jeppsson et al 2007

laeoecology 245 (2007) 115–137www.elsevier.com/locate/palaeo

Palaeogeography, Palaeoclimatology, Pa

High-resolution Late Silurian correlations between Gotland, Sweden,and the Broken River region, NE Australia: Lithologies,

conodonts and isotopes

Lennart Jeppsson a,⁎, John A. Talent b, Ruth Mawson b, Andrew J. Simpson b,Anita S. Andrew b,c, Mikael Calner a,1, David J. Whitford c, Julie A. Trotter c,2,

Olof Sandström a,3, Heidi-Jane Caldon b

a Department of Geology, Sölvegatan 12, SE-223 62 Lund, Swedenb Macquarie University Centre for Ecostratigraphy and Palaeobiology, Department of Earth and Planetary Sciences,

Macquarie University 2109, Australiac CSIRO Petroleum Resources, PO Box 136 North Ryde 1670, Australia

Received 15 February 2005; accepted 2 February 2006

Abstract

High-resolution correlations, partly with a precision better than 10 ka, are achieved between late Ludfordian sequences on Gotland,Sweden (on Baltica), and a section (COG) through the Coral Gardens Formation along the Broken River, northeastern Australia (onGondwana), despite these sections having been on different palaeocontinents facing different oceans. The interval is characterised byrapid, very large faunal, isotopic, and lithologic changes. Lithologies are remarkably similar in the two areas, consisting of, in order frombelow: marls with thin limestone beds, flaggy limestones, oncoidal crinoidal limestone, oncoidal marls, terrigenous clastics (silty clay,mudstone, sandstone), oolite, and cliff-/gorge-forming limestones. Further, independent correlation dates several of the lithologicalchanges as coeval. The δ13C excursion in whole rock carbonates is one of the three largest ones known during the Phanerozoic. Theincrease in δ13C is very similar in the two areas, from below +1‰ to c. +9‰VPDB though two Gotland samples yielded +9.71‰ and+10.54‰ (the corresponding COG interval included fewer carbonate layers than needed to definitely exclude that this difference is dueto a lack of suitable rocks for sampling). Compared with the lithologies and the δ13C curves, the conodont faunas display somedivergence, but key taxa permit precise correlations for much of the studied interval. The conodont assemblages change stepwise fromvery diverse in the Polygnathoides siluricus Zone to a low diversity fauna dominated markedly by a single taxon (Upper IcriodontidSubzone), returning abruptly to a comparatively diverse Ozarkodina snajdri Zone fauna. A method resembling graphic correlation insome respects is used to propose a similarly high-resolution correlation through a longer interval for future testing.© 2006 Elsevier B.V. All rights reserved.

Keywords: Silurian; Conodonts; High-resolution correlation; Carbon isotopes; Sweden; Australia

⁎ Corresponding author. Tel.: +46 46 131299; fax: +46 46 222 4419.E-mail addresses: [email protected] (L. Jeppsson), [email protected] (J.A. Talent), [email protected]

(R. Mawson), [email protected] (A.J. Simpson), [email protected] (A.S. Andrew), [email protected] (M. Calner),[email protected] (D.J. Whitford), [email protected] (J.A. Trotter), [email protected] (O. Sandström).1 Tel.: +46 46 2227379; fax: +46 46 222 4419.2 Petroleum and Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia.3 Helenetorpsgatan 24, SE-214 58 Malmö, Sweden. Tel.: +46 40978020.

0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2006.02.032

mailto:[email protected]









http://dx.doi.org/10.1016/j.palaeo.2006.02.032

116 L. Jeppsson et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 115–137

1. Introduction

The Silurian sequences of Gotland (Sweden) and theBroken River region (northeastern Australia) werelocated on different palaeocontinents: respectivelyBaltica and on the northern Gondwana margin. Al-though faunal differences are evident, conodonts are ofprime importance for correlating the strata of the tworegions. Stable isotopic changes are very similar in bothregions and also useful in the correlations. Unexpected,however, was the very close correspondence in thesequence of lithologies and, based on the various meansof correlation, even synchroneity in the lithologicchanges. The two regions were parts of low latitudeshelf areas on widely separated continents. In ouranalysis we utilise all available data on changes inlithology and conodont faunas and compare these withchanges in ocean chemistry indicated by C, O, and Srisotopic data. Global oceanic cycles – as the origin ofsimilarities and synchronicity of changes – have beendiscussed elsewhere, as has the Lau Event, associatedwith many changes (Jeppsson, 1990, 1998; Aldridge etal., 1993; Jeppsson et al., 1995; Jeppsson and Aldridge,2000; Calner, 2005). Rapid strong oceanic changesduring that event caused synchronous changes in faunas,sediments, and stable isotope ratios that we here use forhigh-resolution correlation of the two sequences. Thetwo successions have not been correlated previously; theidentification of Polygnathoides siluricus in COG wasthe only tie point for correlations of that importantsection.

An anonymous referee asked us to discuss otherfactors controlling the faunal and isotope changes, e.g.facies and dept changes. We completely agree that suchfactors must be eliminated before any conclusions aredrawn; doing that is for us (as it probably is for moststratigraphic colleagues) a routine. Hence, we repeat thatthe faunal changes we use herein occur across the wholerange of facies on Gotland. As an example of our analy-sis we use the most extreme carbonate environmentdiscussed herein, the oolitic facies. It occurred onlytwice, briefly, in the Silurian of Gotland (latestLlandovery to topmost Ludlow), in the Burgsvik(Section 5.8) and in the mid-Homerian Bara OoliteMember (Jeppsson and Calner, 2003). It is similarly rarein the Australian Silurian. Comparisons of the Bara andthe Burgsvik faunas show that: (1) The Bara oolite faunais closely similar to that in the upper Eke Fm (Section5.7.2), but very different from that in the Burgsvikoolite. (2) The Bara fauna is also found in coeval strata(but not below nor above) in the När core in which theWenlock facies typically consists of graptolitic shales

and marls. If facies had controlled the oolite fauna, theBara and Burgsvik oolite faunas should have beenclosely similar, and the När core strata coeval with theBara should have had a very different fauna. Samtlebenet al. (1996, 2000) discussed the relatively minor facieseffects on the isotopic ratios in coeval strata. We notethat the Bara oolite yields δ13C ratios of c. 2–3‰(unpublished data from the Hunninge-1 core, Calner etal.), much lower than that of the Burgsvik oolite, 6.0–10.5‰ (see Section 5.8.3). If the high isotope ratios hadbeen related to the oolitic facies, the Bara oolite wouldbe expected to have similarly high ratios. Other suchcomparisons give similar results, the faunal charactersused herein are not due to the facies changes per se; thatis, coeval changes in lithologies and faunas were not oneof cause and effect; instead they were controlled byanother cause (see Jeppsson, 1990; Jeppsson andAldridge, 2000 for details).

Specification of the interval of a sample is below (−)or above (+) a reference level, and is expressed as+0.00/+0.00 (meters from the reference level, alwayswith the lower boundary first), i.e. the fossil or isotoperatio stated was obtained between the metric intervalsindicated. Conversely, +0.00 – +0.00 specifies bound-aries for an interval having a specific character.

2. Areas studied

2.1. Gotland, Sweden

During the Silurian, the palaeocontinent Baltica waslocated somewhat south of the Equator (Cocks andTorsvik, 2002). The carbonate platform strata of Got-land (Fig. 1) formed in the intracratonic Baltic Basin onthe southern margin of Baltica. The present-day Silurianstrata are an erosional remnant of a major carbonateplatform system that evolved around the basin from thewestern parts of the present-day Baltic Sea northeast-wards to Estonia; it is known from bores between thereand exposures in Podolia (western Ukraine). Strataexposed on Gotland range in age from latest Llandovery(upper Pterospathodus amorphognathoides Zone)through the Ludlow to just above the Ozarkodina crispaZone (Jeppsson, 2006). The entire succession is c. 500–700 m thick depending on where measurements havebeen made. The strata dip 0°–4° to the southeast; theyare neither metamorphosed, strongly folded nor appre-ciably faulted (Figs. 1 and 2). Apart from downdipfacies transitions, the strata also display distinct facieschanges southwestward along strike. Argillaceous lime-stones and marls deposited in an open marine shelfenvironment dominate on western Gotland, whereas

Fig. 1. Studied areas: (A) Gotland and Skåne in Sweden; (B) The position of Bjärsjölagård; (C) Northeastern Australia with position of the BrokenRiver; (D) Location of Fig. 3.

117L. Jeppsson et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 115–137

contemporaneous shoal, reef, and back-reef strataaccumulated episodically toward the northeast. From adepositional point of view, the strata represent a series ofstacked carbonate platforms, with individual platformepisodes being generally some tens of metres thick –from the incipient transgressive surface to the develop-ment of prograding reef complexes – separated byvariably pronounced stratigraphic discontinuities. Theperipheral platform areas and central parts of the basinconsist of marls and graptolitic shales located beneaththe southeastern part of the Baltic Sea, western Lith-uania and Poland. Some of the cyclic changes in sedi-mentation were discovered many years ago (Munthe,1925, Fig. 5, referred to as from [rather, based on datain] Hede, 1921). They correlate with faunal changesincluding extinctions and are now interpreted as reflect-ing global atmospheric and oceanic cyclicity (Jeppsson,1990). Gotland functions as the reference area for thesequence of events and episodes during the latestLlandovery to the end of the Ludlow (Aldridge et al.,

1993; Jeppsson et al., 1995; Jeppsson and Aldridge,2000).

The interval of interest here is the När Formation ofthe Hemse Group, and the Eke, Burgsvik and lowerHamra formations outcropping on southern Gotland(Figs. 1 and 2). Their boundaries have been revisedrecently, and precision in local correlations considerablyincreased (Jeppsson, 2006). For locality descriptions,see Laufeld (1974b) and Jeppsson and Jerre (ms.; copyavailable at Allekvia Field Station). OS did thesedimentology for Bodudd 1 and MC that for otherareas, including the Uddvide 1 bore.

2.2. The Broken River region, northeastern Australia

The Broken River region has an extensive suite ofOrdovician to Early Carboniferous sedimentary units(Withnall et al., 1993; Fig. 3). During the Late Silurian itformed part of the eastern Gondwana margin and layjust south of the Equator (Cocks and Torsvik, 2002).

Fig. 2. Gotland (Sweden) showing localities mentioned in text. Map based on Jeppsson (2006).



The Jack Group, initially referred to as the JackLimestone Member of the Graveyard Creek Formation(White, 1959), conformably overlies the Quinton For-mation, but is inferred to interfinger with it northwards(Withnall and Fleming, 1993). In its type section alongthe Broken River, the Jack Group consists of, frombelow: carbonates (Dark Dog Limestone), clastics (RedBull Formation), interbedded marls and nodular lime-stones (Coral Gardens Formation), and generallymassively-bedded limestones (Jack Hills Limestone;Fig. 4d). The latter two are of special interest here.Munson (1979), Fleming (1986) and Caldon (2003)described lithologies from the type section of JackGroup and in its vicinity. Sloan et al. (1995), Simpson(1999) and Brime et al. (2003) have discussed strati-graphic relations and modifications of nomenclature.

The 100 m thick Dark Dog Limestone (Simpson,1998a) is best exposed near the Jack Hills Gorge. Formany years the age of its base remained uncertain,though a broad Wenlock age had been suggested(Simpson, 1998a). A small conodont fauna, includingthe long-ranging Early Silurian Distomodus staurog-nathoides, was obtained from its base near Six-MileDam 3 km northeast of, and along strike from, the typesection. In the context of underlying pelitic lithologieswith Llandovery (Telychian) graptolites (turriculatus togreistonensis zones; Jell et al., 1993) and conodontsfrom small allochthonous carbonates (Simpson, 1999),the age of the basal Dark Dog Limestone is most prob-ably early Wenlock age, probably Lower Kockelellaranuliformis Zone, with apparently little or no time-break between it and the underlying pelitic sequence.Large Megalomoidea appeared near the middle of theDark Dog as on Gotland in the middle part of the SliteGroup (middle Sheinwoodian). The latest Wenlockconsists widely of carbonates (the Klinte SecundoEpisode, Jeppsson et al., 1995), followed by argilla-ceous early Ludlow sediments (the Sproge Primo Epi-sode; Jeppsson and Aldridge, 2000). By analogy, thebase of the Ludlow in the COG section may be at or nearthe base of the Red Bull Formation and the Dark DogLimestone may represent the whole of Wenlock time.The Red Bull Formation consists of 165 m of thick-bedded, red to purple, medium to coarse micaceousquartzose arenite, succeeded by the Coral Gardens For-mation, 81.2 m in its type section.

Note that sampling limits herein are in meters mea-sured along the ground; true thickness requires multi-plication by 0.974 to compensate for the 77° dip in theCoral Garden Formation. The dip increases to c. 85° inthe middle of the Jack Hills Limestone (246.4 m totalthickness). Above it follow thin-bedded shales and fine-

grained arenites with thin interbedded calcarenites, nowreferred to the Ralph Flint Formation (Talent et al.,2002, 2003; Brime et al., 2003).

3. Conodont collections

3.1. Gotland

Sampling strategy, sample size, processing technique,yield/kg, yield/man-hour, and average number of con-odonts/collection have changed enormously since collect-ing began on Gotland in 1969 (Jeppsson et al., 1985,1999; Jeppsson and Anehus, 1995, 1999; Jeppsson,2005). In some of these variables the increase in yields hasbeen more than a hundred-fold. As a result, most latercollections have at least a few 1000 elements each, evenfor samples that produced only a few 10s to 100s ofelements/kg (for further details of technique see Jeppsson,2005). The samples were processed with the best pH-measured buffered technique known and taken on a63 μm screen (for details see Jeppsson, 2005).

About 10 localities close to the Hemse/Eke boundarywere measured and collected and c. 20 from Eke,Burgsvik, and lower Hamra formations (Jeppsson,2006; Fig. 2). These yielded about 270 collectionsfrom the När, Eke, and Burgsvik formations (c. 60 ofwhich are from the Botvide Member; many morecollections have been studied from older and youngerstrata). The När, Eke and Burgsvik collections producedover 100,000 elements. Most samples from youngerhorizons were less productive than those from the NärFm. In some cases, even samples N100 kg failed toproduce adequate collections (cf. Ward, 1984). For theBotvide Member, most recorded range-ends have anuncertainty of one to a few decimetres.

3.2. The Broken River region

Sampling extended through the Coral Gardens Fm andthe basal 2.9m of the JackHills Limestone. In total, c. 100samples were collected (Fig. 4). Sample spacing forconodonts averaged about 1 m but wasmore concentratedin intervals where lithologic and faunal changes wereapparent. In general, the volume of carbonates acid-leached in quest of conodonts from the Broken Riversequenceswas less than for Gotland. Some age-diagnosticLate Silurian conodont taxa known to occur in rather lowfrequencies may not therefore have been obtained.Conodont yields were generally 2 to 3 elements per kg;the exception, another sample from the bed with COG44.74–44.8 produced 30 elements per kg (Simpson,1983). Considering the similarity in lithologies and


lithologic changes therein and in the thickness of theWenlock–Ludlow sequence in COG and on Gotland, asimilar conodont frequency would be expected. Diver-gence in yields could be due to various factors (or acombination of them). Differing processing methods andbiogeographic factors could only have caused minordivergence in conodont data from the two regions and thedifference in yield is enigmatic. The Broken Riverconodonts have a CAI of 4.5–5.1, are nearly black incolour (Brime et al., 2003) and, unlike the Gotlandconodonts, the white matter cannot be readily observed.This is significant for identification of a few taxa (thewhite matter distribution provides important taxonomiccharacters for some taxa), however, direct comparisons ofthe taxa used for correlations herein have providedenough characters for reliable identifications.

3.3. Conodont zonation

The interval of the main interest here includes theP. siluricus Zone, the Icriodontid Zone (portion ofwhich corresponds, at least partly, with the Icrioduslatialata Zone of Walliser, 1964) and the Ozarkodinasnajdri Zone (Jeppsson, 2006). Problems identif-ied with previous definitions necessitated revisionsof their boundaries. In addition, four new subzoneswere discriminated: Upper P. siluricus Subzone, andthe Lower, Middle and Upper Icriodontid subzones(Figs. 5–8).

4. Sampling and analytical procedures for isotopeanalysis

Whole rocks were chosen to give high-resolutionsampling over intervals of interest. C, O and Sr isotopeanalyses were undertaken on whole rock samples with-out weathered surfaces or veining. For the COG sectionsamples were taken over its 84.1 m (Fig. 5). Goodsections are available on Gotland from the När/Ekeboundary interval. RM and JAT collected samples forisotope analysis from Botvide 1 (Fig. 6); these weresupplemented by pieces from LJ's conodont samples.OS supplied samples from the upper and main part ofBodudd 1 section (Fig. 7). Analyses of the latter werebased on powdered material (an average for each sam-pled interval). Data from younger parts of the sequencerelied on reference pieces from LJ's conodont samples;hence, every one is biostratigraphically dated.

Fig. 3. Broken River region of northeastern Australia showing stratigraphyGardens Formation at the Jack Hills Gorge, and ancillary BRC and MBC secBullock Creek. Geology from Munson (1979), Withnall and Lang (1992), S

Examination of selected sectioned samples by cath-odoluminescence microscopy revealed significant sec-ondary alteration in the COG section, so resultant data areused with caution. Where available, published and newanalyses of secondary layers of brachiopod shell (Samtle-ben et al., 1996, 2000; Wenzel and Joachimski, 1996 andown data) are similar to those from whole-rock samples,suggesting secondary alteration did not significantly alterC and O values. All data about the carbon and oxygenisotopic composition of carbonates is reported in conven-tional δ notation as parts per thousand (‰) relative to theVPDB with calibration against the NBS19 carbonatestandard δ13C=+1.95, δ18O=− 2.20). Replicate analysesof carbonate standard are better than ±0.1 (Stalker et al.,2005). Illustrations were produced electronically from thetables, hence the symbols are placed as recorded;therefore, those values are chiefly used in the text.

Sr isotope analyses were performed mostly on wholerock samples. Strontium was separated using conven-tional cation exchange techniques and the samples ana-lysed on a VG 354 multicollector mass spectrometer atCSIRO North Ryde Laboratories. 87Sr/86Sr ratios havebeen normalised to 86Sr/88Sr=0.1194. All results arenormalised to NBS987 87Sr/86Sr=0.710235. Over theperiod in which the analyses were performed, externalprecision estimated from replicate analyses of theNBS987 standard lay within the range 0.0020–0.0030%(95% confidence limits).

ASA, DJWand JATr performed the isotope analyses.

5. The sequence of changes

5.1. The Early and mid Ludlow

5.1.1. LithologiesOn Gotland, the Hemse Group includes three distinct

units, together ≥200 m (Jeppsson, 2006). (1) A lowermarl unit, Hemse Marl Northwestern Part, ≥105.7 m.(2) Limestone, ≈30 m, dominates in the middle unit. (3)The När Formation, ≥65 m (Jeppsson, 2006), consistsof marl except for limestones in the extreme northeast.The lowermost Hemse Group (as now delimited) hasyielded Ozarkodina bohemica bohemica; hence, isreferable to the lowermost Ludlow.

The early Late Ludlow När Formation consists of inter-bedded marls and argillaceous limestones with a distinctconodont fauna (Jeppsson, 2006). Rounded intraforma-tional pebbles are found at some levels; rare graptolites

in the vicinity of the pivotal COG section (insert) through the Coraltions through the Jack Group at the Broken River Crossing and middleimpson (1995b), and Talent et al. (2002).

Fig. 4. (a) COG section through the Coral Gardens Formation with sample bags for isotopic data along a metric tape, viewed from high on the JackHills Gorge on the left (north) flank of the Broken River, NE Australia. Photo JAT, 1991. (b) Members 2 and 3 of the Coral Gardens Formation,Broken River. The step-up is horizon 43.5 m with an approximate change in relief of 50 cm. The boundary marks the transition from the geologicallyolder interval (on the left and middle part of the figure) with medium-grained mica-rich siltstones with lenticular, nodular or thin-beds of limestone(packstones and bindstones), sometimes biostromal, with abundant stromatoporoids, tabulates and occasional solitary rugose corals (7.3–43.5 m) into(on the right) flaggy limestones with interbedded rudstones, packstones and bindstones with abundant Cyanobacteria, microproblematica, Girva-nella, Sphaerocodium and Wetheredella (43.5–51.4 m). The latter interval has produced the microbial bloom post-dating the initial C excursion.Photo H-JC, 2003. (c) The unconformable boundary between the Hemse Group and the Eke Formation at Botvide 1. The centimetre scale bar rests onthe boundary, which is developed as an abraded hard ground overlain by a conglomerate. The interval is coeval with the topmost step-up in Fig. 4band the immediately overlaying strata. Photo MC. (d) Composite photograph of the Coral Gardens Formation (left part of the figure) and Jack HillsLimestone (right part) viewed from left (north) flank of the Broken River, NE Australia. Photo H-JC, 2003.


occur especially on western Gotland (Hede, 1919b, 1942).In the far northeast, crinoidal limestones cap the hills; theserepresent only the lower part of the formation.

In COG, Member 1 (0–14.1 m) of the Coral GardensFormation consists of thinly interbedded nodular lime-stones, siltstones, and fine-grained micaceous labile

Fig. 5. Stratigraphic column and isotope curves from COG, through the Coral Gardens Formation and the lowermost Jack Hills Limestone. Note thatidentification of zonal boundaries below COG 40 m is less precise than in the interval that is the main focus of this report.


arenites with abundant in situ coral heads (Munson, 1979;Caldon, 2003) and stromatoporoids (Webby and Zhen,1997). Member 1 has a lower percentage of carbonateinterbeds than Member 2 (from 14.1 to 43.5 m). Up to50%of the skeletal allochems in the carbonates consists ofcoral fragments, brachiopods, medium- and high-spiredgastropods, bryozoans, ostracodes, and ossicles ofcrinoids. The thickness of interbeds varies, but bothcarbonate and pelitic beds are generally less than 10 cm.The limestone beds include a variable but relatively highcontent of sand, silt and clay-equating with marls.

5.1.2. ConodontsConodont faunas throughout Members 1 and 2 of the

Coral Garden Formation consist mainly of Ozarkodina

excavata excavata and Panderodus ssp. From the baseof Member 1 to COG 10.5–10.55 m, the frequency ofOzarkodina excavata elements is roughly equal to that ofPanderodus spp. Rare acanthodian scales occur in COG2.75, COG 3.4 and COG 10.5–10.55, and Panderodusgreenlandensis (?n. ssp.) in COG 7.3–7.35, i.e. lower-most in Member 2. From COG 12.8–12.83 through toCOG 15.05 faunas are dominated by Panderodus spp.,with ?Panderodus panderi in COG 12.8–12.83. O.excavata reappeared in COG 15.85 and dominates ameagre fauna with Panderodus spp.; all elements of theformer in this sample have uncharacteristically elongateprocesses in comparison with other populations. FromCOG 16 through to COG 43.1, uppermost Member 2,coniforms predominate, but other taxa occur in small

Fig. 6. Carbon and oxygen isotope analysis of whole-rock samples from three Gotland localities where the basal Eke Formation is developed as anoncoidal crinoidal limestone with a basal conglomerate, Lithological symbols: black=marls; dark grey=marly limestones, partly dolomitic;white= limestones.


numbers, particularly above COG 23.2. These in-clude Oulodus siluricus siluricus from COG 38.1 andCoryssognathus dubius from COG 27.6. C. dubius wasalso obtained from 6 m above the base of Member 1 onthe northern side of the Broken River but was not foundbelow 27.6 m in COG. Panderodontids from this interval

include P. panderi (COG 40.75 and COG 43.1) andPanderodus serratus (COG 42). From the north side ofthe river, Kockelella ortus was obtained from c. 45 mabove the base of Coral Gardens Formation. In the BRCsection, about 5 km east of COG (Fig. 3), the CoralGardens Formation yielded rare elements of Ancoradella

Fig. 7. Carbon and oxygen isotope analysis of whole-rock samples from Bodudd 1, the most distal section on Gotland, where no erosion surface,conglomerate, or crinoidal limestone have been found. Lithological symbols: black=marls; dark grey=marly limestones, partly dolomitic;white= limestones.


ploeckensis, Kockelella variabilis, Oulodus sp. and, nearthe top of this interval,P. siluricus (see Simpson, 2000). Asingle Pa element of Kockelella maenniki was obtainedfrom 27.3 m below the highest find of P. siluricus(Simpson, unpublished data). In the MBC section, alongstrike from the BRC section, A. ploeckensis was found14.4 m below the highest find of P. siluricus (seeSimpson, 1995a). Simpson (2000) discussed the overlapof these two index taxa, noted by many authors (e.g.Klapper and Murphy, 1975). During 2003 a largespecimen of P. siluricus was found on Gotland in a verylarge collection with Kockelella variabilis variabilisand Ozarkodina n. sp. A, i.e. older than those with A.ploeckensis; hence a similar overlap occurs there(Jeppsson, 2006). In contrast, none of the taxa restrictedto the K. variabilis variabilis s.str. Zone and older stratahave been found in COG.

These records permit correlations with, inter alia,Gotland where O. siluricus siluricus appears first inextremely low numbers in the K. variabilis variabilisZone but is typical of faunas between it and the P.siluricus Zone. A. ploeckensis is limited to a middle partof that interval. P. greenlandensis appeared in or justbelow the K. variabilis variabilis s.str. Zone. C. dubius,populations close to K. maenniki, and acanthodians ap-peared somewhat later. Hence the fauna of Member 2indicates that at least a major part of it may be coevalwith the När Formation on Gotland, and that most of

Member 1 is not older than the K. variabilis variabilisZone, as would be expected if the Red Bull Formationrepresents the Early Ludlow.

Globally, an abundant and diverse conodont faunacharacterises the P. siluricus Zone (Jeppsson, 1975).At least 23 species are present in the När Formationon Gotland. Frequent are Ozarkodina confluens, O.excavata, C. dubius, P. serratus (in the marl area),Panderodus dentatus (in distal areas), P. greenlandensisn. ssp. (in the limestone area), and Panderodus gracilis(at most distal localities). Rare but regular are: P.siluricus, Oulodus cf. excavatus and P. panderi (in themarl area), and Silurognathus maximus (Jeppsson,2006; =Gen. et sp. indet. of Jeppsson, 1983; low inthe formation). O. siluricus siluricus and K. ortus aresimilarly restricted but exceedingly rare. Decoriconus,Pseudooneotodus beckmanni, Belodella sp., Belodellamira, and Ozarkodina n. sp. of Aldridge (1985) aresmall, mostly fragile, and rare, except in the 63–125 μmfraction of some collections from low energy environ-ments. The När Formation is early Ludfordian in age(‘late Leintwardinian’) and coeval with the Neo-cucullograptus kozlowskii graptolite Zone (Jeppsson,2006).

5.1.3. IsotopesThe δ13C and δ18O values from the COG section up to

43.5m show a gradual increase in values of about +1‰ in

Fig. 8. Correlation of δ13C curves from COG and Gotland. At this resolution, the slow increase before the Lau Event contrasts with the rapid shiftduring the event followed by a culmination. The rapid fluctuations through time around the 'average trend' merge into a wide band, giving a falseimpression of substantial uncertainties in the data (compare the curve that, with selected data points, is copied from Fig. 8, where resolution is betterand partly adequate). Gotland lithologies are illustrated from the Uddvide 1 core, using its lithologies for correlating it with the conodont and isotopedata from surface sections and exposures; these data are used for correlating with the COG section. The core was scaled linearly so that the (flaggy)top of the Hemse Group correlated with the top of the flaggy interval in COG, and the base of the Hamra with the base of the well-bedded limestone.As a consequence, the top of the (oncoidal) Eke happens to be nearly coeval with the top of the oncoidal interval in COG. Correlations between COGand Gotland below the uppermost När Formation are less certain than those higher up. Grey diamonds=COG data, X and x together with a localityname=data from Gotland, X=selected examples of our data and x=selected literature data (chiefly Samtleben et al., 1996, 2000); see the text forfurther details and for most locality names.


δ13C (c. +0.03‰/m) and +2.5‰ in δ18O (c. +0.04‰/m;Fig. 5). Equivalent data from the several locations inthe Hemse Group, Gotland (chiefly Samtleben et al.,

1996, 2000; Fig. 8) are not stratigraphically continuousbut show a similar increase of about +1‰ in δ13C(c. +0.01‰/m) but a considerable scatter in δ18O values.


Isotope data from exposures dated with conodonts areimportant for our correlations of COG with Gotland andare hence included in our Fig. 8. The δ13C values fromboth sections are similar, ranging from around 0‰ at thebase of the COG section and in theK. variabilis variabilisZone of theHemseGroup to around+1‰ at 40m inCOGand just below the Botvide Mbr. The δ18O values for theHemseGroup (Figs. 6 and 7) are in line with other data forsea-water in Middle Silurian times (Veizer et al., 1997a)whereas the COG data are significantly depleted in 18Osuggesting secondary processes may have modified theoriginal signature.

From 5.1 m in the COG section up to 38.0 m, the87Sr/86Sr ratios decrease from 0.70905 to 0.708737.This range of values is somewhat higher than mostestimates for the seawater Sr curve for the Ludlow andfollows an upsection decrease in 87Sr/86Sr rather thanincrease (McArthur et al., 2001). However, given thefine-scale sampling resolution in this study, this trendmay be significant. Fifty-five samples were analysedbetween 39.85 and 56.16 m (Figs. 5 and 9). Within thisinterval Sr isotope values vary in an apparent cyclicalpattern within a range between 0.70872 and 0.70882,broadly consistent with Middle Silurian data (Veizeret al., 1997b; McArthur et al., 2001), with 0.7 m percycle. Strontium isotope data above 38 m are distinctlydifferent from those of the preceding argillaceoussediments. Whereas the ratios lie within the range ofthose measured in the lower interval, the change fromdecreasing 87Sr/86Sr to increasing 87Sr/86Sr may besignificant. In the COG and Gotland sections, the inter-val with peak in 87Sr/86Sr values above 0.7088 includes

Fig. 9. 87Sr/86Sr curve for the COG interval between 40 and 50 m.

the flaggy limestone (see Section 5.2); it is more resis-tant to weathering and richer in carbonate than theunderlying marls and argillites.

5.2. The upper P. siluricus subzone

5.2.1. LithologiesOn Gotland, the Botvide Member forms the distinct

uppermost part (c. 2.15 m at Botvide 1, Fig. 6) of the NärFormation. Both fauna and lithology change markedlythrough the member. Its upper part is markedly dolo-mitic and rich in coquinas of the brachiopod Dayianavicula (regarding the topmost decimetres of Hemsesee Section 5.3; Munthe, 1902). D. navicula rangethrough the Hemse and the lower Eke (Hede, 1921) butwe have only seen this kind of mass occurrence in thisinterval. Carbonate content decreases distally (towardsthe SWand south). In the most distal outcrop, Bodudd 1(Fig. 7), there are argillaceous mudstones and scatteredthin limestone interbeds, including at least one Dayiacoquina. Higher weathering resistance causes the uppercontact to be exposed in several sections across Gotland.

The COG sequence 43.5–44.9 m of Member 3 con-sists of thinly bedded (c. 5 to 10 cm) dark flaggybioclastic micrites outcropping more prominently thanunderlying strata due to a marked increase in carbonatecontent (Caldon, 2003; Fig. 4b). As on Gotland, thissubzone may have started slightly before the lithologicchange (see Section 5.2.3).

5.2.2. ConodontsInitially, the conodont faunas on Gotland remained

essentially the same as in the main part of the P.siluricus Zone, although large collections reveal somedifferences. Upwards, many of the species becameextinct or disappeared.

On Gotland, P. siluricus occurs regularly in the NärFormation up to c. −2.15 m at Botvide 1. In the BotvideMember, however, its frequency is below one in 10,000.Only scattered specimens are found, possibly due tocompetition with rare Kockelella (see Jeppsson, 2006).Correlations herein show that the youngest P. siluricusfragment from Gotland was collected c. 0.2 m below thelevel that is coeval with that of the last specimen fromCOG, at 44.70 m, i.e. just below the isotope excursion.Other taxa in parts of, or the entire subzone, includeOzarkodina confluens confluens, O. excavata, K. var-iabilis n. ssp. b, K. ortus n. ssp. y, K. aff. maenniki, O.siluricus siluricus, Panderodus unicostatus, P. gracilis,P. panderi, Decoriconus, Belodella, and P. beckmanni.O. cf. crispa is known from occasional collections andsporadic specimens frommost of the Ludlow on Gotland


(in Australia probably identified as Spathognathodus sp.cf. Spathognathodus ranulifornis by Link and Druce,1972). The Pa(sp) element is morphologically close toboth the O. crispa group and the Early Silurian K.ranuliformis. Identification of non-Pa(sp) elements isneeded to settle its taxonomic position.

COG 43.5 was barren, but COG 44.7 has a char-acteristically diverse fauna dominated by panderodon-tids, P. serratus and P. panderi, and a large number ofacanthodian scales. Other taxa include O. excavataexcavata, O. siluricus siluricus and P. siluricus. Thelater two are the youngest occurrences of these taxa inthe Broken River region. One fragmentary element hasbeen tentatively identified as S. maximus. If it is thatspecies, then this is a younger occurrence than thosereported from the lower När Formation on Gotland.More sampling is needed for confirmation. Otherconiform taxa include Decoriconus fragilis, Belodellaresima and Belodella anomalis. A few coniformelements classified tentatively as ?Icriodus n. sp.(Simpson, 1998b) both in the COG and BRC sectionsappear first at this level, but, as no Pa element has beenrecovered, there is some doubt concerning the genericassignment.

5.2.3. IsotopesIn the COG section, the increase in δ13C values

continues at a slightly greater rate (c. 0.45‰/m) from c.43.5‰ to 2.06‰ at 44.7 m (Fig. 10). At Botvide 1, δ13Cvalues in the interval between −2.0 and −0.2 m increasefrom 1.5‰ to 2.4‰ (c. 0.7‰/m) (Fig. 6). At Bodudd 1,apart from a single sample from the base of the section,the δ13C values are constant at +1.7±0.1‰ from −3.22to −2.75 m (Fig. 7).

At COG, δ18O fluctuates between −9.02‰ and−8.61‰. The δ18O values from Botvide 1 show somecyclical variations but with an overall increase from−6.6‰ to−5.8‰. At least one similar cycle is apparent inthe base of the Bodudd section between −3.1 and −2.8 mrising from a base of −4.8‰ to a peak of −4.2‰. Carbonand oxygen isotope data in this interval reflect continu-ation of the conditions of the interval below, with either anincrease in the activity driving the isotopic changes or, lesslikely, a change in sedimentation rate.

The marked fluctuations in 87Sr/86Sr ratios arounda decreasing trend continue through this interval to0.708792 at 44.7 m (Fig. 9). At least some of the scatterin the data may be related to cycle variations and notspikes in the data-set related to alteration. The 87Sr/86Srratios for the Botvide section are all N0.709 (Fig. 6) andare considered unlikely to represent primary seawatervalues.

5.3. The final early Lau Event

5.3.1. LithologiesOn Gotland, in the topmost decimetre(s) of the

Hemse Group, bedding surfaces are often covered withthe brachiopod Shaleria aff. ornatella, except at, e.g.Botvide 1 (Munthe, 1902). Like D. navicula, his taxonhas a longer range on Gotland but mass occurrences arelimited to this interval as far as we know. No differencein lithologies was noted between those formed duringthis interval and those formed during the preceding partof the Botvide Member.

5.3.2. ConodontsIn the COG section at the Broken River there is a

distinct change in the conodont faunas between COG44.7 and COG 44.74–44.8. Panderodontid elements andacanthodian scales are dominant in the former col-lection. In COG 44.74–44.8 and COG 44.8–44.85,O. excavata predominates over all other conodont taxaby a ratio of approximately 22:1, i.e. it has a frequencyof 95.6%. In contrast to Gotland, no elements of O.confluens have been obtained. Elements of O. excavataare smaller than those from underlying strata and haveclosely packed denticulation. Elements of Panderodusfrom this interval are not identifiable at species level. Inthe BRC section, the distinctive Ozarkodina martins-soni auriformis first occurs with the last find of P.siluricus, whereas in the MBC section the firstappearance is well above this level (Simpson, 1995b,2000).

Through the När Formation and most of the BotvideMember on Gotland, the robustO. confluens confluens isusually better represented than Ozarkodina exavata.However, the topmost Hemse Group includes a similarkind of “Lilliput fauna” at five localities (Jeppsson, 2006).Conodonts are rarer, small specimens dominate, and O.exavata is more frequent than O. confluens, e.g. in mostsamples from the interval between −0.21 and 0m at Nyan2. Thus, during this brief interval, environmental condi-tions on both Gotland and northeastern Australia changedto favour O. excavata, although it did not reach a similardominance on Gotland in this interval, as it did in COG.On Gotland,O. excavata is found in frequencies between90% and 98% during two intervals of the Mulde Event; aslight freshening of the shallow coastal waters wasdiscussed as a possible cause (Jeppsson and Calner,2003).

5.3.3. IsotopesOn Gotland, δ13C values suddenly increase slightly

more than 1‰, from c. 2–2.5‰ to c. 3.5‰; in the

Fig. 10. Correlation of sections with strata from the interval from just before the event to the earliest middle part of the event, COG from Australia,Bodudd 1 on western Gotland, Botvide 1 and Nyan 2 on eastern Gotland, supplemented with own and literature data for Lau Backar 1 (see thetext for the literature data). In the four left columns, metric data for each section are given together with indication of some key horizons used toconstrain the isotopic correlations of the sections; only the metre lines of COG are drawn across the isotope diagram. LAD P.p.=last recordof Panderodus panderi during the event, O.ex.=Ozarkodina excavata fauna, FAD P.eq.=oldest identified Panderodus equicostatus duringthe event, onc.=appearance of oncoids visible in the field. Strata older than the O. excavata fauna are shadowed grey, ‘dolomite pattern’ marksstrata with that fauna, and brick pattern (‘limestone’) strata with oncoids. Isotope and conodont samples were collected at different times fromBodudd 1. Only those from the upper part can be correlated precisely, hence, only the extent of oncoids is marked. Vertical lines mark theprobable extent of identified and assumed gaps.


eastern localities. At Nyan 2 the sudden shift occurs c.9 cm down in the Hemse Marl, across a markedly rustysurface; i.e. it post-dates the conodont faunal change by12 cm (Fig. 6).

Bodudd 1 is the southwesternmost locality. Beingfarther offshore and slightly deeper, the sequenceis expected to be more complete than those farthernortheast. The sequence agrees well with this; no


conglomerate was found (see Section 5.4). The suddenshift in δ13C values is c. 25–27 cm below the Hemse/Eke boundary (see Section 5.4). Above that shift istypical ‘Hemse’, 13.9 cm of skeletal wackestone topackstone followed by 8 cm of mudstone (marl; Fig. 7).

In COG there is a similar sudden shift in δ13C fromabout 1.9‰ to 3.7‰ between 44.70 and 44.74/44.80 m.The latter sample yielded the least radiogenic 87Sr/86Srmeasured in the COG section, 0.708686 (the succeedingvalue is also low). These changes were found 15–20 cmbelow the lithologic boundary defined by entry ofoncolitic crinoidal limestone at 44.90 m (see Section5.5).

The sudden δ13C shift takes place between adjacentsamples at Nyan, Bodudd and COG. Distances betweenthese samples are c. 3 cm, 1.3 cm and 7 cm, respectively.Considered together, the sudden shift was probablycompleted over b2 cm. Better precision would requirefinely laminated sediments lacking bioturbation and re-working. With an average rate of sedimentation of 1 m/20,000 years on Gotland (Jeppsson, 1987, 1990, p. 674),3 cm corresponds to about 600 years. The shift precededthe lithologic change (see Section 5.5). The thicknessesare very similar at Nyan (9 cm+eroded strata), Bodudd(25–27 cm) on Gotland and COG (15–20 cm) inQueensland. Thus, there are good reasons to concludethat:

⋅ faunal changes preceded the isotopic shift,⋅ the δ13C isotopic shift was contemporaneous ocean-

wide,⋅ it took at the most a few hundred years to complete,⋅ the lithologic change at the end of this interval was

coeval on the two continents, and⋅ the lithologic changes postdate the δ13C shift by

about 3000–4000 years.

5.4. Erosion

The upper boundary of the Hemse Group is adiscontinuity surface and the basal Eke layer(s) isconglomeratic with rounded pebbles of Hemse Marl(Spjeldnaes, 1950), except in distal areas, as in theBodudd 1 and Uddvide 1 cores. Locally, erosion hasremoved all strata with the O. excavata fauna and theShaleria mass occurrence, and the sudden δ13C shift isfound across the discontinuity surface. This is thesituation at Botvide 1 (Fig. 4c). At Malms 1, the O.exavata fauna was not found, the Shaleria frequency isunstudied, the conglomerate unusually well developed,and the δ13C shift seems to be across the Hemse/Ekeboundary (Fig. 6).

5.5. Early part of the interval with oncolites, the LowerIcriodontid Zone

5.5.1. LithologiesOn eastern Gotland, the lower Eke Formation

consists of oncoidal crinoid limestone (Hede, 1921;Fig. 4c) overlain by the Lau Backar marl. The boundaryis well below the first oncoids more distally, both in theBurgsvik core (re-illustrated by Hede, 1921, the paper inwhich he introduced his subdivision of the Silurian ofGotland, compare that figure with fossil ranges in Hede,1919a), and in more distal exposures although this hassometimes been overlooked. At Bodudd 1, the most distalexposure, δ13C data (compare Figs. 6 and 8) and cono-donts agree that the c. 2.8–3m of lower Eke Fm reaches atleast to +0.35 m, but not above +0.65 m (above the upperreference level). The lower 2.05 m is characterised bypartly dolomitised wackestones and mudstones. Thefollowing 0.50 m, up to the reference level, is char-acterised by skeletal wackestones and packstones. Thisinterval is overlain by, in order, a shell coquina (pack-stone), and mostly argillaceous marls (mudstones).

Limestones between 44.9 and 47.3 m, the lower partof Member 3, in the COG section differ from older onesin being purer and oncolitic, although thin sectionsreveal cyanobacteria forming oncoids from as low as43.6 (Caldon, 2003). Further, these strata contain cri-noid remains between 44.9 and 46.0 m.

5.5.2. ConodontsOn Gotland, the Lower Icriodontid Subzone in the

lower Eke has yielded O. confluens, O. excavata and P.unicostatus. P. beckmanni, Decoriconus, C. dubius, P.gracilis. P. serratus may also be present basally, but re-deposition with HemseMarl pebbles can not be excluded.P. serratus occurs high in the lower Eke Formation withO. confluens, O. excavata, P. beckmanni, P. unicostatus,and Decoriconus. The conodont-based correlation acrossGotland, e.g. between Lau Backar 1 and Bodudd 1 at+0.28/0.35 m (Jeppsson, 2006) is in excellent agreementwith that based on isotopes (see Section 5.3.3).

Conodont yields from the oncolitic unit in the COGsection are markedly poorer than from underlying strata.This interval sees the first appearance of a small numberof taxa and shows a distinctive trend in the relativeabundance of coniform taxa. In COG there is an im-poverished conodont fauna consisting mostly of O.excavata and panderodontids, mostly unidentifiable atthe species level. Unlike the earlier dominance of O.excavata, this taxon is equally represented with coniformsin this interval. The lowest sample of this interval, COG45.7/45.75, retains a population of O. excavata, but these


elements are not so diminutive, having broader denticlesand widely flared basal cavities in comparison with thosefrom the preceding interval. It dominates over pander-dontid elements by a ratio of 4:1 (=a frequency of 80%).A single fragmentary Pb element of ?O. confluens mayrepresent the first appearance of this cosmopolitan speciesin the COG section. COG 46.15–46.2 documents furtherreduction of theO. excavata stock, with numbers roughlyequating with panderdontid elements. P. beckmanni firstappears in this sample.

5.5.3. IsotopesIn COG, δ13C increases about 0.7‰/m up to c.

46.2 m and then slower, reaching c. 6.0‰ at the top ofthis interval. The more rapid increase after the suddenshift (see Section 5.3.3) is very similar on bothpalaeocontinents (Fig. 10).

At Bodudd 1 on Gotland, δ13C increases from 2.5‰just above the sudden shift, to 5.96‰ at +0.31 m(Fig. 7). In five brachiopod collections from 1.7 m ofstrata, δ13C increases from 2.31‰ to 5.8‰ (Samtlebenet al., 2000), i.e. very much the same as through the c.1.9 m between −2.22 m (2.3‰) and −0.31 m (5.8‰) inour section. The difference in thickness, c. 10%, is lessthan the probable uncertainty in measuring wheresurface exposures of strata dip only slightly. Theincrease in our curve is 1.3‰/m. This trend is modifiedby what may be cyclicity with a frequency of c. 1 cycle/m of strata.

At Lau Backar 1 on Gotland, δ13C values of atrypidshells are 6.2‰, 7.0‰, 7.3‰ (three analyses;Wenzel andJoachimski, 1996), c. 5.1‰ to c. 5.9‰ (seven analyses;Samtleben et al., 2000) and, in micrite, 6.30‰ (herein;Figs. 8 and 10). Part of this spread is attributed to standarderrors in analyses (Samtleben et al., 2000, p. 6, suggestedthe analytical precision of their analyses was ±0.4‰), butthe spread is the same as the increase through 1.1 m –from −0.50 m to +0.60 m at Bodudd 1 – except that thehighest value is slightly higher. At least a metre of sectionis exposed in the clay pits at Lau Backar 1. In other words,even if the rate of deposition were not exactly the same,the spread in measurements could reflect a stratigraphicdifference. The δ13C value of our sample is within therange of variation at Bodudd 1, in the strata between+0.21 and +0.51 m. This corresponds with the conodontcorrelation of the sample from +0.28/0.35 m (see above).It follows that our high-resolution isotope record fromBodudd 1 represents isotopic changes through the lower(and lowermost middle) Eke Formation. This record maybe combined with those from Nyan 2 and Botvide 1 in aGotland composite by using the isotope excursion and thedisappearance of P. panderi as tie-points.

The COG δ18O values vary between −8.94‰ and−8.46‰. In the Bodudd section δ18O fluctuates between−5.06‰ and −7.23‰, around a slightly decreasingtrend.

Above one of the least radiogenic 87Sr/86Sr ratios(0.708686) measured in the COG section just below thebase of this interval, the 87Sr/86Sr ratios increasedthrough only 0.2 m to 0.708779, nearly the previousvalue, but then continued decreasing to the lowest valuemeasured in COG, 0.708679 at 47.3/47.4 m (Fig. 9).

5.6. Middle part of the oncoidal interval, the MiddleIcriodontid Zone

5.6.1. LithologiesIn the Middle Eke on Gotland, oncoids spread to

the most distal exposure, Bodudd 1, appearing there atc. +0.4 m. Limestones between COG 47.3 and 50.6 m,the middle part of Member 3, are oncolitic, like sub-jacent strata.

5.6.2. ConodontsPanderodus equicostatus re-appeared at the base of

the Middle Icriodontid Subzone (absent since lateHomerian) occurring with O. confluens, O. excavata,P. serratus, P. beckmanni, Decoriconus and rare Belo-della sp. It was found both on eastern Gotland and atBodudd 1 at +0.65/0.70 m and +1.16/1.20 m.

P. equicostatus first appeared in COG 47.3/47.4 andwas also obtained from COG 47.7 and COG 48.4–48.5.From a comparable stratigraphic level in the BRCsection, i.e. above the last P. siluricus, a singleicriodontid cone, and elements of D. fragilis werefound in a poor, predominantly panderodontid fauna(Simpson, unpublished data).

5.6.3. IsotopesIn COG, δ13C values increase through this interval

from 5.92‰ at 47.3/47.4 m to 6.88‰ at 50.45/50.50 m,about 0.3‰/m. At Bodudd 1 on Gotland, δ13C increasedfrom 5.96‰ just below this interval to a maximum of7.05‰ at +0.61 m, then dropped to 5.57‰ at +1.16/1.20 m, the top of the section.

The δ18O curve for COG maintains the same steadilydecreasing trend as occurred before onset of the event: c.0.07‰/m to about 47.8 m where it reached about−8.2‰ (the highest actually measured value is−7.79‰). Above 47.8 m the trend decreases. In theBodudd section, δ18O fluctuates between −5‰ and−7‰ around a slightly decreasing trend. The 87Sr/86Srratio, however, fluctuates around an increasing trendthrough the same interval in COG.


5.7. The upper oncoidal interval, the Upper IcriodontidZone

5.7.1. LithologiesOn Gotland, proximal oncoidal strata are distally

interbedded with non-oncoidal strata (Laufeld, 1974a);the upper boundary of the Eke Formation has beenrevised using the top of the Icriodontid Zone (Jeppsson,2006).

In the COG section, 50.6–51.4 m is shaly withoncoids (Caldon, 2003). As on Gotland, part of thesucceeding terrigenous interval may also belong here. Ifrelative thickness of the Icriodontid Zone and thesucceeding interval to the top of the oolites were similarin the two areas, then the top of the Icriodontid Zonewould be some metres higher in COG (see Fig. 8).

5.7.2. ConodontsOn Gotland, the Upper Eke includes an Upper

Icriodontid Subzone fauna strikingly dominated by P.equicostatus (typically N90%). Decoriconus, P. beck-manni and Oulodus sp. are regularly present. Rare and/or intermittent are Ozarkodina cf. scanica, Dapsilodusobliquicostatus, and Icriodus sp.

The few samples between COG 50.85 and COG 67have so far proved barren. Lateral equivalents in otherparts of the Broken River region have yielded onlyimpoverished panderodontid faunas. A fragmentary Paelement of ?Pedavis latialata was found in one section(Simpson, 2000).

5.7.3. IsotopesToo few samples were taken between COG 50.85

and 67 to tell if there is a trend in the δ13C values or ifthey fluctuate around an average of c. 6.1‰. δ13Cremained below +7‰, at least up to 60.25 m, but rose toabove +8‰ from 68.9 m. This pattern is very similar tothat at Bjärsjölagård in Skåne, southernmost Sweden(Fig. 1B), where δ13C rose from 6.96‰ just below thetop of the Icriodontid Zone to around 8.36‰ 3 m aboveit (our unpublished data).

On Gotland, Samtleben et al. (2000) measured 7.79‰at Bodudd 2, 8.45‰, 8.59‰, 8.38‰ at Kullunde 3, 6 and8 respectively, and 8.23‰ at Ronehamn 2, where theuppermost Eke is exposed. Wenzel and Joachimski(1996) measured 7.66‰ and 8.45‰ at Kullunde 2. Ourtwo samples from Petsarve 14 and 3 have 8.23‰ and8.80‰, respectively, and our four samples fromRonnings 1 increase upwards from 8.79‰ to 9.17‰.Considering their chitinozoan faunas (Laufeld, 1974a),geographic position and conodont faunas, the order ofthese localities (in this paragraph) may approximate their

age-order. If so, they record an increasing trend throughthis interval. All other Eke analyses of Samtleben et al.(2000) give δ13C within the lower to upper Eke range.

In older intervals there is very close similaritybetween COG and Gotland (see above), whereas inthis interval δ13C is about 2‰ lower in the COG section.Values of δ13C similar to those on Gotland were foundabove 68 m, i.e. in the oolitic interval. Unfortunately,correlation of this interval is based solely on similarityof lithologic sequences. There may be genuine differ-ences or miscorrelation. More data are needed, e.g.conodonts between 68 and 70 m, before choosing aninterpretation.

The δ18O trend changes to decreasing at c. 47.8 m inCOG; i.e. 3 m below the interval discussed in thissection. This trend is maintained through this interval.δ18O is −10.63‰ at 56.1 m and −11.57‰ at 62.70 m.,The trend in δ18O from 47.8 to 56.1 is −0.28‰/m, butfrom 47.8 to 62.7 the decrease is more rapid, −0.22‰/m.On Gotland, δ18O is also lower than in subjacent stratabut not as low as in COG. In our analyses it is between−5.77 (at Petsarve 3) and −6.65‰ (at Petsarve 14).

The rise in 87Sr/86Sr continued and culminated at0.708998 at 55.93–56.00 m, higher than valuesconsidered by McArthur et al. (2001) to representprimary oceanic signatures.

5.8. The interval with terrigenous sediments and oolites

5.8.1. LithologiesOn Gotland, the Burgsvik Formation probably

consists of 47 m (Hede, 1921see Jeppsson, 2006, for adiscussion) of partly hummocky, cross-stratified sand-stone with intercalated sandy claystones and, throughthe uppermost 7.18 m, it is intercalated with, or overlainby a few metres, at most, of oolite (Hede, 1919a, 1921).Bivalves are a salient component of the fauna (Hede,1921, p. 73; Liljedahl, 1994). In the shallowest areas,both lower and higher parts of the formation arerepresented by about 10 m of oolite (Hede, 1925, p.30–33), although unpublished data (MC) indicate thismay need to be revised.

Member 4 of the Coral Gardens Formation, fromCOG 51.4 to 68.15 m, is characterised by micaceoussiltstone to fine sandstone with trilobites, brachiopodsand bivalves (Caldon, 2003). It has the same position asthe terrigenous interval on Gotland. In the absence ofconodonts, accurate identification and correlation of thelower boundary of this interval is not possible. In thissection we discuss the strata above 62–65 m. As onGotland, the sequence changes to sandy, partly ooliticlimestone with some partings, some channelling and


some long-wave, low angle cross-bedding between68.15 and 81.2 m (Member 5) (Withnall et al., 1993).

5.8.2. ConodontsAn abrupt major conodont faunal change marks the

start of the O. snajdri Zone. The diverse incoming faunaoccurs directly above a fauna typical of the UpperIcriodontid Subzone in Bjärsjölagård 2b section inSkåne (LJ unpublished data; Fig. 1B).

The first distinct O. snajdri Zone fauna on Gotlandincludes Ozarkodina wimani, O. snajdri, O. confluens,Oulodus novoexcavatus, P. equicostatus, P. beckmanni,Decoriconus. O. scanica, Icriodus? sp., and D. obliqui-costatus. The first five are all well represented. Species ofPanderodus other thanP. equicostatus are notably absent.So are also O. excavata in the main part of the formationbut it returned near the top (Jeppsson, 2006).

Absence of carbonates prevented conodont samplingbetween COG 57.2 and 68.15 m. One sample on thenorthern side of the Broken River, 66 m above the baseof the Coral Gardens Formation, produced a singleelement of O. excavata; equivalent intervals in the BRCsection produced poor faunas of O. excavata, P.equicostatus, and ?D. fragilis.

5.8.3. IsotopesThere are large intervals between the few limestones

suitable for collecting in COG (Fig. 8). δ13C roseinitially, culminated with 8.98‰ in COG 73.35 m and9.67‰ in COG 75.90–75.95 m and remained above 8‰except perhaps just below the top of the interval, fluc-tuating through the interval.

On Gotland, in the uppermost metre(s) of theBurgsvik Formation in its main area of distribution,Wenzel and Joachimski (1996) measured δ13C values of5.95‰ at Kättelviken 2 and 5.28‰ to 8.10‰ at Uddvide2 (seven analyses). Samtleben et al. (2000) reported δ13Cvalues (SW to NE) of 7.74‰ at Kulhaken 2, 7.08‰ atUddvide 3, 6.98‰ at Ronehamn 4, 8.68‰ at Närs Hamn4, and 8.47‰ at Närs Hamn 3 (Fig. 8). Our sample G72–20LJ from Glasskär 3 has δ13C=9.71‰ and G72–17LJfrom Glasskär 1 10.54‰.

In COG, the decrease in δ18O continued to at leastthe middle of the shaly interval at 62.7 m, where itis −11.57‰. No analyses are available from the upperhalf of this interval, but an increase occurs to −9.39‰at 67.8 m, and −6.96/−7.47‰ at 71.70 m, low in theoolitic interval. From there, δ18O fluctuates, partirregularly, part cyclically, to −7.97‰ near the top ofthis interval.

The few 87Sr/86Sr analyses available indicate asteady drop to 0.708715 at 80.15 m.

5.9. Return to “normal” sediments

5.9.1. LithologiesThe Hamra and Sundre formations on Gotland

aggregate at least 88 m (Jeppsson, 2006). The lower-most Hamra Formation consists of more or less oncoliticstrata, followed by marls and marly limestones in moredistal areas (in the south) and, more proximally, reefsand reef-associated sediments. The latter kinds ofsediments are typical for most of the Silurian of Got-land. That succession was interrupted by the anachro-nistic sediments characterising the Botvide Member toBurgsvik Formation (Calner, 2005).

The COG section continued for 2.9 m (81.2–84.1 m)into the boldly outcropping Jack Hills Limestone, a unitconsisting, in the Jack Hills Gorge, of 2.9 m of well-bedded limestone followed by 210 m of fine grained,massive to thickly/poorly bedded, dark blue-grey, bluff-forming micritic limestone (the walls of the Jack HillsGorge) with, above it, 10 m of well bedded limestonewith large stromatoporoids and corals. It resembles theDark Dog Limestone; both are interpreted as reflectingrestricted marine lagoonal carbonate sedimentation.

5.9.2. ConodontsCollections from the Hamra Formation include, in

addition to the fauna from the Burgsvik Formation(listed above), other surviving taxa, e.g. P. unicostatus.New and old lineages appear higher up, e.g. O. crispa,Oulodus n. sp. p aff. Oulodus elegans, O. elegans, andCtenognathodus confluens.

In the Broken River region, C. dubius reappears andwas foundwithO. excavatus in COG82.65. The 210m ofbluff-forming micrite was barren of conodonts. Theuppermost 10 m of the Jack Hills Limestone produced asmall conodont fauna including O. excavata, O. con-fluens, O. martinssoni auriformis, C. dubius, ?Dvorakiasp. P. unicostatus, and P. equicostatus. The presence ofO. confluens, which became extinct during the Silurianpart of theKlonk Event, straddling the Silurian–Devonianboundary (Jeppsson and Aldridge, 2000; cf. Jeppsson,1988, 1989, 1998; Aldridge and Schönlaub, 1989;Barrick and Klapper, 1992), indicates the last 10 m ofthe Jack Hills Limestone in this area is entirely Silurian.The fauna was interpreted as Ludlow due to presence ofC. dubius (Sloan et al., 1995), but this taxon extendedthrough the Pridoli in Sardinia (Serpagli et al., 1996). Incontrast, an early Lochkovian age is indicated for theuppermost Jack Group carbonates in the BRC section atthe Broken River Crossing (Simpson, 1995a,b, 2000).Ctenognathodus has been found in the Pridoli, high in theJack Hills Limestone in the BRC section.


5.9.3. IsotopesIn the highest COG interval sampled, 81.2–84.1 m,

δ13C fluctuates between 7.78‰ and 8.95‰ (Fig. 8). Thiscompares well with the reported c. 7.60‰ in the HamraFormation at Uddvide 2 on Gotland (Samtleben et al.,2000), where the uppermost Burgsvik Formation andlowermost Hamra are exposed. A subsequent consider-able drop may be indicated a few metres above the baseof the Hamra Formation at Kättelviken 5, where δ13C is4.62‰ in the lowermost sample, rising gradually to5.84‰ at the top of this 4 m section (Fig. 8). However,both the Uddvide 2 and the Kättelviken 5 ratios are closeto the stratigraphically lower ones from Uddvide 2 andKättelviken 2. Hence, either δ13C fluctuated markedlythrough the O. snajdri Zone, or the relative position ofthe sampled strata needs reconsideration. Nonetheless,δ13C values around 8.0‰ correlate with the highestsampled interval in COG. Higher in the HamraFormation at Bankvät 1, 4.06‰ (Samtleben et al.,2000) and 5.87‰ (in G72–2LJ, micrite) were obtained.Samtleben et al. (2000) indicate a major drop in δ13C toas low as 0.52‰ at Klehammarsård 3.

6. Summary of the sequence of changes

The sequence of lithologies is remarkably similar onGotland, Sweden, and in the COG section in northeast-ern Australia: (A) argillaceous strata, (B) more weath-ering-resistant limestones, (C) oncoids and crinoids(oncolitic crinoid limestone or oncolite with crinoids),(D) argillaceous oncolites, (E) terrigenous clastics, (F)oolite, and (G) return to a similar suite of sediments ascharacterising most of the Wenlock and Ludlow in theseareas before B. The δ13C record includes a slow riseduring A, a slightly more rapid rise during B, an abrupt (afew hundred years or less) shift upwards of some-what N1‰, 3000–4000 years before the lithologicchange at B/C, a rapid rise initially during C, stable, highvalues during D, (probably?) an abrupt shift downwardsat the start of E, followed by a rapid rise. A high diversityconodont fauna (including platform-equipped taxa) wasfound during A, stepwise extinctions and disappearancesduring B and C, a notably unbalanced fauna during D,followed by an increase in diversity in E.

7. Discussion

Though based on standard biostratigraphy, combinedwith changes in δ13C and sedimentary sequence, inter-continental correlations of the mid-Ludlow intervalbetween Sweden and northeastern Australia have unex-pectedly high resolution. These changes were closely

synchronous in the two areas despite having been ondifferent palaeocontinents and being more differentfaunally than coeval sequences from other palaeoconti-nents (unpublished conodont data LJ, AJS, macrofaunaldata H-CC, JAT). Resolution of the sequence of somechanges has been possible even when separated by lessthan 10 ka (Section 5.3.3), and then used for very-high-precision correlations. It approaches the time-scale resolu-tion of present-day oceanic mixing time (1000 a; Hollandet al., 1986), though mixing time may have been greater inthe past, e.g. N10 times as long during the mid Cretaceousblack shale events (Bralower and Thierstein, 1984). Blackshales were common and widespread during much of theSilurian (Moore et al., 1998). The precision achieved incorrelation of the COG and Gotland sections may thus becomparable to the scale of oceanic mixing time during theSilurian. Sincemany kinds of signals are spread globally byoceanic water mixing, this phenomenon may place a limiton the resolution that can be achieved. Our sampling wasindependent and largely routine stratigraphic-cum-bio-stratigraphic sampling with centimetric precision, done at atime when rapid lithologic changes were assumed to bemerely local phenomena, and so no extra samples weretaken from the intervals now known to have had thequickest changes. Hence, the precision achievedwas partlydue to the average rate of sedimentation being in the orderof 50 m/Ma in these areas (thickness of the Wenlock+Ludlow/the radiometric duration of that interval). There-fore, the present limit may be related more to the distancebetween the samples and their thickness.

A signal carried by the atmosphere may spreadglobally more rapidly than a signal associated withoceanic mixing, and surface currents may also carry asignal around the globe faster than a complete oceanicmixing, especially if continents do not block equatorialand subtropical circulation as they do today. Hence, anattempt should be made to extend this precision to awider time-interval in pursuit of further refinement.

In Fig. 10 we have used δ13C changes to refinecorrelations of strata between biostratigraphically cor-related points, taking into account uncertainties in thedata, notably the probability that the oldest identifiedspecimen(s) represent the oldest preserved recorddepends on the size of collections and their spacing(Jeppsson, 2005). The test includes only the mostrestricted alternative, that the relative rate of sedimen-tation was linear between two biostratigraphicallycorrelated planes, i.e. if it increased through time inone section, it is hypothesised as doing likewise in theother sections, simultaneously and at the same rate. Wehave, accordingly, stretched or compressed each partuniformly. The sections and the data tied to them are


thus treated as in the standard method of graphiccorrelations. The difference is that graphic correlation(Shaw, 1964) was developed to handle range endswhereas data in the form of curves are better handled byelectronic stretching/compression.

Uncertainty as to how much strata are missing at anerosion surface(s) requires that all sections, except one,must be correlated as two discrete parts – for brevity,referred to herein as lower and upper. As an example ofour approach, the lower parts of Botvide 1 and Nyan 2were correlated, stretching the latter to correlate thedisappearances of P. panderi and the base of theO. excavata faunal interval (missing in Botvide), seeFig. 10. Then the lower part of the COG sequence wasadded and then Bodudd 1. Next, the starting points ofthe upper parts of the COG, Botvide and Nyan sectionswere adjusted upwards until they fitted with the Boduddcurve. This suggested/required two previously unrecog-nised gaps in the COG section (presumably at beddingplanes). The upper parts were stretched to correlate theappearance of P. equicostatus in COG and Bodudd.Judging from field relations, the top of Botvide is wellbelow that appearance. The relative rates of deposition,indicated by these adjustments, make sense sedimento-logically and imply, for instance, that the coarse-grainedcrinoid limestone at Botvide was deposited much fasterthan coeval mudrocks at Bodudd.

The coincidence between the curves is good, e.g.,nearly all pre-erosion data at COG and Botvide arewithin 0.1‰ or 0.2‰ from the line fitted in Fig. 10.The Nyan curve shows the same slight oscillations,although on average it seems to be slightly displacedtowards lower values. Only a small part of the Boduddcurve belongs in this interval. Due in part to too fewbiostratigraphic points of correlation, it fits less well.The Bodudd δ13C data, moreover, are from samplesthat were prepared slightly differently (see Section 4).Additional, minor adjustments could be made, e.g. theappearance of oncoids at Bodudd possibly increasedthe local rate of sedimentation in a minor but signi-ficant way.

The pre-erosion part of the curve oscillates around anincreasing trend, the amplitude of the oscillations beingabout 0.4–0.5‰. There are 4 such oscillations in thepre-erosion curve in Fig. 10. The average rate ofsedimentation on Gotland has been calculated as ‘20 ka/m’ (Jeppsson, 1987) using Hede's (1960) thicknesscalculations (chiefly based on eastern facies types) andradiometric ages. The uncertainty in that rate is dueprimarily to imprecision as regards the duration of theWenlock+Ludlow (9.5± 5.0 Ma; Gradstein et al.,2004). A new calculation of the Ludlow thickness,

including new bore data representing the facies ofwestern Gotland (Jeppsson, 2006), has increased thethickness. However, because Botvide is on easternGotland, and assuming that pre-erosion deposition therewas normal, the average duration (based on 4 cycles in c.2.4 m and 20 ka/m) was c. 12,000 years per cycle,although the uncertainty may be a factor of 2 (6000 to25,000 years or worse). The amplitude of at least someTertiary high frequency δ13C Milankovitch cycles isgenerally b0.4‰, and that of lower frequency cyclesexceeds 0.8‰ (Zachos et al., 1997). Hence, bothamplitude and frequency are what would be expected, ifthese oscillations would represent Milankovitch cycles,although a series long enough to permit spectral analysis,is needed to prove such an origin.

Most correlations in Fig. 10 are within the limitsidentified in the main part of this paper; they ‘only’refine the correlation of the intermediate intervals. Afew correlations diverge in minor details, chiefly due toassuming a gap (see earlier) in the COG section. (Con-firming these gaps would increase the similarities witheastern Gotland sections even further.). High-resolutionstudies of other sections will permit testing which of thealternative correlations is the best, although findingthese oscillations requires that at least isotope datapoints are a few ka apart at the most.

Acknowledgements

The Swedish Natural Science Council and TheSwedish Research Council funded much of LJ's fieldand laboratory work, especially conodont extraction.Ann-Sofi Jeppsson skilfully handled most of the typing,linguistic checking, etc. Rickard Anehus assisted LJduring parts of the fieldwork. Git Klintvik Ahlberg andRickard Anehus processed many of the conodontssamples and picked many of the residues. All this andother help are much appreciated. Research grants fromthe Australian Research Council and Macquarie Uni-versity to RM and JAT enabled sampling of variousBroken River sections, collection of isotope samplesfrom the Botvide section, and funded the isotopeanalyses and the conodont extraction from the Austra-lian sections. They are grateful for field assistance fromNadia Talent in sampling the Botvide section, and fromGlenn Brock, David Mathieson, Rosemary Saul, TerrySloan, and the late Ross Talent in helping sampleBroken River Late Silurian sections. Final drafting ofmost of the figures is by Dean Oliver. Many usefulsuggestions were received from referees Brad Cramerand Michael Joachimski. This study is a contribution toIGCP 421 and 503.


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