Pakerort Stage

H. Heinsalu & V. Viira

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 lowermost Ordovician Pakerort Stage (Table 7) distinguished by Raymond (1916) consists of two different lithotypes - the Obolus Sandstone (the Kallavere Formation, Männil & Rõõmusoks 1984) and Dictyonema Shale (Türisalu Formation, Müürisepp 1958, 1960b). During the last decades, both lithotypes have been studied in particular detail in terms of their industrial use and potential environmental impact. In northern Estonia, the so-called Obolus-Conglomerate (brachiopod coquina) occurs usually at the base of the Obolus Sandstone. The coquina was used as a good lithological marker in fixing the lower boundary of the Pakerort Stage and the Cambrian/Ordovician boundary in Estonia.

The Cambrian/Ordovician boundary (Photo 17) became an object of special international studies in the 1970s, and since then several different stratigraphical levels have been proposed as the lower boundary of the Ordovician system. According to most of stratigraphers, the definition of the Cambrian/Ordovician boundary should be based on conodonts and the horizon chosen should be close to, but below the lowest planktic (nematophorous) graptolites. Three biostratigraphic horizons of conodonts were considered as possible guides for marking the boundary level. These are the base of Cordylodus proavus, of C. intermedius and of C. lindstromi zones. Currently, the attention is focused on the first appearance of the conodont Iapetognathus n. sp. in the lower part of the C. lindstromi Zone just above the first appearance of the planktic graptolites R. praeparabola and R. parabola in the Dayangcha section (China). In Estonia, these conodont zones have been identified in the Kallavere Formation considered preliminarily (Männil & Rõõmusoks 1984) as the oldest part of the Ordovician sequence. Somewhat later the lower boundary of the system and of the Pakerort Stage was tentatively drawn at the level of the first appearance of Cordylodus (base of C. andresi Zone, Fig. 25) in the lower part of the Kallavere Formation (Kaljo et al. 1986, Resheniya… 1987, Männil 1990). But if a higher stratigraphical level (e.g. the base of the C. lindstromi Zone) will be accepted internationally for the boundary between the Cambrian and Ordovician systems, most of the Kallavere Formation must be excluded from the Ordovician (Norford 1991, Miller & Taylor 1995, Fig. 25).

As there are no distinct lithological changes on the boundaries of the conodont zones in most of the sequences, the Obolus Sandstone of the Kallavere Formation will be treated below as an entity (Fig. 26). Within that formation the main attention focuses on the distribution of conodonts, zones of which could serve as guides for the boundary between the systems. The following succession of conodont zones (in ascending order) has been established in the Obolus Sandstone and Dictyonema Shale (Kaljo et al. 1986): Cordylodus andersi, C. proavus, C. intermedius, C. lindstromi and C. angulatus. The Cordylodus andresi Zone has been established only in a few sequences in northern Estonia (Kidaste core in Hiiumaa, outcrops at Turjekelder, Vihula and Toolse). The C. proavus Zone occurs in all sections (except Turjekelder) where conodonts have been studied, and in some sections it is rather thick (Fig. 25). The zone is absent in most of the brachiopod coquina (Obolus-Conglomerate). The morphological variability of the zonal species of the C. proavus Zone suggests that the zone is discontinuous: sometimes the lower, sometimes the upper part is missing. Compared to other conodont zones in the Estonian sequences, the C. proavus Zone has the most distinct lower boundary which coincides or occurs close to the lower boundary of the Kallavere Formation.

The C. intermedius Zone is of limited distribution in Estonia. The index species has been established only in three sections (Mäekalda in Tallinn, Ülgase, Toolse). The C. lindstromi Zone, vice versa, is widespread. It is missing only on the Pakri Cape and possibly also in the Vihula section (Fig. 25). The specimens of C. lindstromi Druce et Jones found in Estonia and Australia (Nicoll 1991), are morphologically very similar (low base, small cavity with one or more pointed secondary tips).

The C. angulatus Zone occurs in all northern Estonian sections, except the Pakri Cape. Its lower boundary is well defined by the appearance of numerous specimens of C. angulatus Pander apparatus. In the western part of Estonia, it coincides with the lower boundary of the Suurjõgi Member, which is a good lithological marker (Fig. 26).

The co-occurrence of conodonts and graptolites differs considerably from area to area (Kaljo & Viira 1989). The Rhabdinopora flabelliformis group makes its first appearance at different levels in relation to the conodont zones. In the vicinity of Tallinn, it occurs in the top of the C. proavus Zone or in the C. intermedius Zone, east of Tallinn in the C. lindstromi Zone, and to the west of it in the C. angulatus Zone. This complicates the use of graptolites in the correlation of sections. Nevertheless, the graptolites serve as the most important group of fossils in establishing the upper boundary of the Pakerort Stage which falls into the organic-rich argillites of the Türisalu Formation.

Besides the above-mentioned conodonts and graptolites, lingulate brachiopods and acritarchs can be used for the subdivision of the Cambrian-Ordovician boundary beds (Kaljo et al. 1986, Puura & Holmer 1993, Mens et al. 1993, Paalits 1995).

Kallavere Formation

The quartzose sandstone with interbeds of dark argillite of the Kallavere Formation are distributed almost all over Estonia. It is missing only in a belt running from southwestern to eastern Estonia (Fig. 27). The formation is at its thickest (more than 17 m) in central Estonia. It consists of the Maardu, Rannu, Katela, Orasoja and Suurjõgi members, replacing each other in space or in time (Heinsalu 1981, 1987, Fig. 26).

Lithologically, the Kallavere Formation is dominated by quartzose sandstone, commonly weakly cemented, with the grain-size of 0.05-0.25 mm. It contains phosphatic brachiopod valves and their fragments, forming a distinct brachiopod coquina layer at the base of the Kallavere Formation (in the Maardu and Rannu members). The thickness of the coquina is only a few cm, except the Maardu (up to 1 m) and Rakvere (up to 4-6 m, sometimes even more) areas. The Kallavere Formation comprises lingulate brachiopods of the Ungula ingrica and Obolus apollinis zones (Heinsalu et al. 1987, Mens et al. 1993). In the conglomerate bed the most common species is Ungula ingrica, accompanied by species of the genera Schmidtites, Keyserlingia and Oepikites.

Dark argillite interbeds, 0.1 mm to 15 cm in thickness, occur generally above the brachiopod coquina. In northwestern Estonia, a 10–30-cm-thick dark argillite (Dictyonema Shale) bed with very thin interbeds of sandstone lies immediately on the lower boundary of the formation. In northwestern Estonia, west of the Kunda - Rakvere line, the uppermost part of the formation is represented by the so-called skeletal detritus layer of the Suurjõgi Member which consists of cross-bedded quartzose sandstone comprising brachiopod fragments with a size of 1-3 mm.The thickness of the Suurjõgi Member is about 1 m, except the Toolse - Vihula area where it exceeds 5 m (Fig. 26).

Türisalu Formation

Up to the 1970s, the Türisalu Formation (Müürisepp 1958, 1960a, b) was considered as the upper part of the Pakerort Stage. The studies of the distribution of graptolites (Kaljo & Kivimägi 1970, 1976) allowed to divide the formation between the Pakerort and Varangu stages. The older part is characterized by the occurrence of graptolites of the Rhabdinopora flabelliformis Zone and the younger part by the graptolites of the Kiaerograptus Zone. West of the Tallinn - Rapla line (Fig. 28) only the older part of the formation is represented. It comprises the Rhabdinopora f. flabelliformis (in the lower part) and R. flabelliformis multithecata graptolite subzones (Kaljo & Kivimägi 1970, 1976), and the upper part of the C. angulatus conodont Zone. The most complete stratigraphical sequences of the Türisalu Formation occur between Tallinn and Tapa where the lower part of the formation belongs to the Pakerort and the upper part to the Varangu Stage. All the sections of the Türisalu Formation east of this area are of Varangu Age.

The Pakerort Stage is represented mainly by dark-brown horizontal laminated graptolite argillite. The lamination is caused by the different content of organic matter (intercalation of darker and lighter laminae) or by different grain-size (Heinsalu 1990a, Kivimägi & Loog 1972). In some cases, wavy or cross-bedded structures or thin (a few cm) interbeds of light, often pyritized, quartzose siltstone occur in the lower part of the formation.

There is no lithological markers for identification of the boundary between the Pakerort and Varangu stages in the limits of the Türisalu Formation and the maps of the distribution of Tremadoc rocks have been compiled by formations (Figs. 27, 28, 29). The thickness of the Türisalu Formation is up to 7 m (Fig. 28).