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A multidisciplinary study of middle Miocene seep-carbonates from the northern Apennine foredeep (Italy)

A multidisciplinary study of middle Miocene seep-carbonates from the northern Apennine foredeep (Italy)
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  A multidisciplinary study of middle Miocene seep-carbonatesfrom the northern Apennine foredeep (Italy) S. Conti a, *, D. Fontana a,1 , A. Gubertini a,2 , G. Sighinolfi a,3 ,F. Tateo  b,4 , C. Fioroni a,5 , P. Fregni a,6 a   Dipartimento di Scienze della Terra, Universita`  di Modena e Reggio, Largo Sant’ Eufemia 19, I-41100 Modena, Italy  ,7   b  Istituto Geoscienze e Georisorse, CNR, c/o Dipartimento di Geologia, Paleontologia e Geofisica,Universita`  di Padova, Via Giotto, 1, I-35137 Padova, Italy Received 18 June 2003; received in revised form 21 January 2004; accepted 31 March 2004 Abstract Several pelitic intervals are intercalated at various levels within the marly-arenaceous turbiditic successions of the middleMiocene northern Apennine foredeep. They range in thickness from 30 to 200 m, and represent sedimentation on top of ephemeral structural highs related to blind faults. Sediments are made up of hemipelagites and fine-grained turbidites, andinclude  13 C-depleted carbonates, related to the rising of methane-rich fluids (hydrocarbon seep-carbonates). Large portions of  pelitic intervals are involved in chaotic masses by soft sediment deformation (slumps, slides, intraformational breccias),revealing an intense sediment instability during middle Miocene. A stratigraphic, mineralogic and geochemical study wasconducted on two of these pelitic intervals (Castagno d’Andrea, Mt Citerna) in order to reconstruct carbonate development, thecomposition of fluids, and to document the connections between fluid seepage and syndepositional tectonics. Thismultidisciplinary approach has allowed us to discriminate between the two examined pelitic intervals in terms of age,depositional rates and conditions, organic carbon and post-depositional processes.Seep-carbonates are characterized by chemosymbiotic fossil communities, autoclastic fractures and brecciation; carbonate bodies show complex facies relationships, as indicative of different stages in chemoherm growth. The compositional studyevidences the peculiar chemistry of chemoherm carbonates (calcite low in Mg and Sr) compared with carbonates in associatedenclosing pelites and with modern chemoherms, in general. The non-carbonate components within the chemoherms areenriched in detrital minerals and depleted in phyllosilicates with respect to the enclosing pelites. The mineralogical changes inthe clay component within the brecciated unit of the Castagno d’Andrea chemoherms suggest authigenic precipitation of theMg-rich phases. 0037-0738/$ - see front matter   D  2004 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2004.03.010* Corresponding author. Tel.: +39-59-2055858.  E-mail addresses: (S. Conti), (D. Fontana), (A. Gubertini), (G. Sighinolfi), (F. Tateo), (C. Fioroni), (P. Fregni). 1 Tel.: +39-59-2055844. 2 Tel.: +39-59-2055863. 3 Tel.: +39-59-2055824. 4 Tel.: +39-49-8272083; fax: +39-49-8272070. 5 Tel.: +39-59-2055854. 6 Tel.: +39-59-2055846. 7 Fax: Geology 169 (2004) 1–19  Isotopic analyses show the distinct carbon signature in the chemoherms from the two examined intervals (Castagnod’Andrea chemoherms more depleted, from   15.8 x up to   41.3 x PDB, than Mt Citerna, from   5.2 x up to   16.7 x ),and the transitional  13 C-depletion trend observed moving from chemoherms to the enclosing pelites (moderately depleted) andTe (Bouma sequence) turbidites (in the range of marine carbonates). A slight but significant enrichment in  d 18 O (up to +1.4%PDB) is observed for all chemoherms when compared to values of carbonate phases present in enclosing pelites.Geochemical data indicate that the brecciated facies of seep-carbonates are related to an explosive release of gaseous fluids probably associated with the rise of deep hypersaline fluids. D  2004 Elsevier B.V. All rights reserved.  Keywords:  Seep-carbonates; Northern Apennines; Miocene foredeep; Fluid seepage; Mineralogy; Geochemistry; Tectonic setting; Sediment instability 1. Introduction Seep-carbonate deposits with very negative  d 13 Cvalues are well-known products of the microbialoxidation of methane-rich fluids extruding diffuselythrough sediment pore space, or focused along con-duits intersecting the sea floor  (Suess and Whiticar,1989; Aharon et al., 1997; Cavagna et al., 1999). In the past decade, they have been documented from varioustectonic settings: convergent plate boundaries off thecoasts of Oregon (Ritger et al., 1987; Bohrmann et al.,1998), Nankai trough off Japan (Sakai et al., 1992), Makran accretionary prism off Pakistan (Von Rad et al., 1996), Peru continental margin (Sample, 1996), Barbados accretionary wedge (Lance et al., 1998),accretionary prism of eastern Mediterranean (Aloisiet al., 2000), northwestern Black Sea (Peckmann et al., 2001b), marginal sea of Okhotsk  (Esikov and Pash-kina, 1990), New Zealand continental slope (Lewisand Marshall, 1996), active transform margin of cen-tral California (Embley et al., 1990; Stakes et al.,1999), and passive continental margins in the NorthSea (Hovland et al., 1987), off Danish coast  (Jørgen- sen, 1992), at the base of the Florida escarpment  (Paullet al., 1995), Louisiana and Texas continental slope(Roberts et al., 1987; Roberts and Aharon, 1994),Baffin island continental shelf  (Matsumoto, 1990).Authigenic carbonates exhibit various mineralogies(Mg-calcite, aragonite and dolomite), and range fromsmall, millimetre- to decimetre-thick concretions, pavements, slabs, nodules, pillars and crusts, to largeirregular lenses, pinnacles and buildups (referred tochemoherms) up to 25–30 m in height (Taviani, 2001and references therein). Carbonate crusts and concre-tions may comprise pore-filling cements of variouslithologies, whereas carbonate lenses and buildups areusually included in fine-grained terrigenous sedi-ments, commonly hemipelagic muds. In many cases,sites of hydrocarbon-rich fluid emission are evidenced by localized flourishing communities of mussel andclam bivalves, gastropods, tube worms, and bacterialmats (Sibuet et al., 1988; Kennicutt et al., 1989;Callender et al., 1990; Callender et al., 1992; Carney,1994; Barry et al., 1996; Corselli and Basso, 1996;Olu et al., 1997; Sibuet and Olu, 1998). Theseinvertebrates rely on sulphide or methane oxidationvia chemoautotrophic endosymbiotic bacteria whichsupply their hosts with energy and nutrients (Kulmet al., 1986; Brooks et al., 1987; Kotelnikova, 2002).In modern convergent margins, tectonic uplift andthrusting cause faults, folds, structural highs andaccretionary ridges, and local extensional fractures,which are a preferential pathway for fluid seepage. In particular, seep-carbonates form in association withfaults (Kulm and Suess, 1990; Chamot-Rooke et al.,1992; Kobayashi et al., 1992; Von Rad et al., 1996),folds and ridges related to blind faults (Lallemandet al., 1992; Tryon et al., 1999; Suess et al., 1999), or at the top of mud volcanoes and diapiric breccias (Jollivet et al., 1990; Lance et al., 1998; Aloisi et al., 2000). Inthe last 10 years, it has emerged that a significant source of hydrocarbon-rich fluids is represented byhydrate dissociation, and possible relations betweengas hydrate destabilization and authigenic carbonate precipitation are reported by many authors (Matsu-moto, 1990; Sakai et al., 1992; Bohrmann et al., 1998;Lance et al., 1998; Woodside et al., 1998; Sassen et al.,1998; Tre´hu et al., 1999; Aloisi et al., 2000; Dillon et al., 2001; Greinert et al., 2001; Kastner, 2001; Roberts,2001; Bohrmann et al., 2002). S. Conti et al. / Sedimentary Geology 169 (2004) 1–19 2  Fossil seep-carbonates are frequently reported(Niitsuma et al., 1989; Goedert and Squires, 1990;Beauchamp and Savard, 1992; Bitter et al., 1992;Campbell, 1992; Gaillard et al., 1992; Campbell andBottjer, 1993; Berti et al., 1994; Clari et al., 1994;Taviani, 1994; Terzi et al., 1994; Kelly et al., 1995;Kauffman et al., 1996; Conti and Fontana, 1999a;Orange et al., 1999; Peckmann et al., 1999a,b;2001a; 2002; Aiello et al., 2001; Campbell et al.,2002) and are important indicators of ancient hydro-carbon seepage.In middle Miocene foredeep basins of the northernApennines, several pelitic intervals include methane-derived carbonates, characterized by negative  d 13 Cvalues, by brecciation and fluid-flow conduits (non-systematic carbonate veins, extensive vuggy fabricsand irregular network of cavities, doughnuts and cir-cular or pipe-like chimneys), associated with slumps,intraformational breccias and olistostromes due tointense sediment instability (Conti and Fontana,2002b). In this paper, a comprehensive approach usingstratigraphy,mineralogyandgeochemistryisappliedtothe study of two pelitic intervals enclosing methane-derived carbonates; in particular we focus on compo-sitional variations throughout the chemoherms, theenclosing pelites and turbidites. These data elucidatethe fluid source and the growth mechanisms of meth-ane-derived carbonates on ephemeral intrabasinalhighs, and illustrate the possible connections betweenfluid venting, slumping of pelitic sediments, and fore-deep tectonics. 2. Northern Apennine geological setting and seep-carbonate bearing sediments The Apennine chain was formed by the collision of the European plate (Corsica–Sardinia block) and theAdriatic microplate after the closure of the Ligurian– Piedmont Ocean, a portion of the Thetys. Foredeep basins formed in front of the actively migrating thrust system (NE–E-vergent), and their sedimentary fill Fig. 1. Geologic sketch of the northern Apennines. The locations of main seep-carbonate outcrops are shown (stars); the studied sections areindicated as red stars and numbers: Castagno d’Andrea (1) and Monte Citerna (2). S. Conti et al. / Sedimentary Geology 169 (2004) 1–19  3  was progressively deformed and accreted to the fold-thrust belt (see Fig. 3 of  Conti and Fontana, 2002b). During the migration of the thrust belt–foredeepsystem, deposition occurred both in the foredeepand in small basins located on top of the migratingfrontal thrust (satellite basins). Satellite basins andtheir infillings (Epiligurian deposits) rest unconform-ably on top of the deformed Ligurian nappe and weretransported piggy-back towards the foreland. Duringthe advancement of the Ligurian nappe and its Epi-ligurian cover deposits, materials constituting olistos-tromes slid off the front of the nappe, and wereintercalated in slope–foredeep sequences (see Contiand Fontana, 1999a, 2002b for more details).Epiligurian satellite and foredeep basins containmany seep-carbonates (chemoherms), concentrated inmiddle–late Miocene pelitic successions (Fig. 1)(Ricci Lucchi and Vai, 1994; Conti and Fontana,1998, 1999b). In the foredeep, chemoherms occur inmarly and clayey hemipelagites and fine-grainedturbidites, located in two different depositional set-tings (Fig. 2): (1) slope-closure deposits, Langhian– early Messinian in age, passing laterally and cappinghuge arenaceous turbidites; and (2) slope depositsdraping ephemeral topographic highs (pelitic inter-vals), and intercalated within the Langhian–earlySerravallian basin plain turbiditic succession. 2.1. Pelitic intervals of the middle Miocene foredeepof the Northern Apennines Pelitic intervals are composite, wedge-shaped bod-ies, elongated along a NW–SE direction for a lengthof 10–30 km, gradually passing into the surroundingarenaceous turbidites (Fig. 2). The thickness average from 30–50 m to a maximum of 200 m. These Fig. 2. Stratigraphy of pelitic intervals and associated seep-carbonates. The following numbers and abbreviations refer to pelitic intervals:(1-CdA)=Castagno d’Andrea; (2-MC)=Monte Citerna; (3)=Suviana–l’Alpe (Conti and Fontana, 2002a); (4)=Acquadalto; (5)=Susanello– Molino Pontevecchio; (6)=Casaglia–Nasseto–Le Caselle; (7)=Peglio–Visignano–Colline–Mondera (Conti, 2001). S. Conti et al. / Sedimentary Geology 169 (2004) 1–19 4  intervals include intrabasinal and subordinately extra- basinal sediments. The former are foredeep basin plainand slope lithofacies, partly involved in spectacular megaslumps; the latter are olistostromes deriving fromthe migrating thrust front, mainly composed of vari-coloured shales, thin bedded dark grey mudstones andgrey limestones.Seep-carbonates irregularly occur from the base tothe top of the pelitic intervals, concentrated in twolithofacies: thin bedded siltstones and arenaceous silt- Fig. 3. (a) Castagno d’Andrea Type 1 chemoherm, the basal part of the lower body is severely brecciated (arrow); (b) Monte Citerna Type 2chemoherm, small scattered chemoherms are marked by white arrows, some of them are involved in soft sediment deformation. S. Conti et al. / Sedimentary Geology 169 (2004) 1–19  5
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