Lower Palaeozoic evolution of the northeast German Basin ...

Originally published as:

McCann, T. (1998): Lower Palaeozoic evolution of the NE German Basin/Baltica borderland. - Geological Magazine, 135, 129-142.

DOI: 10.1017/S0016756897007863

1. Introduction

The northeast German Basin is situated between the sta-ble Precambrian shield area of the Baltic Sea/Scandinaviato the north and the Cadomian/Caledonian/Variscan-influenced areas to the south. It thus straddles two verydifferent tectonic regimes although it was much moreinfluenced by events to the south. Three major north-west–southeast striking deep fault zones occur in thearea, including the suspected Trans-European Fault(TEF) separating Rügen from mainland northernGermany, the Caledonian Deformation Front (CDF)which lies to the north of Rügen and the Tornquist Zone(TZ) comprising the Sorgenfrei-Tornquist Zone (STZ) inthe northeast and the Teisseyre-Tornquist Zone (TTZ) inthe southeast (Thomas et al. 1993) (Fig. 1).

The present study is a broad examination of the LowerPalaeozoic of the region, including the Vendian–Siluriansuccession of the G14 1/86 well. The location of this wellon the southern margin of Baltica (see Section 2) enablesthe contrasting petrographic and geochemical signaturesfor southern Baltica and the northern part of EasternAvalonia to be compared and contrasted.

2. Regional geology and tectonics

The forelands of the North German–Polish Caledonidesare formed by the Precambrian Fennoscandian Shield andits extension beneath the Baltic Depression and the stableEast European Platform. The bulk of the Fennoscandian–Baltic craton was consolidated in pre-Grenvillian time(Ziegler, 1990). The Caledonian orogenic cycle (lateCambrian to earliest Devonian) was governed by the sinistral oblique convergence and collision of theLaurentia–Greenland and Fennoscandian–Baltic cratons(Ziegler, 1989).

The precise location of the southwestern edge ofBaltica (that is, that part of Baltica to the south of theSorgenfrei-Tornquist Zone) is not known. This is largelyas a result of masking by younger sediments (Tanner &Meissner, 1996). Many workers have identified the strike-slip Trans-European Fault, which runs throughparts of northern Germany and south of Rügen to join upwith the Teisseyre-Tornquist Zone in Poland, as thesouthern boundary of Baltica (Berthelsen, 1984;EUGENO-S Working Group, 1988; Franke, 1990a, 1993;Hoffmann, 1990). However, recent ideas favour a terraneaccretional model rather than fault movement to interpretthe observed geology of the region (Ziegler, 1990;Berthelsen, 1992; Torsvik et al. 1993; Meissner,Sadowiak & Thomas, 1994; Tanner & Meissner, 1996).In such a model, the northward convergence and accre-tion of Gondwana-derived continental fragments to thesouthern margin of the newly-forming Laurussian super-continent is envisaged (Ziegler, 1989). These fragmentswould have included Eastern Avalonia, which driftednorthwards and docked with Baltica by Ordovician/Silurian times (Cocks & Fortey, 1990; Thomas et al.1993; Torsvik et al. 1992).

Two possible boundaries have been suggested for thenortheast margin of Eastern Avalonia, the CaledonianDeformation Front (CDF) and the Elbe Lineament(Tanner & Meissner, 1996). The CDF forms the northernlimit of the Caledonian margin thrust belt which devel-oped following the oblique collision between Avaloniaand Baltica, and comprises a series of south–southwestdipping seismic reflectors which are interpreted as havingbeen formed by the thrusting of Caledonian metamor-phosed shelf slope sediments over the Precambrian base-ment of Baltica (Franke, 1990a; Thomas et al. 1993).North of the CDF the undeformed Lower Palaeozoic succession is found in downfaulted and slightly southwest

Geol. Mag. 135 (1), 1998, pp. 129–142. Printed in the United Kingdom © 1998 Cambridge University Press 129

Lower Palaeozoic evolution of the northeast German Basin/Baltica borderland

TOMMY MCCANN

GeoForschungsZentrum (Projektbereich 3.3 – Sedimente und Beckenbildung),Telegrafenberg A26, 14473 Potsdam, Germany

(Received 15 October 1996; accepted 11 July 1997)

Abstract – The Vendian–Silurian succession from a series of boreholes in northeast Germany has been pet-rographically and geochemically investigated. Evidence suggests that the more northerly Vendian andCambrian succession was deposited on a craton which became increasingly unstable in Ordovician times.Similarly, the Ordovician-age succession deposited in the Rügen area indicates a strongly active continentalmargin tectonic setting for the same period. By Silurian times the region was once more relatively tectoni-cally quiescent. Although complete closure of the Tornquist Sea was not complete until latest Silurian times,the major changes in tectonic regime in the Eastern Avalonia/Baltica area recorded from the Ordovician sug-gest that a significant degree of closure occurred during this time.

dipping blocks (Thomas et al. 1993). The G14 1/86 well isdrilled into the North Arkona Block, which is to the northof the Caledonian Deformation Front (CDF) (Fig. 1).

The Lower Palaeozoic succession from G14 1/86 iscomparable to similar successions to the north onBornholm and Scania in terms of, for example, strati-graphic thickness, lithofacies, and faunal contents (Franke,1993; Maletz, 1997; McCann, 1996a). Thus, the regionaround the G14 1/86 well can be assigned to Baltica, sug-gesting that no large-scale strike-slip movements can havetaken place along the Sorgenfrei-Tornquist Zone inPhanerozoic times (Franke, 1990b). In the Rügen area,only an Ordovician succession is recognized, and this isquite different in character from that to the north, compris-ing a thick succession of deep-marine turbidites.

The exact position of the Rügen area on the Avalonianmicrocontinent, however, has not been fully determined.Some authors favour a position on the eastern margin ofthe Avalonia microcontinent (Torsvik et al. 1992, 1993)while others (e.g. Channell, McCabe & Woodcock, 1993,fig. 8) appear to suggest that this part of northeasternGermany was part of a separate Western Avalonia micro-continent. Microfossil data, however, would suggest thatthe Rügen area was unequivocally part of EasternAvalonia (Servais, 1994).

Arguments favouring the Elbe Lineament as the north-eastern boundary of Eastern Avalonia include the evidence

of continuity between Baltica and part of northernGermany (Tanner & Meissner, 1996). Such evidenceincludes the increased crustal velocities in the area extending from the Baltic Shield to the Elbe Lineament(Rabbel et al. 1994) and the observed velocity changesacross the Elbe Lineament between crust derived fromBaltica and that derived from Eastern Avalonia(Abramowitz, Thybo & MONA LISA Working Group, inpress). Furthermore, xenoliths have been recorded fromPermian basalts in northern Germany which show affini-ties to Proterozoic anorthosite massifs of the Baltic Shield (Kämpf, Korich & Brause, 1994). In this model the area between the CDF and the Elbe Lineament is interpreted as a thrust onto the passive margin of Baltica(Tanner & Meissner, 1996). The region between the CDFand the Elbe Lineament is, therefore, a complex one withaffinities to both Baltica and Eastern Avalonia. Indeed,Abramovitz, Thybo & MONA LISA Working Group (inpress) have suggested that the region may be a compositemicro-continent made up of a collage of terranes accretedin front of the original micro-continent of EasternAvalonia. While the palaeontological evidence would suggest that the area is clearly part of Eastern Avalonia,the precise geological setting is unclear, and probablyinvolves a degree of (?low-angle) thrusting. Some of thisuncertainty may be clarified following the shooting ofsome new deep-seismic data in the north German region

130 T. McC A N N

Figure 1. The main tectonic elements in the North German Basin/Baltic Sea region (after Ziegler, 1990; Berthelsen, 1992). The studyarea and relevant wells in northeast Germany are shown on the inset.

in 1996 (Krawczyk, Lück & Stiller, 1997; DEKORPResearch Group, unpub. data.)

3. Lithostratigraphy and distribution of theVendian–Silurian succession

The lithostratigraphy and biostratigraphy of the northeastGerman Basin is based on a network of 63 boreholeswhich were drilled from 1962–86 (Hoth et al. 1993). Afurther four offshore boreholes were drilled between1986–90 by Petrobaltic – a consortium involving workersfrom the (then) German Democratic Republic, PolishPeople’s Republic and Soviet Union (Rempel, 1992).These provide a dense data network facilitating cross-borehole correlation in the Rügen area (McCann, 1996b).

The work for this study is based largely onVendian–Silurian core samples from the G14 1/86 wellwhich is located on the passive margin of Baltica (Figs 1,2). Deposition was in a foreland basin developed to thenorth of the North German–Polish Caledonides. The pre-served Lower Palaeozoic succession is relatively unde-formed and is found in downfaulted and slightlysouthwest dipping blocks (Thomas et al. 1993). Thesesamples were analysed in conjunction with a series ofOrdovician-age samples from the Binz 1/73, H 2 1/90,K 5 1/88, Lohme 2/70, Loissin 1/70 and Rügen 5/66 wellsfrom the Rügen area (Fig. 1).

Economic basement in the northeast German Basincomprises probable Precambrian crystalline basementand Lower Palaeozoic sediments and metasediments. Theoldest recorded rocks in the Rügen area are microcline-rich granites from the G14 1/86 well with a K–Ar age of530–540 Ma, although it is possible that this date wasreset (Piske & Neumann, 1993). Indeed, Franke (1993)suggests that a pre-Cadomian age would be more proba-ble. To the north, K–Ar ages of 1255–1390 Ma have beenrecorded for granites and gneisses from Bornholm Island(Larsen, 1971; Gravesen & Bjerreskov, 1984), while granites from Scania (Bergström et al. 1982) have beendated as 1350–1450 Ma. Lundqvist (1979) reports a vari-ety of ages for granites in the Scania region, ranging from1215–1655 Ma (Rb/Sr dating) with the youngest granitesgiving ages of 890 Ma (Bohus granite, Rb/Sr dating) andrelated pegmatites (910 Ma, U–Pb dating). In Denmark,K/Ar measurements from a hornblende gneiss and abiotite gneiss yielded ages of 815 ± 15 Ma and 870 ± 15 Marespectively (Larsen, 1971). These dates are in agreementwith K/Ar dates obtained in southern Norway and west-ern Sweden (Broch, 1964; Magnusson, 1960). In Poland,Ryka (1982) reported K–Ar ages of 517 Ma for somemagmatic intrusions. The variety of dates in the region islargely as a result of the structural complications resultingfrom Caledonian-age thrusting.

The lowermost G14 1/86 sediments are of possibleVendian age (Franke et al. 1989, 1994) and comprise a55.5 m thick succession of conglomerates and sandstones(Fig. 2). These are overlain by almost 300 m ofCambrian-age sandstones and shales with some lime-

stones occurring close to the top of the succession. Theoverlying Ordovician unit is dominantly clastic and fine-grained although some limestones also occur close to thetop of the unit. Recent work by Maletz (1996, 1997) onthe graptolites from the G14 1/86 well reports that thelowermost preserved Ordovician section is Tremadoc in age. This is overlain by mudstones containingExpansogratpus suecicus and Pseudophyllograptus

The Lower Palaeozoic northeast German Basin 131

Figure 2. Generalized lithostratigraphic log of the Precambrianto Silurian succession from the G14 1/86 well (after Piske &Neumann, 1993).

densus suggesting an Arenig age (Maletz, 1996). Theoverlying thin carbonate unit is followed by tectonizedmudstones which contain graptolites suggesting a lateLlanvirn (Llandeilian) age. Some of the species, forexample, Hustedograptus teretiusculsus andGymnograptus linnarssoni are especially indicative of alatest Llanvirn age (Maletz, 1997). The mudstones areoverlain by c. 30 m of non-fossiliferous shales and subse-quent mudstones, siltstones and fine-grained sandstoneswhich have been included in the Upper Ordovician basedon lithological interpretations (Franke et al. 1994).

In the Rügen area, a thick (in excess of 3000 m)Ordovician succession is recorded. The dominantly marinesediments are Llanvirn–Caradoc in age and thicken to thenorthwest (McCann, 1996b). The Llanvirn succession con-tains a range of ichnofossils belonging to the Nereitesichnofacies (Zagora, 1997). Giese, Katzung & Walter(1994) subdivided the Ordovician section into three parts: a300 m thick basal unit comprising fine-grained sandstonesand shales, the upper part of which has been dated as UpperTremadoc–Lower Arenig in age; a 1000 m thick centralblack shale unit with graptolites indicating an age of LowerCaradoc–Lower Llanvirn; and an uppermost 1800 m thicksuccession of interbedded greywackes and shales with aLlandeilo age suggested by the graptolite assemblages. The section is, however, undoubtedly both incomplete andrepeated as a result of localized structural complexity.

The sedimentology of the Silurian succession of theG14 1/86 well was recently described by McCann(1996a). Graptolite assemblages have also been recordedfrom the five cored units (Maletz, 1996, 1997). The oldestof these (1457.4–1465.8 m) includes Glyptograptustamariscoides, G. fastigatus, Monograptus bjerreskovae,‘Monograptus’ gemmatus, Paradisversograptus runcina-tus, Pristiograptus variabilis, Stimulograptus becki andSpirograptus guerichi. The second cored unit(1376–1384 m) contains Glyptograptus fastigatus,‘Monograptus’ gemmatus, Parapetalolithus altissimus?,P. conicus, P. kurcki, Pristiograptus renaudi, P. variabilis,Rastrites linnei and Streptograptus pseudoruncinatus.The graptolite assemblage of unit three (1306.1–1313.9m) includes Glyptograptus elegans?, G. fastigatus,Parapetalolithus kurcki, Rastrites linnei, Stimulograptusbecki and Torquigraptus planus. Unit 4 (1237–1243.2 m)yielded an assemblage including Normalograptus sp.?,Rastrites linnei and Torquigraptus planus, while the uppermost unit (1224.3–1231.0 m) containedMonograptus sp., Pristiograptus nudus and a small den-droid fragment. All of the identified graptolites wouldsuggest a middle Llandovery age (“Monograptus” gem-matus subzone of the Spirograptus guerichi Biozone inthe early Telychian) for the succession (Maletz, 1996,1997). Overall, the Ordovician- and Silurian-age grapto-lite faunas from the G14 1/86 well have a Baltic affinityand are typically Scandinavian in character (Maletz,1996, 1997).

4. Previous work

Previous work on the provenance region has concentratedon the petrology/heavy mineral spectrum (e.g. Giese,Katzung & Walter, 1994, 1995) and isotopic analysis ofspecific heavy minerals (e.g. Giese et al. 1995). Giese,Katzung & Walter (1994) examined 34 sandstone samplesfrom five boreholes in the Rügen area, concentrating onthe petrology and heavy mineral assemblages from theLoissin 1/70 and Rügen 5/66 wells. The heavy mineralspectrum of the Loissin sandstones is dominated by zir-cons (more than 90 %) with decreasing amounts of pyrite,rutile, apatite and sphene forming the final 10 %. Somesamples show large amounts of Fe-rich carbonates(ankerite, siderite), which are probably derived from thincarbonate veins which cut the sandstones (Giese,Katzung & Walter, 1994). The zircons comprise bothpolycyclic, anhedral forms, derived from reworked sedi-ments and basement rocks and monocyclic, euhedral tosubhedral forms, derived from mantle-dominated mag-matic, and to a lesser extent, crustal melts and associatedmagmatic rocks (Giese, Katzung & Walter, 1994, 1995).

The heavy mineral spectrum from Rügen 5/66 is domi-nated by pyroxene, epidote and chromite, while pyriteand magnetite are also common. Geochemically thepyroxenes are all Ca-rich clinopyroxenes derived fromorogenic tholeiitic basalts, the chromites being derivedfrom alpine peridotites and ophiolite sequences. Theirhigh Al/Al+Cr ratio (> 0.46) suggests derivation fromharzburgites and cumulates of ophiolite sequences (Pober& Faupl, 1988). The epidotes are Fe-rich. Transparentheavy minerals are rarer (often less than 1.0 %) and domi-nantly zircon. The zircons are variously derived frommantle-derived melts, older magmatic sources, or simplyreworked. Rare apatite, sphene, tourmaline and rutile arealso found (Giese, Katzung & Walter, 1994).

5. Petrography

The petrography of 17 sandstones of Vendian–Silurianage from the G14 1/86 well, together with fiveOrdovician-age samples from Rügen and the adjacentmainland, were examined. Medium to coarse sandstoneswere analysed using the Gazzi-Dickinson point-countingtechnique (see Ingersoll et al. 1984) to minimize thedependence of rock composition on grain size. Grainparameters and the recalculated parameters (Table 1) arethose of Ingersoll & Suczek (1979). The majority of thesamples were medium- to well-sorted (rarely poorlysorted) with individual grains being sub-angular to sub-rounded (Fig. 3).

Excluding the cement and matrix the principal con-stituents of the sandstone samples are as follows:

Quartz (Q). Grains of quartz are dominantlymonocrystalline (Qm) and sub-rounded to sub-angular inshape (Fig. 3). Some strained Qm is common in all samples, but there is no common orientation, suggestingthat the straining occurred elsewhere and that the grainswere subsequently transported into the basin. Qm grains

132 T. McC A N N

showing embayments are common, especially in theVendian. Bohm lamellae are rarely observed.

Cambrian-age Qm may have quartz overgrowths,frequently with accompanying fluid-inclusion rims.Inclusions of tourmaline may also be present.

Qm of Ordovician age may contain microliths of zircon and tourmaline. Clay rims are also observed.

Polycrystalline quartz grains (Qp) of > 3 constituentcrystals are more abundant than Qp with 2–3 constituentcrystals (Fig. 3). Contacts between the subgrains arestraight to sutured. Individual subgrains may be variablein size, even within a single composite grain of Qp.

Feldspar (F). Plagioclase is the most abundant varietyof feldspar, and is commonly replaced by calcite oraltered to clay mineral (Fig. 3). It commonly exhibitslath- or plate-like morphology. Occasional grains ofmicrocline are present. Rare perthite is observed in theVendian-, Cambrian- and Ordovician-age samples.

Lithic Fragments (L). Sedimentary lithic fragments(Ls), commonly siltstone and mudstone, dominate thelithic fraction. In the Silurian some microconglomerateshave been noted. These are clast supported, with individ-ual clasts being sub-angular to sub-rounded, up to 2 mmlong and are dominantly composed of intraclastic mud-stones with some bioclastic material (corals, brachiopods)(McCann, 1996a). Carbonate fragments, sometimes bio-clastic, are also recorded from the Ordovician of the G141/86 well.

Volcanic lithic fragments (Lv) (largely trachytic) in theVendian–Silurian succession are rare with some notableexceptions (Fig. 3). Possible glassy fragments have beenrecorded from the Cambrian. Ordovician-age samplescontain a more varied suite of volcanic lithic fragments,including fragments with spherulitic texture (?rhyolite),ignimbrite-type fragments with shard-like textures, possi-ble trachytoid gabbro, and large numbers with trachytic orandesitic textures. The Ordovician-age volcanic rocks aredominantly of intermediate to acid-intermediate origin.

Metamorphic lithic fragments (Lm) are rare, and aredominantly composed of elongate polycrystalline quartzfragments.

Possible igneous lithic fragments are also found, andare largely quartz/plagioclase composite fragments. Theyoccur in most samples of Vendian to Ordovician age. Oneparticular Cambrian-age sample contains a quartz lithicfragment enclosing a microcline microlith.

Accessory minerals are present in all samples andinclude pyroxene, zircon and muscovite in the Vendian,epidote, zircon, pyroxene, chlorite and muscovite in theCambrian, zircon, tourmaline, muscovite, chlorite,pyroxene and epidote in the Ordovician and pyrite fram-boids and chlorite in the Silurian.

Matrix. Excluding carbonate cement, the matrix comprisesfine-grained quartz, feldspar and phyllosilicate fragmentswith significant amounts of clay mineral, and may form upto 41 % of the rock in some cases (averages: Vendian –28.28 %; Cambrian – 1.84 %; Ordovician – 21.0 %;


Tabl

e 1.

Fram

ewor

k gr

ain

mod

e pa

ram

eter

s of

san

dsto

nes

from

the

Ven

dian

–Silu

rian

suc

cess

ion

of N

EG

erm

any

Age

Loc

atio

nn

Q%

F%L

%Q

m%

F%L

t%Q

p%L

vm%

Lsm

%L

m%

Lv%

Ls%

Silu

rian

G14

1/8

63

Mea

n58

.30.

5441

.16

52.9

0.54

46.5

614

.31

2.95

82.7

40

3.66

96.0

7R

ange

43.4

8–80

.53

0.29

–1.0

319

.17–

56.2

339

.7–7

3.7

0.29

–1.0

325

.96–

60.0

6.28

–26.

130–

4.55

69.3

2–93

.72

00–

6.15

90.8

4–10

0St

d D

ev.

19.6

0.42

19.4

718

.26

0.42

18.1

210

.46

2.56

12.3

80

3.24

4.72

Ord

ovic

ian

G14

1/8

61

75.8

71.

7422

.39

65.4

21.

7432

.84

31.8

215

.15

53.0

30

22.2

275

.9O

rdov

icia

nR

ügen

4

Mea

n80

.51

10.5

38.

9666

.92

10.5

322

.56

68.3

510

.84

20.8

24.

6526

.88

39.4

8R

ange

67.5

1–90

.52

6.41

–13.

230–

19.4

952

.54–

76.7

6.41

–13.

2314

.15–

34.4

642

.86–

100

0–28

.69

0–45

.71

0–12

.510

.26–

50.4

18.0

1–61

.54

Std

Dev

.9.

793.

249.

4110

.09

3.24

9.14

29.5

712

.84

20.2

26.

8420

.94

21.7

7C

ambr

ian

G14

1/8

65

Mea

n86

.51

6.28

7.22

76.0

66.

2817

.66

67.8

71.

4830

.65

05.

7189

.33

Ran

ge73

.88–

92.6

43.

65–8

.86

0.65

–22.

4757

.18–

85.6

33.

65–8

.86

6.12

–38.

7918

.37–

91.8

90–

7.4

8.1–

81.3

00–

28.5

746

.67–

100

Std

Dev

.7.

472.

389.

3712

.62

2.38

14.7

529

.85

3.31

30.4

10

12.7

823

.85

Ven

dian

G11

4 1/

868

Mea

n85

.76

11.7

2.55

75.2

911

.713

.01

77.1

42.

1620

.70

19.4

472

.92

Ran

ge79

.18–

100

0–19

.38

0–5.

5768

.21–

81.5

30–

19.3

85.

3–23

.91

46.1

5–10

00–

13.7

30–

53.8

40

0–10

00–

100

Std

Dev

.7.

047.

332.

284.

577.

336.

4521

4.84

22.4

040

.02

43.6

Key

:Q –

tota

l qua

rtzo

se g

rain

s; F

– to

tal f

elds

par g

rain

s; L

– to

tal l

ithic

gra

ins;

Qm

– m

onoc

ryst

allin

e qu

artz

gra

ins;

Lt –

tota

l lith

ic fr

agm

ents

incl

udin

g po

lycr

ysta

lline

qua

rtz

grai

ns; Q

p –

tota

l pol

ycry

stal

line

quar

tzos

e gr

ains

; Lvm

– to

tal v

olca

nic

and

met

avol

cani

c lit

hic

frag

men

ts; L

sm –

tota

l sed

imen

tary

and

met

ased

imen

tary

lith

ic fr

agm

ents

; Lm

– to

tal m

etam

orph

ic li

thic

frag

men

ts; L

v –

tota

l vol

cani

c lit

hic

frag

men

ts;

Lv

– to

tal v

olca

nic

lithi

c fr

agm

ents

; Ls

– to

tal s

edim

enta

ry li

thic

frag

men

ts.

Silurian – 23.6 %). Is is composed of highly altered lithicand feldspathic fragments, finely divided quartz, chlorite,sericite, carbonate and material too fine to be identified.Much of the matrix is primary ‘pseudomatrix’ (seeDickinson, 1970). Some Ordovician-age samples containsignificant amounts of illite (e.g. Rügen 5/66).

Cement. Carbonate cement is common (up to c. 40 %;averages: Vendian – 6.45 %; Cambrian – 21.08 %;Ordovician – 2.68 %; Silurian – 3.8 %). Calcite cementsshow a patchy distribution in the Cambrian-, Ordovician-and Silurian-age samples.

5.a. Quantitative petrography

The analysed data, together with those of Giese, Katzung& Walter (1994), have been plotted on a series of ternarydiagrams (Fig. 4). An initial plot (QFL) shows theVendian samples plottting largely in the CratonInterior/Transitional Continental fields and the Cambriansamples plotting in the Craton Interior/Recycled Arcfields (Fig. 4a). The Ordovician samples show a widerrange of possibilities, with the samples from the currentstudy plotting in the Transitional Continental/RecycledArc fields, while the Loissin 1/70 and Rügen 5/66

134 T. McC A N N

Figure 3. Photomicrographs of selected sandstone samples from the Lower Palaeozoic succession, northeast Germany. (a) Sub-roundedto sub-angular quartz grains in carbonate cement, G14 1/86, Vendian; (b) Polycrystalline quartz grain showing straight to slightlysutured contacts, G14 1/86, Vendian; (c) Plagioclase crystal showing alteration to clay mineral, G14 1/86, Vendian; (d) Subrounded tosubangular quartz grains and sedimentary lithic fragments (mudstone), G14 1/86, Ordovician; (e) Clay peloids and bioclastic frag-ments, G14 1/86, Silurian; (f) Volcanic lithic fragment showing felted texture, Rügen 5/66, Caradoc. All samples are in cross-polarizedlight. Scale bars are 0.2 mm (except (f) which is 0.1 mm).

samples of Giese, Katzung & Walter (1994) plot in theTransitional Continental and Recycled Arc/Dissected Arcfields respectively. The Silurian samples all plot withinthe Recycled Arc field.

On a second plot (QmFLt), where the polycrystallinequartz clasts are counted as part of the total lithic frag-ments, the Vendian samples plot in the QuartzoseRecycled/Transitional Continental fields with theCambrian samples falling mainly within the CratonInterior field (Fig. 4b). As previously, the Loissin 1/70samples fall comfortably within the TransitionalContinental field while the Rügen 5/66 samples fallmainly within the Dissected Arc field. The Ordoviciansamples from the current study plot mostly within theQuartzose Recycled field. The Silurian-age samples fallmostly within the Transitional Recycled field.

Taking just the polycrystalline quartz grains and plot-ting them against the lithic fragments produces a plot(QpLvmLsm) where the Vendian and Cambrian samplesplot largely in and around the Rifted Continental Margin

field (Fig. 4c). Some Ordovician samples from the currentstudy plot in the Mixed Magmatic Arcs and SubductionComplexes field, while an average point from the Rügen5/66 Ordovician samples (Giese, Katzung & Walter,1994) plots in the Mixed Magmatic Arc and RiftedContinental Margin field. The Silurian-age samples plotwithin the Suture Belt field.

A final plot (LmLvLs) where just the lithic fragmentsare used shows the majority of the points falling outsideof the indicated fields (Fig. 4d). Some Cambrian andOrdovician samples plot within the Mixed Magmatic Arcand Rifted Continental Margin (Back-arc) field.

6. Geochemistry

Analysis of the major and trace element geochemistry of44 mudstones was carried out using an automatic wavelength-dispersive XRF (Type: Siemens SRS 303AS). Major and some trace elements were determinedusing a lithium metaborate flux and a sample to flux ratio


Figure 4. Sandstone modal data for the Lower Palaeozoic succession, northeast Germany. QFL – Quartz–Feldspar–Lithic Fragments;QmFLt – Monocrystalline quartz–Feldspar–Total lithic fragments; QpLvmLsm – Polycrystalline quartz–Volcanic and metavolcaniclithic fragments–Sedimentary and metasedimentary lithic fragments; LmLvLs – Metamorphic lithic fragments–Volcanic lithic frag-ments–Sedimentary lithic fragments. Tectonic discrimination fields defined by Ingersoll & Suczek (1979).

of 1:6. Sulphur, carbon and H2O determinations were car-ried out by means of infrared spectrascopy on a LECOanalyser.

The major and trace element compositions of theVendian, Cambrian, Ordovician and Silurian mudstoneswere determined (Table 2). The majority of the mud-stones have a SiO2 range of 40.9–70.64 wt %, low Fe2O3(total Fe as Fe2O3) and MgO contents of between 0.02and 6.61. The Cambrian and Silurian-age samples are therichest in SiO2 while the Ordovician-age samples fromthe Rügen area have less SiO2.

Variations in the major element geochemistry of themudstones show little evidence of discrimination for arange of elements, including TiO2, Al2O3, Na2O, V andCaO. On the Fe2O3, K2O and Ni plots, however, there isclear evidence of stratigraphic discrimination (Fig. 5).This is particularly true of the Ordovician-age samplesfrom the Rügen area which form a tight cluster distinctfrom the other samples. Another cluster, of Cambrian ageand with a higher SiO2 wt %, is also noted, as is a smallerand less defined cluster of some Silurian-age sampleswhich can overlap with the Cambrian-age samples. TheVendian-age samples, together with the Ordovician-agesamples from the G-14 1/86 well plot between these clusters.

The distribution patterns for Ni and Cr are similar tothose of Fe2O3 and MgO (Fig. 5). All of these elements(Cr, Ni, Mg and Fe) occur in basic igneous rocks and thecohesion of the plots, particularly for the Cambrian andVendian samples and to a lesser extent the RügenOrdovician-age samples, suggests that they were derivedfrom such a source. Minimal levels of weathering andalteration are also suggested by the cohesion of the plots;the MgO levels have not been altered too much by claymineral chemistry nor have the Fe2O3 levels been affected by oxides.

The most noticeable differences in trace element con-tents are the relatively high Ba values for the Cambrian(average: 2078 ppm) and G14 1/86 Ordovician (average:2130 ppm) when compared to the other averages of 611ppm for the Rügen Ordovician or 472 ppm for theSilurian. Low Cr, Ni, and V values for the Vendian (aver-ages: 50, 35, 79), Cr and Ni for the Cambrian (averages:67, 37), and Ni for the G14 1/86 Ordovician (average: 43)when compared with the Rügen Ordovician (averages:90, 107, 132) and the Silurian (averages: 97, 66, 107)were recorded. Low Sr values for the Vendian (76),Cambrian (87) and Silurian (94) were also noted. Highlevels of Cr (100–1500 ppm) and Ni (50–600 ppm) (seeFig. 6) have been used to indicate an ultramafic prove-nance for sediments (Haughton, 1988; Wrafter &Graham, 1989), although more moderate levels wouldalso indicate, to some extent, mafic provenance. Cr val-ues of close to 100 are recorded here for the Ordovician(G14 1/86: 133; Rügen: 90) and Silurian (97) succes-sions. In addition, both the Rügen Ordovician andSilurian also show elevated levels of Ni. High Ni contentsare also observed, in mainly the Ordovician-age samples,

with enhanced Mg contents. This indicates a provenancefrom a mafic, or possibly, ultramafic source, particularlyfor the Rügen samples, since the G14 1/86 samples ploton the left-hand side of the general Ordovician trend.

The tectonic settings for the successions may bebroadly distiguished by plotting SiO2 against K2O/Na2O(Roser & Korsch, 1986). The Vendian and Cambriansamples plot comfortably within the Passive Margin(PM) field, while the Ordovician samples from bothRügen and the G14 1/86 well plot largely within theActive Continental Margin (ACM) field (Fig. 6). TheSilurian-age samples plot mostly on the border of theACM and PM fields. Plotting TiO2 versus Ni (after Floyd& Leveridge, 1987) reveals that the Vendian, Cambrianand Silurian sediments were predominantly derived fromacidic rocks while the Ordovician sediments show a moreintermediate derivation, with significant input frommafic/ultramafic sources (see above) (Fig. 6).

7. Discussion

Petrographic evidence suggests that the Vendian andCambrian sedimentary successions from the G14 1/86well were derived from a stable cratonic area. The sedi-ments are relatively quartz-rich and show evidence ofrecycling, suggesting derivation from older granitic orgneissic outcrops and associated platform sediments(Dickinson, 1988). The presence of rare volcanic frag-ments suggests a subordinate volcanic presence. The geo-chemical signature reflects the petrography and is that ofa strongly passive margin (PM). Further petrographic dis-crimination indicates that the majority of the Vendian andCambrian samples plot in the rifted continental marginfield. Palaeomagnetic data suggest that Baltica andGondwana were separated by wide oceanic basins inCambrian times (Torsvik et al. 1990, 1992, 1996) and thatthe area was a relatively stable cratonic area (Fig. 7).During early to middle Cambrian times, however, theBornholm region (to the north of G14 1/86) was upliftedresulting in the erosion and redeposition of older sedi-mentary strata (Vejbaek, Trouge & Poulsen, 1994). Thisperiod of tectonic instability has been related to thechange from a passive margin setting during spreading toan active margin at the start of subduction along thenorthern margin of Baltica (Gee, 1987). Significantly, thepetrography of the G14 1/86 Ordovician sediments areless cratonic and suggest a more recycled origin. There isalso some indication, largely based on lithic fragments, ofa magmatic-arc signature probably related to the onset ofsubduction on the southern margin.

The Ordovician sediments of the Rügen area are quitedifferent from those of the G14 1/86 succession. Here thepetrographic signature is predominantly that of an arc,with the geochemistry also indicating a strong active con-tinental margin setting. Lithic fragments suggest the pos-sibility of input from a subduction zone or riftedcontinental margin. Lithic clasts are predominantly acidand intermediate volcanic rocks although basic rocks are

136 T. McC A N N


Tabl

e 2.

Rep

rese

ntat

ive

X-r

ay fl

uore

scen

ce c

hem

ical

ana

lyse

s of

mud

ston

es fr

om th

e V

endi

an–S

iluri

an s

ucce

ssio

n of

nor

thea

st G

erm

any

Sam

ple

No

Loc

atio

n/A

geSi

O2

TiO

2A

l2O

3Fe

2O3

MnO

MgO

CaO

Na2

OK

2OP2

O5

H2O

+C

O2

Tota

lB

aC

rN

iR

bSr

VY

Zn

Zr

PR20

2G

14 1

/86

– Si

luri

an64

.49

0.81

15.0

27.

940.

461.

660.

90.

82.

290.

23.

571.

7499

.87

402

9154

105

8496

2917

323

0PR

212

G14

1/8

6 –

Silu

rian

70.9

60.

639.

416.

230.

51.

22.

90.

481.

360.

092.

783.

0399

.58

260

9542

6174

6922

7126

9PR

219

G14

1/8

6 –

Silu

rian

53.9

50.

6613

.24

13.1

71.

321.

911.

580.

442.

090.

423.

976.

3699

.11

458

126

9497

8912

030

156

160

PR23

6G

14 1

/86

– Si

luri

an57

.23

1.01

20.8

16.

340.

161.

910.

50.

844

0.07

5.11

2.08

100.

0673

010

266

182

107

147

2352

182

MV

95-4

1R

ügen

5/6

6 –

Car

adoc

57.2

90.

6313

.88

8.83

0.35

6.31

1.89

2.16

1.47

0.09

4.93

1.04

98.6

826

834

521

766

9915

514

8211

1M

V95

-46

Rüg

en 5

/66

– L

lanv

irn

45.5

90.

9923

.610

.37

2.47

2.66

0.83

0.91

3.24

0.1

6.63

2.9

100.

2880

182

172

138

139

145

3012

911

7M

V95

-58

Bin

z 1/

73 –

Lla

nvir

n49

.01

1.05

23.3

39.

271.

282.

110.

660.

913.

540.

155.

712.

7699

.79

714

9490

162

116

150

2810

913

8M

V95

-51

Loh

me

2/70

– L

lanv

irn

47.8

61.

0322

.02

9.34

0.96

2.1

0.87

0.84

3.2

0.26

7.5

2.98

98.9

459

591

9114

512

413

335

9613

8M

V95

-52

Loh

me

2/70

– L

lanv

irn

49.2

30.

9521

.66

9.64

0.77

0.02

0.82

0.84

2.91

0.25

7.03

2.55

98.6

553

896

7813

611

512

232

113

119

MV

95-4

8R

ügen

5/6

6 –

Tre

mad

oc49

.81

1.18

24.5

610

.42

0.84

1.3

0.36

1.1

4.75

0.18

4.27

0.66

99.4

310

2782

7121

923

411

931

9015

0M

V95

-47

Rüg

en 5

/66

– T

rem

adoc

62.3

40.

7412

.49

6.41

1.52

1.77

3.03

0.88

2.56

0.11

2.33

4.82

9957

750

8510

022

169

2572

481

PB24

3G

14 1

/86

– O

rdov

icia

n54

.21.

0522

.15

7.64

0.19

2.11

0.25

0.57

4.85

0.1

0.06

0.77

99.8

713

5610

957

209

7214

723

106

152

PB25

8G

14 1

/86

– O

rdov

icia

n59

.12

0.78

15.2

36.

510.

032.

722.

120.

454.

261.

234.

592.

499

.45

2905

157

2815

936

750

935

4914

3M

V95

-27

G14

1/8

6 –

Cam

bria

n70

.64

0.44

12.2

2.96

0.08

0.53

1.62

0.1

6.64

0.04

1.29

2.89

99.3

322

9630

1015

110

140

1027

290

PR25

8G

14 1

/86

– C

ambr

ian

60.6

50.

8815

.35

2.97

0.06

1.81

4.89

0.75

4.58

1.25

2.87

3.42

99.4

829

0515

728

159

367

509

3549

143

PR30

4G

14 1

/86

– C

ambr

ian

65.1

30.

9416

.45

4.88

0.02

0.86

0.1

0.1

7.34

0.04

2.65

0.42

98.8

312

5342

1818

684

6520

4362

3M

V95

-18

G14

1/8

6 –

Cam

bria

n53

.70.

7112

.73

2.36

0.04

1.52

7.7

0.47

3.67

4.04

3.14

8.4

98.4

927

6220

527

144

451

1318

120

293

125

MV

95-1

9G

14 1

/86

– C

ambr

ian

51.8

0.75

13.3

57.

730.

031.

241.

10.

494.

550.

143.

5524

.99

109.

7141

0570

280

149

104

962

3415

413

7PR

331

G14

1/8

6 –

Ven

dian

56.9

30.

9218

.25

60.

181.

42.

750.

276.

440.

033.

692.

3399

.283

936

2118

158

7529

181

393

MV

95-3

2G

14 1

/86

– V

endi

an57

.37

0.94

21.1

56.

430.

091.

570.

430.

357.

10.

043.

820.

4599

.74

947

4449

212

5383

3023

636

1

Maj

or o

xide

s in

wt%

,tra

ce e

lem

ents

in p

pm. T

otal

Fe

as F

e 2O3.

also important. Input from a mafic/ultramafic source isindicated by the high Ni levels. The dominance of pyrox-ene derived from orogenic tholeiitic basalts and chromitefrom alpine peridotites and ophiolitic sequences in the theRügen 5/66 succession confirms that basic igneous activ-ity was also important (Giese, Katzung & Walter, 1994,1995). Large-scale volcanism in Eastern Avalonia com-menced in the early Ordovician, initially in the WelshBasin and later in the Leinster Basin and Lake District.Indeed, the period of Lake District subduction-relatedvolcanism led to the eruption of significant amounts oftholeiitic basalts and andesites passing southwards intocalc-alkaline andesites and rhyolites (Bevins, Stillmann& Furnes, 1985). The extent of this volcanic activity canbe gauged from the presence from early Ordovician timesof volcanic ash beds, presumably derived from eruptionswithin the Iapetus region of the Caledonides, on thesouthern margins of Baltica (Huff, Bergstrom & Kolata,

1992). This major period of tectonic and associated vol-canic activity may have been associated with the closureof the Iapetus Ocean between Eastern Avalonia andLaurentia, although this arc system may as easily haveformed due to closure of the Tornquist Sea (Pickering,Bassett & Siveter, 1988). Volcanic activity, however, con-tinued intermittently into the early (Bevins, Stillmann &Furnes, 1985) and later Silurian (Teale & Spears, 1986).

Biostratigraphic and palaeomagnetic data suggest clo-sure of the Tornquist Sea between Baltica and Avaloniaby late Ordovician times (Cocks & Fortey, 1982, 1990;Tanner & Meissner, 1996; Torsvik et al. 1990), but this ispoorly constrained (Torsvik et al. 1993). Closure of the

138 T. McC A N N

Figure 5. Harker variation diagrams for the Lower Palaeozoic-age mudstones, northeast Germany. Figure 6. (a) Tectonic discrimination diagram for mudstones

from the northeast German Basin (after Roser & Korsch, 1986).PM – Passive Margin; ACM – Active Continental Margin; OIA– Oceanic Island Arc. (b) TiO2–Ni plot for mudstones from thenortheast German Basin (after Floyd, Winchester & Park,1989). The high Ni contents indicate provenance from a maficor ultramafic source.

Tornquist Sea by late Ordovician times would suggestthat there was a unified sedimentary system spanning theremnant Tornquist Ocean by late Ordovician (Ashgill)times. The actual situation, however, is a little more com-plex. The thick (hundreds of metres) Ordovician succes-sion on Rügen largely comprises turbiditic sandstonesand mudstones, whereas similar age sediments to thenorth on Bornholm and Scania are dominantly blackshales and carbonates (and less than 100 m thick). Asalready noted, both petrographically and geochemicallythe Ordovician successions from both the G14 1/86 welland the Rügen area differ. In addition there is a distinctlack of any Silurian sediments in the Rügen area, unlikethe Llandovery-age succession, correlatable with sectionson Bornholm and Scania, described from the G14 1/86borehole (McCann, 1996a; Maletz, 1997).

Assuming a position along the eastern extension ofAvalonia, the Cambro-Ordovician succession of Rügenwould represent the link of both domains, implying thatthe closure of the Tornquist Sea had already begun bymiddle Ordovician times (Giese, Katzung & Walter,1994). Accepting the approximate Caradoc–Ashgillpalaeogeographic position of Rügen as suggested byTorsvik et al. (1992) would imply the existence of stronglinks, both biological and sedimentological, by this timeat the latest. The lack of continuity has been interpreted asthe result of later left-lateral displacement of the Rügenarea relative to Baltica (Giese, Katzung & Walter, 1994).Recent palaeomagnetic research, however, indicates thatthe mid-Silurian palaeopoles from Eastern Avalonia andBaltica differ (Torsvik et al. 1992, 1996; Trench &Torsvik, 1991), implying that the Tornquist Sea existeduntil late Ordovician times (Scotese, 1984; Oliver, Corfu& Krogh, 1993; Torsvik et al. 1993) and probablybeyond. This is supported by the lack of late Ordoviciantectonothermal and magmatic events in southwest Poland(Oliver, Corfu & Krogh, 1993). It has even been sug-gested that closure may not have occurred until as late asthe latest Silurian or even early Devonian times(Channell, McCabe & Woodcock, 1993) although faunaland palaeomagnetic data both suggest latitudinal closureprior to Wenlock time (Torsvik et al. 1993).

By the late Ordovician, Baltica lay in subtropical lati-tudes and brachiopods exhibit clear faunal differencesfrom those in Gondwana (Harper, 1992). Palaeolatitudinalestimates for Baltica and Eastern Avalonia are compara-ble from early middle Ordovician (Llanvirn) times(Torsvik et al. 1993, 1996). Harper (1992) suggests thatfaunal provinciality in brachiopods between EasternAvalonia and Baltica was still discernable in lateOrdovician times, thus implying a degree of separation.By the mid-Silurian times, faunal evidence suggests thatEastern Avalonia and Baltica were separated by < 1000 km(McKerrow & Cocks, 1986), while palaeomagnetic datasuggest a distance of c. 1000–1500 km (Torsvik et al.1993), largely longitudinal (Torsvik et al. 1993; MacNiocaill & Smethurst, 1994). Thus, differences in the sedimentological successions of Baltica and Avalonia

may be interpreted in terms of longitudinal variationstogether with the variable tectonic environments that pre-vailed in the individual regions. The relative stability ofthe Baltic Shield region, for example, is reflected in theprovenance of the G14 1/86 Vendian–Silurian succes-sion, where sediment provenance was predominantlyfrom sedimentary sources with magmatic input being sig-nificant only during Ordovician times (although Silurianash layers have also been reported from Bornholm). Atthis time the framework grains suggest a magmatic arcsignature possibly related to the onset of subduction-related processes.

On the opposite side of the Tornquist Sea, the


Figure 7. Cambrian to Llandovery palaeogeographic recon-structions for Baltica and Eastern Avalonia (after Pickering,Bassett & Siveter, 1988; Torsvik et al. 1993; Mac Niocaill &Smethurst, 1994; present study).

Ordovician-age Eastern Avalonia succession (Rügenarea) reveals a strong active margin signature althoughrifting and subduction settings are also indicated. Indeed,active continental margin settings are recorded from else-where in Eastern Avalonia (André, 1991; André,Hertogen & Deutsch, 1986; Pharaoh et al. 1991; Pharaoh,Brewer & Webb, 1993; Noble, Tucker & Pharoah, 1993;Oliver, Corfu & Krogh, 1993) while in southern Polandthere is even evidence of two Ordovician-age metamor-phosed ophiolites and one Silurian-age unmetamor-phosed ophiolite (Oliver, Corfu & Krogh, 1993). A returnto relatively stable tectonic conditions is indicated by thequartz-rich G14 1/86 Lower Silurian succession. Overall,these results would suggest that, although complete clo-sure of the Tornquist Ocean was not complete until thelatest Silurian, major changes in tectonic regime in theEastern Avalonia/Baltica area occurred in the Ordoviciansuggesting a significant degree of closure during thistime.

Final closure, when it occurred, was complex. In onemodel, the overthrusting of Eastern Avalonia in a north-east/east–northeast direction led to the formation of theseismic reflectors which are interpreted as the CaledonianDeformation Front (Tanner & Meissner, 1996). Deeperseismic reflectors have been interpreted as remnants of asubduction zone, where oceanic material from theTornquist Sea was subducted beneath an active continen-tal margin, followed by the continent–continent collisionwhich created the thrusts and nappes of the CDF (Tanner& Meissner, 1996). A second model involves initial colli-sion occurring in southwest Poland (Oliver, Corfu &Krogh, 1993) leading to later transpressional closurealong the Elbe Lineament combined with an anti-clock-wise rotation (Torsvik et al. 1993) of Eastern Avaloniaand the main subduction of crust at the Iapetus suture(Tanner & Meissner, 1996). Thus the area between theCDF and the Elbe Lineament could have been incorpo-rated as a micro-terrane. Given the complexity of the situ-ation in southwest Poland where six terranes wereaccreted along the Tornquist suture zone (Oliver, Corfu &Krogh, 1993), it is clearly possible that similar situationsare to be recognized along the length of the Tornquistmargin resulting from the complex amalgamation ofEastern Avalonia and Baltica.

Acknowledgements. I would like to thank Dr W. von Bülow(GLA Mecklenburg-Vorpommern) for allowing access to corematerial. R. Naumann and team (PB 4.2, GFZ) carried out thegeochemical analysis. I would also like to thank Rolf Romer foruseful discussions on the geochemistry. The manuscript benefit-ted greatly from the comments of U. Giese and two anonymousreviewers. Andreas Hendrich is heartily thanked for drafting allof the diagrams.

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142 The Lower Palaeozoic northeast German Basin

Lower Palaeozoic evolution of the northeast German Basin ...

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