Carbon chemostratigraphy of the Cambrian ... - GSA Publications

34 downloads 2151 Views 579KB Size Report
midlatitude mixed platform, Montagne Noire, France. J. Javier ... †E-mail: Jose-Javier. ... The DICE and SPICE events in a midlatitude platform, Montagne Noire.
Carbon chemostratigraphy of the Cambrian-Ordovician transition in a midlatitude mixed platform, Montagne Noire, France J. Javier Álvaro† Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain, and LP3, UMR 8014 CNRS, University of Lille I, 59655-Villeneuve d’Ascq, France

Blanca Bauluz Ignacio Subías Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain

Catherine Pierre Laboratoire d’Océanographie Dynamique et de Climatologie, Université Pierre et Marie Curie, 75252-Paris cedex 05, France

Daniel Vizcaïno 7 rue Jean-Baptiste Chardin, Maquens, 11090-Carcassonne, France

ABSTRACT The Cambrian Drumian Carbon Isotope Excursion (DICE) and the Steptoean Positive Carbon Isotope Excursion (SPICE) have been so far reported in subtropical platforms: the DICE event is a large negative excursion in δ13Ccarb record that nearly coincides with the beginning of the Drumian (Cambrian Series 3), whereas the SPICE event is a large positive shift in δ13Ccarb identified close to the base of the Paibian (Furongian Series). The chemostratigraphic excursions have been recognized in carbonate-dominated platforms, and their application to mid- and high-latitude, siliciclastic-dominated platforms has been problematic. This paper offers, for the first time, a high-resolution stratigraphic analysis of this time span in a temperate-water platform (Montagne Noire, France) that recorded episodes of carbonate productivity in nearshore environments. Two sections were analyzed in detail: a complete Cambrian Series 3–Tremadocian succession, devoid of carbonate interbeds and paleogeographically located in the distal part of the platform (Pardailhan nappe), and a Furongian-Tremadocian succession, which has limestone interbeds and represents the proximal part of the platform (Minervois nappe). The lower part of the La Gardie Formation in the Pardailhan nappe displays background †

E-mail: [email protected].

δ13Corg values of −22‰ punctuated by a large negative, middle Languedocian (regional substage) shift, peaking at –24.5‰. This negative excursion is similar to the DICE event reported close to the base of the Drumian, although the latter is exclusively based on δ13Ccarb values. Stable isotopes of δ13Ccarb and δ13Corg from both sections also indicate a common chemostratigraphic shift at the onset of the Furongian. In nearshore deposits of the Minervois nappe, the δ13Ccarb background values for the interbedded limestone strata of the Val d’Homs Formation gradually increase from −3‰ to −1‰ and are punctuated by a sharp increase in δ13Ccarb values to >3.0‰. By contrast, in basinal shales of the La Gardie Formation in the Pardailhan nappe, the δ13Corg background values decrease from −22‰ to − 27.1‰. This trend is directly controlled by total organic carbon (TOC) contents and is interpreted to have been initiated by changes in the degree of biodegradation of organic matter, sulfate reduction, and methanogenesis before or soon after burial. Despite this overprinting and the very low degree of metamorphism reached by the shales, the δ13Corg background trend is also punctuated by a sharp positive shift in δ13Corg values to >4.0‰, similar to the SPICE event. In addition, the asymmetric shape of the SPICE excursion in the Montagne Noire shows evidence for the HERB event, a late Furongian negative carbon isotope excursion not yet accepted as a worldwide chemostratigraphic anomaly.

The recognition of both chemostratigraphic shifts in mixed and clayey strata opens new possibilities of chemostratigraphic correlation in mid- and high-latitude platforms, where carbonate factories did not widely develop, and strata only contain endemic fauna. This implication has a major consequence because the Global Stratotype Section and Point (GSSP) of the Drumian and Paibian stages occurs near the base of both excursions. Keywords: chemostratigraphy, Cambrian, Ordovician, West Gondwana. INTRODUCTION The formal boundaries of the base and top of the Cambrian Period were defined by the Working Groups on the Precambrian-Cambrian and Cambrian-Ordovician boundaries in 1992 and 1998, respectively (Brasier et al., 1994; Cooper and Nowlan, 1999). In 1998, the International Subcommission on Cambrian Stratigraphy started to tackle the subdivisions of the Cambrian into series, but the only series boundary so far defined has been the base of the 4th Cambrian Series (= Furongian; Peng et al., 2004). One of the reasons for the complicated interregional correlation in the Cambrian is the endemic character of biostratigraphically significant fossils. The bases of the Drumian (ca. 506–503 Ma; Cambrian Series 3) and Paibian (ca. 499– 496 Ma; Furongian) stages are defined by the first appearance of two agnostoid trilobites, Ptychagnostus atavus and Glyptagnostus

GSA Bulletin; July/August 2008; v. 120; no. 7/8; p. 962–975; doi: 10.1130/B26243.1; 7 figures; Data Repository item 2008081.

962

For permission to copy, contact [email protected] © 2008 Geological Society of America

The DICE and SPICE events in a midlatitude platform, Montagne Noire δ13Ccarb (‰ PDB) Age (Ma)

-6

488.3

496

Furongian

492

Stage 10

-4

-2

0

+2

TOCE

Stage 9 Paibian SPICE

510

528

Lower Cambrian

517

521

Series 3

Stage 7 Drumian DICE

Stage 5 ROECE

Series 2

506

Middle Cambrian

499

503

AECE

Stage 4

MICE

Stage 3

CARE

Stage 2

+4

Figure 1. Composite δ13C curve and global standard chronostratigraphic Cambrian scale showing the isotopic shifts DICE (Drumian carbon isotope excursion) and SPICE (Steptoean positive isotopic carbon excursion) discussed in the text (modified from Zhu et al., 2006). BACE— Basal Cambrian Carbon Isotope Excursion; ZHUCE—Zhujiaqing Carbon Isotope Excursion; SHICE—Shiyantou Carbon Isotope Excursion; CARE— Cambrian Arthropod Radiation Isotope Excursion; MICE— Mingxinsi Carbon Isotope Excursion; AECE—Archaeocyathid Extinction Carbon Isotope Excursion; ROECE— Redlichiid-Olenellid Extinction Carbon Isotope Excursion; DICE—Drumian Carbon Isotope Excursion; SPICE—Steptoean Positive Carbon Isotope Excursion; TOCE—Top of Cambrian Excursion.

SHICE ZHUCE

Series 1

reticulatus, respectively (Peng et al., 2004; Babcock et al., 2007). Both taxa are widely distributed throughout low-latitude platforms; however, their absence in other mid- to highlatitude platforms hampers any possible correlation of the newly erected Drumian and Paibian stages with other classic “middle-upper” Cambrian, mixed to siliciclastic strata rich in endemic fossil taxa. Despite the scarcity of well-constrained carbon isotope excursions in the Cambrian (for a recent synthesis of regional δ13Ccarb shifts, see Zhu et al., 2006; Fig. 1), the potential of carbon isotope excursions as another tool for interregional correlation has been successfully applied to the base of the Drumian and Paibian in lowlatitude, carbonate-dominated platforms. The Drumian Carbon Isotope Excursion (DICE) is recognized as a large negative δ13Ccarb excursion (peak at –2.4‰ δ13C) that nearly coincides with the beginning of the Drumian stage (Babcock et al., 2007). The shift has been recognized close to the base of the P. atavus zone in the Great Basin (Laurentia), eastern Siberia, NW Hunan (China), and the Georgina Basin (Australia) (Brasier and Sukhov, 1998; Montañez et al., 2000; Zhu et al., 2004, 2006). By contrast, the Steptoean Positive Carbon Isotope Excursion (SPICE) is identified as a positive δ13Ccarb shift (peak at ~+5‰), located close to the base of the Paibian, in carbonate-dominated successions of Lesser Karatau (Kazakhstan), Queensland (Australia), Great Basin of North America (Laurentia), and North and South China (Saltzman et al., 1998; Peng et al., 2004; Zhu et al., 2004). The presence of both δ13C shifts has not yet been tested in mid- to high-latitude platforms. The mixed character of the uppermost Cambrian Series 3–Tremadocian strata of the Montagne Noire (southern France), composed of limestone-shale alternations, offers a key opportunity to test the record of chemostratigraphic anomalies at midlatitude settings (for paleolatitudinal precisions, see, e.g., Cocks and Torsvik, 2002; Fortey and Cocks, 2003; Fig. 2). Thus, the aim of this paper is fourfold: (1) to test the identification of the DICE and SPICE events in shales and limestone-shale alternations recorded in a midlatitude, temperate-water platform characterized by episodes of carbonate productivity; (2) to undertake a comparative analysis of Ccarb and Corg signatures in a proximal-to-distal transect of the Montagne Noire platform in order to test the record of carbon isotope excursions in basinal shale–dominated substrates typical of deeper and higher-latitude platforms (as the Global Stratotype Section and Point [GSSP] of the Drumian and Paibian occur near the base of both excursions); (3) to propose a chemostratigraphic correlation of

Stage 1 BACE 542

both stages in mid- to high-latitude platforms, where the index cosmopolitan agnostoid trilobites that coincide with their bases are absent; and (4) to offer a discussion about the primary mechanisms that potentially triggered the DICE and SPICE excursions. GEOLOGIC AND STRATIGRAPHIC SETTING The Montagne Noire is located in the southern prolongation of the French Massif Central, and it consists of three main structural domains: (1) a metamorphic axial zone made up of complex domes of gneiss and migmatites surrounded by mica schists; (2) a northern flank composed of imbricated tectonic nappes bearing Cambrian to Silurian rocks; and (3) a southern flank made up of large nappes involving Cambrian to Carboniferous strata. Cambrian and Lower Ordovician strata are well constrained lithostratigraphically and biostratigraphically in the Minervois and Pardailhan nappes (Fig. 3) of the southern Montagne Noire.

The southward drift of the Mediterranean margin of West Gondwana during Cambrian times is supported by paleobiogeographic, paleomagnetic, and paleoclimatic arguments, including the latitudinal distribution of climatically sensitive facies. Although these latitudinal belts are relative to the modern “icehouse” situation, and not to the Cambrian “greenhouse” model (Berner, 1990), they offer key information to estimate relative rates of drifting. The changing paleogeographic distribution of the arid subtropical belt located in the Southern Hemisphere, gradually migrating from northwest Africa to southeast Europe throughout the Neoproterozoic-Cambrian, has been used to infer the paleolatitudinal southward drift of this margin (Álvaro et al., 2000). The drift was also accompanied by the migration of trilobite diversity patterns, demonstrating a direct relation between warm waters and peaks in trilobite diversity (Álvaro et al., 2003a). The oldest paleomagnetic data of the region were provided by Nysæther et al. (2002), who located the Cabrières wildflysch of the Montagne Noire at

Geological Society of America Bulletin, July/August 2008

963

Álvaro et al. ARMORICA Cabos - C

u

a

r

Oville/Barrios - O/B

SIBERIA

Valtorres/Valconchán - V/V La Gardie/Val d'Homs - LG/VH Alum Shales

Cabitza - Ca ˇ trasice - S ˇ S

Kara

kerogenous black shales

30 °

S

e

q

to

60° S

BALTICA

O/B

C

Monks Park St. George

LAURENTIA

A

lo va

LG/VH

SPICE event S-CHINA

SOUTH POLE

TARIM

IRAN

Awatag

Mila

GONDWANA

ARABIA AFGHAN

Eastern Cordillera Lampazar/ Sta. Rosita /Iscayachi

INDIA

Famatina

?

Volcancito

Chatsworth

AU ST RA LI A

?

ANTARCTICA

Minaret

high southerly latitudes (68° +17/–15) during Middle Ordovician times (Llanvirn to early Caradoc, ca. 470–458 Ma). In the Montagne Noire, carbonate factories were active from Cambrian Series 2 to early Cambrian Series 3 times, as in neighboring platforms, such as Morocco, the Iberian Peninsula, and Sardinia. Although the Mediterranean region lacked major carbonate factories until Late Ordovician times, the Montagne Noire displays an anomalous episode of carbonate productivity (from Furongian to middle Tremadocian times) related to deposition of common storm-induced coquinas and development of pelmatozoan-sponge meadows on horsts (Álvaro et al., 2003b, 2007). These temperate-water limestones have yielded a rich and diversified fossil fauna composed of trilobites, echinoderms, linguliformean brachiopods, sponge spicules, and conodonts, which improve currently understood biodiversity patterns at midlatitude platforms. The upper part of Cambrian Series 3 and the Furongian are recognized in the laterally equivalent La Gardie and Val d’Homs Formations. Both

964

reefs

Oujeft

SOUTH AMERICA

Precordillera

carbonates

Mt - Mexican terranes

Huaqiao

Jbel Lmgaysmat

nia

Mt

La Flecha

Aisha-Bibi Seamount

Ca ˇ S

Dolgellau

Conococheague Hoyt/Whitehall

V/V ?

KAZAKHSTANIA Sibumasu

are lithostratigraphically differentiable by the absence or presence of limestone strata, respectively. The La Gardie Formation, 200–500 m thick, is composed of green shale with centimeter- to decimeter-thick sandstone intercalations, whereas the Val d’Homs Formation, 60–300 m thick, consists of green and purple shale that contains centimeter- to meter-thick, white, reddish and purple, lenticular to bedded dolostone and limestone. The biostratigraphic subdivision of both formations is based on the stratigraphic ranges of polymeroid trilobites (Álvaro and Vizcaïno, 1998; Vizcaïno and Álvaro, 2003): the middle and upper Languedocian (Cambrian Series 3), “regional Upper Cambrian,” and Tremadocian are defined by the first appearance of the endemic trilobites Bailiaspis souchoni, Eccaparadoxides macrocercus, Palaeadotes latefalcata, and Proteuloma geinitzi, respectively. Recently, Álvaro et al. (2007) reported the presence of the conodont Paltodus deltifer in the middle part of the Val d’Homs Formation, which allows identification of the middle Tremadocian Paltodus deltifer zone, and significantly reduces the thickness of

Figure 2. Global Furongian paleogeographic reconstruction (modified from Álvaro et al., 2007) and setting of subtropical- and temperate-water carbonate factories, terrigenous belts, and black shales. Note the subtropical position of the platforms where the DICE and SPICE events had been so far reported.

the “regional Upper Cambrian” in the Montagne Noire. The base of the latter does not coincide with the base of the Furongian Series because each is defined by the first appearance of different trilobite species with different ranges. Both the La Gardie and Val d’Homs Formations are overlain by the progradational conglomerates and sandstones of the La Dentelle Formation and the transgressive Saint-Chinian shales, which episodically recorded progradational shifts of sandstone shoal systems (Álvaro et al., 2003b). We selected two reference sections for chemostratigraphic analysis: the Campelou hill and the village of Sallèles-Cabardès (Fig. 3). The Cambrian Series 3–Tremadocian of Campelou, situated in the Pardailhan nappe, exclusively displays siliciclastic facies (La Gardie Formation): offshore homogeneous shales episodically interrupted by intraformational slumping and convolution representing substrate instability of the distal part of the intraplatform seafloor. By contrast, the Furongian–middle Tremadocian of the Sallèles-Cabardès section, located in the Minervois nappe, consists of limestone-shale

Geological Society of America Bulletin, July/August 2008

The DICE and SPICE events in a midlatitude platform, Montagne Noire

Rouergue

Najac

C

50 km

FRANCE Albigeois

Albi

NORTHERN SIDE

Lacaune mts

Pre-Hercynian outcrops

AXIAL ZONE

Agout

Castres Soré zois

A

SOUTHERN SIDE Nore Cabardè s

St. Pons

Pardailhan nappe Minervois nappe

Poussarou

Roquebrun

B

Carcassonne

Pardailhan Coulouma

Campelou

St. Chinian

Ferrals Vé lieux

Rieussec

Barroubio St. Jean M. Silurian to Devonian

Minerve Lastours

N

PE N AP Sallèles-Cabardès

IS MINERVO

NAPPE

AN

AILH PAR D

Ordovician Coulouma to Val d'Homs fms. Pardailhan to Tanque fms.

10 km

Marcory Fm.

Figure 3. Geologic setting of the Montagne Noire (A), the Minervois and Pardailhan nappes of the southern Montagne Noire (B), and the Sallèles-Cabardès and Campelou sections studied in this work (C).

alternations (Val d’Homs Formation) and represents the proximal part of the platform. There, two kinds of limestone facies were recognized by Álvaro et al. (2003b): storm-induced coquinas and stratigraphically condensed shorefaceto-offshore tops of tectonically induced paleohighs (the so-called “griotte” facies, which consists of alternating nodular- and lenticularbedded wackestone to packstone and argillaceous laminae and thin beds). SAMPLES AND METHODS Limestone samples from the Sallèles-Cabardès section are mostly shallow-water wackestones to packstones. Stable carbon and oxygen isotope analysis was performed on bulk rock samples from fresh limestone intervals devoid of skeletons, cements, and burrowing. Although all of the following have the potential for retaining original seawater chemistry and serving as proxies, in decreasing order of importance and reliability, unaltered calcite-walled brachiopods, pristine cement, and micritic whole-rock material (Brand, 2004),

our choice of the microsparitic matrix for analysis was based on: (1) the virtual absence of well-preserved calcite-walled brachiopods, currently preserved as sparry mosaics; and (2) the low percentage (in volume) of cements, mostly represented by scattered granular sparry calcite cements occluding shelter cavities and syntaxial overgrowths of echinoderm ossicles. Thus, clean microsparite matrix was drilled with a dental microdrill, and samples from cores were finely ground for whole-rock carbonate analyses. The powdered samples were digested in 100% phosphoric acid at 25 °C, and the resulting CO2 gas was analyzed using a triple collector mass spectrometer (VG-Sira 9) at Laboratoire d’Océanographie Dynamique et de Climatologie (LOCEAN), University of Pierre et Marie Curie, Paris. The isotopic compositions of calcite are expressed in the conventional δ notation relative to the Vienna Peedee belemnite (V-PDB) reference (Craig, 1957); δ= [(Rs/Rr) – 1] × 1000, where R = 18 O/16O or 13C/12C, respectively, in the sample and in the reference. Analytical reproducibility (1σ) is 0.1‰ for δ18O and δ13Ccarb.

Shale samples from the Sallèles-Cabardès and Campelou sections, ground to a fine powder in an agate mortar and pestle to form a homogenized sample, were checked by X-ray diffraction (XRD) for carbonates. Samples containing calcite were treated with 3N HCl acid until reaction of all the carbonate occurred. Aliquots of the powdered samples were analyzed by infrared adsorption using a Fisons Instruments, EA 1108 CHNS-O elemental analyzer and placed in tin capsules for subsequent combustion in an HCNOS EURO EA 3000 elemental analyzer. The gas sample was subsequently purified and passed through an ISOPRIME mass spectrometer for isotopic analysis at the University of Salamanca. Organic carbon isotope ratios were measured against international and internal standards and expressed in the usual delta notation. Analytical reproducibility of replicate samples using the aforementioned methods was better than ±0.2% and ±0.1‰, respectively. Shale mineralogy was determined by X-ray diffraction following the methods described by Bauluz et al. (1998). Diagenesis was constrained by the crystallinity index of

Geological Society of America Bulletin, July/August 2008

965

Álvaro et al.

?

50

40

30

20

10

REGIONAL UPPER CAMBRIAN

60

interbedded limestones

interbedded shales δ 13Corg (‰ PDB)

XRD (%) 0

25

75

50

100

-29

-21

-13

δ13Ccarb (‰ PDB)

TOC (%) 0.05 0.15

-6

0.25

-2

2

3

δ18O (‰ PDB) -16

-12

-8

3

?

VAL D'HOMS VAL D'HOMS FORMATION

70

FURONGIAN

80

ABC

LITHOLOGY

TREMADOC low mid

FACIES ASSOCIATIONS

SALLÈLES-CABARDÈS (MINERVOIS NAPPE)

SPICE EVENT

2

1

0m MC

-6

-2

2

3 FACIES ASSOCIATIONS 2

A

offshore shales

B

storm-induced limestones

C

'griotte'-type limestone/shale couplets

1

-16

3

base of Paltodus deltifer Zone (middle Tremadocian)

-12

-8

XRD analysis

chemostratigraphic base of SPICE EVENT shift

quartz

chlorite

feldspar

calcite

base of regional Upper Cambrian (FAD of Palaeadotes latefalcata)

illite

Figure 4. Stratigraphic log of the Sallèles-Cabardès section (Minervois nappe) with biostratigraphic and chemostratigraphic data. XRD— X-ray diffraction; TOC—total organic carbon; PDB—Peedee belemnite; FAD—first appearance datum; SPICE—Steptoean Positive Carbon Isotope Excursion.

illite (IC), measured as recommended by the International Geoscience Programme (IGCP) 294 IC Working Group (Kisch, 1991), and the obtained values were transformed using the interlaboratory standards (C.I.S.) provided by Warr and Rice (1994). In this context, the limits of the anchizone are 0.42 and 0.25°2θ. RESULTS Sallèles-Cabardès Section We analyzed 42 samples for carbonate δ13Ccarb and 20 samples for interbedded shale δ13Corg, spanning a mixed stratigraphic interval of 55 m from the Val d’Homs Formation (see GSA Data Repository1). This section was previously biostratigraphically controlled, where the bases of the regional Upper Cambrian and the middle Tremadocian were reported in detail, based on trilobites, phosphate-walled linguliformean bra-

966

chiopods, and conodonts (Álvaro et al., 2003b, 2007). The δ13Ccarb background values for this section gradually increase from −3‰ to −1‰, as shown in the lower and upper parts of the section. However, this gradual trend is interrupted by a sudden positive shift of δ13Ccarb values to >3.0‰, after which the values decrease sharply over the next 8 m until about –5.3‰ and finally increase gradually over the next 12 m to reach background values (Fig. 4). The δ18O background values show two trends: a lower trend displaying a broad increase upsection from −14.3‰ to −9.1‰, and an upper irregular trend ranging from −11.9‰ to −16.51‰. The bound1 GSA Data Repository Item 2008081, 3Ccarb, δ18O, Corg, TOC values, and clay composition from the Sallèles-Cabardès and Campelou samples analyzed in this work, is available at www.geosociety.org/ pubs/ft2008.htm. Requests may also be sent to [email protected]. 13

ary between both trends is sharply located at the end of the aforementioned positive excursion in δ13Ccarb values. The “griotte” limestones in Sallèles-Cabardès have total organic carbon (TOC) values less than 0.02 wt%, beyond analytical reproductibility. By contrast, the TOC content over most of the interbedded shales is low, although it gradually increases upsection from 0.11% to 0.23%. Both TOC and δ13Corg values were not analyzed in interbedded shales rich in calcite content. Campelou Section The Campelou section dominantly consists of fine-grained siliciclastic strata, and most analyzed samples lack carbonate content. This section was previously biostratigraphically controlled, where the base of the middle and upper Languedocian (Cambrian Series 3), the regional Upper Cambrian, and the Proteuloma geinitzi

Geological Society of America Bulletin, July/August 2008

The DICE and SPICE events in a midlatitude platform, Montagne Noire and Shumardia (Conophrys) pusilla zones (Tremadocian) were reported in detail, based on trilobites and acritarchs (Álvaro et al., 2003b, 2007; Vizcaïno and Álvaro, 2003). We analyzed 27 samples of shale δ13Corg over the middle Languedocian–regional Upper Cambrian, siliciclastic stratigraphic interval of 200 m, and 12 samples for the overlying lower Tremadocian succession (200 m thick; Fig. 5). In addition, samples were selected at meter scale close to chemostratigraphic anomalies. Total organic carbon (TOC) contents were determined from the same clayey samples analyzed isotopically, and they indicate a progressive increase upsection, from values of 0.06% to 0.2%. The middle Languedocian to lower Tremadocian shales are characterized by a large-scale increase in organic-matter content associated with a gradual decrease in δ13Corg values. Across the middle Languedocian–late Tremadocian interval, the δ13Corg background values show a progressive decrease from ~–23‰ to −28‰, which is coeval with a gradual increase in TOC concentrations, from 0.05 wt% to 0.15 wt%. The latter is drastically constrained by the onset of two regressions recorded as prograda-

FACIES

pusilla zone 0m

CAMBRIAN

100

MIDDLE CAMBRIAN

200

SAINT-CHINIAN

13

XRD (%)

6

25

50

75

100

-26

-23

-20

FACIES ASSOCIATIONS

TOC (%)

Corg (‰ PDB)

-29

0.05

0.15

0.25

A

offshore shales

B

sliding and slumping structures

C

shoreface sandstones

SHOAL PROGRADATION

geinitzi zone

(Languedocian) FURONGIAN up middle

?

Both in the Sallèles-Cabardès and Campelou sections, the mineral composition of shales consists of quartz and clay (illite and chlorite), minor feldspar, and accessory calcite (Figs. 4 and 5). No significant trends or variations of the mineral associations are observed in Sallèles-Cabardès, whereas the Campelou section displays a gradual increase in the (illite + chlorite)/(quartz + feldspar) ratio. This modification can be interpreted as a consequence of both weathering (feldspar transforming into illite) and decreased siliciclastic input related to deepening. The IC

glauconitic marker bed

6 DENTELLE

300

Mineralogy and Metamorphic Degree of Shales

values range between 0.29 and 0.44°2θ (mean value: 0.36°2θ ± 0.04), corresponding to the late diagenesis to lower anchizone transition (or very low metamorphic degree; the limits of the anchizone are 0.42 and 0.25°2θ). This is in agreement with the described clay mineralogical association formed by illite and chlorite. The range of variation in IC values is probably due to the coexistence of detrital and authigenic illites, since the occurrence of detrital micas may produce a decrease in IC values. The coexistence of clays with different origins reflects the lack of chemical equilibrium at very low metamorphic conditions. The Tremadocian conodonts found in the overlying limestones sampled in Sallèles-Cabardès (Álvaro et al., 2007) have a color alteration index (CAI) of 5, which fits well with the anchizonal degree supported by the aforementioned IC values (Ferretti and Serpagli, 2007, personal commun.). In summary, the transgressive middle Languedocian–middle Tremadocian Val d’Homs (Sallèles-Cabardès) and La Gardie (Campelou) Formations are characterized by: (1) A progressive increase of δ13Ccarb (from −3‰ to −1‰ in limestone interbeds) coeval

CAMPELOU (PARDAILHAN NAPPE) 0

SHOAL PROGRADATION

5

3

5 4

4

base of Shumardia (Conophrys) pusilla Zone, Tremadocian base of Proteuloma geinitzi Zone, Tremadocian occurrence of Langyashania (Furongian)

3

SPICE

2 LA GARDIE

ORDOVICIAN

TREMADOCIAN

400

ABC

tions of the La Dentelle and intra–Saint Chinian coarse-grained sandstone shoals and channels. Despite the inverse δ13Corg-TOC relationship and the very low degree of metamorphism reached by the shales, the isotope background values are punctuated by two sharp shifts of δ13Corg: a middle Languedocian negative shift to values 4.0‰ close to the Languedocian–regional upper Cambrian boundary (see Fig. 5).

2 1 DICE

chemostratigraphic base of SPICE EVENT shift base of upper Languedocian (occurrence of Bailiella cf. Bailiella griffei) base of middle Languedocian (FAD of Bailiaspis souchoni)

1

Figure 5. Stratigraphic log of the Campelou section (Pardailhan nappe) with biostratigraphic and chemostratigraphic data. XRD—Xray diffraction; TOC—total organic carbon; PDB—Peedee belemnite; FAD—first appearance datum; DICE—Drumian Carbon Isotope Excursion; SPICE—Steptoean Positive Carbon Isotope Excursion.

Geological Society of America Bulletin, July/August 2008

967

Álvaro et al. with a decrease in δ13Corg (from −22‰ to –27.5‰ in shales). (2) A gradual increase in the (illite + chlorite)/ (quartz + feldspar) ratio in shales. Although the gradual increase in TOC concentrations led to an increase in the preservation of organic matter (a trend interrupted by the onset of the La Dentelle and intra–Saint Chinian regressions), the subsequent relative increase in TOC did not affect the benthic shelly communities, which were abundant and diverse. (3) A large-scale increase in organic-matter content that can be divided into three trends, in ascending order: a first and stable, impoverished content less than 0.15 wt%; a subsequent fluctuation that reaches peaks of 0.15–0.25 wt% and rapidly decreases close to the base of the prograding La Dentelle Formation; and a third fluctuation that reaches a peak of 0.20 wt% and decreases again close to the following distinct intra–Saint Chinian shoal progradation. RELIABILITY OF ISOTOPE DATA The diagenetic processes capable of causing changes in the oxygen and carbon isotope composition of marine carbonate components include neomorphism and recrystallization, reequilibration with fluids of different isotope composition, and precipitation of isotopically different diagenetic carbonate phases (e.g., Glumac and Walker, 1998, and references therein). The postdepositional alteration of limestone precursors generally leads to lighter δ13C and δ18O values (Veizer et al., 1997). Oxygen Isotopes from Carbonates Fluctuations of δ18O values in the limestones of Sallèles-Cabardès range from −9‰ to −16‰, whereas similar analyses compiled from Cambrian tropical low-Mg calcite-walled brachiopods (which probably yield the closer proxy to real seawater chemistry) vary from −7‰ to −12‰ (Wadleigh and Veizer, 1992). However, these differences must not be exclusively related to diagenetic processes. Bruckschen et al. (1999) pointed out that δ18O variations in modern tropical brachiopods at shelf depths are ~4‰, and, for specimens sampled in temperate climates, they span ~8‰. Oxygen isotopic differences can also be triggered by metabolic effects or seasonality (Grossman et al., 1991) and may reflect superimposed vital fractionation effects (Qing and Veizer, 1994) or recrystallization of the shell calcite after deposition (Veizer et al., 1986). On average, the difference between bulk rock and skeletal calcite (e.g., calcitewalled brachiopods) δ18O values amounts to ~2‰ (Schrag et al., 1995; Veizer et al., 1999).

968

Another minor difference that may explain δ18O differences with previously compiled data from the same time interval is their different paleolatitude: the assumed temperature difference between Holocene mid- and low-latitude sites can also produce an offset of 0.5‰ (Schrag et al., 1995). Τhe δ18O data from Sallèles-Cabardès show more oscillations that the δ13C data; this fact and their depleted character can be reasonably ascribed to diagenetic alteration. This is consistent with the burial history of the Minervois and Pardailhan nappes of the southern Montagne Noire, since the anchizonal degree could be achieved by increasing lithostatic pressure related to temperatures close to 250 °C (Frey, 1987). Micritic recrystallization would have caused δ18O values to decrease. Therefore, the depleted and variable values of δ18O in SallèlesCabardès reflect a diagenetic resetting and postdepositional alteration of the Val d’Homs limestone samples, probably primarily controlled by thermal burial temperatures. Carbon Isotopes from Carbonates As explained already, the limestone interbeds of Sallèles-Cabardès exhibit a progressive increase of δ13Ccarb values from −3‰ to −1‰, interrupted by a positive shift peak at +3.02‰. These values are slightly depleted in comparison with the background δ13Ccarb values analyzed in Cambrian tropical brachiopods (ranging from −2.2‰ to +1.0‰; Wadleigh and Veizer, 1992). Possible controls on 13C depletion can be related to three factors: the proportion of oxidized kerogen incorporated in the host limestone (Schumacher, 1996); the influence of riverine carbon in nearshore environments (Koch et al., 1992); and diagenesis. The carbon isotope value of marine organic matter is ~25‰ more negative than inorganic bicarbonate (Marshall, 1992); thus, degradation of organic matter has the potential to significantly alter the carbon isotope composition of marine carbonate sediment (Glumac and Walker, 1998). The condensed “griotte” limestones of Sallèles-Cabardès are reddish-colored, a feature influenced by the oxidation state of iron and manganese, and they have been sedimentologically interpreted as having been deposited on the tops of tectonically induced paleohighs submitted to the action of waves (Álvaro et al., 2003b). Because organic-matter preservation is directly related to rapid burial (preserving it from oxidation and scavengers), the scarcity (or virtual absence) of kerogen in the aforementioned limestones may be related to irreversible, synsedimentary to early diagenetic oxidation of organic matter. In addition, δ13C values

of riverine carbon are shifted to more negative values than oceanic values, which may have somewhat impacted the carbon isotopic values of the shoreface-to-offshore griotte limestones. The wide variability of δ18O values and lack of covariance between δ13C and δ18O values (see coefficient of correlation = 0.23; Fig. 6) suggest that the studied limestones were not significantly submitted either to meteoric waters or to marine-meteoric waters of the mixing zone (Allen and Matthews, 1982). As a result, despite the depleted character of the 18O and 13C curves, the lack of δ13Ccarb/δ18O covariance in Sallèles-Cabardès suggests similar diagenetic behaviors for the whole section. Although the absolute isotopic values were probably depleted by burial temperature, they were affected in a similar way throughout the whole section, and the δ13Ccarb fluctuations (not the absolute values) were not predominantly induced by diagenetic alteration. Therefore, despite the depleted isotope carbon values of the Sallèles-Cabardès δ13C curve, its baseline and excursion are well constrained, and they potentially preserve the original chemostratigraphic tendencies. Carbon Isotopes from Shales Two apparent paradoxes displayed in the Campelou section need discussion: (1) the gradual increase of δ13Ccarb values in the inner platform coeval with a gradual decrease in δ13Corg values in the outer platform; and (2) the TOC/ δ13Corg covariance (coefficient of correlation = 0.9; Fig. 6). The difference δ13Ccarb – δ13Corg = Δδ13C is generally considered to separate secondary fractionation and source effects from secular variations in the δ13CDIC (dissolved inorganic carbon) signal. Constant Δδ13C values (e.g., approaching 28.5‰ ± 2.0‰ for the Neoproterozoic in Knoll et al., 1986) were attributed to global changes in δ13CDIC, whereas changes in Δδ13C are currently interpreted as changes in the origin of the organic matter, fractionation processes, or diagenesis (Hayes et al., 1989; Magaritz et al., 1992). Although the analysis of δ13Ccarb and δ13Corg values could not be made in the same samples of the Sallèles-Cabardès section, their respective background trends indicate an upsection modification of Δδ13C. The direct TOC-δ13Corg relationship explains the modification in Δδ13C because the upward increase in TOC concentrations controlled the coeval decrease in δ13Corg values. This relationship also has been observed in some Proterozoic strata and can be related to changes in the degree of biodegradation of organic matter, sulfate reduction, and methanogenesis before or soon after burial (Hayes et al., 1983; Kaufman and Knoll, 1995).

Geological Society of America Bulletin, July/August 2008

The DICE and SPICE events in a midlatitude platform, Montagne Noire

SHALE VALUES

LIMESTONE VALUES - 16

r = 0.23

2

Campelou, r = 0.83 Sallèles, r = 0.07

0

δ 13C (‰ PDB)

δ 13C (‰ PDB)

- 18

-2

- 20

Figure 6. Cross plots of samples taken across the Middle-Upper Cambrian transition from Sallèles-Cabardès (δ13Ccarb [‰] vs. δ18O [‰]) and Campelou (δ13Corg [‰] vs. total organic carbon [TOC]); r = coefficient of correlation. DICE—Drumian Carbon Isotope Excursion; SPICE—Steptoean Positive Carbon Isotope Excursion.

- 22

-4 - 24 -6 - 14

- 16

- 12

- 10 - 26

δ O (‰ PDB) 18

Limestone Sallèles-Cabardès

- 28

Shale Sallèles-Cabardès

0.0

0.1

0.2

0.3

Campelou (DICE

and SPICE

data from Campelou)

There is another explanation that can take into account the fact that Ccarb and Corg production and burial may have been dominated by decoupled processes in the development of water-column stratification and bottom anoxia affecting shelf biota (Bartley and Kah, 2004): although biological productivity and carbonate production are primarily attributed to benthic shelly metazoans, resulting in strong coupling of Corg and Ccarb production and burial, during an environmental crisis (e.g., related to water-column stratification), open-water production dominated by nonskeletal plankton would provide a potential source of Corg production and burial uncoupled from Ccarb production and burial. The difference in δ13CDIC between surface and bottom waters, related to the “photic pump,” may have strongly affected 13C abundance in surface and deep waters without necessarily altering average seawater isotopic composition (Calver, 2000). However, despite the uncoupled Corg/Ccarb signatures exhibited in comparisons of the nearshore Sallèles-Cabardès and the basinal Campelou sections, the chemostratigraphic signal related to a synchronic positive shift of δ13Ccarb and δ13Corg values located close to the

TOC (wt %)

base of the regional upper Cambrian is still preserved in both sections. Other overprinting modifications must be considered. The δ13C values of zooplankton in the modern oceans are generally the same or within 1‰ of the δ13C values of the phytoplankton, so that a higher proportion of zooplankton in total organic matter would not act as a major control on the isotopic composition of TOC (Joachimski, 1997). The thermally induced loss of hydrocarbons, which can represent an important factor affecting the stable isotopic composition of organic carbon (Hayes et al., 1983; Strauss et al., 1992), may have been significant in the Campelou area because a 13C enrichment is generally observed with increasing thermal alteration. Thus, the higher δ13Corg values might be explained by a thermally driven loss of hydrocarbons in TOC-poor samples, as pointed out by Joachimski (1997). In summary, the thermal burial temperatures achieved during the very low metamorphic degree of the Minervois and Pardailhan nappes probably affected the Cambrian strata in a homogeneous way, leading to a depletion of δ13Ccarb and δ18O values in limestones, and an increase of δ13Corg values in shales.

IMPLICATIONS Correlation with the DICE Excursion In the Drumian Stratotype Ridge section, a relatively uniform δ13Ccarb signal (slightly ranging from 0‰ to +0.5‰) is interrupted by a small, weakly negative peak (−1.3‰ δ13Ccarb), which coincides with the base of the P. atavus zone (= base of the Drumian). A second, more pronounced peak (−2.4‰ δ13Ccarb), which is easily traceable interregionally, occurs at a horizon corresponding to the acme of P. atavus in the Drum Mountains. Similar sharp negative δ13Ccarb excursions close to the base of the P. atavus zone were recorded by Brasier and Sukhov (1998) from the Great Basin (USA), eastern Siberia, and the Georgina Basin (Australia), although the peak of that excursion is illustrated as slightly below the base of the P. atavus zone. A similar negative δ13Ccarb excursion was also recorded from the base of the P. atavus zone in northwestern Hunan (China; Zhu et al., 2004). As recorded in the Drum Mountains, the post-DICE excursion reaches peak values of about +1.7‰ δ13Ccarb, at a position corresponding roughly to

Geological Society of America Bulletin, July/August 2008

969

Álvaro et al. Chrono-

Bio(trilobites)

Lithostratigraphy conodonts

SW

NE

Lithology

proximal Minervois

distal Pardailhan

mfs Shumardia (C.) pusilla

Proteuloma geinitzi

Paltodus deltifer

MOUNIO

TREMADOCIAN

open-shelf offshore

SAINT-CHINIAN Fm.

unnamed FURONGIAN

glaucon

itic pelo

Mounio horsts

ids

ONLAP

La Dentelle progradation LA DENTELLE Fm.

SB tempestites

LA GARDIE Fm.

VAL D'HOMS Fm.

mfs

unstable substrates with slope-related facies

SPICE

LANGUE DOCIAN middle upper

MIDDLE CAMBRIAN

~ 50 m

'griotte' horsts Eccaparadoxides SALLÈLES macrocercus Mb.

ONLAP

retrograda

-6

tion

tidal-flat

Bailiella souchoni

2

(‰ PDB) Minervois nappe

Ferrals shoal progradation Jincella convexa

-2

δ 13Ccarb

ONLAP

DICE

SB

FERRALS Fm. Coulouma shales

-29

-20

δ 13Corg (‰ PDB)

'Regional Upper Cambrian' predating the Furongian FAD of Palaeadotes latefalcata

3rd-order sequences

Pardailhan nappe

Figure 7. Sketch showing the stratigraphic relationship among relative sea-level fluctuations, chronostratigraphy, and chemostratigraphy (modified from Álvaro et al., 2007); mfs—maximum flooding surface; FAD—first appearance datum; SB—sequence boundary; DICE— Drumian Carbon Isotope Excursion; SPICE—Steptoean Positive Carbon Isotope Excursion.

maximum flooding of the Cordilleran margin of the Laurentian shelf. The middle Languedocian (Cambrian Series 3) chemostratigraphic data from the shale-dominated basinal part of the Montagne Noire platform (Campelou section) reflect the onset of a negative δ13Corg shift, ~25 m thick, from a background of −22‰ to a negative peak of −24.5‰ (Fig. 7). Although it is the first time that a negative middle Cambrian Series 3 excursion is tentatively identified on the basis of δ13Corg values, the shift fits well with the aforementioned patterns of the DICE δ13Ccarb excursion. This chemostratigraphic correlation needs to be tested in other subtropical shale-dominated platforms that have the index fossil P. atavus. Correlation with the SPICE Excursion The regional upper Cambrian chemostratigraphic data from nearshore and basinal parts

970

of the Montagne Noire platform display a common positive isotope excursion: (1) the limestone interbeds of Sallèles-Cabardès exhibit a progressive increase of δ13Ccarb values from −3‰ to −1‰, interrupted by a positive shift peak at +3.02‰; and (2) the biostratigraphically coeval shift in the 10-m-thick Campelou section is characterized by a sharp increase in δ13Corg of 2.3‰ followed by a sharp decrease of 4.4‰, reaching a peak close to –21‰ (Fig. 7). In both sectors of the platform, the regional upper Cambrian excursion is unaffected by the influence of the La Dentelle regression, and it mimics the SPICE event. Data sets from micritic analysis of the SPICE excursion in carbonate successions (Saltzman et al., 1998, 2000) yield parallel trends, but they broadly rise from −0.5‰ to +1.5‰ and peak at +4.5‰ to +5‰. Therefore, the δ13C values from Sallèles-Cabardès are ~3‰ more negative in comparison to the published values from

micrites of carbonate platforms. Fluctuations of δ18O values in the limestones of Sallèles-Cabardès range from −9‰ to −16‰. The δ18O data sets from the SPICE excursion analyzed in the micritic fraction of Furongian carbonate-dominated successions (Saltzman et al., 1998, 2000) range from −7.77‰ to −13.14‰ (Nevada), −6.23‰ to −10.19‰ (Malyi Karatau region, Kazakhstan), or −5.45‰ to −10.83‰ (Hunan, China). As a result, the measured δ18O values in Sallèles-Cabardès are also shifted ~3‰–4‰ more negatively in comparison to the reported values from micrites of carbonate platforms. Despite the “depleted character” of the δ13C SPICE excursion in the Montagne Noire, other “depleted SPICE excursions” have been described in dolomitized and highly altered rocks hosting ore deposits (He, 1995). The survival of the positive carbon isotope excursions in these successions suggests that the SPICE excursion represents a perturbation in the global

Geological Society of America Bulletin, July/August 2008

The DICE and SPICE events in a midlatitude platform, Montagne Noire (not regional) cycling of carbon. The similarity between our bulk-rock record and several other records of the SPICE event from different basins and paleoenvironments indicates that diagenesis did not significantly alter the carbon isotope record in the Montagne Noire, and that they carry a signal of global significance: we consider that diagenesis has not erased the primary secular trend of the δ13C signal in the Montagne Noire. The proposed SPICE excursion in SallèlesCabardès differs from other coeval shifts in carbonate-dominated platforms in its asymmetric shape: after the positive peak, the carbon isotope values decrease sharply until values about −5.3‰, and then they finally increase gradually to reach background values. This younger negative peak is not preserved in other SPICE-related carbonate successions. The stratigraphically condensed character of the griotte limestone in Sallèles-Cabardès (the whole Furongian would be represented by ~25 m in thickness; see Fig. 4) suggests that other later Furongian chemostratigraphic shifts can be included in these strata. A similar negative δ13Corg excursion, predating the Cambrian-Ordovician boundary, is also identifiable in Campelou, ~15 m above the end of the SPICE event: the negative shift is ~7 m thick and reaches a negative peak at −27.5‰. Although no further chemostratigraphic excursions are included in Figure 1 (Zhu et al., 2006), Thompson (2003) and Miller et al. (2006) have recently proposed another negative δ13Ccarb excursion (coined the HERB event) close to the base of the highest Furongian (still unnamed) stage, the latter of which is proposed at the first occurrence of the conodont Cordylodus andresi. Evidence of the HERB event has been found in Australia, Newfoundland, and western North America (Miller et al., 2006, and references therein), where it is characterized by a negative peak at −3‰ to −4‰, but the shift needs to be confirmed worldwide. The DICE and SPICE excursions can be used as chemostratigraphic tools for chronostratigraphic correlation of the lower part of the Drumian and the base of the Paibian in the midlatitude Montagne Noire platform, where chronostratigraphically significant trilobites, useful for interregional correlation, are absent. In the Montagne Noire, both excursions took place in a middle Languedocian–middle Tremadocian transgressive mixed succession associated with the stepwise immigration of East Gondwanan trilobites. Based on the aforementioned correlations, the Drumian-Paibian interval of the La Gardie–Val d’Homs Formations in the Montagne Noire corresponds to a broad transgression bracketed by the pre-Drumian (Cambrian Series 3) and middle Tremadocian progradations of the Ferrals and La Dentelle shoal sandstones,

respectively. Because the first appearance of the index trilobite Palaeadotes latefalcata, which marks the base of the regional upper Cambrian, occurs stratigraphically below the SPICE excursion, the base of the regional upper Cambrian is still pre-Furongian in age (see Fig. 7). POTENTIAL MECHANISMS TRIGGERING THE DICE AND SPICE EXCURSIONS: A DISCUSSION The DICE Negative Shift A negative carbon cycle event can be associated with a balance of carbon input (such as δ13C marked land and marine depositional weathering, degassing of volcanic areas, light carbon from organic matter in the ocean basin) and carbon output (burial of organic matter that links with primary productivity). Dramatic depletion of δ13C in a short interval is mostly associated with a sharp drop of primary productivity, whereas eustatic changes would mostly be expressed as longer intervals (Holser, 1997; Kump and Arthur, 1999). The base of the P. atavus zone in the Cordilleran region of Laurentia (Babcock et al., 2004; Howley et al., 2006) and elsewhere (Peng et al., 2004) is associated with the early part of a transgression. Overall, the Wheeler Formation represents a deepening-upward succession deposited during a single third-order cycle (Howley et al., 2006). Comparative work on sections near Paibi and Wangcun, Hunan Province (China; Peng and Robison, 2000; Peng et al., 2001, 2004), shows that P. atavus first appeared along the eastern Gondwanan slope in an early stage of a transgressive event. The earliest occurrence of P. atavus in Baltica (Alum Shale) has also been interpreted to have been related to transgression (Ahlberg et al., 2007). Sea-level variation has been suggested to control the rate of organic-matter oxidation in the ocean, which in turn controls the δ13C variations of marine carbonate. Specifically, during sea-level fall, an increase of organic-matter oxidation is expected as a result of exposure of continental shelves or development of a shallower ocean. Carbonate deposition during sea-level fall is expected to be depleted in 13C due to a decrease of ocean bicarbonate δ13C caused by organic-matter oxidation (Gao and Land, 1991). However, the relative sea-level tendency of the middle Languedocian–middle Tremadocian in the southern Montagne Noire (La Gardie–Val d’Homs Formations), in which both the DICE and SPICE excursions are included, is not representative of this possible triggering factor because it is transgressive. In fact, the delayed progradation of foreshore and shoreface deposi-

tional systems (upper Tremadocian La Dentelle Formation) was probably the result of subsidence perturbations: these are indicated by the abundance of slope-depositional facies in the La Gardie, Val d’Homs, Mounio, and Saint-Chinian Formations, and sharp lateral variations in thickness that favored an episodic development of paleotopographies (Álvaro et al., 2003b, 2007). The δ13C negative excursions are also interpreted either as a result of a pCO2 pulse or as episodes of methane release from gas hydrate reservoirs (e.g., Menegatti et al., 1998; Hesselbo et al., 2000). In some cases, short-term negative carbon isotope excursions are followed by large positive excursions (e.g., Aptian, Toarcian), leading to the intriguing proposal that methane degassing may trigger oceanic anoxic events (Wang et al., 2004). However, in our case study, there is no geological evidence for largescale dissociation of methane by hydrate delivering CO2 to the atmosphere. Finally, intense volcanism or enhanced hydrothermal activity at the scale of large igneous provinces is also thought to influence pCO2. A possible source for depleted carbon isotopes is represented by the Delamerian orogeny: in the South Australian (Adelaide fold belt) domain, the Delamerian orogen commenced at 514 ± 3 Ma and persisted for ~24 m.y. (Foden et al., 2006, and references therein). The beginning of the Cambrian Series 3 subduction-related magmatism recognized in New Zealand, Victoria, South Australia, New South Wales, and Tasmania preceded the onset of the DICE event and may represent a triggering factor for its negative carbon isotope excursion. The SPICE Positive Shift In Laurentia, the beginning of the SPICE excursion is close to a major benthic community replacement that marks the Pterocephaliid biomere, and its peak precedes the initiation of regression (Osleger and Read, 1993; Saltzman et al., 2000). In the Jiangnam slope belt of Hunan (China), the base of the G. reticulatus zone coincides with the initial stages of a transgression (Yang and Xu, 1997). Therefore, the base of the Furongian also coincides with a sea-level rise, which has been reported in the Gushan region (China; Zhu et al., 2004) and the Appalachians (Osleger and Read, 1993) as well. This was succeeded by a highstand phase and then a shallowing that is recorded in North and South China (Yang and Xu, 1997) and Laurentia (named the Sauk II–Sauk III hiatus; Osleger and Read, 1993; Saltzman et al., 2004). In some subtropical platforms, the start of the SPICE excursion coincides with a mass extinction of trilobites at the base of the G. reticulatus

Geological Society of America Bulletin, July/August 2008

971

Álvaro et al. zone (Saltzman et al., 2000). In Australia, this extinction event was first recognized by Öpik (1966), who noted that only a few genera known from the preceding zone of Glyptagnostus stolidotus, and none of the 80 species of trilobites, survived into the overlying G. reticulatus zone. However, Brock et al. (2000) explained the same event as a major facies-controlled faunal reorganization, in which the endemic shallowwater shelf carbonate trilobites were sharply replaced by more cosmopolitan, less diverse, outer-shelf assemblages (Shergold, 1996). In Laurentia, Palmer (1965) documented a drop in generic diversity of nearly 50% at the equivalent horizon, whereas the rise of the SPICE excursion was a time of faunal diversification, at least in Laurentia (Rowell and Brady, 1976; Palmer, 1984). Stitt (1975) and Palmer (1984) noted that early Steptoean trilobites appear to have been adapted to cool waters typical of continental-slope settings and suggested that the extinctions recorded near the base of the Steptoean were caused by cooling resulting from ocean overturn. However, Pratt (1992, 1998) documented a similar mass extinction of indigenous trilobites in a deep-ramp setting, which is inconsistent with Stitt’s hypothesis. The base of the G. reticulatus zone also coincides with turnovers in polymeroid trilobites in South and North China, Kazakhstan, and Siberia (Peng et al., 2004, and references therein), although no distinct extinction patterns are identified there. The nature of the pre-SPICE extinction mechanism in these platforms is unknown. Other Phanerozoic mass extinctions associated with positive excursions may have been controlled by different factors: e.g., the Cenomanian-Turonian boundary extinction began soon after the onset of a positive δ13C excursion but continued through its peak (Jarvis et al., 1988), whereas the rise of the SPICE excursion was a time of faunal diversification, at least in Laurentia (Rowell and Brady, 1976; Palmer, 1984). The excursion must likely reflect the enhanced burial of organic matter and carbon and the development of low-oxygen conditions on the platform (Saltzman, 1999), leading to a fall in atmospheric pCO2 (Kump and Arthur, 1999). The peak of the SPICE event coincides with a time of maximum regression (Saltzman et al., 2000), and the subsequent increased weathering and erosion rates during relative sea-level fall (Sauk II–III) would increase the burial fraction of organic carbon. However, Glumac and Walker (1998) pointed out that, in the southern Appalachians, the excursion most likely reflects the enhanced burial of organic carbon produced by ocean stratification, a warm nonglacial climate, and a sea-level maximum during the beginning of the Furongian. Similar

972

transgressive conditions are documented in the Montagne Noire, where the positive carbon isotope excursion is recorded both in reddish (oxidized) limestone and shale substrates of the inner and outer platform; these substrates record the development of shelly nonreefal meadows and burrowing, reflecting normal oxygenic conditions. This analysis of the Montagne Noire allows us to identify increasing rates of burial of organic matter, which did not overpass values of 0.15 wt%, without development of low-oxygen conditions on the platform associated with the broad transgressive trend recorded across the Cambrian Series 3–Furongian transition. In some cases, major positive carbon isotope excursions are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas (some Silurian examples are synthesized in Cramer and Saltzman, 2007). Models of the global carbon cycle during positive excursions in the carbon isotope ratio of marine waters have currently concentrated on the importance of sequestration and burial of 12C in the form of organic matter (Kump and Arthur, 1999). This model has been supported by the frequent recognition of close stratigraphic associations between transient positive carbon isotope excursions and widespread organic-rich black shale deposition in epeiric sea settings: the sequestration and burial of 12C in epicontinental black shales should leave an isotopically enriched water mass in epeiric sea settings. However, in many instances where individual isotopic excursions were previously attributed to specific intervals of epicontinental black shale deposition, the peak in carbon isotope values significantly postdates the end of black shale deposition in epeiric seas (e.g., Menegati et al., 1998; Racki and Wignall, 2001). As stated by Saltzman et al. (2000), the oceanic anoxic event (OAE) model of Arthur et al. (1987) and the Monterey model of Vincent and Berger (1985) involve enhanced removal of organic matter during intervals of high productivity and/or anoxia. Both models require the formation of copious amounts of kerogen-rich black shales near sites of upwelling. However, the SPICE event cannot be related to the sharp occurrence of worldwide black shales, associated with oxygen-depleted conditions. A general drowning of platforms bordering the Baltic, Avalonian, and European margins of Gondwana took place from the middle part of Cambrian Series 3. These are lithostratigraphically represented, for instance, by: (1) the Cambrian Series 3 Manuels River Formation and the upper Cambrian Series 3–Lower Ordovician Elliott Cove Formation in western Avalonia (Landing, 1996), and their counterparts in eastern Avalonia represented by the Mancetter, Outwoods, and

Mons Park Formations (Rushton et al., 1999); (2) the lower Cambrian Series 3–Lower Ordovician Alum Shales in Baltica (Ahlberg and Bergström, 1998); and (3) the middle Furongian– Lower Ordovician Lampara, Santa Rosita, and Iscayachi Formations in eastern Cordillera and northwest Argentina (Mángano et al., 1996; Aceñolaza and Aceñolaza, 2000). By contrast, the Cambrian Series 2–early Cambrian Series 3 starved basins that developed on the Siberian Platform disappeared during the Mayan (Cambrian Series 3) –Furongian (Pegel, 2000). As a result, the OAE model does not seem to be adequate as a primary triggering factor of the SPICE event. Although not directly related to the onset of black shales, Weissert et al. (1998) proposed that choking of carbonate production on rapidly drowning carbonate platforms contributed to the observed positive shifts in the marine carbon reservoir; e.g., Wortmann and Weissert (2000) documented that the collapse of a Valanginian carbonate platform coincided with the onset of a positive carbon excursion. A middle Cambrian Series 3 generalized drowning of platforms has been recorded in southwestern Europe, which led to the gradual disappearance of carbonate factories (Álvaro et al., 2003a). However, a close link between carbon isotope events and carbonate platform–drowning episodes is in contradiction with the occurrence of the griotte limestones in the Montagne Noire at the onset of the SPICE event. Based on evidence for sea-level fall and erosion of the Laurentian platforms during the SPICE excursion, Saltzman et al. (2000) considered the event to be a positive excursion similar to the Late Ordovician event of Kump et al. (1999). Although some positive stable isotopic excursions are distinctly associated with global cooling (e.g., the Late Ordovician glaciation; Middleton et al., 1991), the SPICE event is not associated with the buildup of glaciers (Saltzman et al., 2004). Many major Paleozoic positive carbon isotope excursions, such as the Silurian Ireviken excursion, have been found to occur during intervals of increased carbonate production rather than organic carbon burial in epeiric seas (Bickert et al., 1997; Kump et al., 1999; Saltzman, 2001, 2002; Munnecke et al., 2003). Although high organic carbon burial is still considered to be the driving mechanism of the positive excursions, Cramer and Saltzman (2007) suggested that epeiric black shale production immediately preceded the positive carbon isotope excursion, and that the positive δ13C excursion was coincident with an expansion of carbonate platform environments throughout epeiric seas, while increased organic carbon

Geological Society of America Bulletin, July/August 2008

The DICE and SPICE events in a midlatitude platform, Montagne Noire burial was restricted to deep basins and the deep ocean. Thus, the SPICE excursion may be associated with an expansion of carbonate platform environments throughout midlatitude epeiric seas (in our case, confirmed by the occurrence of the griotte limestone just at the onset of the SPICE event) and a decrease in organic carbon burial along high-latitude continental margins. In this case, the intense organic carbon burial would be restricted to the deep ocean and the deepest of intracratonic basin settings. CONCLUSIONS The Cambrian Series 3 to middle Tremadocian strata of the Montagne Noire, represented by the Val d’Homs and La Gardie Formations, were deposited on a passive continental margin of West Gondwana. During this interval, the Montagne Noire platform records episodic tectonic activity reflected by the development of a paleotopography composed of paleohighs bearing carbonate factories and lows with shaledominated deposition. This carbonate productivity is unique in temperate waters of the western Gondwana margin, and it was progressively deteriorated related to the final drowning of paleohighs, and the southward drift of this Gondwanan margin to subpolar positions. Several geochemical parameters showing stratigraphic variation have been employed as an aid to chemostratigraphic correlation and as an index of changing paleoceanographic conditions across the Cambrian Series 3–Furongian transition, e.g., total organic carbon (TOC) values of clay-rich sediments, carbon isotope values of organic carbon and carbonates, and oxygen isotope ratios of microsparitic-rich limestones. Stable isotopes of Corg and Ccarb from two synchronous sections of the La Gardie and Val d’Homs Formations provide a record of a negative middle Languedocian δ13Corg excursion and a coeval positive δ13 Ccarb and δ13Corg excursion at the onset of the Furongian. The δ13C fluctuations do not correlate with sea-level variations but were constrained by diagenetic alteration: (1) the light δ18O values of the analyzed limestones suggest that they underwent diagenetic alteration, probably caused by oxidation of organic matter and burial temperature; and (2) the δ13Corg background values from basinal shale strata of the La Gardie Formation show a gradual decrease from −22‰ to −27.1‰, which was directly controlled by TOC contents and is interpreted to have been controlled by changes in the degree of biodegradation of organic matter, sulfate reduction, and methanogenesis before or soon after burial. Despite the diagenetic overprinting, and the anchizonal degree homogeneously shown by the shales, the δ13Corg

background trend is also punctuated by the aforementioned sharp shifts of δ13Corg values. As a result, the survival of the primary carbon isotope signature is a clear indication that the DICE and SPICE events are valuable tools for precise intercontinental correlation. The asymmetric shape of the SPICE excursion in the Montagne Noire shows evidence of the HERB event, a late Furongian carbon isotope negative excursion not yet accepted as a worldwide chemostratigraphic anomaly. The recognition of these events in mixed and shale strata opens new possibilities of chemostratigraphic correlation in other mid- and high-latitude platforms where carbonate factories did not widely develop and strata only contain endemic fauna. ACKNOWLEDGMENTS

The authors thank the constructive criticism and proposals made by Loren Babcock, A. Hope Jahren, Karl E. Karlstrom, Karlis Muehlenbachs, Brendan Murphy, Matt Saltzman, and two anonymous referees, who have greatly improved the ideas expressed in this work. This paper is a contribution to the Working Group on Cambrian Geochemical Correlation of the International Subcommission on Cambrian Stratigraphy, and CGL 2006-13533/BTE Project financed by the Spanish Ministerio de Educación y Ciencia and Fonds Européen de Développement Régional. REFERENCES CITED Aceñolaza, G.F., and Aceñolaza, F., 2000, The Cambrian System in northwest Argentina, in Aceñolaza, G.F., and Peralta, S., eds., Cambrian from the Southern Edge: Instituto Superior de Correlación Geológica, Miscelánea, v. 6, p. 46–50. Ahlberg, P., and Bergström, J., 1998, The Cambrian of Scania, in Ahlberg, P., ed., Guide to excursions in Scania and Västergötland, Southern Sweden: Lund Publications in Geology, v. 141, p. 20–23. Ahlberg, P., Axheimer, N., and Robison, R.A., 2007, Taxonomy of Ptychagnostus atavus: A key trilobite in defining a global stage boundary: Geobios, v. 40, 709-714. Allen, J.R., and Matthews, R.K., 1982, Isotopic signature associated with early meteoric diagenesis: Sedimentology, v. 29, p. 791–817, doi: 10.1111/j.1365-3091.1982. tb00085.x. Álvaro, J.J., and Vizcaïno, D., 1998, Révision biostratigraphique du Cambrien moyen du versant méridional de la Montagne Noire (Languedoc, France): Bulletin de la Société Géologique de France, v. 169, p. 233–242. Álvaro, J.J., Rouchy, J.M., Bechstädt, T., Boucot, A., Boyer, F., Debrenne, F., Moreno-Eiris, E., Perejón, A., and Vennin, E., 2000, Evaporitic constraints on the southward drifting of the western Gondwana margin during Early Cambrian times: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 160, p. 105–122, doi: 10.1016/S0031-0182(00)00061-4. Álvaro, J.J., Elicki, O., Geyer, G., Rushton, A.W.A., and Shergold, J.H., 2003a, Palaeogeographical controls on the Cambrian trilobite immigration and evolutionary patterns reported in the western Gondwana margin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 195, p. 5–35, doi: 10.1016/S0031-0182(03)00300-6. Álvaro, J.J., González-Gómez, C., and Vizcaïno, D., 2003b, Paleogeographic patterns of the Cambrian-Ordovician transition in the southern Montagne Noire (France): Preliminary results: Bulletin de la Société Géologique de France, v. 174, p. 23–31. Álvaro, J.J., Ferretti, A., González-Gómez, C., Serpagli, E., Tortello, M.F., Vecoli, M., and Vizcaïno, D., 2007,

A review of the Late Cambrian (Furongian) palaeogeography in the western Mediterranean region, NW Gondwana: Earth-Science Reviews, v. 85, p. 47–81, doi: 10.1016/j.earscirev.2007.06.006. Arthur, M.A., Schlanger, S.O., and Jenkyns, H.C., 1987, The Cenomanian-Turonian oceanic anoxic event: II. Palaeoceanographic controls on organic-matter production and preservation, in Fleet, A.J., and Brooks, J., eds., Marine Petroleum Source Rocks: Geological Society [London] Special Publication 26, p. 401–420. Babcock, L.E., Rees, M.N., Robison, R.A., Lagenburg, E.S., and Peng, S.C., 2004, Potential global stratotype section and point for a Cambrian stage boundary defined by the first appearance of the trilobite Ptychagnostus atavus, Drum Mountains, Utah, USA: Geobios, v. 37, p. 149–158, doi: 10.1016/j.geobios.2003.03.007. Babcock, L.E., Robison, R.A., Rees, M.N., Peng, S., and Saltzman, R., 2007, The global boundary stratotype and point (GSSP) of the Drumian Stage (Cambrian) in the Drum Mountains, Utah, USA: Episodes, v. 30, p. 84–94. Bartley, J.K., and Kah, L.C., 2004, Marine carbon reservoir Corg-Ccarb coupling, and the evolution of the Proterozoic carbon cycle: Geology, v. 32, p. 129–132, doi: 10.1130/ G19939.1. Bauluz, B., Fernández-Nieto, C., and González López, J.M., 1998, Diagenesis-very low-grade metamorphism of clastic Cambrian and Ordovician sedimentary rocks in the Iberian Range (Spain): Clay Minerals, v. 33, p. 373–393, doi: 10.1180/000985598545697. Berner, R.A., 1990, Atmospheric carbon dioxide over Phanerozoic time: Science, v. 249, p. 1382–1386, doi: 10.1126/science.249.4975.1382. Bickert, T., Pätzold, J., Samtleben, C., and Munnecke, A., 1997, Paleoenvironmental changes in the Silurian indicated by stable isotopes in brachiopod shells from Gotland, Sweden: Geochimica et Cosmochimica Acta, v. 61, p. 2717–2730, doi: 10.1016/ S0016-7037(97)00136-1. Brand, U., 2004, C, O and Sr isotopes in Paleozoic carbonate components: An evaluation of original seawater and glaciations: Geological Society of America Bulletin, v. 110, p. 1499–1512. Brasier, M.D., and Sukhov, S.S., 1998, The falling amplitude of carbon isotopic oscillations through the Lower to Middle Cambrian: Northern Siberian data: Canadian Journal of Earth Sciences, v. 35, p. 353–373, doi: 10.1139/cjes-35-4-353. Brasier, M., Cowie, J., and Taylor, M., 1994, Decision on the Precambrian-Cambrian boundary stratotype: Episodes, v. 17, p. 95–100. Brock, G.A., Engelbretsen, M.J., Jago, J.B., Kruse, P.D., Laurie, J.R., Shergold, J.H., Shi, G.R., and Sorauf, J.E., 2000, Palaeobiogeographic affinities of Australian Cambrian faunas: Memoir of the Association of Australasian Palaeontologists, v. 23, p. 1–61. Bruckschen, P., Oesmann, S., and Veizer, J., 1999, Isotope stratigraphy of the European Carboniferous: Proxy signals for ocean chemistry, climate and tectonics: Chemical Geology, v. 161, p. 127–163, doi: 10.1016/ S0009-2541(99)00084-4. Calver, C.R., 2000, Isotope stratigraphy of the Ediacaran (Neoproterozoic III) of the Adelaide Rift Complex, Australia, and the overprint of water column stratification: Precambrian Research, v. 100, p. 121–150, doi: 10.1016/S0301-9268(99)00072-8. Cocks, L.R.M., and Torsvik, T.H., 2002, Earth geography from 500 to 400 million years ago. A faunal and palaeomagnetic review: Journal of the Geological Society [London], v. 159, p. 631–644. Cooper, R.A., and Nowlan, G.S., 1999, Proposed global stratotype section and point for the base of the Ordovician System: International Working Group on the Cambrian-Ordovician Boundary, March 1999, p. 1–28. Craig, H., 1957, Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analyses of carbon dioxide: Geochimica et Cosmochimica Acta, v. 12, p. 133–149, doi: 10.1016/0016-7037(57)90024-8. Cramer, B.D., and Saltzman, M.R., 2007, Fluctuations in epeiric sea carbonate production during Silurian positive carbon isotope excursions: A review of proposed palaeoceanographic models: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 245, p. 37–45, doi: 10.1016/j.palaeo.2006.02.027.

Geological Society of America Bulletin, July/August 2008

973

Álvaro et al. Foden, J., Elburg, M.A., Dougherty-Page, J., and Burtt, A., 2006, The timing of duration of the Delamerian orogeny: Correlation with the Ross orogen and implications for Gondwana assembly: The Journal of Geology, v. 114, p. 189–210, doi: 10.1086/499570. Fortey, R.A., and Cocks, L.R.M., 2003, Palaeontological evidence bearing on global Ordovician-Silurian continental reconstructions: Earth-Science Reviews, v. 61, p. 245–307, doi: 10.1016/S0012-8252(02)00115-0. Frey, M., 1987, Low Temperature Metamorphism: Blackie, Glasgow, 51 p. Gao, G., and Land, L.S., 1991, Geochemistry of Cambrian-Ordovician Arbuckle Limestone, Oklahoma: Implications for diagenetic δ18O alteration and secular δ13C and 87Sr/ 86Sr variation: Geochimica et Cosmochimica Acta, v. 55, p. 2911–2920, doi: 10.1016/0016-7037(91)90456-F. Glumac, B., and Walker, K.R., 1998, A Late Cambrian positive carbon-isotope excursion in the southern Appalachians: Relation to biostratigraphy, sequence stratigraphy, environments of deposition, and diagenesis: Journal of Sedimentary Research, v. 68, p. 1212–1222. Grossman, E.L., Zhang, C., and Yancey, T.E., 1991, Stableisotope stratigraphy of brachiopods from Pennsylvanian shales in Texas: Geological Society of America Bulletin, v. 103, p. 953–965, doi: 10.1130/0016-7606( 1991)1032.3.CO;2. Hayes, J.M., Kaplan, I.R., and Wedeking, K.W., 1983, Precambrian organic geochemistry, preservation of the record, in Schopf, J.W., ed., The Earth’s Earliest Biosphere: Its Origin and Evolution: Princeton, Princeton University Press, p. 93–134. Hayes, J.M., Popp, B.N., Takigiku, R., and Johnson, M.W., 1989, An isotopic study of biogeochemical relationships between carbonates and organic carbon in the Greenhorn Formation: Geochimica et Cosmochimica Acta, v. 53, p. 2961–2972, doi: 10.1016/0016-7037(89)90172-5. He, Z., 1995, Sedimentary Facies and Variation of Stable Isotope Composition of Upper Cambrian to Lower Ordovician Strata in Southern Missouri: Implications for the Origin of MVT Deposits and the Geochemical and Hydrological Features of Regional Ore-Forming Fluids [Ph.D. thesis]: Rolla, University of Missouri, 124 p. Hesselbo, S.P., Gröcke, D.R., Jenkyns, H.C., Bjerrum, C.J., Farrimand, P., Morgans Bell, H.S., and Green, O.R., 2000, Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event: Nature, v. 406, p. 392–395, doi: 10.1038/35019044. Holser, W.S., 1997, Geochemical events documented in inorganic carbon isotopes: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 132, p. 173–182, doi: 10.1016/S0031-0182(97)00070-9. Howley, R.A., Rees, M.N., and Jiang, C.Q., 2006, Significance of Middle Cambrian mixed carbonate-siliciclastic units for global correlation, southern Nevada, USA: Palaeoworld, v. 15, p. 360–366, doi: 10.1016/j. palwor.2006.10.010. Jarvis, I., Carson, G.A., Cooper, M.K.E., Hart, M.B., Leary, P.N., Tocher, B.A., Horne, D., and Rosenfeld, A., 1988, Microfossil assemblages and the Cenomanian-Turonian (Late Cretaceous) oceanic anoxic event: Cretaceous Research, v. 9, p. 3–103, doi: 10.1016/0195-6671(88)90003-1. Joachimski, M.M., 1997, Comparison of organic and inorganic isotope patterns across the Frasnian-Famennian boundary: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 132, p. 133–145, doi: 10.1016/ S0031-0182(97)00051-5. Kaufman, A.J., and Knoll, A.H., 1995, Neoproterozoic variations in the carbon isotopic composition of seawater: Stratigraphic and biogeochemical implications: Precambrian Research, v. 73, p. 27–49, doi: 10.1016/0301-9268(94)00070-8. Kisch, H.J., 1991, Illite crystallinity: Recommendations on sample preparation, X-ray diffraction settings, and interlaboratory samples: Journal of Metamorphic Geology, v. 9, p. 665–670, doi: 10.1111/j.1525-1314.1991. tb00556.x. Knoll, A.H., Hayes, J.M., Kaufman, A.J., Swett, K., and Lambert, I.B., 1986, Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard

974

and East Greenland: Nature, v. 321, p. 832–838, doi: 10.1038/321832a0. Koch, P.L., Zachos, J.C., and Gingerich, P.D., 1992, Correlation between isotope records in marine and continental carbon reservoirs near the Palaeocene/ Eocene boundary: Nature, v. 358, p. 319–322, doi: 10.1038/358319a0. Kump, L.R., and Arthur, M.A., 1999, Interpreting carbonisotope excursions: Carbonates and organic matter: Chemical Geology, v. 161, p. 181–198, doi: 10.1016/ S0009-2541(99)00086-8. Kump, L.R., Arthur, M.A., Patzkowsky, M.E., Gibbs, M.T., Pinkus, D.E., and Sheehan, P.M., 1999, A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 152, p. 173–187, doi: 10.1016/S0031-0182(99)00046-2. Landing, E., 1996, Avalon: Insular continent by the latest Precambrian, in Nance, R.D., and Thompson, M.D, eds., Avalonian and related Perigondwanan terranes of the circum-North Atlantic: Geological Society of America Special Paper 304, p. 29–63. Magaritz, M., Krishnamurthy, R.V., and Holser, T., 1992, Parallel trends in organic and inorganic carbon isotopes across the Permian/Triassic boundary: American Journal of Science, v. 292, p. 727–739. Mángano, M.G., Buatois, L.A., and Aceñolaza, G.F., 1996, Trace fossils from a Late Cambrian–Early Ordovician tide-dominated shelf (Santa Rosita Formation, northwest Argentina): Implications for ichnofacies models and shallow marine successions: Ichnos, v. 5, p. 53–88. Marshall, J.D., 1992, Climatic and oceanographic isotope signals from the carbonate rock record and their preservation: Geological Magazine, v. 2, p. 143–160. Menegatti, A.P., Weissert, H., Brown, R.S., Tyson, R.V., Farrimond, P., Strasser, A., and Caron, M., 1998, High resolution δ13C stratigraphy through the early Aptian “Livello Selli” of the Alpine Tethys: Paleoceanography, v. 13, p. 530–545, doi: 10.1029/98PA01793. Middleton, P.D., Marshall, J.D., and Brenchley, P.J., 1991, Evidence for isotopic changes associated with Late Ordovician glaciation from brachiopods and marine cements of central Sweden, in Barnes, C.R., and Williams, S.E., eds., Advances in Ordovician Geology: Geological Survey of Canada Paper 90 (9), p. 313–323. Miller, J.F., Ethington, R.L., Evans, K.R., Holmer, L.E., Coch, J.D., Popov, L.E., Repetski, J.E., Ripperdan, R.L., and Taylor, J.F., 2006, Proposed stratotype for the base of the highest Cambrian stage at the first appearance datum of Cordylodus andresi, Lawson Cow section, Utah, USA: Palaeoworld, v. 15, p. 384–405, doi: 10.1016/j.palwor.2006.10.017. Montañez, I.P., Osleger, D.A., Mack, L.E., and Musgrove, M., 2000, Evolution of the Sr and C isotope composition of Cambrian oceans: GSA Today, v. 10, no. 5, p. 1–7. Munnecke, A., Samtleben, C., and Bickert, T., 2003, The Ireviken Event in the Lower Silurian of Gotland, Sweden— Relation to similar Palaeozoic and Proterozoic events: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 195, p. 99–124, doi: 10.1016/S0031-0182(03)00304-3. Nysæther, E., Torsvik, T.H., Feist, R., Walderhang, H.J., and Eide, E.A., 2002, Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (southern France): Earth and Planetary Science Letters, v. 203, p. 329–341, doi: 10.1016/S0012821X(02)00847-6. Öpik, A.A., 1966, The early Upper Cambrian crisis and its correlation: Journal and Proceedings of the Royal Society of New Wales, v. 100, p. 9–14. Osleger, D.A., and Read, J.F., 1993, Comparative analysis of methods used to define eustatic variations in outcrop: Late Cambrian interbasinal sequence development: American Journal of Science, v. 293, p. 157–216. Palmer, A.R., 1965, Trilobites of the Late Cambrian Pterocephaliid Biomere in the Great Basin, United States: U.S. Geological Survey Professional Paper 493, p. 1–105. Palmer, A.R., 1984, The biomere problem: Evolution of an idea: Journal of Paleontology, v. 58, p. 599–611. Pegel, T.V., 2000, Evolution of trilobite biofacies in Cambrian basins of the Siberian Platform: Journal of

Paleontology, v. 74, p. 1000–1019, doi: 10.1666/00223360(2000)0742.0.CO;2. Peng, S., and Robison, R.A., 2000, Agnostoid biostratigraphy across the middle-upper Cambrian boundary in Hunan, China: Palaeontological Society Memoir, v. 53, p. 1–104. Peng, S.C., Babcock, L.E., Lin, H.L., Chen, Y.G., and Zhu, X.J., 2001, Potential global stratotype section and point for the base of an upper Cambrian series defined by the first appearance of the trilobite Glyptagnostus reticulatus, Hunan Province, China: Acta Palaeontologica Sinica, v. 40, supplement, p. 151–172. Peng, S.C., Babcock, L.E., Robison, R.A., Lin, H., Rees, M.N., and Saltzman, M.R., 2004, Global standard stratotype-section and point (GSSP) of the Furongian Series and Paibian Stage (Cambrian): Lethaia, v. 37, p. 365–379, doi: 10.1080/00241160410002081. Pratt, B.R., 1992, Trilobites of the Marjuman and Steptoean stages (Upper Cambrian), Rabbitkettle Formation, southern Mackenzie Mountains, northwest Canada: Palaeontographica Canadiana, v. 9, p. 1–179. Pratt, B.R., 1998, Probable predation on Upper Cambrian trilobites and its relevance for the extinction of soft-bodied Burgess Shale–type animals: Lethaia, v. 31, p. 73–88. Qing, H., and Veizer, J., 1994, Oxygen and carbon isotope composition of Ordovician brachiopods: Implications for coeval seawater: Geochimica et Cosmochimica Acta, v. 58, p. 4429–4442, doi: 10.1016/ 0016-7037(94)90345-X. Racki, G., and Wignall, P., 2001, Eutrophication by decoupling of the marine biogeochemical cycles of C, N, and P: A mechanism for the Late Devonian mass extinction: Geology, v. 29, p. 469–470, doi: 10.1130/0091-76 13(2001)0292.0.CO;2. Rowell, A.J., and Brady, M.J., 1976, Brachiopods and biomeres: Brigham Young University Geology Studies, v. 23, p. 165–180. Rushton, A.W.A., Owen, A.W., Owens, R.M., and Prigmore, J.K., 1999, British Cambrian to Ordovician Stratigraphy: Peterborough, Joint Nature Conservation Committee, Geological Conservation Review Series 18, xxi + 435 p. Saltzman, M.R., 1999, Upper Cambrian carbonate platform evolution, Elvinia and Taenicephalus zones (pterocephaliid-ptychaspid biomere boundary), northwestern Wyoming: Journal of Sedimentary Research, v. 69, p. 926–938, doi: 10.1306/D4268ABD-2B26-11D78648000102C1865D. Saltzman, M.R., 2001, Silurian δ13 C stratigraphy: A view from North America: Geology, v. 29, p. 671–674, doi: 10 .1130/0091-7613(2001)0292.0.CO;2. Saltzman, M.R., 2002, Carbon isotope δ13C stratigraphy across the Silurian-Devonian transition in North America: Evidence for a perturbation of the global carbon cycle: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 187, p. 83–100, doi: 10.1016/S00310182(02)00510-2. Saltzman, M.R., Runnegar, B., and Lohmann, K.C., 1998, Carbon isotope stratigraphy of Upper Cambrian (Steptoean Stage) sequences of the eastern Great Basin: Record of a global oceanographic event: Geological Society of America Bulletin, v. 110, p. 285–297, doi: 10. 1130/0016-7606(1998)1102.3.CO;2. Saltzman, M.R., Ripperdan, R.L., Brasier, M.D., Lohmann, K.C., Robison, R.A., Chang, W.T., Peng, S., Ergaliev, E.K., and Runnegar, B., 2000, A global carbon isotope excursion (SPICE) during the Late Cambrian: Relation to trilobite extinctions, organic-matter burial and sea level: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 162, p. 211–223, doi: 10.1016/ S0031-0182(00)00128-0. Saltzman, M.R., Cowan, C.A., Runkel, A.C., Runnegar, B., Stewart, M.C., and Palmer, A.R., 2004, The Late Cambrian SPICE (δ13C) event and the Sauk II– Sauk III regression: New evidence from Laurentian basins in Utah, Iowa, and Newfoundland: Journal of Sedimentary Research, v. 74, p. 366–377, doi: 10.1306/120203740366. Schrag, D.P., DePaolo, D.J., and Richter, F.M., 1995, Reconstructing past sea surface temperatures: Correcting for diagenesis of bulk marine carbonate: Geochimica et Cosmochimica Acta, v. 59, p. 2265–2278, doi: 10.1016/0016-7037(95)00105-9.

Geological Society of America Bulletin, July/August 2008

The DICE and SPICE events in a midlatitude platform, Montagne Noire Schumacher, D., 1996, Hydrocarbon-induced alteration of soils and sediments, in Schumacher, D., and Abrams, M.A., eds., Hydrocarbon Migration and its Near-Surface Expression: American Association of Petroleum Geologists Memoir 66, p. 71–89. Shergold, J.H., 1996, Cambrian, in Young, G.C., and Laurie, J.R., eds., An Australian Phanerozoic Timescale: Melbourne, Oxford University Press, p. 63–76. Stitt, J.H., 1975, Adaptive radiation, trilobite paleoecology and extinction, Ptychaspid biomere, Late Cambrian of Oklahoma: Fossils and Strata, v. 4, p. 381–390. Strauss, H., DesMarais, D.J., Hayes, J.M., and Summons, R.E., 1992, The carbon-isotope record, in Schopf, J.W., and Klein, C., eds., The Proterozoic Biosphere: A Multidisciplinary Study: Cambridge, Cambridge University Press, p. 117–128. Thompson, R., 2003, Sedimentologic, Sequence Stratigraphic and Stable Isotopic Study of the Late Cambrian Conococheague Formation, Strasburg, VA [Ph.D. thesis]: Baltimore, University of Maryland, GEOL 394, Series, 31 p. Veizer, J., Fritz, P., and Jones, B., 1986, Geochemistry of brachiopods: Oxygen and carbon isotopic records of Paleozoic oceans: Geochimica et Cosmochimica Acta, v. 50, p. 1679–1696, doi: 10.1016/0016-7037(86)90130-4. Veizer, J., Bruckschen, P., Pawellek, F., Diener, A., Podlaha, O.G., Jasper, T., Korte, C., Carden, G.A.F., Strauss, H., Azmy, K., and Ala, D., 1997, Oxygen isotope evolution of Phanerozoic seawater: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 132, p. 159–172, doi: 10.1016/S0031-0182(97)00052-7.

Veizer, J., Ala, D., Azmy, P., Bruckschen, P., Buhl, D., Brhun, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O., and Strauss, H., 1999, 87Sr/86Sr, δ13C, and δ18O evolution of Phanerozoic seawater: Chemical Geology, v. 161, p. 59–88, doi: 10.1016/S0009-2541(99)00081-9. Vincent, E., and Berger, W.H., 1985, Carbon dioxide and polar cooling in the Miocene: The Monterey Hypothesis, in Sundquist, E.T., and Broecker, W.S., eds., The Carbon Cycle and Atmospheric CO2: Natural Variations from the Archean to the Present: American Geophysical Union, Geophysical Monograph Series 32, p. 455–468. Vizcaïno, D., and Álvaro, J.J., 2003, Adequacy of the Lower Ordovician trilobite record in the southern Montagne Noire (France): Biases for biodiversity documentation: Transactions of the Royal Society of Edinburgh, Earth Sciences, v. 93, p. 1–9. Wadleigh, M.M., and Veizer, J., 1992, 18O/16O and 13C/12C in lower Paleozoic brachiopods: Implications for the isotopic composition of sea water: Geochimica et Cosmochimica Acta, v. 56, p. 431–443, doi: 10.1016/0016-7037(92)90143-7. Wang, W., Cao, C., and Wang, Y., 2004, The carbon isotope excursion on GSSP candidate section of LopingianGuadalupian boundary: Earth and Planetary Science Letters, v. 220, p. 57–67. Warr, L.N., and Rice, A.H.N., 1994, Interlaboratory standardization and calibration of clay mineral crystallinity size data: Journal of Metamorphic Geology, v. 12, p. 141–152, doi: 10.1111/j.1525-1314.1994.tb00010.x.

Weissert, H., Lini, A., Föllmi, K., and Kuhn, O., 1998, Correlation of Early Cretaceous carbon isotope stratigraphy and platform drowning events: A possible link?: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 137, p. 189–203, doi: 10.1016/S0031-0182(97)00109-0. Wortmann, U.G., and Weissert, H., 2000, Tying platform drowning to perturbations of the global carbon cycle with a δ13Corg-curve from the Valanginian of DSDP Site 416: Terra Nova, v. 12, p. 289–294, doi: 10.1046/j.1365-3121.2000.00312.x. Yang, J., and Xu, S., 1997, The second-order sequence division and sea level fluctuation in Cambrian on the border of Sichuan, Guizhou and Hunan: Earth-Science Journal of China University of Geosciences, v. 22, p. 466–470. Zhu, M.Y., Zhang, J.M., Li, G.X., and Yang, A.H., 2004, Evolution of C isotopes in the Cambrian of China: Implications for Cambrian subdivision and trilobite mass extinctions: Geobios, v. 37, p. 287–301, doi: 10.1016/j.geobios.2003.06.001. Zhu, M.Y., Babcock, L.E., and Peng, S.C., 2006, Advances in Cambrian stratigraphy and paleontology: Integrating correlation techniques, paleobiology, taphonomy and paleoenvironmental reconstruction: Palaeoworld, v. 15, p. 217–222, doi: 10.1016/j.palwor.2006.10.016. MANUSCRIPT RECEIVED 13 APRIL 2007 REVISED MANUSCRIPT RECEIVED 26 NOVEMBER 2007 MANUSCRIPT ACCEPTED 8 DECEMBER 2007 Printed in the USA

Geological Society of America Bulletin, July/August 2008

975