Porkuni Stage

L. Hints & T. Meidla

Original text from: Raukas, A., Teedumäe, A. (eds). 1997. Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 436 pp. ISBN 9985-50-185-3. Available online at: sarv.gi.ee/geology.

The Porkuni Stage (Borkholm’sche Schicht by Schmidt 1881) represents the topmost Ordovician stage (Raymond 1916, Bekker 1922). Up to the 1960s, the stage was included to the Silurian System (Öpik 1930b, 1934, Aaloe et al. 1958, Alichova 1960, Rõõmusoks 1960). In Estonia, the Porkuni Stage is represented by variable deposits of shallow-water facies (Männil 1966; Oraspõld 1975b, 1982b; Rõõmusoks 1983), with a thickness of about 10 m in northern and up to 18 m in southern Estonia (Fig. 61). In northern Estonia, the stage is supposedly represented by its older part only, because afterwards, during late Porkuni time, this area turned into dry land as a result of the glacioeustatic sea-level lowering (Oraspõld 1975b).

In northern Estonia, the Porkuni Stage is represented by the Ärina Formation comprising a succession of dolomites (Röa Member), stromatoporoid-tabulate reefs (with surrounding facies) and oolitic or sandy limestones (Kamariku Member) in the top. The assignment of the Röa Member (0.5-5.5 m of dolomites) has been problematic over the years. Some researchers have assigned it to the Pirgu Stage (Rõõmusoks 1991), others to the Porkuni Stage (Rosenstein 1943, Jaanusson 1956, Resheniya… 1987). The unit, usually poor in fossils, yields some species common with the Pirgu Stage (Rõõmusoks 1989). In many sections the lower boundary of the member is lithologically sharp, except the areas where the topmost part of the Adila Formation is dolomitized. The upper boundary is transitional. Here, the assignment of the Röa Member to the Porkuni Stage is conventional.

Small reef bodies, recorded in the middle part of the Ärina Formation (2-3 m high, up to 20 m wide, traditionally treated as the Tõrevere Member), yield the tabulate corals of the Palaeofavosites rugosus community (Klaamann 1986) and the stromatoporoids Clathrodictyon mammillatum (Schmidt), Ecclimadictyon porkuni (Riabinin) a.o. The reefs are surrounded by skeletal limestones (Vohilaid Member, up to 3.7 m) and kerogenous limestones (Siuge Member, up to 2.6 m; see Oraspõld 1975b, Rõõmusoks 1983), apparently representing the pre-reef and inter-reef facies, respectively. In the western part of mainland Estonia, the Vohilaid Member, which is often represented by pure skeletal sand in sparry (?) calcite matrix, contains thin (up to 20 cm) layers of oolitic limestone, whereas the ooids make up 10-45% of the rock volume (Oraspõld 1975b).

In core sections, the common succession of the three lithotypes of the “reef complex” begins with skeletal limestones which are overlain by kerogenous and reef limestones (Fig. 62). The rocks contain a rich and diverse macrofauna of corals, brachiopods, gastropods etc., (more than 150 species and subspecies; Männil 1962, Rõõmusoks 1970). The associations characteristic of the particular lithotypes have many species in common: rugose corals Konodophyllum rhizobolon (Dybowski), Streptelasma (Streptelasma) giganteum (Kaljo), brachiopods Streptis undifera (Schmidt), Schmidtomena acuteplicata (Schmidt), trilobite Platylichas mastocephalus (Öpik) and others. Among microfossils, ostracodes are abundant (Meidla 1996), whilst conodonts are extremely rare (Männik 1992b).

South of the distribution area of the Ärina Formation, distinction of the Porkuni Stage is complicated. In some sequences (Ohesaare core) the Porkuni Stage is obviously missing. In many sections in central Estonia (Are and Kahala etc., Fig. 62), the topmost part of the Ordovician sequence is represented by 1- 2m-thick dolomites, which may correspond to some part of the Ärina Formation (?Röa Member). The distribution area of these dolomites coincides roughly with the area of the pre-Silurian (early Silurian?) channeling where the erosion reached the pre-Porkuni rocks (Perens 1995).

In southern Estonia, the Porkuni Stage is represented by the peripheral parts of the Kuldiga and Saldus formations (Ulst & Gailite 1982). The Kuldiga Formation of bioclastic limestones and marls, overlain by the silty and sandy limestones of the Saldus Formation, comprises the cosmopolitan Hirnantia fauna (Rong & Harper 1988). Hirnantia sagittifera (M’Coy), Dalmanella testudinaria (Dalman), Plectothyrella crassicosta (Dalman), typical elements of that fauna, have been identified in the core sections of Ruhnu, Ikla and Taagepera. These species appear in the lower part of the Kuldiga Formation roughly on the level where the ostracodes common with the Ärina Formation and a zonal chitinozoa Spinachitina taugourdeaoui (Eisenack) disappear. The new ostracodes appearing in the Kuldiga Formation seem to have an extraordinarily wide geographical distribution and probably form a part of the Hirnantia fauna sensu lato (Meidla 1996).

The youngest Ordovician deposits corresponding to the Glyptograptus persculptus graptolite Zone are identified only on the western coast of the East Baltic (Ulst 1992). There is no certain evidence on the occurrence of shallow-water deposits of the persculptus Zone in Estonia, although they may be present as the unfossiliferous topmost Ordovician or even in strata assigned to the lowermost Silurian (Kaljo & Hints 1996).

Concluding the data on the terminal Ordovician in Estonia, it should be mentioned that the Porkuni Stage, in the presents limits, comprises rocks of different age. The oldest part of the stage is present in the stratotype area in northern Estonia, while the most complete sequences presumably occur in southern Estonia. The appearance level of the Hirnantia fauna, which may be correlated with the lower boundary of the Hirnantia Stage in Scandinavia, lies seemingly in the lower part of the Porkuni Stage in the East Baltic.