Middle and Upper Ordovician stable isotope stratigraphy across the ...

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Trends in the oceanic carbon stable isotopic compo- sition (δ13C) are considered to be an indicator of dif- ferent oceanographic and environmental changes.

WOGOGOB-2004 CONFERENCE MATERIALS. Edited by O. Hints & L. Ainsaar

Ainsaar, L., Meidla, T. and Tinn, O.

Middle and Upper Ordovician stable isotope stratigraphy across the facies belts in the East Baltic Leho Ainsaar, Tõnu Meidla and Oive Tinn Institute of Geology, University of Tartu, Estonia , ,

Trends in the oceanic carbon stable isotopic composition (δ13C) are considered to be an indicator of different oceanographic and environmental changes. Stratigraphic variations in the carbon isotopic value of marine carbonates have become an important tool in regional and global correlation of sedimentary successions. Here we present new carbon isotope data from the Billingen to Porkuni stages (Hirnantian) from the Kadriorg outcrop, and Männamaa (F-367), Ruhnu (500) and Jurmala (R-1) drill cores. Our stable isotope data set covers almost completely the Middle and Late Ordovician, altogether 30 million years. Using these data in combination with published materials, we can distinguish at least six positive carbon isotope excursions, which can be followed in more than one section in Estonia or Latvia. All of them, except for the end-Ordovician δ13C peak, have a relative value around 1–2‰. 1. The Middle Darriwil excursion (see Meidla et al. this volume) is documented from the Segerstad Formation (Fm) in the Ruhnu and Jurmala core sections. The Segerstad Fm is correlated with the Aseri Stage (Nõlvak 1997). The Middle Darriwil excursion has not been found in northern Estonian sections (Männamaa, Kadriorg), probably because the boundary beds of the Kunda and Aseri stages are missing in the middle–upper shelf facies. Thus, the exact stratigraphic age of the excursion (late Kunda or Aseri) remains unknown. 2. The Middle Caradoc excursion is documented from the upper part of the Kahula Fm in the Ristiküla and Tartu drill cores in southern Estonia (Ainsaar et al. 1999). It has late Keila age and correlates with the sedimentary gap on the boundary of the Keila and Oandu stages in the upper shelf sections (e.g. Männamaa). The excursion is weakly expressed in the basinal sections (Blidene Fm in the Valga, Jurmala and Ruhnu core sections), probably due to condensed sedimentation and low carbonate content of sediments. 3. The Early Late Caradoc excursion is document-

ed from the lower part of the Rägavere Fm, Rakvere Stage, in the Rapla (Kaljo et al. 1999) and Männamaa sections. The excursion is well developed in micritic limestones of the middle–upper shelf, but not in basinal sections, probably due to condensed sedimentation and prevalation of clay-rich sediments. 4. The Late Caradoc excursion is documented from the Saunja Fm (upper Nabala Stage) in the Ristiküla, Valga, Jurmala, Ruhnu and Männamaa sections. Like the previous one, it is developed in micritic limestones, but can be traced across the whole facies profile. 5. The Early Ashgill excursion is documented from the lower part of the Jonstorp Fm in the Kaugatuma (Kaljo et al. 1999), Ruhnu and Jurmala sections, and from the lower part of the Moe Fm in the Rapla (Kaljo et al. 1999) and Männamaa sections. It has early Pirgu age and can be also traced across the whole facies profile. 6. The end-Ordovician (Hirnantian) excursion, a prominent glaciation-driven isotope event, is found in many sections in the area (Kaljo et al. 2001; Brenchley et al. 2003). The comparison of the Jurmala, Ruhnu and Männamaa curves shows an increase in the end-Ordovician stratigraphic gap towards the upper shelf. The Jurmala section is probably the most complete section of the Ordovician–Silurian boundary beds in the area, being closest to the Hirnantian δ13C composite model of Brenchley et al. (2003). The Ordovician carbon isotope excursions have probably different origins. Some of the excursions (Middle Darriwil, Middle Caradoc, end-Ordovician) can be correlated with sedimentary gaps in the upper shelf and may be related to (glacioeustatic) sea level falls. Others seem to be related with sea level highstands or transgressions and need different environmental interpretations. The study was supported by the Estonian Science Foundation Grant 4574 and Paleostudies Programme at the University of Bremen.

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WOGOGOB-2004 CONFERENCE MATERIALS. Edited by O. Hints & L. Ainsaar

JURMALA R-1 MÄNNAMAA F-367 2

3

4

5

RUHNU-500 Fm.

0

1

2

3

4

5

6

600

Kuldiga

760

780

6 Jonstorp 620

4

120

2

RAKVERE

3

Jonstorp 5 Fjäcka Saunja Mõntu Mossen 640 Blidene

13

d C (‰, PDB) -3

-2

-1

0 10

180

VOL. BILL.

0m

KADRIORG (TALLINN)

1

Stirnas

1

2

4

Variku

660

2 420

Fjäcka Saunja Mõntu Mossen Blidene Adze

5 800

4 2 820

Dreimani

Fm.

RISTIKÜLA-174

Taurupe 160

0

Tudul. Mõntu 400

Adze Dreimani

140

KEILA HALJALA KUKRUSE UHAKU LASNAMÄGI ASERI KUNDA VOLKHOV PAKERORT

Paroveja

Jelgava

Saldus

80

Saunja Fm. NABALA Paekna Fm.

CARADOC

Kuili

Õhne (Silurian)

100

OANDU

6

13 d C (‰, PDB)

60

PIRGU Moe Fm.

740

Kuldiga

6

VORMSI

V. DARR.

5

Saldus

6

5

~C-O1

4

40

JUURU PORKUNI Adila Fm.

ASHGILL

1

Kahula

SIL.

Reg.Stage

0

3

720

d13C (‰, PDB) -1

d13C (‰, PDB)

0 1 2 Fm. Staciunai (Silurian)

Taurupe

840

Stirnas

680

Segerstad

1

Segerstad

860

Baldone Šakyna 700 Kriukai Zebre

1

Baldone Šakyna

880

Kriukai

900

Zebre

920 m

720

Positive carbon isotope excursions: 4 - Late Caradoc 1 - Middle Darriwil 5 - Early Ashgill 2 - Middle Caradoc 6 - End-Ordovician (Hirnantian) 3 - Early late Caradoc

Fig. 1. Stable carbon isotope stratigraphy in the East Baltic area.

References Kaljo, D., Hints; L., Hints, O., Martma, T. & Nõlvak, J. 1999: Carbon isotope excursions and coeval environmental and biotic changes in the late Caradoc and Ashgill of Estonia. Acta Universitatis Carolinae Geologica 43, 507–510. Kaljo, D., Hints, L, Martma, T. & Nõlvak, J. 2001: Carbon isotope stratigraphy in the latest Ordovician of Estonia. Chemical Geology 175, 49–59. Nõlvak, J. 1997: Ordovician. Introduction. In Raukas, A. & Teedumäe, A. (eds): Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 52–55.

Ainsaar, L., Meidla, T. & Martma, T. 1999: Evidence for a widespread carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia. Geological Magazine 136, 49– 62. Brenchley, P.J., Carden, G.A., Hints, L., Kaljo, D., Marshall, J.D., Martma, T., Meidla, T. & Nõlvak, J. 2003: High-resolution stable isotope stratigraphy of Upper Ordovician sequences: Constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation. GSA Bulletin 115, 89–104.

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