Messinian alternating selenitic gypsum and marls; Ec = Messinian evaporitic dolostones and breccias; Mc = methane-derived carbonates. PCC: Parona chaotic complex. Quaternary deposits are left blank.

bioclastic limestone blocks of the PCC (object of this study) crop out (Fig. 3). Lower Pliocene outer shelf silts and marls, unconformably resting on the Messinian me´lange, close the succession.

3. The Parona chaotic complex The PCC is a me´lange (Lash, 1987; Orange, 1990) that is made up of cemented blocks floating in a weakly consolidated ‘‘matrix’’ consisting of mud

breccias (Fig. 4). It crops out in an area of about 1 km2. Both geometry and relationships with surrounding sediments are obscure, due to very poor outcrop conditions outside the PCC itself. However, detailed mapping allow to interpret the PCC as a lenticularshaped sedimentary body enclosed in the larger Messinian me´lange. The blocks composing the PCC range in size from some decimeters up to several tens of meters, and their shape is often angular or, less commonly, rounded. Each block shows its own attitude, revealing the chaotic arrangement of the deposit that lacks any internal organization.

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Fig. 4. Parona chaotic complex: two cemented carbonate blocks (a) floating in an unconsolidated matrix consisting of mud breccias (mb) are recognizable.

3.1. The blocks Different lithofacies have been recognized among the blocks (Fig. 5): (1) (2) (3) (4)

Bioclastic rudstones and coquinoid grainstones; Monogenic breccias; Polygenic conglomerates; Micritic limestones.

In the following sections, the macroscopic and microscopic features of each facies will be described in detail. 3.1.1. Bioclastic rudstones and coquinoid grainstones Blocks represented by this lithofacies are quite abundant. Bioclastic rudstones show a sand-sized matrix consisting of a skeletal debris of echinoids, corallinae algae, bryozoa, bivalves, serpulids, benthic foraminifers (Elphidium aculeatum, Miliolids), which contains larger, whole fossils that are preserved as internal (most of the gastropods and bivalves) or external (some gastropods, ahermatypic corals) molds or, in rare cases, still preserve the shell. Preliminary paleontological studies have shown that among the gastropods, Turritella sp. is present, while the bivalves

consist mainly of oysters, Modiolus sp. and Cardium sp. Coquinoid grainstones consist of whitish, laminated porous coquinas that contain oligotypic assemblages of small gastropods (always preserved as internal molds) and brackish-water bivalves referable to the genus Cerastoderma (Pavia, personal communication, 2000), mixed with normal marine shallowwater biota that are represented, as in the previous lithofacies, by fragments of echinoids, corallinae algae (Tenarea sp.), serpulids, bryozoa, bivalves (Modiolus sp.), and benthic foraminifers (Elphidium sp.). Rare dasycladacean algae (Acicularia sp.) have also been observed (Fig. 6.1). Many of the skeletal grains have been bored (Fig. 7.1). In addition, both bioclastic rudstones and coquinas show evidence of selective dissolution of aragonitic skeletal grains (gastropods, most of the bivalves) that preserve their original shape because of the presence of a thin, turbid rim of early dolomitic cement (Fig. 6.2 and 6.3). On the contrary, calcitic and high Mg calcitic biota (echinoids, oysters, corallinae algae, benthic foraminifers) still preserve their fabric or show only minor evidence of dissolution. The degree of dissolution sharply increases from bioclastic rudstones to the coquinas that are sometimes entirely composed of dissolved

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Fig. 5. Detailed geological map of a part of the Parona chaotic complex showing the recognized blocks and the enclosing matrix. The grey line is a wall bordering the San Carlo private property.

bioclasts. The turbid dolomitic cement rimming the dissolved aragonitic grains or filling intragranular pores shows a spherulitic growth habit. SEM observations and EDS analyses have revealed that this cement is entirely composed of microcrystals of dolomite showing more calcitic growth zones and often a calcitic core (Fig. 7.2 and 7.3). Blocky calcite cements growing on the dolomitic rim in the intragranular pores represent the last phase of cementation. Some coquina layers are characterized by a coarser size of the skeletal fraction (Fig. 8.1) by low angle cross-lamination and by a more complex cementation. Also in this case, the bioclasts are rimmed by an isopacous turbid dolomitic cement that is followed by several generations of aragonitic fans covered by internal sediments that fill both the intergranular and the intragranular pores (Fig. 8.2 – 8.4). After these phases of cementation, selective dissolution of the

aragonitic shells occurred. The voids left by the original tests have been partially filled by sparry calcite. In one block of larger dimension (Novellone block, see Fig. 5), the facies described above are vertically arranged to compose a shallowing-upward stratigraphic succession that starts with bioclastic rudstones, with abundant oysters and corallinae algae, sharply followed by laminated coquinoid grainstones that contain 2-dm-thick interbeds of brownish, cemented coquinas displaying a sharp, erosional base, normal grading, and a badly preserved cross-stratification. At the top of the block, dolomitized peloidal wackstones with gastropods, benthic foraminifers (Elphidium sp.), anellid worms, and rare corallinae algae crop out. 3.1.2. Monogenic breccias This lithofacies is represented in many blocks and consists of matrix-supported, poorly sorted breccias

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Fig. 6. (1) Coquinoid grainstones: skeletal grainstone composed of fragments of bivalves (b), corallinae algae (c), echinoid spines (e) benthic foraminifers (Elphidium sp., el). Dasycladacean algae (Acicularia sp., a) are also present. (2) Coquinoid grainstones: skeletal grainstone composed of gastropods, bivalves (b), corallinae algae, and echinoid (e) fragments. Note the selective dissolution that affected only aragonitic skeletal grains. (3) Detail of the previous image, showing the dissolved test of a gastropod, whose shape is preserved, thanks to a turbid, rim of spherulitic dolomitic cement (arrow).

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Fig. 7. (1) SEM image showing boring in a dissolved skeletal grain. Borings are filled by dolomite. (2) SEM image showing the spherulitic dolomitic cement filling the body cavity of a gastropod. Later, blocky calcite cement fills the cavity left by dissolution of the shell. (3) Detail of the previous image showing the spherulitic growth habit of the dolomite crystals.

(Fig. 9.1). The clasts, which range in size from several meters to few centimeters, are made up of coquinoid grainstones showing a completely dissolved skeletal fraction (Fig. 9.2) and by rare peloidal grainstones. All the clasts show an angular shape and float in a turbid, dolomitic matrix that also contain scattered single fragments of benthic foraminifers (Elphidium sp.), corallinae algae, echinoids, bivalves, and serpulids. SEM observations and EDS analyses have revealed that this matrix is also composed of dolomite showing a spherulitic-zoned growth (Fig. 9.3 and 9.4). 3.1.3. Polygenic conglomerates This lithofacies consists of clast-supported polygenic conglomerates containing rounded clasts ranging in size from few millimeters to some decimeters (Fig. 10.1). An abundant skeletal fraction (mainly bivalves, bryozoans, corallinae algae, and echinoids), interpreted as loose bioclastic debris resulting from erosion of coquinoid grainstones and bioclastic rudstones, is also present. It is worth noting that in this bioclastic detritus, fragments of the alga Halimeda have also been observed (Fig. 10.3). As for the clasts, the following types have been observed. (a) Planktic foraminifer and glaucony-rich wackestones and mudstones (Fig. 10.2).

(b) Sponge spicula-rich wackestones. (c) Coquinoid grainstones and bioclastic rudstones (Fig. 10.2). (d) Rare clasts of volcanic rocks of extrabasinal origin. (e) Clasts of aragonitic cements (Fig. 10.2). (f) Unfossiliferous vuggy dolomitic peloidal mudstones showing prismatic voids probably resulting from dissolution of gypsum crystals (Fig. 10.4). (g) Dolomitic peloidal grainstones with ‘‘nests’’ of very elongated, submillimetric, rod-like micritic pellets showing a near-circular cross-section (Fig. 10.5). Their lengths range from 0.5 to 1 mm, whereas the diameter is of about 60 Am. Quite similar micritic grains have been recognized in some Cenozoic hypersaline deposits and have been interpreted as coprolites of brine shrimps (Decima et al., 1988; Finkelstein et al., 1999). Both clasts (f) and (g) are remarkably similar to the evaporitic carbonates described in many Messinian basins of the Mediterranean region (e.g. Decima et al., 1988; Van de Poel, 1991) where they appear as the shallow-water, lateral equivalent of the evaporites deposited during the main phases of the salinity crisis. Clasts of the polygenic conglomerates lack the turbid dolomitic rim that characterizes the abovedescribed lithofacies. Instead, both clasts and skeletal

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Fig. 8. (1) Coarse-grained, decimeter-thick cemented bed of coquinas (a) resting on fine-grained coquinas composed of dissolved bioclasts. Novellone block. (2) Photomicrograph showing well-developed aragonitic fans (a) growing on a dark rim of dolomitic cement (arrow) that outline the dissolved test of a gastropod; the resulting cavity has been occluded by sparry calcitic cement (b). Note the dark sediment (c) that partially fills the intergranular pore and the late dog tooth (meteoric?) calcite cement. (3) SEM image of the previous sample showing welldeveloped aragonitic crystals with blunt terminations (a) growing on a dolomitic rim (d). (4) Detail of the previous image showing the spherulitic dolomite, followed by the aragonite crystals.

fragments are enclosed by blocky calcitic cement that fills both the intergranular and intragranular pores (Fig. 10.6). Cementation likely occurred during an early phase of the diagenesis, as suggested by the lack of compaction in the rock. 3.1.4. Micritic limestones Two small (some tens of centimeters) blocks of this peculiar lithofacies have been found in the PCC. The characteristics of these sediments are quite different from the ones of the so-far-described carbonate blocks. In fact, they consist of extremely hard brownish micritic limestones, crossed by a network of millimeter- to centimeter-sized, cement-filled fractures. In the thin section, this lithofacies is a planktic foraminifer wackestone. All the fractures display sharp boundaries, revealing that the rock was already cemented when the cracks formed. They show a complex filling by laminated carbonate cements con-

sisting of alternating turbid and clear layers. The final plug of the fractures consists of large euhedral calcite crystals. In some fractures, internal sediments, composed of a peloidal grainstone, are present. Finally, some isolated, large fecal pellets have been observed within the fractures (Fig. 11). 3.2. Matrix of the PCC The matrix enveloping the blocks consists of a structureless, non-bedded, weakly consolidated mud breccia showing a brownish silt-sized matrix that contains rounded to angular clasts of whitish and grey marls ranging in size from few millimeters to some centimeters (Fig. 12). Sheared textures have been observed in some places. In order to establish the age of the mud breccias, detailed micropaleontological analyses on foraminiferal assemblages have been carried out on 10 sam-

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Fig. 9. (1) Monogenic breccias: angular clasts of partly dissolved coquinas (a) floating in a whitish dolomitic matrix. Polished slab. (2) Photomicrograph showing a clast of coquina, entirely composed of molds of skeletal grains, encased by a turbid dolomitic micrite. (3) SEM image of the matrix of the monogenic breccias composed of dolomite with spherulitic growth habit. (4) Detail of the previous image showing the spherulitic growth habit of the dolomite.

ples, including both the mud breccias and some larger, isolated marly clasts. In the analyzed residues, an inorganic fraction consisting of quartz grains, mica flakes, and glaucony has been detected. The organic content is made up of echinoid spines, fecal pellets, small shark teeth, ostracods, sponge spicula, and Bollboforma spp. The foraminiferal assemblages are scarce and badly preserved. However, a marked faunal mixing has been evidenced in the mud breccias and in one isolated clast (Table 1). In fact, Upper Tortonian – Lower Messinian planktic foraminifers (e.g. Globigerinoides obliquus extremus, Neogloboquadrina acostaensis, N. humerosa, Globorotalia menardii, G. mediterranea/conomiozeagr., G. saphoae) are mixed with deformed and recrystallized Lower – Middle Miocene (Paragloborotalia siakensis, P. acrostoma, Praeorbulina glomerosa glomerosa), Oligocene (Gloobigerina tripartita, Paragloborotania opima opima), and Eocene (Morozovella sp., Turborotalia cerroazulensis gr.) forms. Instead, poorly preserved assemblages of benthic foraminifers (e.g. Elphidium spp.,

Sigmoilopsis schlumbergeri) have been detected in two of the isolated clasts that were studied (Table 1). These mixed assemblages suggest that the Parona mud breccias were sourced from large portions of the stratigraphic succession of Monferrato.

4. Isotopic data Preliminary isotopic analyses have been performed on different portions of the carbonate rocks of the PCC. The carbonate fraction of selected samples has been analyzed following the classical method (McCrea, 1950). The isotopic ratios are expresses as d13C and d 18O per mil versus the PDB standard; the analytical error is F 0.5x and F 0.1x for d13C and d18O, respectively. The following lithofacies have been analyzed and the results are shown in Fig. 13. (1) Dolomitized bioclastic rudstones and coquinoid grainstones (Fig. 6.2 and 6.3) are indicated as group 1

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Fig. 10. (1) Polygenic conglomerates: polished slab showing the clast-supported texture of the deposit. An elongated clast composed of aragonitic cement (a) is recognizable. (2) Photomicrograph showing different kinds of clasts: (a) planktic foraminifer wackestone; (b) skeletal wackestone; (c) aragonitic cement. (3) Halimeda. Intragranular porosity is partially filled by dog-tooth calcitic cement. (4) Peloidal dolomitic mudstones with millimeter-sized prismatic void (arrows), interpreted as the result of dissolution of gypsum crystals. Clast in the polygenic conglomerates. (5) Dolomitized grainstones with elongated, rod-like micritic pellets, showing a near-circular cross-section. Brine shrimp fecal pellets? (6) SEM image showing the blocky calcite cement partly filling an intergranular pore in the polygenic conglomerate. Crystal size increases towards the center of the pore.

(Fig. 13). Two different kinds of calcite fractions have been detected. The first, with d 13C and d 18O values around 0x , suggests an isotopic equilibrium with relatively normal marine water; this is interpreted as the organic calcite of bioclasts. The second one shows more depleted d13C and d 18O values that reveal the meteoric origin of portion of the calcite fraction; this

could be referred to the late blocky cement within the bioclasts (Fig. 7.2). In all these samples, both calcite and dolomite fractions have been analyzed. The lines connecting calcite and dolomite of the same sample describe a common trend: dolomites are all shifted towards more positive d 18O and d 13C values. These data could fit

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Fig. 11. Photomicrograph of a micritic limestone showing different fractures filled by laminated carbonate cements consisting of alternating turbid and clear layers. Large fecal pellets are also recognizable (arrows).

well with the classical calcite – dolomite oxygen isotope fractionation: in low temperature range, the oxygen fractionation between calcite and dolomite tends to enrich the latter in the heavy isotope (Degens and Epstein, 1964). However, as two different kinds of calcite (organic and late diagenetic) are present, this trend could be better explained by dolomite growth during diagenesis in isotopic equilibration with waters that have been enriched in 18 O by evaporation (McKenzie et al., 1979). For what concerns d 13C

values, an experimental study (Sheppard and Schwartz, 1970) demonstrates that dolomite should be slightly enriched in 13C relative to cogenetic calcite. Group 2 are cemented coquinas with well-developed aragonitic fans (Fig. 8.2 – 8.4) showing both d 13 C and d 18O positive values ( + 1/ + 5x and + 2/ + 7x , respectively). These results are inconsistent with the traditional calcite – aragonite fractionation pattern that predicts the enrichment of 18O in calcite relative to aragonite, as reported for both organic

Fig. 12. Mud breccia of the matrix of the PCC. Light-colored marly clasts floating in a silt-sized matrix are clearly recognizable.

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Table 1 Foraminiferal content of the mud breccias and clasts

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explained, also for these samples, by isotopic equilibration with waters that have been enriched in 18O by repeated evaporation (Aharon et al., 1977).

5. Discussion The data exposed in the previous sections evidence that the PCC is the result of complex geological processes that involved: (1) deposition of sediments presently preserved in the blocks; (2) their dismemberment and the emplacement of mud breccias. 5.1. Depositional history Fig. 13. Oxygen and carbon isotopic composition of Parona samples. Groups 1, 2, and 3 are described in the text.

(Epstein et al., 1953; Tarutani et al., 1969) and inorganic precipitates. The positive d 18O values of the analyzed samples could probably be explained by isotopic equilibration with waters that have been enriched in d 18O after evaporation (McKenzie et al., 1979). No considerations can be done on 13C values since d 13C of inorganically precipitated aragonite – calcite mixtures display different enrichment factors depending on the rate of precipitation (Turner, 1982). Group 3 are strongly cemented micritic limestones, with sharp depletion in d 13C values ( 28.51x/ 10.16x) and positive d 18O values ( + 5.65x/ + 1.85x). Strongly negative d 13C values can be explained by invoking a contribution of CO2 derived from oxidation of methane seeping upwards from the deeper sediments (Claypool and Kaplan, 1974), as have already been observed in many anomalous carbonate masses of Monferrato that have been interpreted as the result of peculiar diagenetic processes that took place around methane-rich fluids emission on the sea floor (Clari et al., 1994; Cavagna, 1999). d 18 O values of samples enclosed in group 3 are different from those of already known Monferrato methanederived carbonates that generally display d 18O values around 0x; such positive d 18O values could be

Reconstruction of the original depositional architecture of the basin is hampered by the loss of stratigraphic relationships among the sediments preserved as blocks. However, composition of sediments allows one to sketch a possible sedimentary evolution that occurred according to the following steps (Fig. 14). 5.1.1. Deposition of bioclastic rudstones and of coquinoid grainstones Bioclastic rudstones are indicative of marine shallow-water carbonate sedimentation, not affected by terrigenous influx. Coquinoid grainstones suggest a high-energy, shallow-water environment where normal marine and brackish biota (such as the bivalve Cerastoderma) were mixed. The decimeter-thick, cemented aragonitic coquina beds that, in the Novellone block, are interbedded in this facies can be interpreted as tempestitic layers resulting from episodical, high-energy storms that concentrated the coarsest biota. One major problem regards the age of the bioclastic carbonates of Parona. A general Late Miocene age (Upper Tortonian – Messinian) is suggested by the malacofauna (Pavia, personal communication, 2000) and by the occurrence of Elphidium aculeatum whose first occurrence is reported in the Tortonian (AGIP Mineraria, 1982). However, a Messinian age for these sediments is suggested here on the base of the following points.

Note to Table 1: Grey cells represent reworked species. Stratigraphic distribution of the species is reported after D’Onofrio et al. (1975), Bolli and Saunders (1985), and Iaccarino (1985).

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Fig. 14. The three phases of Messinian sedimentation, as inferred by composition of the blocks composing the Parona chaotic complex. Dark grey areas represent basinal sectors that are not recorded in the Parona outcrops. (a) Pre-evaporitic phase with deposition of shallow water carbonates; (b) Evaporitic phase: two cycles, separated by an erosional surface and corresponding to deposition of monogenic breccias and of evaporitic vuggy dolostones, are represented; (c) post-evaporitic phase with deposition of fan delta (?) polygenic conglomerates. For further explanations, see text.

(a) Strong compositional and textural similarities with Upper Miocene carbonate platform facies cropping out all over the Mediterranean region, from northern Africa (Saint Martin and Corne´e, 1996; Corne´e et al., 1996) to Spain (Esteban, 1996), Sicily (Pedley, 1996), and peninsular Italy (Pedley and Grasso, 1994; Bossio et al., 1996) till the northern Apennines (Barrier et al., 1994). The shallower parts of these platforms are made up of bioclastic sediments rich in corallinae algae, bryozoa, and mollusks. They are considered as the product of an Early Messinian transgressive phase (Esteban, 1996) and are correlatable to deep-water, restricted sediments (Tripoli-like diatomites) deposited in the more distal part of the

marginal Mediterranean basins (Rouchy and Saint Martin, 1992). Most of these Messinian platforms are typically rimmed by narrow fringing coral reefs built by oligospecific assemblages dominated by the coral Porites that until now has not been found in the Monferrato outcrops. Nevertheless, Messinian bioclastic carbonates associated to reefoidal Porites limestones that are strongly comparable to those of Parona are known from NW Sicily (Aruta and Buccheri, 1976; Di Stefano and Catalano, 1976) and from northern Apennines (Barrier et al., 1994). The occurrence of fragments of the alga Halimeda, reworked in the polygenic conglomerates of Parona, supports this comparison, as Halimeda is a common

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component of the Mediterranean Messinian carbonate platforms (e.g. Saint Martin and Corne´e, 1996; Bossio et al., 1996). (b) The diagenetic overprint. The Parona sediments show the same diagenetic overprint of the Mediterranean Upper Miocene carbonates that consists of selective dissolution of aragonitic skeletal grains and pervasive dolomitization that precedes precipitation of calcitic cements (e.g. Sun, 1992; Sun and Esteban, 1994; Pomar et al., 1996; Mankiewicz, 1996; Bossio et al., 1996). This peculiar diagenetic pattern has been related to the flushing of the sediments by hypersaline brines undersaturated with respect to the aragonite (Oswald et al., 1991a,b; Sun and Esteban, 1994). Also, preservation of thick crusts of aragonitic cements that has been observed in some of the coquinas at Parona is a characteristic feature of Upper Miocene carbonates of the Mediterranean (e.g. Sicily, Esteban and Prezbindowski, 1985) and has been imputated to the encasement of the aragonite by subsequent gypsum cements. Moreover, the results of the isotopic analyses performed on these sediments (Fig. 13) reveal d18 O values that could be produced by isotopic equilibration with water that have been enriched in 18O by repeated evaporation. On the base of sedimentological, compositional, diagenetic, and geochemical characteristics, the Parona carbonates are interpreted as Lower Messinian carbonate platform sediments. They could represent shallow-water facies of a pre-evaporitic basin, whose distal sediments likely consisted of restricted diatomaceous marls that presently crop out East of the studied area (Sturani and Sampo`, 1973; Pavia, 1989). However, no evidence of this inferred lateral facies change is preserved in the blocks of the PCC. It is worth noting that the Parona carbonates, together with the examples reported in Central Italy (Bossio et al., 1996) and in the northern Apennines (Barrier et al., 1994), represent one of the more northern evidence of Messinian carbonate platform facies known in the Mediterranean. 5.1.2. Deposition of the monogenic breccias The bad sorting and the coarse size of the angular clasts suggest rapid deposition of the detritus, while the dolomitic spherulitic matrix is indicative of a peculiar nonmarine environment of deposition, likely a hypersaline lagoon. These sediments probably

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formed during the evaporitic phase of the Messinian that was accompanied by a relative sea-level drop and by subaerial exposure and erosion of the previously deposited bioclastic carbonates. Depositional and diagenetical processes during the Messinian evaporitic phases are also suggested by the occurrence of clasts of evaporitic carbonates (brine shrimps peloidal grainstones and unfossiliferous vuggy dolostones, Fig. 10.4 and 10.5) preserved within the polygenic conglomerates. The genesis of these carbonates is still debated since they are considered either as primary carbonate deposits resulting from algally controlled precipitation of CaCO3 in a hypersaline environment (Decima et al., 1988) or as the product of microbial transformation of sulphate (Pierre and Rouchy, 1988) or as the results of distinct diagenetic phases involving, first, the microbial sulphate reduction in oxygen-poor conditions and, second, an important fresh-water diagenetic phase (Van de Poel, 1991). Anyway, in the basins where the original depositional architecture is preserved, the evaporitic carbonates are seen to be the shallow-water, lateral equivalent of the evaporites (e.g. Decima et al., 1988). 5.1.3. Deposition of the polygenic conglomerates Composition of the clasts suggests that these deposits were sourced from: (a) the pre-Messinian succession of Monferrato (clasts of planktic foraminifer- and glaucony-rich wackestones); (b) the Messinian pre-evaporitic deposits (clasts of coquinoid limestones and fragments of shallow water biota such as the alga Halimeda); (c) the evaporitic sediments (clasts of peloidal grainstones and vuggy dolostones); (d) some volcanic rocks of extrabasinal provenance whose primary position is still unknown. As for the age of these deposits, the occurrence of clasts of evaporitic carbonates allows one to refer them to the Late Messinian post-evaporitic phase. The depositional environment is more difficult to assess since no relationships with surrounding sediments can be observed and no clear sedimentary structures have been identified within the blocks. However, taking into account their textural and compositional characteristics and the diagenetic overprint (blocky calcite meteoric cements have been observed among the clasts, see Fig. 10.6), we interpret them as fan delta continental facies analogous to the Upper Messinian coarse-grained deposits (Cassano Spinola

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Conglomerates) cropping out south of Monferrato (e.g. Ghibaudo et al., 1985). We can hypothesize that the polygenic conglomerates record a prominent erosional phase that resulted in an ‘‘intra-Messinian’’ unconformity, separating evaporitic deposits from post-evaporitic ones, as also reported in northern Italy (Ghibaudo et al., 1985; Gelati et al., 1987; Roveri et al., 1998, 2001) and in all the Mediterranean region (e.g. Esteban, 1996). 5.2. Dismemberment of the succession An intriguing question concerning the PCC is the mechanism that dismembered this peculiar succession and gave rise to the present-day chaotic sedimentary body (me´lange). In order to suggest a reliable hypothesis on this subject, a brief summary of the contrasting ideas about the genesis of me´langes is presented. 5.2.1. Genesis of me´langes Although me´langes are not restricted to any particular tectonic setting, they are a common component of ancient and present-day accretionary complexes. Three main processes have been suggested for their genesis (e.g. Lash, 1987; Orange and Underwood, 1995). (a) Tectonic disruption and mixing of an original coherent sedimentary succession (e.g. Hsu¨, 1968) giving rise to mappable rock bodies bounded by tectonic contacts and consisting of blocks that were wholly derived by the in situ disruption of layered sediments. The resulting blocks show a preferred orientation that is parallel to a pervasive scaly cleavage that characterizes the matrix (Cowan, 1985; Pini, 1999). (b) Gravity-driven debris flow or olistostrome (Beneo, 1956; Abbate et al., 1970; Page and Suppe, 1981), which has been proposed since the 1950s as the most common mechanism generating chaotic deposits in the stratigraphic record. The products are sedimentary bodies that are interbedded with normal sediments through depositional contacts and are composed of blocks that are randomly distributed in the matrix. The blocks can range in size from some millimeters to hundreds of meters and are composed by a great variety of rocks, depending on the composition of the source area. Furthermore, the matrix does not show any evidence of tectonic deformation although postdepositional deformation may imprint a scaly cleavage

that is generally less pervasive and spaced than the one in the above-discussed case (e.g. Pini, 1999). (c) Diapiric rise of overpressured, fluid-permeated mud. Investigations on present-day accretionary complexes have revealed that mud diapirism is a very effective mechanism that is able to generate huge volumes of chaotic deposits (Barber et al., 1986; Camerlenghi et al., 1995; Harris et al., 1998). It gives rise, at depth, to mud diapirs and ridges and, at the surface, to mud volcanoes erupting large volume of chaotic sediments consisting of viscous mud enclosing more competent clasts (mud breccias). The latter can form lens-shaped bodies interbedded with normal sediments. Quoted criteria for recognition of diapiric me´langes include the high-angular and discordant contact with surrounding sediments, the presence of exotic clasts sampled during the uprise of the mud, the phacoidal shape of the blocks that often display thin fractures filled by the matrix mud that was ‘‘injected’’ in the cemented blocks as a consequence of the high-pressure conditions. In the matrix, a well-developed scaly cleavage, whose intensity increases towards the contact with encasing sediments, occurs (Barber and Brown, 1988; Orange, 1990). All these criteria are useful only if original characteristics of sedimentary bodies are not later modified by sedimentary or tectonic processes, a circumstance that rarely takes place in the highly mobile geodynamic contexts where these deposits form (Cavagna et al., 1998, 1999; Conti and Fontana, 1999). A long-lasting feature suggesting a diapiric origin for a me´lange is the presence of methane-derived carbonates, as evidenced both in present-day environments (Lance et al., 1998; von Rad et al., 2000) and in few fossil examples (Cavagna, 1999). These carbonate masses form because the migration of hydrocarbon-rich fluids towards the surface during shale diapirism generates localized cementation of sediments and the proliferation of peculiar chemosymbiotic assemblages. 5.2.2. Interpretation of the PCC The geometry and the internal characteristics of the PCC allow one to rule out a tectonic origin, as no evidence of tectonic structures obliquely bounding neither the complex nor the single blocks has been recognized. An origin linked to gravity sliding is instead consistent with the lenticular shape of the PCC and

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the totally chaotic arrangement of the deposits that contain angular to subangular blocks with a wide range of sizes (from few centimeters up to several decameters) that are randomly distributed in the matrix without any preferred orientation. Moreover, this interpretation is fully compatible with the overall characteristics (geometry, stratigraphic lower and upper boundaries) of the larger Messinian me´lange of which the PCC represents a smaller portion. However, two peculiar features of the PCC allow one to envisage that also mud diapirism contributed to the genesis of this particular chaotic body. They are as follows: (a) The faunal mixing observed in the mud breccias enveloping the blocks in which upper Miocene (Tortonian and lower Messinian) planktic foraminifers are mixed with Middle Miocene, Oligocene, and Eocene forms. This is a very common characteristic of the present-day and fossil diapiric mud breccias (Staffini et al., 1993; Kohl and Roberts, 1994; Kopf et al., 2000). Moreover, it should not be forgotten that extensive reworking has been recorded only in the mud breccias, whereas all the blocks of the PCC are referable to the Messinian. This evidence is in contrast with a gravitative origin for the mud breccias because, in this case, gravitational processes would have involved large portions of the succession of Monferrato, and sediments older than the Messinian should also be expected as larger blocks in the PCC, which is not the case. As a consequence, we interpret the mud breccias of the PCC as due to diapiric phenomena, which involved upward movement of overpressured mud that incorporated and sampled, during its rise, portions of the crossed succession. Sheared texture and scaly cleavage, which are commonly considered to be diagnostic features of diapiric mud breccias (Barber et al., 1986; Orange, 1990), are not common at Parona and have been recognized only locally. However, the involvement of the mud breccias in later gravity sliding could have concealed these characteristics. (b) The occurrence of blocks of micritic limestones of methane-related origin that are commonly associated both in present-day settings and in some fossil examples to mud diapirs and mud volcanoes (Lance et al., 1998; Cavagna et al., 1998; Cavagna, 1999). The above-described characteristics hence provide compelling evidence that large-scale gravity-driven phenomena were responsible for the emplacement of

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the major part of the carbonate blocks and were accompanied by mud diapirism and upward migration of methane-rich fluids that resulted in the extrusion of mud breccias and in the generation of methane-related carbonates. These last sediments were, in turn, involved in gravity sliding after their emplacement. Mud diapirism and fluid expulsion at the surface are often related to large-scale gravity-driven mass movements. Recent studies suggest that two mechanisms, both involving fluid expulsion, can trigger catastrophic sediment failure: (1) tectonic oversteepening of previously deposited sedimentary sequences and sediment dewatering related to thrust development (e.g. Brown and Westbrook, 1988); (2) sudden gas hydrate destabilization induced, in turn, by rapid sea-level drop (McIver, 1982; Henriet and Meniert, 1998). Both mechanisms are consistent with the geodynamic and paleoenvironmental framework of Monferrato at the time of the emplacement of the PCC. In fact, during the Messinian, on one hand, outward migration of the North-vergent Padane thrust front occurred (e.g. Piana, 2000) and, on the other, large oscillations of the sea level, possibly causing gas hydrate dissociation, took place in the Mediterranean (e.g. Esteban, 1996). The role played by each of these factors is difficult to evaluate. However, the occurrence of a pronounced angular unconformity at the base of the Monferrato Messinian me´lange (where the PCC is enclosed) suggests that tectonic deformation was the driving factor for both fluid expulsion and gravity-induced phenomena. The latter resulted in catastrophic mass wasting, which involved, at the same time, a previously deposited Messinian succession as well as the products of mud diapirism, giving rise to the presentday complex and polygenic chaotic sedimentary body.

6. Conclusions The PCC is a unique window on the Late Miocene evolution of Monferrato and provides valuable informations on the complex events that affected this region during the Messinian.

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The carbonate sediments preserved as blocks clearly suggest a three-fold subdivision of the Messinian succession, as had already been recognized in other sectors of the TPB by Sturani (1973, 1976). However, in Monferrato, this succession show completely different features and is composed of: (a) Lower Messinian bioclastic sediments suggesting marine carbonate platform sedimentation with episodic brackish incursions; (b) Messinian evaporitic breccias and carbonates passing basinward to selenitic gypsum deposits; (c) Upper Messinian polygenic conglomerates representing post-evaporitic fan delta facies that underline an important erosional phase that has involved Messinian as well as pre-Messinian facies. The Lower Messinian bioclastic sediments are of great interest for the Late Miocene paleogeography of the Mediterranean region because they represent one of the more northern examples of carbonate platform facies that are known till now. After deposition, this succession was dismembered by complex processes that resulted in the emplacement of an areally extensive me´lange of which the PCC represents only a small part. An origin related to gravity-driven and diapiric processes, both triggered by tectonic deformation, may be envisaged. Mud diapirism could have played a role also in the generation of the large volumes of Messinian chaotic deposits that crop out extensively in NW Italy. In order to verify this hypothesis, a careful search of the geological products of mud diapirism, which in present-day environments is one of the most effective mechanisms for the genesis of me´lange, is thus needed.

Acknowledgements The authors would like to thank G. Pavia for the preliminary paleontological analyses of the malacofauna, and L. Martire and F. Piana for their support during the field work. A. d’Atri is thanked for the critical reading of the manuscript. W. Schlager and an anonymous reviewer are thanked for the constructive reviews. Isotopic analyses and SEM observations

were performed in the laboratories of the Dipartimento di Scienze delle Terra, University of Torino. Analytical work was divided up as follows: field analyses, mapping, and sampling: F. Dela Pierre and P Clari; petrographic and SEM analyses: S. Cavagna, P. Clari, F. Dela Pierre; isotopic analyses: S. Cavagna; micropaleontology: E. Bicchi. Research was funded by MURST grants to P. Clari and by CNR, c.s. geodinamica delle catene collisionali, Torino.

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