Age of Estonian kukersite oil shale--middle or late Ordovician?

Introduction

The kukersite oil shale is composed of the stratigraphically widespread mat-forming cyanobacterium Gloeocapsomorpha prisca Zalessky. In Estonia, Ordovician kukersite-bearing strata are assigned to the Kunda to Porkuni regional stages. However, commercially important kukersite beds and the Estonia oil shale deposit belong to the Kukruse Regional Stage.

As a stratigraphic unit, the Kukruse Stage was established already in the late 19th Century: "Kuckerssche Schicht" by F. Schmidt [1] and has been widely used and thoroughly studied since then. The boundary between the Uhaku and Kukruse stages has usually been drawn at the base of the lowest commercially important kukersite bed A in NE Estonia (Fig. 1), as proposed by H. Bekker [2].

The Kukruse Stage belongs to the Viru Regional Series, which represents the middle part of the Ordovician succession in the East Baltic area [3]. Traditionally the Viru Series has been considered to be synonymous with Middle Ordovician [e.g. 4] and hence the oil shale deposits have also been viewed as Middle Ordovician [e.g. 5, 6].

[FIGURE 1 OMITTED]

The terms Lower, Middle and Upper Ordovician have been used in very different meanings and contexts worldwide, which makes them difficult to follow and compare [see 7, 8]. Since the mid-1990s, however, the International Commission on Stratigraphy and numerous research groups have been building a more universal and globally usable time scale with carefully selected key events and type sections [see reference 9 and references therein]. The new global Ordovician time scale, which has already received wide acceptance, is now composed of seven formally defined stages and three series, the Lower, Middle and Upper Ordovician.

Apparently the content and geologic time span of the newly defined global series diverge from those of the Baltic regional series. Particularly the global volume of the Upper Ordovician has been expanded compared to its former usage in Estonia. In order to avoid ambiguity of basic terminology, which in turn may easily lead to biased conclusions and generalisations, a clear distinction between the regional and global units must be made. In effect, these two separate sets of stratigraphic units should not be combined, but only compared and correlated to each other. It follows that the terms "Lower", "Middle" and "Upper" should not be used in informal context and regionally the Oland, Viru and Harju series cannot be synonymous with the Lower, Middle and Upper Ordovician, respectively.

Consequently, the principal question about the age of Estonian oil shale depends only on the correlation between the Estonian succession and the new international time scale.

Definition of the Upper Ordovician

In modern chronostratigraphy, series are defined by the lowest stage they contain. Each global stage, on the other hand, is formally defined by its lower boundary as specified by its Global Stratotype Section and Point (GSSP).

The Sandbian Stage, which is the lowermost stage of the Upper Ordovician Series, is established by its GSSP in the Fagelsang section in Scania, southern Sweden, where succession of deep shelf black shales supposedly represents a continuous record of geological time across the Middle and Upper Ordovician boundary. The boundary level itself was chosen to coincide with the level of the first appearance of a globally distributed index fossil--the graptolite Nemagraptus gracilis Hall [10].

Middle Ordovician-Upper Ordovician boundary in Estonia

Precise time-correlation in the Ordovician can be achieved by biostratigraphical methods, particularly using graptolite, conodont and chitinozoan distribution (biozones). Although global geochemical markers can also be found in the Upper Ordovician boundary interval [11], their resolution is currently inferior to that of biostratigraphy. Hence fossils have to be used for correlating the Upper Ordovician boundary worldwide. To improve the correlation of the Upper Ordovician boundary in Estonia, new sections in the Kohtla opencast and Viru underground oil shale mines in northeastern Estonia have been recently studied for microfossils (Fig. 1) and some other collections were re-examined. The results are briefly discussed below.

In Estonia, the Kukruse Regional Stage has been considered roughly coeval with the Nemagraptus gracilis Biozone [12]. This would mean that the oil shale deposit belongs entirely to the Upper Ordovician. In details the problem is more complicated, since graptolites, including N. gracilis are very rare in shallow shelf carbonate successions like that of Estonia. Recent taxonomical studies have shown that the first reliable finds of N. gracilis in Estonia come from the middle part of the Kukruse Stage [13]. In the outcrop area, attempts to find N. gracilis have been unsuccessful, possibly due to a stratigraphic hiatus in the upper part of the Kukruse Stage in northern Estonia [14, 15]. On the other hand, studies from the global stratotype show that in the lower part of its range, below the "Fagelsang Phosphorite", finds of N. gracilis are very rare [10]. Thus it is also possible that the early specimens of this graptolite species were not distributed into carbonate facies, or they have hitherto remained undiscovered there due to low frequency.

However, based solely on actual finds N. gracilis, the Estonia oil shale deposit (kukersite beds A-[F.sub.1]) falls in the topmost part of the Middle Ordovician, whereas the stratigraphically younger Tapa occurrence (bed III) and the overlying beds [6, 14] belong to the Upper Ordovician (see Fig. 2).

Conodont studies from the Fagelsang stratotype [10] show that the boundary between the Pygodus anserinus and Amorphognathus tvaerensis biozones lies well above the appearance level of N. gracilis, that is, above the global Upper Ordovician boundary. New finds of conodonts in northern Estonia (Kohtla opencast, Fig. 1) indicate that the A. tvaerensis Biozone starts very close to the base of the Kukruse Stage (bed A) [16]. Accordingly, the P. anserinus-A. tvaerensis biozone boundary lies in the upper part of the Uhaku Stage and thus the Upper Ordovician boundary could be drawn within the upper part of the Uhaku Stage [16], which in turn would mean that the Estonia oil shale deposit and Tapa oil shale occurrence are Upper Ordovician. However, based on conodont data from two boreholes, the biozone boundary could be traced within the lower part of the Kukruse Stage [17, 18].

Chitinozoans, particularly the Eisenackitina rhenana Sub-biozone, have proved very useful for correlation of the base of the Kukruse Stage in southern and western Estonia, where kukersite-rich beds are missing or cannot be confidently identified. New finds of E. rhenana very close to the base of kukersite bed A in the Viru mine section (Fig. 1) further emphasize the utility of this species. In the Fagelsang stratotype section, E. rhenana appears slightly below N. gracilis and has been proposed as a good approximation of the lower boundary of Upper Ordovician [19]. Thus, according to chitinozoans, the global Middle-Upper Ordovician boundary lies probably within the Kukruse Stage, very close to its lower boundary.

[FIGURE 2 OMITTED]

Age of the oil shale

It is not uncommon that different fossil groups may lead to slightly different stratigraphic conclusions, especially if studied at fine temporal scale as in the present case (Fig. 2).

There is a potential to solve this biostratigraphical ambiguity by obtaining more fossil material from the stratotype section in Sweden. In this respect, two successive chitinozoans are especially promising. Conochitina "savalaensis" and C. "viruana" [20] (corresponding to Conochitina sp. 1 and C. sp. 2 of Nolvak [21]) occur in many East Baltic successions (including Sirgala opencast mine and Savala drill core, northwestern Estonia), but have hitherto not been recovered in the Fagelsang stratotype section. Moreover, further taxonomical work on conodonts and chitinozoans, both in Estonia and Sweden, might help to resolve different opinions.

Based on currently available biostratigraphical data, however, the global Upper Ordovician boundary in Estonia could be drawn at the lower boundary of the Kukruse Stage. The kukersite oil shale deposits of Estonia are therefore of Late Ordovician rather than Middle Ordovician age. In regional stratigraphy, however, main kukersite oil shale belongs to the Viru Regional Series and can be classified as of Viruan age.

Acknowledgements

The authors are grateful to the reviewers John R. Dyni and Tonu Meidla for their invaluable comments and corrections. Dimitri Kaljo is acknowledged for discussion on the early draft of this paper. Our study is a contribution to IGCP Project 503 and was supported by the Estonian Science Foundation grants 5920, 5922 and 6127.

Presented by A. Raukas

Received February 22, 2007

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O. HINTS *, J. NOLVAK, V. VIIRA

Institute of Geology at Tallinn University of Technology Ehitajate tee 5, 19086 Tallinn, Estonia

* Corresponding author: e-mail [e-mail]