The Ladinian San Giorgio Dolomite and Meride Limestone of Monte San Giorgio (Switzerland, WHL UNESCO)

Home , Besano Formation, Dolomites, Monte San Giorgio

Sedimentology (2013) 60, 239–269 doi: 10.1111/sed.12021

Palaeoenvironmental signiﬁcance of organic facies and stable isotope signatures: the Ladinian San Giorgio Dolomite and Meride Limestone of Monte San Giorgio (Switzerland, WHL UNESCO)

RUDOLF STOCKAR*†, THIERRY ADATTE*, PETER O. BAUMGARTNER* and € KARL B. FOLLMI* *Institut des sciences de la Terre, Universite de Lausanne, Batiment^ Geopolis, 1015, Lausanne, Switzerland †Museo Cantonale di Storia Naturale, Viale Cattaneo 4, 6900, Lugano, Switzerland (E-mail: rudolf. [email protected])

Associate Editor – Helmut Weissert

ABSTRACT The over 600 m thick Ladinian carbonate section of Monte San Giorgio (World Heritage List, United Nations Educational, Scientific and Cultural Organization, Switzerland), including the San Giorgio Dolomite and the Meride Limestone, was analysed with respect to its sedimentology, organic- matter content (Rock-Eval and palynofacies) and stable carbon and oxygen- isotope composition. Application of geochemical proxies and optical data (transmitted light microscopy, epifluorescence, cathodoluminescence and scanning electron microscopy) allowed the assessment of the relative sea- level trend and the characterization of the organic-matter content. Three main organic-matter assemblages were defined according to their composition and stratigraphic position. Overall, results suggest immature organic matter, predominantly of marine bacterial origin with an upsection-increasing land plant-derived contribution. Forcing factors controlling organic-matter accumulation include changes in sea-level, productivity and runoff which, in turn, were probably promoted by periods of rainfall following explosive volcanic activity. Enhanced productivity during sea-level highstands is considered to have played a key role in black-shale formation under anoxic– sulphidic conditions (mainly in the Besano Formation). In contrast, sea-level lowstands, coupled with intensified runoff, resulted in increased basin restriction and in deposition of laminated limestone, mainly under lower dysoxic to anoxic conditions (chiefly in the Lower Meride Limestone). Under the latter conditions, benthic microbial activity produced most of the hydrogen- rich organic matter, contributed to carbonate precipitation and also played a major role in taphonomic control on vertebrate fossil preservation. In more general terms, the Monte San Giorgio section proved to be an excellent testing ground, making it possible to compare diverse approaches with each other and, more specifically, to relate optical evidence to geochemical signatures. Keywords Ladinian, microbial carbonates, Monte San Giorgio, organic matter, oxygen and carbon stable isotopes.

INTRODUCTION was situated at a northern intertropical latitude of about 15° to 18° (Muttoni et al., 2004) and The Middle Triassic carbonate succession of was strongly influenced by monsoonal circula- Monte San Giorgio (Southern Alps, Switzerland– tion (Preto et al., 2010). This passive continen- Italy; Fig. 1A and B) was inscribed in the tal margin open to the western Neo-Tethys was UNESCO World Heritage List (WHL) because of progressively submerged by a long-term trans- its unique palaeontological value. It is, in gression from the east. The marine ingression particular, world-famous for the exceptionally reached the eastern South-Alpine domain in the well-preserved fossil fishes and marine reptiles Late Permian and the westernmost (i.e. west of (Rieber, 1973a; Kuhn-Schnyder, 1974; Burgin€ Lake Como) South-Alpine domain in late et al., 1989; Sander, 1989; Furrer, 1995; Etter, Anisian times. The increasing differentiation of 2002; Stockar, 2010; Stockar & Renesto, 2011). depositional environments occurring from then The formations bearing the vertebrate fossils onward resulted in a rapidly changing pattern (Besano Formation and Meride Limestone) are of carbonate platforms, locally oxygen-depleted also regarded as the immature equivalents of the intraplatform and open-marine pelagic basins source rocks of one of the most important hydro- (Brack & Rieber, 1993). Coeval volcanic activity carbon petroleum systems of southern Europe produced high volumes of material stored in the (Bernasconi, 1994; Fantoni & Scotti, 2003). This basins as volcanic tuffs (‘Pietra verde’ type includes the Villafortuna-Trecate oil field in the layers). This marginal location of the Monte San western Po Plain, 60 km south of the Monte San Giorgio basin resulted in a peculiar sedimentary Giorgio at a depth of around 5000 m (Riva et al., succession and in at least temporarily dysoxic 1986; Mattavelli & Novelli, 1990; Picotti et al., to anoxic bottom water conditions (Bernasconi, 2007). In order to provide a more comprehensive 1994; Furrer, 1995; Rohl€ et al., 2001; Etter, interpretation of the environmental conditions 2002; Stockar, 2010). The Middle Triassic and depositional processes that prevailed in this succession at Monte San Giorgio (Fig. 1B) starts basin during Ladinian times, the over 600 m with fluvio-deltaic deposits (Bellano Formation, thick Ladinian carbonate section of Monte San Illyrian; Sommaruga et al., 1997) unconformably Giorgio was logged and analysed with respect to overlying a Lower Permian volcanic basement. its sedimentology, organic-matter content and The upper Anisian sediments testify to the stable-isotope composition. The main objectives progressive transgression and to the initiation of of this study are to: carbonate platform growth (Lower Salvatore 1 test widely used different analytical meth- Dolomite/Esino Limestone; Zorn, 1971). While ods by comparing results from both the optical in the north and in the east shallow-water sedi- (petrographic and palynofacies) approach [trans- mentation continued during the latest Anisian mitted light, epifluorescence, cathodolumines- and Ladinian, in the Monte San Giorgio area cence (CL), scanning electron microscopy the formation of an intraplatform basin with (SEM)] and geochemical signatures (Rock-Eval restricted circulation resulted in the deposition parameters, d13Candd18O values); of the Besano Formation, the San Giorgio Dolo- 2 determine the composition of preserved mite and the Meride Limestone. The Besano organic matter (OM) throughout the section and Formation (‘Grenzbitumenzone’; Frauenfelder, to highlight the processes involved in its 1916) directly overlies the Lower Salvatore production and preservation; Dolomite and is composed of a 16 m thick alter- 3 characterize palaeoenvironmental changes nation of black shales and laminated dolostone. and to identify the main trends underlying the Its uppermost part includes the Anisian/Ladi- depositional history of the basin; and nian boundary (equivalent to the base of the E. 4 highlight the most significant implications curionii Ammonoid Zone; Brack & Rieber, 1993; for the taphonomic history of the vertebrate Brack et al., 2005). Most of the spectacular ver- fossils. tebrate fossils (reptiles and fishes), together with important index fossils including ammonoids and daonellid bivalves, come from this forma- GEOLOGICAL SETTING tion (e.g. Rieber, 1969, 1973b; Kuhn-Schnyder, 1974; Burgin€ et al., 1989). The Besano Forma- The Monte San Giorgio belongs to the western tion grades upwards into the San Giorgio termination of the Southern Alps (Fig. 1C). In Dolomite and the Meride Limestone, together Middle Triassic times, the South-Alpine domain constituting a 614 m thick sequence in total.

A Carnian Pizzella Marls B E

' ?

7 Lake Lugano

5 °

8 Kalkschieferzone Lake Lugano Monte San Giorgio 'Site D'

Serpiano Upper

2 i m e s t o n M e r i d L 1 45°54' N Ladinian 'Dolomitband' Porto 3 Cassina beds Cava superiore beds Ceresio 4 Val Cava inferiore beds Lower Serrata 5 tuff Besano 6 San Giorgio Dolomite Meride r i a s c T M i d l e Besano Fm. Lower Salvatore Dolomite Switzerland Italy Bellano Fm. Anisian

1 km Rhyolites and associated volcaniclastics

Meride Limestone Permian Reptiles Fishes San Giorgio Dolomite Switzerland Ticinosuchus Chondrichthyes 100 m Nothosauria Crossopterygii Besano Formation Protorosauria Actinopterygii Lower 0 m Placodontia (other than Saurichthys) Salvatore Dolomite Italy Ichthyosauria Saurichthys

C Periadriatic lineament

50 km Seceda Dolomites

Insubric lineament

Perledo- Varenna

Bagolino Giudicarie lineament Tretto Monte 'Buchenstein' San pelagic sediments Giorgio Recoaro Platform carbonates Po Plain Intra-platform Milano organic-rich sediments

Fig. 1. (A) Simplified location map of the Monte San Giorgio showing the Middle Triassic carbonate succession. Numbered circles indicate the location of the partial sections: (1) Val Porina; (2) Valle della Cassina; (3) Val Sceltrich; (4) Val Serrata; (5) Fontana Fredda; (6) Val Mara. (B) Stratigraphic section of the Middle Triassic sediments in the Monte San Giorgio area with the classic fossil-vertebrate levels (modified and updated from Furrer, 1995). (C) Distribution of the lower Ladinian sediments in the Southern Alps (E. curionii Ammonoid Zone). From Brack & Rieber (1993), simplified.

Fig. 2. Measured composite stratigraphic log of the Ladinian succession at Monte San Giorgio, with position of partial sections and sampled beds. The 0 m level corresponds to Bed 85 of the Besano Forma- tion (N. secedensis Ammonoid Zone) in the standard profile at locality Mirigioli (Site P. 902; Rieber, Ammonoid zone Metres Formation Lithology U-Pb age (Ma) Stable isotopes Palynofacies Geochemestry Stage Substage 1973b). Biostratigraphic subdivisions after Stockar Partial sections et al. (2012a). U–Pb single-zircon ages after Stockar et al. (2012a) for the Ladinian Stage and Mundil et al. (2010) for the Anisian Stage: ‘Be’: Besano Formation; ‘SGD’: San Giorgio Dolomite; ‘LMe’: Lower Meride Limestone; ‘DB’: Dolomitband; ‘LKSZ’, ‘MKSZ’ and ‘UKSZ’: Lower, Middle and Upper Kalkschieferzone. + 0·15 239·51 – 0·15 Sections in Val Porina and Val Serrata show that the San Giorgio Dolomite results from early and late diagenetic dolomitization, the latter cutting Mara Fontana Fredda - Val across stratification and affecting the original limestone in an irregular pattern up to a major volcaniclastic bed (‘Val Serrata tuff’). The Lower Meride Limestone consists of well-bedded micritic limestone, laminated limestone and volcaniclastic layers. Three intervals, mainly consisting of finely laminated limestone, yielded different vertebrate fossil assemblages (Peyer, 1931; Sander, 1989; Furrer, 1995; Stockar, 2010; Stockar & Renesto, 2011). These classic fossil-vertebrate Serrata Val intervals are informally known as ‘Cava inferiore beds’ (ca 1Á5 m thick), ‘Cava superiore beds’ (ca 10 m thick) and ‘Cassina beds’ (ca 3 m thick). The top of the Lower Meride Limestone is defined by a very discontinuous dolostone horizon (‘Dol- omitband’; Frauenfelder, 1916) resulting from late and early diagenetic dolomitization of the Meride + 0·13 Sceltrich Val Limestone. The overlying Upper Meride Lime- 240·63 – 0·13 stone is a sequence of alternating well-bedded mi- + 0·13 critic limestone and marlstone. The uppermost 241·07 part comprises the 120 m thick ‘Kalkschieferz- – 0·13

one’ (Senn, 1924), made up of thinly-bedded, della Cassina Valle mostly laminated limestone and marlstone with peculiar faunas of ﬁshes, crustaceans and insects (Burgin,€ 1995; Furrer, 1995; Krzeminski & Lom- bardo, 2001; Bechly & Stockar, 2011). Consistent with Wirz (1945) and Furrer (1995), the Kalks-

chieferzone is subdivided into three subunits Porina Val (Lower, Middle and Upper Kalkschieferzone). The top of the Lower Kalkschieferzone contains the laminated interval cropping out at ‘Site D’ + 0·6 (Val Mara) which yielded the richest and 242·1 best-preserved fossil ﬁsh fauna of the entire – 0·6 Cherty Micritic Kalkschieferzone (Lombardo, 2002; Lombardo & dolostone limestone Marlstone Micritic Thin-bedded Marlstone and Tintori, 2004; Tintori & Lombardo, 2007). micritic limestone – dolostone micritic limestone The east west extension of the Monte San Gior- Laminated dolostone Laminated Volcanic ash layers and black shale limestone gio basin is estimated to have been about 10 km, Laminated Micritic limestone Not exposed or up to 20 km if it was part of the basin in which dolostone and marlstone the Perledo–Varenna Formation cropping out to

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 Ladinian organic-rich carbonate rocks of Monte San Giorgio 243 the east of Lake Como was deposited (Fig. 1C; and 717′900/83′700) and Fontana Fredda–Val Gianotti & Tannoia, 1988; Bernasconi, 1994). Mara (717′050/83′350). Both the widespread Maximum basin depths are regarded as varying occurrence of volcanic ash beds, as well as the between 130 m (Monte San Giorgio basin; Zorn, lateral persistence of bedding patterns allow for 1971; Rieber, 1973a; Bernasconi, 1994) and an unambiguous correlation of the studied sec- 260 m (Perledo–Varenna Formation; Gaetani tions. The resulting composite section is illus- et al., 1992). Further to the east, the depositional trated in Fig. 2. Besides the world-famous fossil- area of the Perledo–Varenna Formation was, in vertebrate intervals, the position of a new hori- turn, separated from the pelagic deposits of the zon is reported here. This laminated interval, ‘Buchenstein’ facies by shallow-water barriers around 30 cm thick, is here informally intro- (Esino Platform; Fig. 1C), which were only a few duced as ‘Sceltrich beds’; it is very rich in verte- metres deep (Gaetani et al., 1992). brate fossils and will be described in a For this study, stratigraphic successions have forthcoming paper. been studied at the centimetre-scale at the following localities (Fig. 1C; approximated Swiss National Coordinates refer to the centre of the LITHOFACIES TYPES sections): Val Porina (716′600/85′100), Valle della Cassina (717′150/84′950), Val Sceltrich The most representative lithofacies, traceable (717′000/84′400), Val Serrata (717′850/84′150 throughout the section, were sampled for the

A B

10 mm TOC: 3·1 HI: 898 10 mm TOC: 1·1 HI: 509

C D F TOC: 0·1 10 mm HI: 108

TOC: 0·2 HI: 291 10 mm

10 mm TOC: 0·4 HI: 302 10 mm Fig. 3. Lithofacies types, Monte San Giorgio section. (A) Laminated limestone with storm-generated concentrations of platform-derived skeletal grains (Upper Meride Limestone, Sceltrich beds, sample VSC3, 253Á4 m). (B) Laminated limestone. The arrow indicates a ﬁsh scale (Lower Meride Limestone, Cassina beds, sample Ca18e, 213Á2 m). (C) Laminated limestone, cracked microbial mat (Middle Kalkschieferzone, sample VM47, 559Á7 m). (D) Laminated limestone (Upper Meride Limestone, sample VM11, 585Á0 m). (E) Laminated limestone with enterolithic laminae (for example, arrow: Middle Kalkschieferzone, sample VM4, 573Á3 m). (F) Bioturbated laminated limestone, arrows indicate two examples of burrows (Lower Kalkschieferzone, sample VM34, 502Á5 m). (A), (B), (D), (E) and (F): polished section. (C): outcrop upper bedding-plane surface. TOC (Total Organic Carbon) in weight %, HI (Hydrogen Index) in mg HC/g TOC.

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 244 R. Stockar et al. present study. They include the following intercalations at most. The exception is a sections. centimetre-thick bed occurring in the San Giorgio Dolomite (VP6, 39Á9 m in the section). Less common lithofacies include the following Micritic carbonate rock ones: Well-bedded, fine-grained, dark limestone and dolostone, with sharp bases and tops, and a high Thin-shelled bivalve limestone lateral depositional continuity occur throughout the succession. Zoophycos is a trace fossil occa- Rock-forming monospecific mass occurrences of sionally occurring in the lower part of the Upper thin-shelled bivalves belonging to the Posidonia- Meride Limestone. Peribositria group occur in the lower part of the Upper Meride Limestone and in the Dolomit- band. Valves are usually articulated and pre- Laminated carbonate rock served in open (‘butterfly’) position. Laminated limestone, more or less organic-rich, mainly occurs in the Lower Meride Limestone Bentonite (Cava inferiore beds, Cava superiore beds, Cas- sina beds), in the lowermost part of the Upper Intercalations of volcanic ash layers occur Meride Limestone (Sceltrich beds) and in the throughout the section. Distinctly high concen- Kalkschieferzone (Fig. 3). Laminated dolostone trations, however, cluster in the Besano Forma- is frequent in the Dolomitband. Only in some tion and in the 170 to 220 m interval; in beds of the Kalkschieferzone the occurrence of addition, a significant volcaniclastic event is dehydration cracks (Fig. 3C), enterolithic folds recorded at the top of the Lower Kalkschiefer- (Fig. 3E) or biological reworking (Fig. 3F) affects zone. The ash layers are airborne tuffs altered to the original lamination pattern. In the 170 to bentonite. Somewhat different is the so-called 253 m interval and around the 426 m level, ‘Val Serrata tuff’ (Lower Meride Limestone), a coarse carbonate lenses up to 8 cm in size, con- tuffite with a calcite cement representing one of taining densely-packed skeletal grains of shal- the most reliable marker beds of the sequence. It low-water benthic biota are common (Fig. 3A); consists of a lower and an upper bed, up to 4 m fossil remains here include dasycladalean algae, and 3 m thick, respectively, separated by the sponges, echinoderms, bivalves, foraminifera ‘Cava inferiore’ fossiliferous interval. and ostracods (Stockar, 2010, fig. 9). While lacking a benthic macrofauna, this lithofacies may bear an in situ monotypic meiofauna ANALYTICAL METHODS (Stockar, 2010). Microfacies characterization of the laminated Marlstone and micritic carbonate rocks Marlstone mainly occurs in the upper part of Rock microstructures and organic components the Lower Meride Limestone and in the Upper were studied by optical petrographic microscopy Meride Limestone (including the Kalkschiefer- (177 thin sections), blue ultra violet (UV) epifluo- zone). It is organized in beds up to 1 m thick rescence, CL and SEM. Blue-UV epifluorescence (exceptionally 3 m), usually structureless. Lami- and CL observations were performed on thin sec- nated or bioturbated marlstone (Chondrites and tions polished with 0Á25 μm diamond-impreg- Planolites trace fossils), however, occurs in the nated pads. Fluorescence was induced with a Kalkschieferzone. Hg-high-pressure vapour lamp attached to a Leitz Laborlux 11 microscope (Ernst Leitz Wetzlar GmbH, Wetzlar, Germany) equipped with a wide Black shale bandpass filter (BP 450 to 490 nm/LP 520 nm; Organic-rich claystone characterized by high blue-light UV epifluorescence). All thin sections total organic carbon (TOC) values are the were checked for fluorescence to reveal the hallmark of the Besano Formation, where the distribution of the OM (Dravis & Yurewicz, 1985; thickness of single layers ranges from a few Neuweiler & Reitner, 1995). Cathodolumine- millimetres to 15 cm. Up-section, black shales scence observations were performed with a are very rare and occur as millimetre-thick Technosyn Model 8200 Mk II Cold

A B C

20 μm 20 μm 20 μm

D E F

20 μm 20 μm 20 μm

G H I

20 μm 20 μm 20 μm

Fig. 4. Particulate organic matter (POM) categories, Monte San Giorgio (transmitted light micrographs). (A) Amor- phous organic matter (AOM) group: AOM with pyrite inclusions. (B) Phytoclast group: lath-shaped translucent phytoclast (gymnosperm tracheid). (C) Phytoclast group: equidimensional translucent phytoclast derived from tracheid tissue. (D) Palynomorph group: bisaccate pollen grain (Triadispora stabilis). (E) Palynomorph group: spore (Echinitosporites iliacoides). (F) Palynomorph group: bisaccate pollen tetrad. (G) Palynomorph group: acritarch. (H) Palynomorph group: algal cyst (Dictyotidium tenuiornatum). (I) Palynomorph group: trochospiral foraminiferal test lining.

Cathodoluminescence Unit (Technosyn Ltd, Cranbury, NJ, USA) was used as mounting med- Cambridge, England). Micromorphologies and ium. Observations were carried out under trans- chemical compositions were analysed by SEM mitted light microscopy and blue-UV (Tescan Mira LMU equipped with an energy-dis- epiﬂuorescence microscopy. The latter also persive X-ray spectrometer; Tescan USA Inc., proved to be an excellent means of detecting the Cranberry Twp, PA, USA) on both slightly etched presence of palynomorphs shrouded by amor- (0Á05% HCl, 1 min) polished surfaces and unet- phous organic matter (AOM). To determine the ched fresh rock subsamples. afﬁnity of the AOM, the cleaning up process of the palynological residue was followed (Batten, 1981, 1996a; Batten & Morrison, 1983). Palynofa- Optical characterization of the organic matter cies data were obtained by counting at least 300 Seventy-one samples were selected for palynofa- organic particles per sample. The particulate cies analysis and treated according to standard organic matter (POM or ‘kerogen’) was subdi- palynological techniques (Traverse, 2007). The vided into three major groups (Tyson, 1995; unoxidized residue was sieved with a 10 μm Fig. 4). The AOM group consists of aggregates mesh screen and strew mounted on palynologi- that appear structureless at the scale of light cal slides; NOA 61 (Norland Products Inc., microscopy (Fig. 4A); it is usually regarded as

A B

100 µm

C 10 µm

5 µm 5 µm

E F

10 µm 20 µm

Fig. 5. Laminated limestone (Lower Meride Limestone, Cassina beds, sample Ca18e, 213Á2 m). (A) Wavy lamination. Dark laminae drape light laminae and lenses made up of detrital grains showing graded bedding (thin section, plane polarized light). (B) Boundary between a dark lamina (lower) and a light lamina (upper) encasing unetched dolomite crystals (acid-etched surface, SEM micrograph). (C) Alveolar network typical of EPS and lamina (arrow) of densely-packed clay minerals (acid-etched surface, SEM micrograph). (D) Pyrite framboids rooted in a thin EPS layer (arrow) and covered by clay minerals (acid-etched surface, SEM micrograph). (E) Micro- porous calcite grains (arrows) encased within polygonally-framed EPS (acid-etched surface, SEM micrograph). (F) Micrite grains organized into a clotted fabric within an entanglement of organic ﬁlaments interpreted as relics of EPS matrix (acid-etched surface, SEM micrograph). derived from algae or bacteria (Lewan, 1986; C) consists of structured particles derived from Tyson, 1995). The phytoclast group (Fig. 4B and terrestrial macrophytes; they are subdivided into

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 Ladinian organic-rich carbonate rocks of Monte San Giorgio 247 opaque and translucent phytoclasts and, based RESULTS on the morphology, into equidimensional and blade-shaped particles. The palynomorph group Microfacies characterization of carbonate (Fig. 4D to I) includes the sporomorph subgroup rocks (land-derived, further subdivided in spores and pollen grains), the zoomorph subgroup (inclu- Laminated limestone and dolostone show two ding only foraminiferal test linings and thus main types of lamination. The first type (‘wavy marine-derived) and the phytoplankton sub- lamination’; Figs 3A, 3B, 5A and 6A) is com- group (marine-derived, including algal cysts and posed of thin dark wavy organic-rich laminae, acritarchs). Finally, 10 organic-rich laminated which coat single grains or drape light laminae samples, representative of the AOM encountered and lenses made up of silt-sized to sand-sized across the section, were selected for a high- grains. The coarse and light laminae (Fig. 5B) resolution SEM study. consist of carbonate grains which are usually strongly recrystallized and/or dolomitized. Graded bedding is usually recognizable. Frag- Organic geochemistry mented, reworked bioclasts, mostly including Fifty-five samples were selected for the geo- foraminifera and ostracods, are frequent. The chemical characterization of the organic matter. dark laminae (up to around 100 μm) are mainly The total organic carbon (TOC) content, the total composed of an organic alveolar network inorganic carbon (MINC) content, as well as the (Figs 5C and 6C). Calcite grains are sparsely source and thermal maturation of the organic embedded within the network; they show mi- matter were determined using a Rock-Eval VI cropores (Fig. 5E) and are organized into a clot- instrument (Vinci Technologies, Rueil-Malmai- ted fabric (Fig. 5F). Skeletal grains (foraminifera, son, France). The IFP 160000 standard was used ostracods and thin-shelled bivalves) may also be to calibrate the measurements. Rock-Eval pyroly- present within the organic matrix (Fig. 6A, B sis (Espitalie et al., 1977, 1985) involves the and D). The same organic network may also measurement of parameters such as free hydro- expand into the granular light laminae (Fig. 5B). carbons (S1), residual petroleum potential (S2), Occasionally, high-magnification SEM observa- generate CO2 (S3) and the temperature of maxi- tions reveal the occurrence of spheroidal bodies, mum hydrocarbon evolution from kerogen around 200 nm in size (Fig. 6E). Thinner (20 to μ (Tmax). In turn, diagnostic indices such as the 40 m) denser seams (Figs 5C to E) are always hydrogen index (HI) and the oxygen index (OI) associated with the alveolar-structured dark lam- can be calculated from these parameters. The inae; they mainly consist of densely packed clay Rock-Eval organic matter characterization allows minerals. Finely disseminated pyrite grains, con- four types of kerogen to be identified by means sisting of framboids up to 8 μm in size (Figs 5D of cross-plots (Type I to IV; Tissot et al., 1974; and 6C), are common throughout the dark lami- Tissot, 1984). nae. The second type of lamination (‘subplanar lamination’; Fig. 3D) is mainly characteristic of Stable isotopes the Kalkschieferzone and consists of thin, planar One hundred and seventy-seven samples to subplanar, smooth isopachous dark laminae selected for carbon and oxygen isotope analyses and thicker light laminae (Fig. 6F). The optically were drilled (Proxxon microdriller, 0Á4mm lighter laminae, usually 300 to 500 μm thick, tungsten drillbit; Proxxon GmbH, Foehren, show a clotted fabric of peloidal grains ranging Germany) to produce a fine powder. Cracks, from 10 to 50 μm in diameter (Fig. 6G and H). veins and weathered surfaces have been Under transmitted light (Fig. 6G), peloids show avoided. Measurements were performed using a an internal microporous micritic core followed Thermo Fisher Scientific GasBench II prepara- by an exterior rim of clear sparry calcite. The CL tion device interfaced to a Thermo Fisher Scien- analysis (Fig. 6H) reveals two phases of cemen- tific Delta Plus XL continuous flow isotope ratio tation, consisting of an earliest bright lumine- mass spectrometer (IRMS; Thermo Fisher Scien- scent overgrowth on peloids followed by a later tific Inc., Waltham, MA, USA). Results are generation of dull luminescent calcite. Under reported in the delta (d) notation as the per mil blue-UV light (Fig. 6I), the internal core shows a (&) deviation relative to the Vienna PeeDee bright fluorescence, indicating the presence of Belemnite standard (VPDB). well-preserved residual organic matter. After

A B

100 µm 100 µm

C D E

5 µm 20 µm 1 µm

F G H

200 µm 50 µm 50 µm

I L

20 µm 10 µm

Fig. 6. (A) to (E) Laminated limestone (Lower Meride Limestone, Cava inferiore beds, sample RS08-2b, 170Á0 m). (A) Wavy lamination. Dark laminae drape light-coloured detrital lenses and coat skeletal grains (nodosariid foraminifera, longitudinal section, arrow) (thin section, plane polarized light). (B) Same field of view as in panel (A), cathodoluminescence microscopy. The uniserial structure of the foraminiferal tests (arrow) becomes evident (thin section, CL micrograph). (C) Alveolar network typical of EPS including a pyrite framboid (arrow, bottom left-hand corner) (acid-etched surface, SEM micrograph). (D) Foraminiferal test, coated by EPS matrix (acid-etched surface, SEM micrograph). (E) Nanospheres (arrow) in the EPS matrix (acid-etched surface, SEM micrograph). (F) to (L) Laminated limestone (Middle Kalkschieferzone, sample VM47, 559Á7 m). (F) Subplanar lamination, defined by very thin isopachous dark laminae and thicker lighter laminae with clotted peloidal fabric (thin section, plane polarized light). (G) Clotted peloids showing a cloudy micritic centre and an exterior rim of clear spar cement (thin section, plane polarized light). (H) Same field of view as in (G), cathodoluminescence microscopy (thin section, CL micrograph). (I) The internal core of peloids shows a bright fluorescence, indicating the presence of well- preserved residual organic matter (blue-UV epifluorescence micrograph). (L) Filamentous to rod-like organic bodies, protruding outwards from the microporous cores of peloids (arrows); they are interpreted as relics of bacterial clumps. No micropores (and no bacterial fossils) occur in the exterior rim (acid-etched surface, SEM micrograph).

etching, under the SEM filamentous to rod-like feature of the organic-matter assemblages (Fig. 9) organic bodies occur that protrude outwards is the very high content of AOM, except for the from the pores of the internal cores (Fig. 6L). uppermost part (Upper Kalkschieferzone). The Replacement dolomitization occurring in the AOM is usually strongly fluorescent (Fig. 10F San Giorgio Dolomite and Dolomitband may and H), rather granular and inclusion-rich. Dis- roughly preserve the laminated structure persed pyrite grains are common (Fig. 4A), both (Figs 7A and B). Dark laminae include loose as euhedral grains and small framboids. After a aggregates of euhedral to subhedral ferroan dolo- short oxidation, hardly any amorphous matter mite; in the light laminae, the latter forms a was disaggregated. Scanning electron micro- mosaic of interlocking anhedral to subhedral scopy investigations reveal an alveolar network crystals. Unlike laminated carbonate rocks, black (Fig. 8B). The distribution of both the pal- shales exhibit a discontinuous fabric (Fig. 7C) ynomorph and phytoclast groups is chiefly con- consisting of parallel-oriented clay minerals and trolled by AOM dilution. The palynomorph organic-matter enrichments. group is largely dominated by sporomorphs, The fabric of well-bedded micritic limestone mainly represented by bisaccate pollen grains, and dolostone (Fig. 7D) is characterized by inter- whereas spores average only 6%. Foraminiferal locking calcite and dolomite grains, showing linings are the most abundant marine palyno- large variability in crystal size. In places, grains morphs; acritarchs and algal cysts are rare. As a show traces of internal microporosity (Fig. 7E) whole, marine palynomorphs are usually well and traces of graded bedding are preserved. below 1% of POM. Across this 600 m thick Reworked shallow-water skeletal grains are com- AOM-dominated section, the stratigraphic mon. Scattered thin flake-like black chips, up to pattern of the POM assemblage displays some a few millimetres in size, occur frequently prominent variations (Figs 9 and 10): (Fig. 7D); they show an alveolar structure under 1 Palynofacies of the lower part of the the SEM (Fig. 8A), despite the fact that the sur- section (up to 170 m, Besano Formation, San face was strongly deformed and moulded by the Giorgio Dolomite and lower part of the Lower recrystallization of the surrounding sediment. Meride Limestone) are characterized by high AOM percentages, while sporomorphs and phytoclasts average 12% and 5%, respectively. In Optical characterization of the organic matter the latter fraction, small equant-shaped particles Palynomorphs show a yellow-orange colour are common. (Fig. 4D to F), suggesting a value of 2 of the 2 In the 170 to 253 m interval (upper part of thermal alteration scale (TAS) of Batten (1996b). the Lower Meride Limestone and lowermost part This provides evidence for the lack of any sig- of the Upper Meride Limestone), the total AOM nificant thermal alteration and for an immature occasionally falls below the 50% of POM and it state of the organic matter, which is further is sometimes composed of non-fluorescent supported by the prevailing bright fluorescence aggregates. Sporomorphs may contribute up to of the organic particles. The most striking 52% to the POM while phytoclasts, which are

sometimes relatively abundant (up to 26%), are A B dominated by opaque particles. Abundance of tetrads, consisting of clusters of pollen grains and spores, reach a peak value here. A further interval, characterized by an increase of terrestrial organic matter fraction, occurs at around 300 to 330 m in the section; here the palynofacies shift signal is, however, much less marked and it is not paralleled by any increase in pollen tetrads. 3 In the upper half of the Upper Meride Limestone (from 403 m upwards), the percentage of the non-fluorescent AOM begins to increase (Fig. 10C and D) and rises steadily upwards to the top of the Middle Kalkschieferzone. The increase 100 µm 100 µm of non-fluorescent AOM (from 2 to 87%) is paralleled by a progressive decrease in fluorescent C AOM (from 83 to 4%). Two AOM peaks occur which are particularly significant. The peak value of non-fluorescent AOM is recorded in a laminated limestone with enterolithic structures at the top of the Middle Kalkschieferzone (VM4, 573Á3 m; Fig. 3E). Within this major interval (403 to 573 m), a second interval with high abundances of AOM occurs. This interval, marked by fluorescent AOM, corresponds to the fossil-vertebrate ‘Site D’ level (sample VMD10, 531Á5 m). 500 µm 4 Thereafter, sporomorph contents abruptly increase and in the Upper Kalkschieferzone D E land-derived organic particles strongly dominate the POM (Fig. 10A and B). Sporomorphs, frequently grouped into tetrads, here reach percentages of up to 65% of POM. Phytoclasts are mainly composed of opaque particles and contribute up to 44% to the POM; their size and shape show a high variability with a significant amount of large, blade-shaped fragments. The relative abundances of some taxa provide fur- 200 µm 10 µm ther information. Circumpolles pollen grains, forms with coniferal affinity mainly represented Fig. 7. (A) and (B) Laminated dolostone (Dolomit- here by the genera Camerosporites and Dupli- Á band, sample VSC6, 242 6 m). (A) Replacement dolo- cisporites (Scheuring, 1978; Stockar et al., mitization by ferroan dolomite roughly preserves the laminated structure (thin section, plane polarized 2012a), continuously increase in percentage light). (B) Same field of view as in panel (A), but under from 403 m upwards. The Ovalipollis group cathodoluminescence microscopy (thin section, CL shows a first-order gradual increase throughout micrograph). (C) Black shale (Besano Formation, sam- the section. Bisaccate pollen grains (other than ple VP113, 2Á4 m). Discontinuous microfabric of alter- Ovalipollis) include, besides the dominant nating dark seams, enriched in organic matter and clay Triadispora group, chiefly Staurosaccites spp., minerals, and light detrital lenses. (D) and (E) Micritic Striatoabieites spp., Lunatisporites spp., Inferno- limestone (Lower Meride Limestone, Cassina beds, sample Ca55, 212Á4 m). (D) Lime mudstone including pollenites spp. and Parillinites spp. Lower scattered thin black flake-like chips, interpreted as rip- percentages are recorded at the onset of the lam- up clasts derived from microbial mats (thin section, inated interval of the Lower Meride Limestone plane polarized light). (E) Despite the heavy recrystal- (170 m), followed by a moderate increase and a lization, some calcite grains show internal porosity subsequent general decreasing trend in the (fresh unetched surface, SEM micrograph). upper part of the section. The spore/bisaccate

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 Ladinian organic-rich carbonate rocks of Monte San Giorgio 251 ratio usually remains very low throughout the section and only moderately increases in the A laminated interval of the Lower Meride Lime- stone and in the upper part of the section. The OP/TR ratio (opaque/translucent phytoclasts) shows fluctuations including two major peaks, the first centred in the upper part of the Lower Meride Limestone (around 200 m), the second, less conspicuous, in the Kalkschieferzone. In several intervals, the reliability of the OP/TR ratio is, however, questionable due to the statistically poorly significant phytoclast content.

The distribution of the three main POM 200 µm groups is somewhat lithology-dependent (Fig. 9; Table 1). The palynomorph group and the phyto- B clast group are relatively more abundant in marlstone (mean around 34% and 16%, respectively) compared with micritic carbonate rocks (mean around 21% and 7%) and laminated carbonate rocks (mean around 13% and 6%). By contrast, the AOM group displays the highest abundances in the black shales (where it is vir- tually the only component) and in the laminated carbonate rocks (usually over 80%).

Organic geochemistry 20 µm Total organic carbon (TOC) values of the laminated carbonate rocks range from 0Á08 to 4Á26% Á = Fig. 8. Amorphous organic matter from palynological (mean value: 1 09%, n 27; Fig. 11). However, residues (SEM micrographs). (A) AOM particle from a stratigraphy-related differences are evident, with non-laminated limestone. Despite the moulded samples from the upper half of the Upper Meride surface, due to the recrystallization of the surrounding Limestone (also including the Kalkschieferzone) sediment, a vacuolar EPS-like arrangement is still vis- being poorer in organic matter by a factor of six ible (Lower Meride Limestone, Cassina beds, sample Á (mean value: 0Á24%, n = 9) compared with the Ca55, 212 4 m). (B) AOM particle from a laminated lower part of the section (mean value 1Á44%, limestone, displaying a vacuolar structure suggestive = of EPS arrangement (Lower Meride Limestone, Cava n 20). Micritic carbonate rocks are usually inferiore beds, sample RS08–2b, 170Á0 m). organic-poor and average 0Á24% TOC (range 0Á04 to 0Á87%). Marlstone averages 0Á42% TOC (range 0Á09 to 0Á93%); the maximum value is reached in plotting them, they have been subdivided into the ﬂuorescent-AOM peak at 531Á5 m in the sec- two subsets (0Á1% < TOC <0Á3% and TOC tion (‘Site D’). The highest TOC values are shown 0Á3%). Results from the organic-poorer subset by the two black-shale samples, one from the Be- (0Á1% < TOC <0Á3%) should, however, be con- sano Formation (2Á4m,26Á32% TOC) and the sidered with caution. The HI values show a other from the lowermost part of the San Giorgio stratigraphic trend (Fig. 11) with higher values Dolomite (39Á9m,29Á7% TOC). in the interval between the upper part of the Owing to the ‘mineral matrix effect’ (Tyson, Lower Meride Limestone and the lower part of 1995), in organic-poor samples (TOC values the Upper Meride Limestone. However, diffe- <0Á1to0Á3%; Espitalie et al., 1985), the resulting rences are not as marked as for the TOC content, HI and Tmax values may be too low and too given that the mean HI value for the laminated high, respectively. Accordingly, Rock-Eval data carbonate rocks from the Kalkschieferzone still from samples with TOC 0Á1% have not been exceeds 300 mg HC/g TOC (n = 6). The HI considered herein. By contrast, values from sam- values of the laminated carbonate rocks range ples with TOC >0Á1% are all discussed but, by from 108 (Kalkschieferzone) to 934 mg HC/g

239–269 , 60 , Sedimentology Sedimentologists, of Association International 2012 compilation Journal Authors. The 2012 © © bnacso olngan r xrse spretgso h oa prmrhasmlg.Nme fple erd ee otettlcut( count total the to refer tetrads pollen of Number assemblage. sporomorph total the of percentages as expressed are grains pollen of abundances i.9. Fig.

uniaiedsrbto atr fpriuaeogncmte costeMneSnGogoscinadifre eaiesalvlted Relat trend. sea-level relative inferred and section Giorgio San Monte the across matter organic particulate of pattern distribution Quantitative Metres Formation Non-fluorescent Fluorescent AOM Lithology A l l s Translucent Phytoclasts Opaque a m p l e s carbonates Laminated PALYNOFACIES carbonates Foraminiferal linings Sporomorphs Marine phytoplancton Micritic M a r l s t o n e s tetrads Pollen

Marine palynomorphs CLASTS PHYTO- OP/TR

Not reliable Circumpolles Ovalipollis SPOROMORPHS Bisaccates Bisaccates Spores/ Rising the reach Relative sea-level Within Fluctuating hypersaline waves storm conditions of trend n Falling = 300).

ive

.Sokre al. et Stockar R. 252 Ladinian organic-rich carbonate rocks of Monte San Giorgio 253

All samples Transmitted white-light Incident blue-UV-light fluorescence Metres Formation Lithology microphotographs microphotographs

A B

100 µm 100 µm

C D

50 µm 50 µm

E F

50 µm 50 µm

G H

50 µm 50 µm

Fig. 10. Selected palynofacies under transmitted white-light microscopy (A), (C), (E) and (G) and incident blue- UV-light ﬂuorescence microscopy (B), (D), (F) and (H). (A) and (B) Upper Kalkschieferzone, sample VM17, 607Á3 m. (C) and (D) Lower Kalkschieferzone, sample VM34, 502Á5 m. (E) and (F) Dolomitband, sample VSC6, 242Á6 m. (G) and (H) Lower Meride Limestone, Cassina beds, Ca11, 213Á4 m. Key to palynofacies log as in Fig. 9.

TOC (Lower Meride Limestone) (mean value: shales yielded HI values ranging between 445 501 mg HC/g TOC, n = 27). The HI values are and 515 mg HC/g TOC. lower in the micritic carbonate rocks (mean The S2-TOC (Fig. 12A and B) and HI-OI value: 310 mg HC/g TOC) and they further (Fig. 12C) diagrams indicate prevailing Type-II decrease in the marlstone, where they average kerogen. However, a lithology-related analysis 167 mg HC/g TOC. The two analysed black reveals some differences in the organic-matter

; distribution. In the black shales, Type-II kerogen max C is the only kerogen present. In the laminated ° T carbonate rocks from the upper part of the

/ Lower Meride Limestone and lowermost part of 2 the Upper Meride Limestone, Type-II kerogen is clearly dominant but, in addition, exceptionally OI (mg CO gTOC) high HI values (up to 934 mgHC/g TOC) are also recorded, which are suggestive of Type-I kerogen. In the laminated lithologies from the Kalkschieferzone, a contribution from Type-III

HI (mgHC/ gTOC) kerogen, characterized by relatively low HI values is chieﬂy limited to organic-poor samples. Compared with the laminated carbonate rocks, micritic carbonate rocks display a shift towards a Type-III signature, which becomes even more evident in the marlstone samples. The kerogen from the marlstone samples from the top of the section plots in the Type-III ﬁeld. In the stratigraphic context, the evolution from Type-II(I) kerogen to a mixed Type-II/Type-III kerogen is well traceable (Fig. 11). Finally, Tmax values cluster around values of about 410 to 415° C for organic matter from all lithologies (Table 1).

Stable isotopes Bulk-rock carbonate stable-isotope data (d13C and d18O) are plotted in Fig. 13. The d13C curve shows an abrupt initial increase from À4Á15 (Besano Formation) to +1Á49& (San Giorgio Dolo- mite, 20 m in the section), followed by a plateau between 20 m and 156 m with values averaging +1Á43&. The plateau ends with a negative spike

Non-fluo AOM Phytoclasts Palynomorphs TOC MINC at 170 m (À4Á69&, Cava inferiore beds), followed by an increase to values of around À0Á59&, which represents the mean value for the interval (%) (%) (%) (%) (wt%) (wt%) Fluo AOM from 175 to 253 m (Sceltrich beds). Thereafter, from 253 to 345 m, the d13C values increase up + Á & 1 100.0 0.0 0.0 0.0 26.3 2.5 515 5 414 1 82.8to 0.01 94 with 1.6 minor 15.5 fluctuations. A 0.4 subsequent 12.0 552 78 414 n plateau extends between 345 m and 450 m, with most values clustering around +1& and with two negative peaks (373 m: À0Á88&; 395 m: (Me) À0Á43&). In the upper part of the section (from 450 m upwards), d13C values gradually drop by 4& from around +1Á5& to around À2Á5& (579 m). The top of the section (579 to 622 m, Upper Kalkschieferzone) is characterized by negative, however, highly variable, values, ranging between À4Á44& and À0Á92& (mean (Besano Fm.) À2Á36&). Comparison between the average results from Rock-Eval pyrolysis and palynofacies composition, grouped by lithofacies. Only samples for which The d18O values in the lower part of the section (up to 156 m) are highly scattered, ranging between À7Á73& and À1Á73&. From 156 m Table 1. both palynofacies and Rock-Eval values were determined have been reported. SGD: San Giorgio Dolomite; Me: Meride limestone;Unit KSZ: or lithozone Kalkschieferzone Fluo AOM: fluorescentpercentages amorphous occur organic in matter; organic-poor samples Non-fluo due AOM: toLithofacies non-fluorescent the mineral amorphous matrix organic effect. matter. Low HI values associated with high AOM Laminated carbonatesSGD and Me (withoutKalkschieferzone KSZ) (KSZ)Micritic carbonatesSGD and Me (withoutKalkschieferzone(KSZ) KSZ)Marlstones 17 22SGD and Me (withoutKalkschieferzone KSZ) 76.5 5 58.7Black shales 10 11.1Thin-shelled 22.4 bivalve limestones 11 9.1 67.4 1 58.2upwards, 64.3 4 5.1 7.6 33.3 6.2 7.1 66.1 2.6 6.1 10 6 6.5they 4.1 12.6 7.3 7.1 43.3 12.3 25.1 increase 26.7 6.7 9.2 8.7 18.5 21.5 51.8 1.1 1.4 and 16.4 22.1 20.5 10.1 0.3 reach 9.5 0.3 502 12.3 34.0 0.3 44.8 0.2 561 12.5a 315 12.2peak 0.2 12.6 342 78 66 340 328 7.4 118 0.4 0.6 176 413 121 171 416 123 6.9 6.1 402 415 206 171 197 414 405 142 411 83 410 408

TOC (wt%) HI Laminated Micritic ( mgHC g–1 TOC) All samples carbonates carbonates Marlstones HI/OI diagrams Metres Formation Lithology

1000

I Kalkschieferzone 800 II 600 HI 400

200

III IV 0 1000 OI

I 800 Upper Meride II Limestone 600 HI 400

200

III IV 0

1000 OI Dolomitband I 800 Lower Meride Limestone II San Giorgio Dolomite 600 HI 400

200

III IV 0

1000 OI

I 800 Besano Fm. II 600 HI 400

200

III IV 0 0 200 OI 400 600

Fig. 11. Rock-Eval results plotted against stratigraphy. In the HI plot, blue, black, red, white and green dots indicate black shales, laminated carbonate rocks, micritic carbonate rocks, bivalve limestone and marlstone, respectively. Key to HI/OI plots as in Fig. 12C. of À1Á66& at 182 m (Cava superiore beds), INTERPRETATION followed by a decreasing trend, which culmi- nates in a negative spike of À7Á60& (243 m). Microfacies characterization of carbonate Thereafter, between 243 m and 531 m, the d18O rocks values ﬁrstly rapidly increase and then ﬂuctuate around values of À4&. From 531 m onwards, The alveolar network common to the optically d18O values drop by over 3& reaching a negative dark laminae is indicative of exopolymeric sub- spike at 553 m (À6Á21&). Thereafter, they gradu- stances (EPS; Decho, 2000; Pacton et al., 2007; ally increase across the Middle and Upper Gorin et al., 2008) and, accordingly, it suggests Kalkschieferzone to around À3&. an origin from bacterial activity within microbial

40 1000 AC 35 I 900 30 ) –1 25 I 800 0·1%

HI (mg HC g 400

5·0 4·5 B 300

4·0 I

) 3·5 200 –1 3·0

2·5 II 100 2·0 III IV S2 (mgHC g 1·5 0 0 100 200 300 400 500 1·0 –1 OI (mg CO2 g TOC) 0·5 III 1000 0 E 0·1%0·3% Vitrinite TOC (wt%) reflectance I 800 0·5

700 1000 TOC)

600

900 D –1 II 800 500

700 400 TOC) HI (mg HC g

–1 600 300 500 Black II+III Shales 400 200

HI (mgHC g 300 1·3 100 III 200

100 0 380 390 400 410 420 430 440 450 460 470 480 0 0 0·5 1·0 1·5 2·0 2·5 3·0 3·5 4·0 4·5 Immature Oil zone Gas zone TOC (wt%) Tmax (°C) Fig. 12. (A) and (B) S2-TOC diagrams (Langford & Blanc-Valleron, 1990). (A) For high TOC values (black-shale values plot outside the fields and are not shown). (B) For low TOC values [see dashed area in (A)]. (C) Hydrogen index (HI) versus oxygen index (OI) plot [‘modified Van Krevelen plot’ (Van Krevelen, 1993); field boundaries after Westermann et al., 2010 and Gertsch, 2010]. (D) Hydrogen index (HI) versus TOC plot (laminated carbonate rocks). Note how HI values start to level off at around 0Á8% TOC. This breakpoint indicates the attainment of optimal preservation conditions. (E) Hydrogen index (HI) versus Tmax plot, indicating an immature state of the organic matter. Field boundaries according to Westermann et al. (2010) and Ruhl et al. (2009).

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 i aiinscin lc os aiae abnt ok.Rddt:mcii abnt ok.Gendt:marl- dots: Green rocks. carbonate micritic curves. dots: average Red six-point rocks. to carbonate correspond laminated lines Grey dots: stones. Black section. Ladinian gio © 13. Fig. 02TeAtos ora compilation Journal Authors. The 2012 hl okcroaecro ( carbon carbonate rock Whole

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Stage Substage Formation © 02ItrainlAscaino Sedimentologists, of Association International 2012 d 13 Lithology Giorgio San Monte of rocks carbonate organic-rich Ladinian )adoye ( oxygen and C) d 18 –5 )iooercr ( record isotope O) –4 –3 –2 (‰ VPDB) δ

–1 13

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Sedimentology –4 –3 –2 –1 , 60 239–269 , 257 258 R. Stockar et al. mats (Potz, 1994; Dupraz et al., 2009 and refe- the saturation index and, consequently, in rences therein). Photoautotrophic cyanobacteria mediating carbonate precipitation (Kazmierczak are generally recognized as the most important et al., 1996; Sprachta et al., 2001; Dupraz et al., EPS producers but the potential role of hetero- 2004, 2009). The EPS matrix itself plays a key trophic bacteria has been demonstrated as well role in organomineralization, because it provides (Braissant et al., 2007). The presence of dissemi- organic templates acting as carbonate mineral nated framboidal pyrite grains may well reflect nucleation sites (Dupraz & Visscher, 2005; Dup- the activity of sulphate-reducing bacteria in the raz et al., 2009). The later generation of dull microbial community. luminescent calcite is probably related to burial The nature of the sub-micrometre-sized spher- diagenesis. Consistent with this interpretation, ical bodies (‘nanospheres’, here around 200 nm the filamentous to rod-like organic bodies, some- in size; Fig. 6E) is still debated and controver- times protruding from the pores and showing a sial. These bodies have been interpreted as arte- bright fluorescence, would represent relics of facts derived from acid etching (Kirkland et al., bacterial clumps around which carbonate pre- 1999), fossilized bacteria-like dwarf life forms cipitated. Similar microporous calcite grains, (Vasconcelos et al., 1995; Folk & Chafetz, 2000; organized into clots, also occur within the EPS Sprachta et al., 2001; Russo et al., 2006; Perri & structure in the wavy laminated fabric where, in Tucker, 2007) or carbonate nanospheres that addition, they conform to the typical polygonal progressively replaced EPS structures (Reitner shape of EPS pits (Fig. 5E) and not the converse. et al., 1995; Sprachta et al., 2001; Dupraz et al., This indicates mineral precipitation within the 2004; Aloisi et al., 2007) and later coalesced to pits themselves rather than a covering of allo- form peloids (Dupraz et al., 2004, 2009). chthonous grains by EPS sheats. Therefore, a The laminated carbonate facies includes two microbial origin may be inferred for these pe- main lamination patterns, probably reflecting loids as well. different environmental conditions, as follows: The different lamination patterns may reflect 1 The wavy laminated fabric (Figs 5A and different environmental conditions, possibly 6A) results from trapping and binding of mainly also resulting in different microbial communi- platform-derived sediment including shallow- ties. Hydrodynamics (for example, storm wave water skeletal grains (Stockar, 2010) by the EPS action) associated with changing sediment input structure (light laminae, Fig. 5A) and from might have resulted in perturbations of micro- microbially-mediated growth of the micrite-rich bial growth, giving rise to irregular laminae (and organic-rich) laminae (dark laminae). The (wavy lamination). The more continuous and concentrations of coarser platform-derived regular subplanar laminated fabric, typical of bioclastic material (Fig. 3A) are interpreted as the Kalkschieferzone, might have been related to storm-generated accumulations resulting from more quiet and more saline conditions com- episodic storm-wave sea floor agitation. pared with the wavy lamination (Pope et al., 2 In the subplanar laminated fabric, the inter- 2000). nal microporosity of peloids may have different In micritic carbonate rocks, the presence of origins, representing, for instance, the disorga- reworked platform-derived bioclasts, scattered nized structure of faecal pellets (Gerdes et al., flake-like chips and traces of graded bedding 1994) or a diagenetic product (Cantrell & Hager- suggests that much of the sediment was allo- ty, 1999). The clotted fabric of the peloids and chthonous and deposited by mechanical the occurrence within bindstones, however, processes. The flake-like black chips may be point to a microbially induced origin of the interpreted as mineralized microbial-mat rip-up grains (Flugel,€ 2004). Accordingly, peloidal clasts, probably released by desiccation and ero- grains may be interpreted as bacterially induced sion from intertidal environments (e.g. Gerdes, precipitates (‘microbial peloids’), micropores 2007). These non-laminated lithologies are sug- being the result of the subsequent decay of bac- gested to represent carbonate mud stirred up teria (i.e. bacterial moulds), followed by the pre- from the shallow-water Salvatore-Esino platform cipitation of a clear spar cement around the system. Transport into the basin most probably exterior of the grain (Chafetz, 1986; Folk & occurred by lime mud-dominated turbidity cur- Chafetz, 2000). Metabolic pathways of both rents which, in the presence of water stratifica- autotrophic cyanobacteria and heterotrophic tion, could flow along the top of the pycnocline. sulphate-reducing bacteria play a key role in Resulting detached turbid layers could drop alkalinity changes, resulting in an increase of

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 Ladinian organic-rich carbonate rocks of Monte San Giorgio 259 their load as a suspension cascade over a broad imality recorded in this interval might have been area (Bernasconi, 1994; Wignall, 1994). Major enhanced by recurrent storm events, resulting in turbidity currents could have been able to cross increased input of land plant-derived organic the pycnocline and to transiently oxygenate the matter into the basin (Stockar & Kustatscher, bottom waters. The occasional presence of 2010). Also, increased continental runoff should Zoophycos-type bioturbations may result from be taken into consideration which might have this process. been promoted by periods of rainfall following volcanic eruptions (Dawson et al., 1997; Szulc, 2000; Lamoureux et al., 2001; Todesco & Todini, Optical characterization of the organic matter 2004); in this respect, the occurrence of volcanic The dominance of glossy, inclusion-rich and ash layers concentrated in this stratigraphic strongly fluorescent AOM suggests prevailing interval may provide a supporting line of dysoxic to anoxic bottom water conditions with evidence. Starting around the 400 m level in the high preservation potential of lipoid-rich material section (upper part of the Upper Meride Lime- (readily degraded under oxic conditions but stone; Fig. 9), a much more definite distal to refractory to anaerobic bacteria), potentially proximal trend is substantiated by the features derived from planktonic organic matter or micro- of the POM. These include the progressive dete- bial mat material (Batten, 1982; Tribovillard & Go- rioration of the AOM preservation (suggested by rin, 1991; Tyson, 1995; Waterhouse, 1998). The weak to absent fluorescence; Fig. 10C and D) presence, under the SEM (Fig. 8B), of a common and the increase in terrestrial organic matter, alveolar network is indicative of EPS (see above) which is paralleled by the rise in Circumpolles and, accordingly, suggests an origin from bacte- percentages. Such trends indicate a regressive rial activity. Commonly occurring pyrite inclu- phase and a shift towards more oxic, however sions, probably derived from the activity of still oxygen-depleted, bottom water conditions. sulphate-reducing bacteria, support this assump- In this context, the occurrence of prevailing opa- tion. Such features are typical of oxygen-deficient que phytoclasts may well be interpreted as basins (and particularly of those belonging to car- resulting from an in situ (bio-)oxidation effect bonate systems), which show ideal conditions for (Tyson, 1993; Rameil et al., 2000) rather than AOM preservation and where terrestrial POM from sorting processes (Tyson, 1993; Bombardi- only dominates in the immediate vicinity of fluvi- ere & Gorin, 1998). o-deltaic inputs (Tyson, 1993, 1995). Amorphous Two AOM peaks occur which are particularly organic matter contents of 65% (Æ5%) or more significant. The first occurs at the 531Á5 m level are typical of most dysoxic–anoxic source rock (‘Site-D’ level) where the strong fluorescence facies (Tyson, 1995); only in the Upper Kalkschie- displayed by the totality of AOM suggests tem- ferzone AOM do the percentages fall below this porarily increased preservation rates, probably threshold (Figs 9, 10A and B). due to bottom-water oxygen depletion. Land- Relative sea-level trends may be inferred by derived organic matter, however, still greatly qualitative and quantitative distribution patterns contributes to the total POM. By contrast, the of POM (Fig. 9). The first shift towards a more second peak (573Á3 m) almost entirely consists of land-influenced sedimentation occurs in the non-fluorescent AOM. Enterolithic laminae, sim- Lower Meride Limestone at around 170 m in the ilar to those characterizing this sample (Fig. 3E), section. It is substantiated by an increase in both have been interpreted as pseudomorphs after sporomorphs, including a first tetrad peak, and contorted anhydrite layers (Furrer, 1995) and are phytoclasts, the latter being mainly opaque and regarded as an indicator of low rainfall and high heterogeneous in size and shape. The AOM is evaporation conditions (Mutti & Weissert, 1995). abundant but, besides prevailing strongly fluo- The AOM could well be derived from microbial rescent aggregates, a relevant contribution of mats, which are today well-known from modern non-fluorescent AOM is recorded also, which hypersaline environments (Dupraz & Visscher, may indicate degraded marine AOM. Even 2005; Dupraz et al., 2009; Pacton et al., 2009), though it probably reflects fluctuating conditions also in association with anhydrite enterolithic resulting in a succession of different pulses, this folds (Gerdes et al., 2000). The lack of fluores- first interval might have lasted until the lower- cence may derive from dehydration and oxida- most part of the Upper Meride Limestone, tion of the mats, which is suggested by the where, in the Sceltrich beds, a major input of occurrence of shrinkage cracking in the Middle degraded AOM occurs. The signal towards prox- Kalkschieferzone (Fig 3C; Furrer, 1995). The pre-

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 260 R. Stockar et al. dominance of terrestrial organic matter in the analysis. The preservation of Type-I and Type-II Upper Kalkschieferzone (Fig. 10A and B) is typi- kerogen is supposed to require anoxic/lower dys- cal of a low sea-level deposit. Phytoclasts, among oxic conditions. By contrast, owing to its low which opaque particles are dominant, show oxygen demand, Type-III (and IV) kerogen can important size-variety with common large blade- accumulate under upper dysoxic to oxic condi- shaped particles, as usually observed in proximal tions (Tribovillard & Gorin, 1991; Wignall, 1994; settings (Steffen & Gorin, 1993). Pollen tetrads Tyson, 1995; Batten, 1996b). The lithology- reach the highest peak of the section. Marine pal- related differences in the geochemical signal may ynomorphs are no longer recorded. However, reflect different sources feeding the basin, even both the excellent preservation of sporomorphs though a bias due to the mineral matrix effect and the presence of a subordinate, but strongly should be taken into account. Marlstone samples, fluorescent, AOM fraction suggest not fully oxic in particular, yield a more terrestrial signature bottom water conditions (Tyson, 1995; Batten, (Type-II/III kerogen) compared with the micritic 1996a). The increasing amount of Circumpolles lithologies and even more if compared with the pollen grains, most probably derived from a laminated lithologies and black shales. (xerophytic) coastal pioneer vegetation (Roghi An assessment of the optimal conditions pro- et al., 2010), and the decrease of the bisaccate moting preservation of organic matter in the basin Triadispora group represent further evidence for may be achieved by a within-lithology cross-cor- the regressive trend (Tyson, 1995, and references relation of HI and TOC values (Tyson, 1995; therein) which started in the Upper Meride Fig. 12D). After a first steady increase in concert Limestone. The lack of any significant rise in the with TOC, HI values start to level off at TOC val- spore percentage probably reflects prevailing arid ues around 0Á8% (HI = 500 to 600 mg HC/g TOC). climatic conditions, which would be consistent This behaviour suggests the reach of optimal with the increase in the xerophytic Circumpolles preservation conditions resulting from increased (Heimhofer et al., 2006; Roghi et al., 2010). AOM percentages and the achievement of suffi- cient depletion of oxygen levels to prevent degradation of anaerobically refractory lipoids (Tyson, Organic geochemistry 1995). Once this threshold is reached, any further The Rock-Eval data from organic-rich samples increase in TOC may simply result from (laminated carbonate rocks and black shales) are decreased dilution by (carbonate) sediment. consistent with the optical characterization of the Black-shale samples (n = 2) from the Besano For- organic matter (Figs 9 and 10). High to very high mation and the San Giorgio Dolomite show TOC HI values typify organic-rich samples whose pal- values being ten times higher than those of the ynofacies are characterized by dominant amounts laminated carbonate sediments whereas HI val- of fluorescent AOM; their geochemical signatures ues are well below the trend displayed by the lat- fall into the Type-I and Type-II kerogen fields, ter. This might result from a moderate oxidation thus corroborating an AOM origin from in the water column, slightly reducing the poten- (hydrogen-rich) marine algal/bacterial matter. tially fossilizing hydrogen-rich fraction during Organic-matter assemblages with only subordi- deposition of the black shales. nate (fluorescent) AOM content, like those In a HI/Tmax diagram (Fig. 12E), Tmax values characterizing the Upper Kalkschieferzone, plot in the field of immature organic matter (see may still yield relatively high HI values also Bernasconi, 1994 for the Besano Formation), (ca 300 mg HC/g TOC; Fig. 11), provided that which is consistent with the TAS value shown brightly fluorescent (well-preserved) sporo- by palynomorphs. Consequently, a low degree of morphs overwhelmingly contribute to the assem- thermal alteration can be inferred and, according blage (Fig. 10B). This suggests a lipoid-rich to the modified correlation chart of Hunt (1995) sporomorph composition, resulting in HI values in Scotti (2005), burial temperatures averaging increasing towards Type-II kerogen (see also 60°Cto70°C may be assumed for the studied Wignall, 1994). By contrast, high contributions section (see also Fantoni & Scotti, 2003). from the (mainly opaque) phytoclast fraction result in substantial lowering of HI values and Stable isotopes generate a typical terrestrial Type-III kerogen signature (Fig. 11, uppermost part of the section). Thermal alteration indices from palynomorphs Increased TOC values coincide with more (see above) and conodonts (CAI = 1; R. Stockar, reducing conditions inferred from palynofacies 2012), together with Tmax values (Table 1,

Fig. 12E) provide evidence for the lack of a progressive increase of 12C in the marine carbon thermal overprint deeply altering the chemo- reservoir. This trend departs from the global stratigraphic record. The lack of a statistically d13C record which, through Ladinian times, dis- significant positive correlation between d18O plays slightly increasing positive values without and d13C values (r2 = 0Á009) also excludes a any clear excursion (Payne et al., 2004; Korte marked diagenetic alteration of the primary et al., 2005; Tanner, 2010). Many studies sup- isotope signature (Allan & Matthews, 1982; port a close relation between carbonate carbon Korte et al., 2004; Mutterlose et al., 2009; Fio isotope record and sea-level changes, which et al., 2010). result in variations of the partitioning between Consistent with the palynofacies results, the organic and inorganic (carbonate) carbon sinks curve of the carbonate carbon and oxygen stable (Renard, 1986; Baud et al., 1989; Weissert & isotopes in the studied section is interpreted to Lini, 1991; Weissert et al., 1998; Berner, 2006; reflect mainly the sea-level evolution of the Herrle et al., 2010; Nunn & Price, 2010; Her- depositional environment. However, the relation mann et al., 2011). A sea-level fall, in particular, between sea-level changes and isotope curves is results in oxidation of marine organic matter at not straightforward and calls for the influence of exposed continental shelves, erosion and sea- concurrent effects. ward transport of the carbon-rich sediments; this The lowermost part of the section (Besano induces the release of previously stored 12C- 13 Formation) shows highly depleted d C values, enriched CO2 and therefore generates a negative ascribed to organic matter diagenesis (Bernas- d13C shift. Supply of (12C-enriched) terrestrially coni, 1994). Up-section (20 to 156 m; San Gior- derived carbon as a result of intensified erosion gio Dolomite and lowermost Meride Limestone) and weathering (Fio et al., 2010) would also be the complete overlap in d13C of limestone and consistent with palynofacies showing increasing dolostone indicates that the d13C value of the amounts of degraded AOM and land-derived replacement dolomite was inherited directly POM. This process may, therefore, act as a con- from its calcite precursor (see also Carmichael curring cause of the observed d13C depletion, et al., 2010). In this stratigraphic interval, the which was associated with the shallowing- influence of open-marine Tethyan waters may upward trend strongly suggested by both sedi- be inferred from d13C values, which form a very mentological and palaeontological evidence. stable plateau clustering around values of Depleted, however highly variable, d13C values +1Á4&, comparable with the d13C signature of recorded in the uppermost part of the section the Ladinian open marine Tethys basins (Veizer (upper half of the Middle Kalkschieferzone and et al., 1999; Korte et al., 2005). The interval Upper Kalkschieferzone) are consistent with a between 170 m and 345 m shows overall marginal depositional setting. Owing to poten- depleted d13C values. The lighter d13C values of tially considerable nutrient inputs from the land, the carbonate parallel increased TOC values and primary production can be significant there, are most probably related to partial recycling of while differences in metabolic pathways and organic matter. These values are consistent with preservation conditions of organic matter in bot- early diagenetic sulphate reduction and, hence, tom sediments may result in d13C values becom- anaerobic bacterial activity within the sediment ing more negative and highly variable (Armstrong 12 releasing C-enriched CO2 into the pore waters & Brasier, 2005; Goevert & Conrad, 2008). (Kempe, 1990; Hopf et al., 2001). The following Strongly negative d13C signals, for instance, are plateau (345 to 450 m), with carbon-isotope known for carbonates from Triassic hypersaline ratios being rather stable and clustering around environments of the Germanic Basin (À6& < d13C Ladinian sea water values (Veizer et al., 1999; < À5&; Szulc, 2000) and are typical of modern Korte et al., 2005), suggest more stable condi- hypersaline lagoons (Armstrong & Brasier, 2005). tions and higher sea-level. The most important Even under hypersaline conditions, however, the negative d13C shifts correlate with increased distinctly depleted d13C values (down to À4Á4&) TOC values and thus probably reflect partial strongly suggest a contribution from organically recycling of organic matter. The scatter of d18O derived carbon. The concurrent slight increase of values may be related to small-scale variations d18O might represent a characteristic isotopic sig- in temperature and salinity, or more probably to nature of evaporitic environments where 16Ois diagenetic processes. preferentially removed by evaporation (Thunell The gradual, yet significant, 4& drop of d13C et al., 1987; Marshall, 1992; Joachimski, 1994; values in the 450 to 583 m interval requires a Szulc, 2000; Jaffres et al., 2007).

DISCUSSION unusually high hydrogen indices, sometimes exceeding 900 mg HC/g TOC, thus suggesting high preservation of the labile lipoid-rich mate- Organic matter distribution and preservation rial. Composition and preservational patterns of the 3 The youngest OM-rich interval (Kalkschie- organic matter vary throughout the section. ferzone) is characterized by strongly increased Based on both optical and geochemical charac- percentages of terrestrially derived organic terization, three main OM-rich stratigraphic matter, including sporomorphs and phytoclasts, intervals are distinguished, each controlled by a associated with abundant, yet mostly degraded, characteristic suite of environmental parameters. AOM. The increased presence of pollen grains and spores still grouped into tetrads provides 1 The organic matter content of the black evidence for shallowing and short-distance shales (mainly in the Besano Formation) is transport. A near-shore depositional setting is represented by fluorescent AOM, which exclu- also supported here by the recent find of excep- sively belongs to marine-derived Type-II tionally well-preserved apterigote insects, which kerogen. Earlier geochemical studies indicate an are regarded as terrestrial coast dwellers (Bechly important bacterial contribution; Bernasconi & Stockar, 2011). Also in this interval, laminated (1994) suggested an origin from mid-water mats limestone shows an origin from benthic micro- while Chicarelli et al. (1993) detected a cyano- bial activity, already envisaged by Furrer (1995). bacteria-derived fraction based on biomarkers Notwithstanding the high percentage of the ter- and nitrogen isotope contents. Black shales, restrially-derived fraction, the organic matter however, do not bear conclusive microscopic does not always display a definite Type-III kero- evidence of benthic microbial mats, while HI gen signature. Abundance of well-preserved values seemingly support an origin within the (lipoid-rich) sporomorphs often shifts the signal water column. Indeed, despite very high TOC to the (typically marine) Type-II kerogen field. values, HI values are never as high as in the By contrast, as expected, the lowest TOC and HI laminated carbonate rocks of the 170 to 253 m values are associated with either laminated interval (Lower Meride Limestone and lower- limestone which bears evidence of burrowing, most part of Upper Meride Limestone), for thus suggesting reworking under oxic conditions which a prevailing benthic origin of organic (lowest TOC values) or organic-matter assem- matter may be postulated. Therefore, it is con- blages dominated by opaque phytoclasts (lowest ceivable that the original biomass, before being HI values). buried in sea floor sediments under fully anoxic conditions, underwent a moderate oxidation during settling through a partly oxic water col- Palaeoenvironmental changes and umn, slightly reducing the potentially fossilizing depositional history of the basin hydrogen-rich fraction. Permanent anoxic–sulphidic (euxinic) condi- 2 By contrast, benthic microbial activity tions in the water column were only dominant accounts for the microfabrics observed in the during deposition of the black shales preserved laminated carbonate rocks of the Lower Meride in the lower part of the section (Besano Forma- Limestone and lowermost part of the Upper Me- tion and lower part of the San Giorgio Dolomite; ride Limestone, including clotted-peloidal mi- see also Bernasconi, 1994). Black shales may be crite and EPS-like structures. Consistent with interpreted as a product of episodes of enhanced the palynofacies features, showing a dominance bioproductivity and organic-matter flux. Tran- of fluorescent AOM and geochemical signatures, siently enhanced primary productivity in the indicating Type-II (sometimes even Type-I) lower part of the section is also supported by the kerogen, it is suggested that benthic bacterial evidence of radiolarian blooms in the Besano biomass largely contributed to the organic matter Formation and in the San Giorgio Dolomite (Ber- composition in the 170 to 253 m interval. Acting nasconi, 1994; Stockar et al., 2012b). These indi- as a protective agent in the sorptive preservation cate surface but not deep-water open-marine pathway (Pacton et al., 2009), clay minerals connections with the basins of the western resulting from increased runoff most probably Tethys, which is also supported by d13C values contributed to the high degree of preservation of being very close to those of Ladinian sea water the bacterially derived organic matter (Kennedy (Veizer et al., 1999; Korte et al., 2005). A higher et al., 2002). Consequently, the latter may show

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 Ladinian organic-rich carbonate rocks of Monte San Giorgio 263 sea-level probably enabled the enhanced See also Stockar, 2010). Lower dysoxic condi- exchange of water masses between the Monte tions were even able to allow episodic sea floor San Giorgio basin and the open-marine western colonization by posidonioids, as testified by Tethys. Based on structural considerations, max- organic-rich thin-shelled bivalve limestone. imum basin depth was ca 130 m (see Bernasco- These flat clam palaeocommunities are regarded ni, 1994). Episodic volcanic activity, increased as being autochthonous opportunistic popula- terrigenous supply and upwelling should be tions, representing the result of single immigra- taken into account as possible sources of an tion events with subsequent rapid reproduction, excessive input of nutrients. In the Besano For- typical of r-strategists (McRoberts, 2010). How- mation explosive volcanic activity is docu- ever, both lower dysoxic and anoxic bottom- mented by dozens of ash layers intercalated water conditions ruled out higher macrobenthos, within the carbonate sequence, which also occur including scavengers, and resulted in complete in the lower San Giorgio Dolomite (Wirz, 1945; oxygen depletion within the sediment. This Muller€ et al., 1964; Bernasconi, 1994). Moreover, allowed the fine laminated fabric to be preserved. intensified terrigenous supply causally linked to Lack of evidence for long-term anoxic–sulphidic a volcanically-induced increase in precipitation conditions, along with evidence for transient ben- cannot be ruled out. Enhanced local upwelling thic colonization events by opportunistic species in South-Alpine basins during Early Ladinian and for occasional storm wave base impingement, times was postulated by Preto et al. (2005) as are consistent with recurrent dysoxia–anoxia being a consequence of strong seasonal trade related to water stratification at shallower depths. winds related to the dominating monsoonal cli- Therefore, the main variable that controlled mate (see also Brack et al., 2005). Regardless of organic-matter accumulation was probably rela- the cause, increased surface bioproductivity tive sea-level change, with lower sea-levels could have resulted in oxygen depletion in the resulting in diminished water-mass mixing and, bottom waters, in turn promoting more efficient thus, in conditions favouring improved organic- phosphorus regeneration under anoxic condi- matter preservation. Based on both physical tions (Ingall et al., 1993; Mort et al., 2007), parameters and a strong monsoonal activity, Ber- finally forcing a rising of the zone of sulphate nasconi (1994) suggested a value of 31 m for the reduction into the water column. Cessation of maximum depth of storm wave base in the Monte this process was probably forced by the deposi- San Giorgio basin. Such a value, or slightly tion of carbonate resediments stirred up from the greater depths, may therefore be assumed to be adjoining carbonate platforms. representative of this stratigraphic interval. The Higher up-section, laminated carbonate rocks establishment of dysoxic to anoxic conditions from the Cava inferiore–Sceltrich beds interval was most probably triggered by enhanced runoff (170 to 253 m, Lower and Upper Meride Lime- and input of degraded marine and terrestrial stone) show TOC values which are an order of organic matter into the basin, causing increasing magnitude lower than the black shales. Moreover, oxygen-deficiency in the water column and in pyrite framboid size up to 8 μm exceeds the bottom waters. An increased runoff is consistent critical threshold indicative of formation within with the palynofacies data, indicating higher ter- the water column due to sulphidic (euxinic) restrial input. Recent macrofloral finds, implying conditions (Wignall & Newton, 1998). These intensive fresh water input, also support this lithologies call for a different scenario involving a inference (Stockar & Kustatscher, 2010). Episodi- much more dynamic depositional environment, cally increased supply of nutrients, associated with redox conditions of bottom waters fluctuat- with enhanced runoff, may well have contributed ing between lower dysoxia and anoxia. An anoxic to increase productivity and organic-matter flux. threshold (TOC around 0Á8%) lower than usually Water-mass stratification and intensified runoff reported from oxygen-deficient settings (1Á5to may have been favoured by the monsoonal pre- 2%, Espitalie et al., 1986; Tyson, 1995; Algeo & cipitation regime, which was probably associated Maynard, 2004; Westermann, 2010), probably with increased precipitation due to enhanced expresses the physiographic restriction of this volcanic aerosol concentrations, and by the marginal basin resulting in high potential for restricted character of the basin, small and partly organic-matter preservation. The transient coloni- surrounded by emerged areas covered with vege- zation of the sea floor by opportunistic foramini- tation. feral meiobenthos is consistent with episodic In the overlying part of the Upper Meride lower dysoxia (suboxia; Tyson & Pearson, 1991. Limestone, a relative sea-level rise, suggested by

© 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 264 R. Stockar et al. sedimentological, geochemical and palynofacies CONCLUSIONS data, did not result in a major exchange with open-marine water masses, as indicated by pal- Both optical and geochemical results suggest that aeontological evidence. The subsequent gradual a major part of the organic matter (OM) in the sea-level fall resulted in a final moderate studied section was produced by bacteria. High- increase in organic-matter preservation, observed est total organic carbon (TOC) values and Type-II in the upper part of the section, including the kerogen are shown by black shales (mainly in the Kalkschieferzone. There, wider, however short- Besano Formation) from the lower part of the sec- lived, redox fluctuations occur which comprise, tion. For these lithologies, an origin from mid- for the first time in the section, intervals which water bacterial plates is assumed, followed by are oxygenated enough to allow for recurrent preservation under anoxic–sulphidic conditions, macrobenthic infaunal colonization. in agreement with Bernasconi (1994). Black shales formed during a highstand interval and open-marine surface connections, at times of Implications for vertebrate taphonomy increased primary productivity. By contrast, ben- The results of the present study have significant thic microbial activity occurring during sea-level implications for the reconstruction of the ecology lowstand intervals produced most of the amor- and taphonomy of the vertebrate fossils which, phous organic matter preserved in the laminated in the Meride Limestone, appear mainly articu- limestone from the upper part of the Lower Me- lated and excellently preserved (Peyer, 1931; ride Limestone and the lower part of the Upper Kuhn-Schnyder, 1974; Rieppel, 1985; Sander, Meride. Alveolar structures typical of exopoly- 1989; Furrer, 1999; Renesto, 2005; Stockar, meric substances (EPS) secreted by bacteria 2010). All the world-known vertebrate fossil- within microbial mats, along with nanospheres bearing levels of the Meride Limestone prove to encased in EPS matrix, have been documented be associated with low sea-level intervals and, for the first time from Monte San Giorgio. More- thus, with progradation of the coastline. This is over, a microbial contribution to carbonate consistent with the features of both the fish fau- (peloidal) precipitation was substantiated by nas, showing evidence of niche-partitioning means of scanning electron microscopy (SEM), (Burgin,€ 1996; Lombardo & Tintori, 2004; Stoc- cathodoluminescence (CL) and epifluorescence kar, 2010), and the reptiles, showing merely a microscopy. Laminite-dominated, organic-matter low degree of adaptation to an aquatic life rich intervals show, however, different features. (Kuhn-Schnyder, 1974; Renesto, 2005). The In the interval between the upper part of the assumption of a shallower setting disagrees with Lower Meride Limestone and the lower part of earlier studies, which postulated the necessity of the Upper Meride Limestone, sea-level fall was a high hydrostatic pressure (and thus of a high paralleled by increased runoff, possibly enhanced water column) to avoid disarticulation of car- by the aftermath of recurrent explosive volcanic casses on the sea floor and subsequent dispersal activity. This is considered as a trigger of the drop of skeletal elements (Sander, 1989). This of bottom-water ventilation resulting in severely argumentation led earlier authors to regard the dysoxic to anoxic, yet non-sulphidic, conditions transition from the Besano Formation to the which, in turn, allowed accumulation and preser- Meride Limestone as possibly recording a deep- vation of organic matter. Compared with this ening-upward sequence (Bernasconi, 1994). It is organic-matter rich interval, TOC values of lami- proposed that, coupled with widespread oxygen- nated limestone from the upper part of the section depletion excluding benthic scavenger organ- (Kalkschieferzone) drop by an order of magnitude isms, rapid coating of skeletons by benthic and hydrogen indices halve, generating a Type-II/ bacterial mats described in this paper (‘microbial III kerogen signature. shroud’ effect; Gall, 2001) played the key role in The underlying causes are two-fold. On one protecting the carcasses from decay and in hold- hand, an increased terrestrial fraction contrib- ing skeletal elements together (see also Furrer, uted to these organic-matter assemblages; how- 1995, 1999; Stockar, 2010). This proposal is ever, a significant contribution from bright- corroborated by the frequent disarticulation of fluorescent sporomorphs to the hydrogen-rich vertebrates in the black shales (Besano Forma- fraction is observed. On the other hand, owing tion) which, though formed under fully anoxic to fluctuating, sometimes even oxic, redox con- conditions, lack conclusive evidence for the ditions, the more labile marine organic matter growth of benthic microbial mats. underwent partial degradation. This youngest

OM-rich interval developed during a gradual rel- Aloisi, G., Gloter, A., Kruger,€ M., Wallmann, K., Guyot, M. ative sea-level fall, finally resulting in a shallow- and Zuddas, P. (2007) Nucleation of calcium carbonate on – water, at times hypersaline, setting. The onset of bacterial nannoglobules. Geology, 34, 1017 1020. Armstrong, H.A. and Brasier, M.D. (2005) Microfossils. this trend and its subsequent development are Blackwell, Oxford, 296 pp. clearly recorded by both the composition of the Batten, D.J. (1981) Palynofacies, organic maturation and sedimentary organic matter and the steady source potential for petroleum. In: Organic Maturation depletion of the heavy carbon isotope. It can Studies and Fossil Fuel Exploration (Ed. J. Brooks), pp. – confidently be placed at around the 400 m level 201 223. Academic Press, New York. Batten, D.J. (1982) Palynofacies, palaeoenvironments and in the section where, by contrast, no appreciable petroleum. J. Micropalaeont., 1, 107–114. lithofacies and palaeontological changes occur. Batten, D.J. (1996a) Chapter 26a. Palynofacies and Relative sea-level changes in a monsoonal palaeoenvironmental interpretation. In: Palynology: regime are regarded as the main variable driving Principles and Applications (Eds J. Jansonius and D.C. – the distribution of inorganic and organic facies. McGregor), pp. 1011 1064. AASP Foundation,Dallas. Batten, D.J. (1996b) Chapter 26B. Palynofacies and Other factors, however, such as volcanism and petroleum potential. In: Palynology: Principles and changing wet/arid conditions in its aftermath, Applications (Eds J. Jansonius and D.C. McGregor), pp. interacted to enhance (or to buffer) the predomi- 1065–1084. AASP Foundation, Dallas. nantly bathymetry-controlled environmental and Batten, D.J. and Morrison, L. (1983) Methods of ecological variations. 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(1994) Geochemical and microbial controls ACKNOWLEDGEMENTS on dolomite formation in anoxic environments: a case study from the Middle Triassic (Ticino, Switzerland). Contrib. Sedimentol., 19,1–109. € We particularly acknowledge Neria Romer who Berner, R.A. (2006) Carbon, sulphur and O2 across the assisted during all of the first author’s field work. Permian-Triassic boundary. J. Geochem. Explor., 88, 416– The SEM and CL work considerably benefited 418. from the assistance of Pierre Vonlanthen and Bombardiere, L. and Gorin, G.E. (1998) Sedimentary organic matter in condensed sections from distal oxic Claudia Baumgartner-Mora, respectively (both environments: examples from the Mesozoic of SE France. Institute of Earth Sciences, University of Lau- Sedimentology, 45, 771–788. sanne). Stable-isotope analysis was carried out by Brack, P. and Rieber, H. 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