The development of dark shales from the middle and late Cambrian to early Ordovician on the East European Platform – with focus on Gotland

ABSTRACT

By compiling data from literature and unpublished reports a more detailed description is presented on the geographical and stratigraphic distribution of the Alum Shale Formation (ASF) and correlateable units on the East European Platform. In the northern part of Gotland, downfaulted patchy beds of the ASF indicate a former wider extension of the formation. Lower Ordovician (Tremadocian) examples of downfaulted patchy beds are the contemporaneous Sepopol and Nivenskaya formations in northeastern Poland and the Kaliningrad area, as well as the Salantai Formation in the east Baltic area. It is indicated that Furongian-Tremadocian beds, contemporaneous with the Kallavere, Türisalu, Tosna and Koporye formations, in the area of northern Estonia and the northwestern part of the Moscow Basin, extended to the Gotland and the South Bothnian Basin areas. South of Gotland, in the Swedish sector of the Baltic Basin, drill cores show evidence of tectonic movements through the presence of erosional surfaces indicating occasional subaerial exposure. In this region, variations in the areal extent and thickness of the ASF and coeval formations are suggested to be the result of epeirogenic and tectonic block movements. Tremadocian ASF is also indicated to be present south of Gotland. On Gotland, at least 5 m of the ASF is presumed to have been eroded. The Moscow Basin contains 19 m of dark shales (Koporye Formation) which is significantly thicker than in surrounding areas.

KEYWORDS:

• Cambrian
• Miaolingian
• Furongian
• Ordovician
• Tremadocian
• ASF
• Gotland
• Baltic basin
• Åland basin
• South bothnian basin
• Moscow basin
• East European platform

Introduction

Alum Shale Formation (ASF) is known for its high concentration of combustible hydrocarbons and industrially important metals (Andersson et al. 1985), which has made the formation economically interesting. Generally, the ASF comprises dark shales with interbedded bituminous limestones and sandstones. The amount of limestone and sandstone was dependent on the distance to the palaeoshore (Nielsen & Schovsbo 2007).

Within Sweden, the formation is exposed in different parts of the country such as Skåne, Öland, Östergötland, Närke, Västergötland and along the Caledonian Mountain Chain (Andersson et al. 1985; Nielsen & Schovsbo 2007). Submarine occurrences are also found in the southern Bothnian Basin (Thorslund & Axberg 1979) as well as in the central Baltic Basin (Erlström et al. 2014; Sopher et al. 2016; Sopher et al. 2019; unpublished internal reports of the Svenska Petroleum Exploration AB).

ASF was formed in oxygen depleted environments during the Middle Cambrian (Miaolingian) to the Early Ordovician (Tremadocian) from 510 to 480 million years ago (Thickpenny 1984).

On the island of Gotland, Sweden (Fig. 1) the only exposed sedimentary rocks are of Silurian age, while in the subsurface, Cambrian sedimentary sequence are represented by the File Haidar Formation (Bergström & Gee 1985), the Borgholm Formation (Hagenfeldt 1994; Nielsen & Schovsbo 2007) and the ASF (Gee 1972; Nielsen & Schovsbo 2007; Figs. 2, 3). Over time, these stratigraphic units have been investigated by researchers including Hedström (1923), Thorslund & Westergård (1938), Thorslund (1958), Flodén (1980), Bergström & Gee (1985), Andersson et al. (1985) Ahlberg (1989), Hagenfeldt (1988, 1989a, 1989b, 1989c, 1994), Hagenfeldt & Bjerkéus (1991), Buchardt et al. (1997), Nielsen & Schovsbo (2007, 2011, 2015), Sopher et al. (2016), Slater et al. (2017), Guilbaud et al. (2017) and Erlström & Sopher (2019).

Figure 1. Geographic view of the Baltic, Åland, South Bothnian and the Moscow Basins. The extent of the Miaolingian-Lower Ordovician is shown by the green colour. The Finngrundet-1 and Västra Banken-1 cores in the South Bothnian Basin area are marked by (1) and (2), respectively (Thorslund & Axberg 1979). The lithological sections for traverses A1–A6 and B1–B6 are shown in Figs. 2 and 3. TT on the map marks the Teisseyre-Tornquist Line. Revised from Petrov & Kirikov (2006).

In the Gotland area, the ASF comprises the Guzhangian Stage, parts of the Furongian Series and the Tremadocian Stage (Andersson et al. 1985; Ahlberg 1989; Figs. 2 and 3). In the ASF, a set of intraformational sandstone beds occur, such as the informally named “Obolus sandstone”, also referred to as the Skåningstorp Sandstone Beds (Puura & Holmer 1993, Nielsen & Schovsbo 2007). The accumulation of the ASF is believed to have taken place on the flat bottom of a marine basin below wave base. The presence of siliciclastics is believed to result from submarine currents (Nielsen & Schovsbo 2007). The Guzhangian (Upper Miaolingian) to Tremadocian (Lower Ordovician) interval has been reported to represent a tectonically inactive period in this region (Nielsen & Schovsbo 2011). However, a report by Westergård (1922) suggests regional tectonic movements during the Tremadocian in the province of Östergötland (Fig. 1), indicated by the presence of Skåningstorp Sandstone beds in the Tremadocian ASF. This was the result of local block movements producing raised horst structures above sea level (Westergård 1922) in turn resulting in subaerial erosion of crystalline rocks that generated the siliciclastics found in the succession, such as the Skåningstorp Sandstone. Similar structural features were reported by Tuuling (2019) influencing the sedimentary bedrock sequence in the east Baltic area at several occasions during the Middle Cambrian to Early Ordivician. Furthermore, Sopher et al. (2016) report that the faults on the East European Platform were active during the Miaolingian to Tremadocian. These movements are connected with the last phase in the break-up of the Rodinia Supercontinent. Thereafter, a passive margin developed in the west of Baltica which rotated anticlockwise (Sopher et al. 2016). Other tectonic activities during the Cambrian have been observed in the Verkegard-1 core on northern Gotland (Fig. 4) where a 32.5 m thick breccia is present (Nielsen & Schovsbo 2015; internal unpublished report, Svenska Petroleum Exploration AB). The breccia could be the result of activity within the nearby Fårösund tectonic lineament (Flodén 1980; Sopher et al. 2019; Fig. 4).

On the Åland Islands (Fig. 1), siliciclastic fissure fillings (Bergman 1982), dated as Furongian using brachiopods and acritarchs (Martinsson 1968; Tynni 1982; Puura & Holmer 1993), indicate tectonic movements here. On the coast of East Sweden, near the Västervik area (Fig. 1), undated fissure fillings with a similar orientation, i.e., SW–NE (Bergman 1982), could also be the result of Furongian tectonic activity (Friese et al. 2011). Similar sandstone fillings have also been found in east-central Sweden (see reviews by Hagenfeldt 1994 and Söderberg & Hagenfeldt 1995). These fissure fillings indicate tectonic movements of a Cambrian age, although the majority of these are not well dated, their orientations suggest this age (see Martinsson 1968). Teve & Lindström (1988) and Ahlberg et al. (2016) reported subaerial erosion in sedimentary rocks of the Acerocarina Superzone (Figs. 2 and 3) in south-central Sweden as evident by karst weathering, which in turn indicates tectonic uplift. On Öland (Fig. 1), Dahlqvist et al. (2013) reported similar conditions creating incisions of Floian limestones in the Tremadocian “Obolus sandstone” (Fig. 2).

Figure 2. A simplified correlation chart of the Miaolingian-Lower Ordovician in the southern Baltic Basin and the East Baltic area. Yellow – sandstones, green – silt and shales, grey – dark shales and blue – limestones. See Fig. 1 for a location of the investigated sections (A1–A6). Abbreviations: C. = Cordylodus, P. = Paltodus (conodonts); A. = Agnostus, E. = Eccaparadoxides, P. = Paradoxides (trilobites), Fm = Formation. Revised from various sources (see text for details). In the Baltic Basin, the base of the Miaolingian is drawn at the base of the Kybartai Formation, following Nielsen & Ahlberg (2019). The stratigraphic chronology of the Ülkase and Tsitre Formations follows Meidla (2017).

Figure 3. A simplified correlation chart of the Miaolingian – Lower Ordovician in south-central Sweden, northern Baltic Basin, East Baltic area and the Moscow Basin. Yellow – sandstones, green – silt and shales, grey – dark shales and blue – limestones. See Fig. 1 for the location of the investigated section (B1–B6). Abbreviations: C = Cordylodus P. = Paltodus (conodonts); A. = Agnostus, E. = Eccaparadoxides, P. = Paradoxides, (trilobites), Fm = Formation. Revised from various sources (see text for details). The base of the Miaolingian in the Baltic region is drawn at the base of the Kybartai Formation, following Nielsen & Ahlberg (2019). The stratigraphic chronology of the Kallavere Formation in western Lithuania follows Meidla et al. (2022).

The aim of this paper is to tie together dark shale areas on the East European Platform during the Miaolingian, Furongian and Tremadocian. The area stretches from the Baltic Basin in the west to Lake Onega in the east (Fig. 1). Additionally, we show regional events that can be correlated between the different areas, with a focus on tectonic and epeirogenic activity, especially erosion. Gotland is a key area for the understanding of the development of dark shales on the East European Platform.

Material and methods

The present investigation is based on data such as trilobite, conodont and acritarch biostratigraphy, lithostratigraphy, geochemical data (stable C-isotopes), lithological descriptions, petrophysical and wireline logs (gamma-ray wireline logs, reflection and refraction seismics), and in the Gotland area, from onshore and offshore drill cores (see Figs. 4 and 5 and Appendices 1, 2 in Tables) compiled from Hagenfeldt (1994) and unpublished internal reports of the Svenska Petroleum Exploration AB.

Figure 4. The extent of the ASF in the Gotland area is shown by the grey area (unpublished internal reports, Svenska Petroleum Exploration AB). Downfaulted beds of ASF in the northern part of Gotland are found in the Långvät-1, Norderskogen-1, Mölnerbunker-1 and Kvarnmyrskogen-1 well cores. The Friggsarve-1 core (5 on the map), is another isolated area of ASF on southern Gotland. Well cores mentioned in the text are 6, Viklau-1; 7, När-1; 8, Grötlingbo-1; 9, File Haidar-1; 10, Visby-1; 11, Verkegard-1 and 12, Lajrhagen-1. For the thickness and depths of the ASF in the cores see Tables 1 and 2.

Figure 5. A. Isopach map of the ASF and coeval siliciclastics strata in the Baltic Basin area. Modified from unpublished internal reports of the Svenska Exploration AB, Buchardt et al. (1997) and Erlström (2014, 2016). The extension of the ASF west of the Baltic Basin is proposed by Buchardt et al. (1997, fig. 7). The most prominent structure of the ASF is the Slupsk Bank Depression, which is the result of tectonic activity just north of Poland (Modliński et al. 1999, fig. 4). TT on the map marks the Teisseyre-Tornquist Line. In the upper right corner of the picture is the western part of the Türisalu Formation. In B., a three-dimensional extension of the Slupsk Bank Depression.

Table 1. Drill cores on Gotland which have penetrated the whole Cambrian sequence.

Drill core	Thickness (m)	Interval (m)
Ala-1	2.0	401.0–403.0
Alby-1	–	–
Ale-1	–	–
Alnäsa-1	–	–
Alvskog-2	–	–
Ansarve-1	2.0	348.0–350.0
Arhagen-1	–	–
Audungs-1	–	–
Augstens-1	–	–
Backen-1	–	–
Bjärges-1	2.4	327.9–330.3
Bofride-1	4.3	439.7–444.0
Botvide-1	–	–
Brutmyr-1	–	–
Bunge-1	–	–
Bäcks-1	–	–
Dalsviden-1	1.0	452.0–453.0
Djauby-1	2.0	481.4–483.4
Faludden-1	–	–
Faludden-2	–	–
Flygfältet-1	–	–
File Haidar-1	–	–
Friggsarve-1	2.0	481.4–483.4
Furilden-1	–	–
Galtmyr-1	–	–
Gannorträsk-1	4.0	458.0–462.0
Gervalds-1	–	–
Gervalds-2	–	–
Gisslause-1	–	–
Glisslause-4	–	–
Gläves-1	3.0	459.0–462.0
Grunnet-1	–
Grunnet-2		–
Grunnet-3		–
Grötlingbo-1	–
Grötlingbo-2	–	–
Gutehajd-1	–	–
Hajdhagen-1	–	–
Hallbjäns-1	–	–
Hammaren-1	1.0	472.0–473.0
Hamnudden-1	–	–
(Gotska Sandön)
Hamra-1	–	–
Hamra-2	–	–
Hamra-3	–	–
Hamra-4	–	–
Hamra-5	–	–
Hamra-8A	–
Hamra-10	–	–
Hamra-10A	–	–
Hangre-1	–	–
Hemse-1	3.0	422.0–425.0
Horsan-1	–	–
Kauparve-1	–
Klasen-1	–	–
Knuts-1	–
Kvarne-1	–	–
Kvarne-2	–	–
Kvarnmyrskogen-1	2.0	271.0–273.0
Kvinnegården-1	–	–
Kyllarhajdar-1	–	–
Källstäde-1	–	–
Kännungs-1	–	–
Lajrhagen-1	5.0	452.0–457.0
Lau-1	4.5	455.5–459.0
Leviken-1	–	–
Libbensarve-1	–	–
Lilla Ire-1	–	–
Lillolje-1	–	–
Linhatte-1	–	–
Lukse-1	3.0	462.0–465.0
Långvät-1	1.0	283.0–284.0
Lärbro-1	–	–
Maldes-1	1.0	473.0–474.0
Mattise-1	–	–
Myrhaga-1	–	–
Mästermyr-1	–	–
Mästermyr-2	–	–
Mölnerbunker-1	<1.0	284.0–284.0
Norderskogen-1	2.0	283.0–285.0
Norrbys-1	–	–
Nors-1	–	–
Nystugu-1	–	–
Nyudden-1	2.0	473.0–475.0
Nyudden-2	1.0	474.0–475.0
När-1	2.2	468.2–470.0
Näs-1	–	–
Ollajvs-1	–	–
Pavals-1	–	–
Petes-1	–	–
Rasnar-1	–	–
Rausikar-1	–	–
Rings-1	–	–
Rojrviken-1	–	–
Ronehamn-1	1.0	471.0–472.0
Rums-1	–	–
Rutemyr-6	–	–
Salthamn-1	–	–
Sanda-1	2.0	290.0–292.0
Sandviken-1	–	–
Sigdes-1	3.0	451.0–454.0
Sikhagen-1	–	–
Silte-1	–	–
Sindarve-1	–	–
Skymnings-1	–	–
Skåls-1	1.0	447.0–448.0
Skäggs-1	–	–
Skärbod-1	–	–
Släktmyr-1	–	–
Smiss-1	–	–
Stajnskogen-1	–	–
Stengrinde-2	–	–
Stenstugu-1	–	–
Storms-1	–	–
Stucks-1	–	–
Suderskogen-1	–	–
Sundre-1	–	–
Trosings-1	–	–
Tuer-1	–	–
Utskogen-1	–	–
Valleberg-1	–	–
Vamlingbo-1	–	–
Verkegard-1	–	–
Vike-1	–	–
Viklau-1	2.0	346.0–348.0
Visby-1	–	–
Vistäde myr-1	–	–
Vätskog-1	–	–
Vännes-1	–	–
Västergården-1	–	–
Västerhagen-1	–	–
Östergarn-1	–	–
Östris-1	–	–

Notes: Depth and interval are denoted where the ASF is found. Remaining cores without the ASF are marked by an ‘–’. Data are collected from internal reports from Svenska Petroleum Exploration AB and Hagenfeldt (1994).

Table 2. Offshore drill cores within the Swedish Economic Zone south and west of Gotland.

Drill core	Thickness (m)	Depth (m)
B-3	6.0	757.0–763.0
B-3A	4.0	767.0–771.0
B-5	5.0	736.0–741.0
B-6	6.3	817.0–823.3
B-7^a	6.1	851.7–857.8
B-9	3.8	1021.0–1024.8
B_10	6.4	432.5–438.9
B-11	–	–
B-12	–	–
B-20	–	–
BO-13	–	–
BO-21	–	–
Yoldia-1^a	46.5	737.0–783.5

Notes: Thickness and interval are noted where the ASF is found. Data is from internal reports from Svenska Petroleum Exploration AB. The drill cores, situated east of Gotland, and the whole offshore area outside the Baltic coastline lack the ASF. Recent findings of 0.3 m of glauconitic sandstone from equivalents to the Leetse and Cirma formations of a late Tremadocian Stage, have been reported from the interval of 874.7–875.0 m in the E6-1 drill core off the coastal area of Latvia.

^a Proven presence of Tremadocian ASF.

In addition, data covering the Miaolingian, Furongian and Tremadocian in the south Bothnian Basin, east-central Sweden, the Baltic Basin and the East European Platform are included. The correlation here is mainly based on a combination of biostratigraphic and lithological characters.

The geographic distribution of the ASF in the Gotland area is shown in Fig. 4, together with the position of the main cover and downfaulted patchy beds of the formation. The whole extension and thickness of dark shales in the Baltic Basin area are shown in Fig. 5(a and b). The Tremadocian Stage partly contains siliciclastics of the Sepopol and Nivenskaya Formations in northeastern Poland and the Kaliningrad area. They are contemporaneous with the Tremadocian part of the ASF (Modliński et al. 1999; Lukyanova et al. 2011; Zytner & Fenin 2010; Figs. 1 and 2). This indicates a regional development on Baltica with erosion and faulting of earlier sedimentary sequences creating downfaulted remnants of patchy beds also stretching into the Gotland area. In all, the whole eastern margin of the ASF in the Baltic area seems to be developed in this way. However, there are factors that limit this interpretation. The uneven geographic distribution of drill core data, and that the thickness of the ASF is below seismic resolution make the use of seismic data impossible (Flodén 1980). Sopher et al. (2016) and Erlström & Sopher (2019) also describe the problem of separating the ASF from overlying glauconitic limestone beds using Gamma-Ray (GR) readings from several wells.

The extension of the ASF west of the East European Platform is outside the scope of the present investigation, but a possible extension is shown in Buchardt et al. (1997, fig. 7). Moreover, Erlström et al. (2014; fig. 7) show the extension of the ASF within the Swedish economic zone southwest of Gotland.

On Gotland, the ASF includes the Upper Guzhangian Stage (Figs. 2 and 3), following the assignment of the Agnostus pisiformis Zone to the lowermost part of the ASF. The formation then continues with the Furongian Series. In the Tremadocian ASF, the deposits continue with dark shales (Terfelt et al. 2008; Nielsen et al. 2014).

The sedimentary rocks surrounding the study area, but coeval with the ASF, are included in the regional evaluation; for summaries, see Mens et al. (1990), Buchardt et al. (1997) and Nielsen & Schovsbo (2007).

We follow the description by Nielsen & Schovsbo (2007, pp. 82–87) of the ASF. We also believe, as the authors mentioned, that the ASF stretches out to the east and south of Sweden. This because the description of formations in this region is consistent with the ASF. For the sake of simplicity it is proposed to informally subdivide the ASF into a Cambrian and Ordovician part, following Nielsen et al. (2018), where the Cambrian part of the ASF include the Miaolingian and Furongian of the ASF in Sweden, the Slowinski Formation and part of the Piasnica Formation in northern Poland, the Mlynary Formation in the Kaliningrad District and parts of the Tolbukhino Formation in the Moscow Basin, as mentioned in the present article.

The Ordovician part comprises the Tremadocian of the ASF. In northern Poland, the dark shales of the Piasnica Formation continue from the Cambrian to the Tremadocian. In northern Estonia, the Türisalu Formation includes Tremadocian dark shales that are diachronously distributed with the oldest layers in the west. Here, older names, such as Dictyonema Shale are omitted in favour of the Türisalu Formation, see also Nielsen & Schovsbo (2007). Finally, the Koporye Formation replaces older names such as Dictyonema Shale in the Leningrad District and the Moscow Basin.

Geological setting

During the Mesoproterozoic or Middle Riphean (Ectasian Period), rift basins were developed in the Bothnian Bay, South Bothnian Basin, Åland Basin, Baltic Basin, the Baltic States and the Moscow Basin (Flodén 1980; Wannäs 1989; Söderberg 1993; Grigelis 1991; Amantov 1992; Fig. 1). In the Neoproterozoic Era, the Rodinia Supercontinent broke up, resulting in the development of a passive margin along the western edge of Baltica and further west a spreading zone creating the Iapetus Ocean (Andréasson 1994; Torsvik et al. 1996; Cocks & Torsvik 2005; Torsvik & Cocks 2017).

During the Cambrian, Baltica moved north towards the equator from about 60° south (Torsvik et al. 1996; Cocks & Torsvik 2005; Torsvik & Cocks 2017), along strike slip faults and insignificant rotation, see fig. 5.1.a–c in Torsvik & Cocks (2017). Baltica had rotated counterclockwise 120° by the Middle Ordovician (Torsvik et al. 1996; Cocks & Torsvik 2005; Torsvik & Cocks 2017), and had finally rotated more than 160° by Early Devonian times (Torsvik et al. 2013). The accretion of Avalonia, and later of Laurentia, to Baltica caused tectonic movements (the Finnmarkian Event), which commenced during the late Cambrian (Andréasson 1994). The Cambrian is considered to be tectonically quiet period in the Baltica region (Nielsen & Schovsbo 2011). However, Artyushkov et al. (2000) and Kheraskova et al. (2005) have suggested active tectonic movements on Baltica. In the Moscow Basin, the Camodian and Salair tectonic structures indicate movements during the Furongian resulting in an influx of sediments into the basin (Kheraskova et al. 2005).

During the Cambrian and Tremadocian, epeirogenic movements, such as transgressions and regressions, occurred on Baltica (Mens et al. 1990; Hagenfeldt 1994; Söderberg & Hagenfeldt 1995; Buchardt et al. 1997; Nielsen & Schovsbo 2007, 2011, 2015; Kheraskova et al. 2015). Apart from these events carbon isotope excursions indicate a shift in δ¹³C values as recorded from the organic material (Erdtmann 1986; Ahlberg et al. 2009, 2018). Investigations of the ASF in the western part of Baltica exhibit at least two main carbon isotope excursions during the development of the formation: (1) the positive Strepotean Carbon isotope Excursion (SPICE) represented in the Lower ASF (Guzhangian Stage; Ahlberg et al. 2009); and the negative Top of Cambrian Excursion (TOCE) occurring in the Lower Furongian (Terfelt et al. 2014; Ahlberg et al. 2018). The excursions indicate changes of the faunal components, such as depletion and immigration of new species, and changes in the water depth (Ahlberg et al. 2009). There could be a link between the change of trilobite faunas and positive values of the carbon isotopes in the beginning of the Furongian (Ahlberg et al. 2009).

Transgressive and regressive events are: (1) the Acerocare Regressive Event (ARE) during the Furongian (Erdtmann 1986) and (2) the Peltocare Regressive Event (PRE) in the Tremadocian (Erdtmann 1986). The Ceratopyge Transgressive Event (CTE; Erdtmann 1986) marks the end of ASF sedimentation and the start of the deposition of glauconitic shale, sandstone and limestone during the late Tremadocian and Early Floian Stages (Sturesson et al. 2005; Kheraskova et al. 2005).

Results

Gotland area

On Gotland, many drill cores are too shallow to give any information on the presence or absence of subsurface Cambrian sedimentary rocks. However, 131 drill cores contained the whole Cambrian succession (Table 1). Of these, the ASF was detected in 27 cores (Table 1). The preserved thickness of the ASF ranges from maximally 5 m, present in the Lajrhagen-1 core (Fig. 4; unpublished internal reports Svenska Petroleum AB), to less than one meter (Table 1). The bulk of the formation is distributed under central and southern Gotland (Fig. 4). However, in the northern part of the island, downfaulted patchy beds of the ASF are found in the Långvät-1, Norderskogen-1 and Mölnerbunker-1 cores in the northeast and Kvarnmyrskogen-1 core further west-northwest (Fig. 4; Table 1). These cores indicate the presence of two restricted areas of the ASF. An additional outlier is indicated in the area of the Friggsarve-1 core in the far south of the island (Fig. 4; Table 1).

Many drill cores lack the ASF. In these cores, the underlying Miaolingian Faludden Member of the Upper Borgholm Formation (Figs. 2 and 3; see also Hagenfeldt 1994, fig. 4; Nielsen & Schovsbo 2007, fig. 3) is instead covered by a glauconitic conglomerate and limestone of Late Tremadocian-Early Floian age (e.g., Hedström 1923; Thorslund & Westergård 1938; Hagenfeldt & Bjerkéus 1991).

An example of a core missing the ASF is in the Grötlingbo-1 well (Hagenfeldt 1989a, 1989b; Hagenfeldt & Bjerkéus 1991; Nielsen & Schovsbo 2015; Fig. 4). Here, the Faludden Member of the Borgholm Formation is covered by a conglomerate with fragments of brachiopods, together with glauconite and phosphorite pebbles. Acritarchs revealed a late Tremadocian age for this unit (Hagenfeldt & Bjerkéus 1991).

In the När-1 core (Ahlberg 1989; Fig. 4), the Faludden Member of the Upper Borgholm Formation forms the base of the ASF and consists of a porous sandstone with a fauna attributed to the Eccaparadoxides oelandicus Superzone. The Miaolingian ASF is here developed as a dark shale with dark grey bituminous limestone (stinkstone) intercalated with sandstones and grey shales (Ahlberg 1989). The fauna firmly indicate the Agnostus pisiformis Zone of the Guzhangian Stage (Terfelt et al. 2008; Figs. 2 and 3).

In the Viklau-1 core, Furongian and Tremadocian strata (Thickpenny 1984; Andersson et al. 1985; Fig. 4) are indicated by the presence of Ctenopyge flagellifera Zone index taxa. The Tremadocian ASF was earlier informally named the Dictyonema shale, and is indicated here by the high values of vanadium (1867 ppm; Andersson et al. 1985) and the presence of Dictyonema sp. The combination of cross-bedded sandstones within the Tremadocian ASF resembles the Suurjõgi Member of the Upper Kallavere Formation in the region of northern Estonia (Fig. 3; Heinsalu et al. 2003, fig. 2; Hints et al. 2014; fig. 1C).

The B-7 core south of Gotland (Figs. 6 and 7) corresponds well with the development of the ASF on Gotland. The 41 m thick Faludden Member of the Upper Borgholm Formation is here covered by a sequence of dark shales, conglomerates and sandstones (Figs. 8 and 9). This indicates at least nine transgressional and regressional events, the latter creating deep incisions in the ASF, suggesting subaerial conditions (internal report, Svenska Petroleum Exploration AB). This part of the succession probably belongs to the Furongian, due to the high levels of radioactivity shown by the high gamma response on the wireline logs (Andersson et al. 1985; Nielsen et al. 2018). The following dark shale is, in turn, attributed to the Tremadocian by high levels of vanadium, as well as Ordovician conodonts and other faunal elements (internal report Svenska Petroleum Exploration AB).

Figure 6. Offshore and near shore drill cores in the Baltic Basin used in the present investigation: Swedish Economical Zone: 3, Hamnudden-1, Gotska Sandön; 4, BO-12; 5, BO-13; 6, BO-20; 7, BO-21; 8, B-11; 9, B-5; 10, B-3, B-3A; 11, B-6; 12, B-9; 13, B-7; 14, B-10; 15; Yoldia-1 (Indoor reports Svenska Petroleum Exploration AB), Polish Economical Zone: 16, B4-1; 17, B6-2; 18, B3-1; 19, B7-1; 20, B6-1; 21, B8-1; 22, B16-1 (Modliński et al. 1999; Kosakowski et al. 2016). Latvian Economical Zone: 23, E-5 (internal reports, Svenska Petroleum Exploration AB). Nr 1 and 2 denote well cores in the South Bothnian Basin (Fig. 1). TT on the map marks the Teisseyre-Tornquist Line.

Figure 7. Reconstruction of facies belts during the Early Tremadocian. Revised from Männil (1966), Heinsalu (1986), Heinsalu and Bednarczyk (1997), Modliński et al. (1999) and Popov et al. (2019): A. striated, grey areas denote dark shales and deep marine facies; B. dotted, yellow areas indicate siliciclastics and more near shore conditions. Black streaks in the sandstone denotes thin intercalations of dark shales in the sandstone sequence. The picture shows the tentative facies distribution of the Tremadocian ASF and contemporaneous dark shales and siliciclastics before the onset of the Jelgava Depression (Modliński et al. 1999, fig. 5). TT on the map marks the Teisseyre-Tornquist Line. On Gotland, finds of Tremadocian ASF have been reported from the drill core Viklau-1 (Andersson et al. 1985). In the upper right corner of the picture is the western part of the Türisalu Formation.

Figure 8. Photo on part of the B-7 core showing a conglomerate separating a fine-grained, well sorted sandstone from the overlying ASF. The diameter of the core is 42 cm. The core B-7 is stored at the collections of the Geological Survey of Sweden in Uppsala. Photo: Courtesy of Erling Siggerud, Trondheim, Norway.

Figure 9. Photo on part of the B-7 core showing well sorted sandstone with mud clasts and erosional surfaces. The distance between 857.5 and 857.65 is approximately 15 cm. The younging direction of the core is upwards. For more information see Fig. 8. Photo: Courtesy of Erling Siggerud, Trondheim, Norway.

In the Gotland area, the geographic distribution of the Miaolingian – Furongian and the Tremadocian ASF, is uncertain. This is because the coring was limited to certain stratigraphic intervals targeted for oil prospecting with the result that parts of the cores, belonging to the ASF, were not sampled. When present, it is also not possible to separate the Furongian and the Tremadocian within the ASF by way of the core logs. Apart from earlier reports of diagnostic fossils (Andersson et al. 1985; Ahlberg 1989), new finds in the B-7 drill core indicate the presence of the Furongian and the Tremadocian also in the Swedish economic zone of the central Baltic Basin (unpublished internal reports, Svenska Petroleum Exploration AB; Figs. 6 and 7). A continuous extension of the Furongian and the Tremadocian deposits is here proposed from the offshore economic zone of Poland into the Swedish one (Figs. 6 and 7).

Öland area and Lake Hummeln

On Öland, and in Lake Hummeln (Fig. 1), a crater structure in the eastern part of the province of Småland, mainland Sweden (Lindström et al. 1999), the Miaolingian ASF correlates to the Agnostus pisiformis Zone (Linnarsson 1878; Artyushkov et al. 2000; Ahlberg et al. 2018). At the same time tectonic movements result in c. 300 m of subsidence of the crystalline basement in the area of southern Öland (Artyushkov et al. 2000). The thickness of the ASF decreases to the north and finally ends with a condensed conglomerate on northern Öland (Westergård 1922). This conglomerate comprises two parts of which the older belongs to the Miaolingian with the Paradoxides forchammeri Superzone, and the younger to the Furongian with the Leptoplastus and Parabolina Superzones. Within the conglomerate are also Furongian-Tremadocian faunal elements such as Obolus appolinis Popov 1976. The condensed sequence is probably the result of the ARE(Erdtmann 1986). The event has influenced the main part of south-central Sweden, east-central Sweden and the Baltic region. The extension of the ASF to the east and south is proven by the presence of the Tremadocian ASF in the Swedish Yoldia-1 and B-7 offshore well cores (Figs. 6 and 7), as well as in the Polish offshore well cores B7-1, B6-2, B6-1, B4-1 and B3-1 further to the south (Modliński & Szymański 1997).

North Estonian area

In the Gotland and northern Estonian areas, the absence of parts of the Miaolingian Series could possibly be an effect of erosion during the Furongian and Tremadocian (Mens et al. 1999, fig. 5). Reworked acritarchs in phosphorite conglomerates, situated at the base of the Furongian Ülgase and Kallavere Formations (Fig. 3), indicate the former presence of now eroded Miaolingian beds (e.g., Mens et al. 1996). The conglomerate at the base of the Ülgase Formation contains eroded parts of the former sedimentary sequence including Cambrian 2 and the Miaolingian, maybe due to the Hawke Bay regression Event (Nielsen & Schovsbo 2015). The conglomerate at the bottom of the Kallavere Formation shows a spectrum of parts of the sedimentary sequence belonging to the Cambrian 2, Miaolingian and Furongian Series (Mens et al. 1999, fig. 5). It is believed that the ARE caused the erosion of the sedimentary sequence (Erdtmann 1986; Terfelt et al. 2014; Ahlberg et al. 2018). The Furongian and Tremadocian parts of the Kallavere Formation typically consist of siliciclastic facies with streaks of dark shales in the northern Estonian area (e. g. Mens et al. 1996).

The acritarchs and conodonts of the Ülgase Formation correspond to the Olenus and Parabolina Superzones of the Furongian (Mens et al. 1999, Terfelt et al. 2008; Nielsen et al. 2014). The covering Kallavere Formation consists of five parts (see Heinsalu 1987; Heinsalu et al. 2003, fig. 2; Hints et al. 2014, fig. 1C) of which the lower, the Maardu Member (Heinsalu et al. 2003, fig. 2; Hints et al. 2014, fig. 1C), is assigned to the Acerocarina Superzone. The upper Suurjõgi Member (Heinsalu et al. 2003, fig. 2; Hints et al. 2014; fig. 1C) correlates to the Tremadocian. The members are diacronous with the oldest deposite in northwestern Estonia and the youngest further east (Fig. 2; Heinsalu et al. 2003, fig. 2; Hints et al. 2014, fig. 1C).

In the northern Estonian area, the Ülgase and Kallavere Formations show a pattern of redeposited phosphatic shell fragments of Furongian inarticulate brachiopods (Mens et al. 1996; Puura & Holmer 1993). The Ülgase Formation and the Maardu Member of the Lower Kallavere Formation, both of Furongian age, contain well-preserved brachiopods, while the Suurjogi Member of the lower Tremadocian contains fragmented shells of phosphatic inarticulate brachiopods (Heinsalu et al. 2003).

The Ülgase and Kallavere Formations include at least two erosive events, spanning the Olenus, Parabolina and Acerocarina Superzones (Heinsalu et al. 1987; Mens et al. 1993, 1999; Terfelt et al. 2008; Nielsen et al. 2014).

The transition between Furongian and Tremadocian strata is diacronous. In NW Estonia the boundary is at the base of the Cordylodus lindstromi Conodont Zone in the Maardu Member of the Kallavere Formation; while in NE Estonia, it occurs at the base of the Paltodus deltifer Conodont Zone, in the top of the Orasoja Member of the Kallavere Formation (see also Hints et al. 2014, fig. 1.A.). The faunal components, such as conodonts, brachiopods, graptolites and acritarchs, also confirm the transition from the Furongian to the Tremadocian ages (Mens et al. 1996; Nemliher & Puura 1996). The Suurjõgi Member has a sharp contact with the overlying Türisalu Formation (Hints et al. 2014; Fig. 2). The faunal components such as conodonts, brachiopods, graptolites and acritarchs also confirm the transition to the Tremadocian (Mens et al. 1996; Nemliher & Puura 1996). The upper part of this formation ends with a hiatus as a result of a regression (Dronov et al. 2011). Uranium has been commercially extracted from the Türisalu Formation in the Sillamäe area, northern Estonia (Vyalov et al. 2013).

The Varangu Formation of northern Estonia consists of glauconitic greyish silty clay. This indicates that the anoxic condition, present during the development of the Türisalu Formation ceased, and with this also the deposition of dark shales (Kaljo et al. 1986).

Åland Islands

On the Åland Islands, fissure fillings with brachiopods and acritarchs give ages of both Cambrian Series 2 and Furongian (Martinsson 1968; Tynni 1982; Puura & Holmer 1993). The fissures have been filled in by sandstones on at least two occasions (Bergman 1982). These can be dated by acritarchs to Cambrian 2 and Furongian, respectively (Tynni 1982), and indicate tectonic activity on Åland. Presumably there has been a continuation between the sedimentary basins surrounding Åland, but these have since eroded. An example is the Cambrian of northern Estonia.

South Bothnian Basin

In the South Bothnian Basin area, north of the Åland Islands, the Tremadocian ASF has been encountered in the Finngrundet-1 and Västra Banken-1 cores (Thorslund & Axberg 1979; Figs. 1 and 3). The sequence consists of dark shales, cross-bedded sandstone and stinkstone. The maximum thickness is one metre. The age is estimated to be Tremadocian based on Dictyonema sp. and brachiopods (Thorslund & Axberg 1979; Puura & Holmer 1993). High concentrations of uranium (190–240 ppm) and vanadium (1700–2100 ppm) also indicate the presence of the Tremadocian in the cores (Andersson et al. 1985). Glauconitic limestone attributed to the Floian Stage overlays the sequence (Tjernvik & Johansson 1980).

The Tremadocian ASF rests on the Miaolingian Borgholm Formation (Hagenfeldt 1989a; Nielsen & Schovsbo 2007), which forms the base for the Tremadocian sequence (Thorslund & Axberg 1979). However, as shown in Fig. 2, it is here proposed that earlier equivalents to the Furongian were present between the Miaolingian and the Tremadocian strata. This assumption is based on erratics found further south and presumed to have originated in the South Bothnian Basin and dated to the Furongian using conodonts (Löfgren & Laufeld 2007; Löfgren & Viira 2007).

There are no reports of in situ Furongian aged ASF beds in the South Bothnian Basin area. This could be explained by the fact that the Finngrundet-1 and Västra Banken-1 cores are situated on a horst and, therefore, represent a thinner sedimentary sequence (Thorslund & Axberg 1979). However, an additional 11 m of a presumed Cambrian sequence is registered in the seismic reflection and refraction diagrams of the area, as compared to the length of the Cambrian in the Finngrundet-1 and Västra Banken-1 cores (Thorslund & Axberg 1979). These strata may be the source of siliciclastic erratics attributed to the Furongian found along the coast in east-central Sweden and the South Bothnian Basin areas. The estimation of the in situ origin of the erratics is calculated by the geographical distribution caused by Pleistocene ice transport (Wiman 1903; Hagenfeldt 1995).

Stockholm Archipelago and the Åland Basin

Along the coastal area of the Stockholm Archipelago, sandstone erratics rich in bitumen indicate a former presence of the ASF in the Åland Basin and South Bothnian Basin areas (Wiman 1903). Erratics consisting of calcareous sandstone and siltstone originating in the coastal area of the South Bothnian Basin have yielded conodonts attributed to the Furongian Cordylodus proavus and C. intermedius Conodont Zones (Löfgren & Laufeld 2007; Löfgren & Viira 2007). It is uncertain whether typical specimens of Cordylodus lindstromi Druce & Jones 1971 are present. A revision is necessary of the species (see also Heinsalu et al. 2003). Therefore we consider the Cordylodus lindstromi Conodont Zone not to be valid. Only the Furongian is indicated to be present. These layers in the South Bothnian Basin may be coeval with parts of the Kallavere Formation (Maardu Member) in the northern Estonia (Löfgren & Viira 2007).

 In the Åland Basin area, dark sandstone erratics with numerous inarticulate brachiopod specimens indicate the presence of the Furongian (Lars Holmer pers. comm. 2019). Furthermore, Puura & Holmer (1993) report brachiopods of a Furongian age from erratics on the island of Fantom in the Singöfjärden Bay, Stockholm Archipelago; Fig. 1). This brachiopod assemblage differs from that encountered in the Finngrundet-1 core by being of a Furongian age, as compared with a Tremadocian age in the Finngrundet-1 core. However, seismic investigations by Söderberg & Hagenfeldt (1995, fig. 3B; Fig. 1) in the Singöfjärden Bay area indicate in situ occurrence of sedimentary rocks here which may be the origin of the erratics.

Additionally, the presence of Acerocare sp. (Fig. 10) preserved in a coquina limestone erratic from the Stockholm Archipelago area suggests the possibility that the Furongian Acerocarina Superzone may have once been present in the Åland Sea Basin area, but has since eroded.

Figure 10. A cephalon of Acerocare sp., housed in an erratic with Furongian coquina limestone and pyrite crystals, found in the Stockholm Archipelago area. The specimen, 4.0 × 3.5 cm in size, is stored at the Swedish Museum of Natural History, Stockholm, Sweden, accession number NRM-PZ Ar 46074A. Identifier Jan Bergström. Photo credit: Stefan Hagenfeldt.

As stated by Söderberg & Hagenfeldt (1995), most of the Furongian and Tremadocian are believed to be missing in the Åland Sea Basin and the Stockholm Archipelago. However, the total thickness of the Lower Palaeozoic sedimentary sequence in the Åland Basin is about 360 m, of which 120 m is Cambrian in age. This estimation is based on reflection and refraction seismics (Söderberg 1993). Approximately, the same thickness was recorded in the South Bothnian Basin area (Thorslund & Axberg 1979). This fact indicates that parts of the Cambrian may also be present in the Åland Basin as well. The more loosely consolidated erratics of Cambrian age (Miaolingian and Furongian) were probably destroyed during ice transport. The sequence is overlain by Lower Ordovician limestone (Söderberg & Hagenfeldt 1995). The absence of loosely consolidated erratics of Cambrian age may also be due to a steep bottom relief in the Åland Basin (Söderberg 1993), that increases the ability of the moving ice to erode (Gillberg 1967). This means that the erratics consisting of more consolidated rocks will be enriched in the Åland Basin and the Stockholm Archipelago, for instance the Mesoproterozoic (Stenian) quarzitic sandstone of the Söderarm Formation and the Upper Ordovician Baltic Limestone (Hagenfeldt 1995).

St Petersburg area

In the St. Petersburg area (Fig. 1), the Miaolingian, Furongian and Tremadocian strata are represented by a series of sandstones and dark shales of the Ladoga, Tosna and Koporye Formations (Kaljo et al. 1986; Mens et al. 1990; Puura & Holmer 1993; Heinsalu & Bednarczyk 1997; Artyushkov et al. 2000; Berthault et al. 2011; Tyranova & Platonov 2014; Fig. 3). In general, the three series here form a continuation from the northern Estonian area to the Moscow Basin. A summary of these strata at the Tosna River in the St. Petersburg area is shown in Fig. 3.

The local Sablinka Formation (Fig. 3) of Miaolingian age comprises the Upper Wuliuan, Drumian and Lower Guzhangian stages, and is separated from the overlying Ladoga Formation by an intraformational conglomerate.

The following Ladoga Formation, in part spanning the Agnostus pisiformis Zone (Volkova 1990) and the Leptoplastus and Peltura Superzones of the Furongian (Mens et al. 1990; Figs. 2 and 3), consists of medium to coarse quartz sandstone with numerous shells of well-preserved inarticulate brachiopods, i.e., Ralfia ovata Pander 1830, Ungula convexa Pander 1830, Keyserlingia reversa de Vernéuil 1845 and paraconodonts such as Prooneotodus aff. gallatini Müller 1959 and Problematoconites perforata Müller 1959 (Popov et al. 2019).

The transition to the Tremadocian is marked by a conglomerate indicating a regional unconformity, maybe connected with the ARE (Erdtmann 1986). The lowest lithological unit is the Tosna Formation (Fig. 3) is a medium grained sandstone containing cross-bedding and phophatic shell fragments of inarticulate brachiopods redeposited from the Ladoga Formation. However, well-preserved specimens of the brachiopods Obolus apollinis Eichwald 1829 and Helmersenia ladogensis Jeremejew 1856 occur, as well as the conodont Cordylodus proavus Müller 1959 (Popov 1976) indicate a Furongian age. At the top of the Tosna Formation, Cordylodus lindstromi Druce & Jones 1971, show an Early Tremadocian age. Furthermore, detritical zircons indicate that the sandstone is a remnant of eroded granites from the mountain chain developed during the Timanid orogeny (710–510 Ma) at the eastern border of Baltica, thus proving the transport of sedimentary material from the east during the Cambrian-Ordovician (Orlov et al. 2011).

The overlying Koporye Formation consists of dark shales and extends to Lake Onega (Kaljo et al. 1986; Fig. 1). The formation has not yielded any graptolites, and is covered by a glauconitic sandstone where fragments of conodonts have been found which indicate a transition to the glauconitic Leetse Formation (Viira et al. 2006) of the Floian Stage in the St. Petersburg area (Fig. 3). Dronov et al. (1995, fig. 1) list six Cambrian-Early Ordovician eustatic events for this area.

Moscow Basin

The Miaolingian, Furongian and Tremadocian strata in the Central Moscow Basin is represented by the Urdom, Tolbukhino, Nikolskoye, Pestovo, Bugino and Ukhra formations (Mens et al. 1990; Dmitrovskaya et al. 1983; Heinsalu & Bednarczyk 1997; Kheraskova et al. 2005; Kirikov 2016).

The Miaolingian Urdom Formation consists of fine-grained, weakly consolidated, planar laminated to cross-bedded sandstones, with a light greyish colour. These facies are interfingered by dark grey, greenish silt- and claystone (Mens et al. 1990; Kirikov 2016). The only fossils so far found are acritarchs, including Dictyotidium sp. and Multiplicisphaeridium sp., and several fragments of inarticulate brachiopods (Mens et al 1990; Kirikov 2016). Based on acritarchs the estimated age comprises the Paradoxides paradoxissiumus and P. forchammeri Superzones (Mens et al. 1990). The Urdom Formation is 18–40 m thick in the Moscow Basin (Mens et al. 1990; Kirikov 2016).

The following Tolbukhino Formation is present across the Moscow Basin. It consists of quartz sandstone, and dark grey to greenish siltstone (Mens et al. 1990; Kirikov 2016). Dark shales are also present (Kirikov 2016), which may be equivalent to the Miaolingian part of the ASF, indicating the temporary development of anoxic conditions during the Miaolingian in the Moscow Basin. Trace fossils of the type commonly called “crow rock” (in Swedish kråksten) is recognized in this unit (Mens et al. 1990). Amongst the fossils, Agnostus subsulcatus Westergård 1946 is found (Mens et al. 1990). Brachiopods are represented by rare occurrence of Paldiskia and Westonia (Mens et al. 1990; Kirikov 2016). Two acritarch assemblages indicate a Miaolingian age by findings of Aranidium, Cristallinium dubium Volkova 1990, Dictyotidium aff. hasletianum Vanguestaine 1974, Ovulum, Timofeevia janischewski = T. phosphoritica sensu Volkova 1990 and T. lancare Vanguestaine 1978. The youngest assemblage is represented by Cymatiogalera, Pirea orbicularis Volkova 1990, Raphesphaera spinilifera Volkova 1990. R. turbata Volkova 1990 and Vulcanisphaera which point to an Early Furongian age (Mens et al. 1990; Kirikov 2016). The thickness of the formation varies from 11 to 48 m (Mens et al. 1990; Kirikov 2016).

Next is the Nikolskoye Formation situated in the central part of the Moscow Basin. The thickness is 30–50 m and decreases eastwards, as a result of erosion before the onset of the Ordovician (Mens et al. 1990; Kirikov 2016). The formation consists of weakly cemented quartz sandstone, light grey in colour, intersected by thin layers of dark grey siltstone (Mens et al. 1990; Kirikov 2016). Occurrences of brachiopods as Westonia and Lingulella, together with acritarchs indicate a Furongian age (Kirikov 2016).

The Pestovo Formation consists of blue-grey clays with dark siltstones. Interbedded fine-grained quartz sandstone beds with pebble horizons are also present. The formation is present in the central parts of the Moscow Basin. The thickness is highly variable, with a maximum thickness of 40 m (Mens et al. 1990; Kirikov 2016). In the basal part of the formation, remains of the brachiopod Obolus occur (Kirikov 2016). Fossils include the trilobites, Parabolina lobata rossica Balashova 1963, P. longicornus Westergård 1922, P. pestovensis Balashova 1963 together with inarticulate brachiopods such as Westonia and Lingulella all indicating a late Furongian age (Mens et al 1990; Kirikov 2016). This is further supported by the acritarch assemblage named Vk-1, following Volkova (1980).

The following Bugino Formation is present in the western part of the Moscow Basin. The thickness increases eastwards to a maximum of 90 m (Mens et al. 1990). The lowest part consists of a mixture of sandy-silty strata with carbonate and glauconitic cement. The rest of the formation contains dark grey and brownish clays with interbedded silt- and sandstone. The base of the formation consists of a layer rich in phosphoritic pebbles (Mens et al. 1990). Trilobites such as Parabolina sp. and Parabolina jaroslavica Suvorova 1976 (Shestakova et al. 1976) have been identified (Mens et al. 1990; Kirikov 2016), as well as the inarticulate brachiopods Obolus, Lingulella, Westonia, Paldiskia and Acrotreta (Mens et al. 1990; Kirikov 2016). Acritarch assemblages contain Arbuculidium which correlates it with the Peltura Superzone (Mens et al. 1990). Finds of Acanthodiacrodium angustum Combaz 1967 and Dicrodiacrodium ramusculum Volkova 1990 suggest that the middle part of the formation correlates to the Acerocarina Superzone of the late Furongian (Mens et al. 1990). A transition into Ordovician strata has not been detected, so the Bugino Formation probably straddles the Furongian–Lower Ordovician boundary (Kirikov 2016). This is suggested by finds of graptolites indicating the presence of the Tremadocian (Rybnikova & Strikovskaya 1984). Thus the boundary between the Cambrian and the Ordovician is tentatively placed in the uppermost part of the Bugino Formation.

The Ukhra Formation is present in the western part of the Moscow Basin (Kirikov 2016) whereas the lower part of the Ukhra Formation is missing in the southwest and the eastern part of the Moscow Basin (Kirikov 2016). The Ukhra Formation consists of dark grey sandstones, greyish fine-grained quarts and siltstones. Thin layers of intervenient clays are present. The Middle and Late Tremadocian Ukhra Formation contains graptolite shales with Bryograptus ramosus Brögger 1882, B, kjerulfi Lapworth 1880 and Kiaerograptus kiaeri Monsen 1925 (Kirikov 2016). In the eastern part of the Moscow Basin Middle Tremadocian conodonts have been recorded including Cordylodus lindstromi Druce & Jones 1971, C. intermedius Furnish 1938, C. proavus Müller 1959 and Variabiliconus variabilis Lindström 1955 (Kirikov 2016). The Ukhra Formation also contains acritarchs and phosphatic brachiopods (Kirikov 2016). The thickness of the Ukhra Formation varies between 10 and 30 m. It is overlain by the Floian age Sementsovo Formation (Kirikov 2016).

The lower part of the Ukhra Formation is present in the Pestovo-1 well core (Fig. 1). The transition between the Cambrian and Lower Ordovician strata is similar to that in the Baltic Basin, e.g., Gotland (Ahlberg 1989; internal reports, Svenska petroleum AB). The transitional beds contain greyish quartz sandstone with occurrences of Obolus cf. apollinis Eichwald 1829. The thickness is only 20 cm. The following 19 m, attributed to the Koporye Formation, consists of dark shales with interbeds of siltstone containing gypsum and pyrite. The occurrence of Dictyonema sp. suggests an Early Tremadocian age of this part of the succession, which is situated at the top of the Bugino Formation (Alikhova 1971). During the Cambrian and Ordovician, the Moscow Basin was occasionally separated from the Baltic Basin by the Novgorod High (Fig. 1; Kheraskova et al. 2005). The difference between the St. Petersburg area in the west and the Moscow Basin in the east can be studied on either side of the Ladoga-Porkhov line (Fig. 1; Mens et al. 1993). The dominant sandy beds of the Furongian-Tremadocian strata in the St. Petersburg area contain thin streaks of dark shale along the western basin edge of the Moscow Basin, from Lake Ladoga (Fig. 1), along the Ladoga-Porkhov line, to northeastern Poland (see Mens et al. 1993, fig. 1). The dark shale probably represents the peripheral influence from the stagnant conditions in the Baltic Basin.

On the eastern side of the horst, in the Moscow Basin, such conditions did not evolve due to the influx of oxygenic water from the area which is now occupied by the Ural Mountains. Instead, more deep-marine silt and shales developed here. However, exceptions to the oxygenated conditions might be the presence of lateral equivalents to the Furongian ASF in the Miaolingian-Lower Furongian Tolbukhino Formation (Kirikov 2016) and the Koporye Formation occurring in the Upper Tremadocian part of the Ukhra Formation; the latter unit is contemporaneous with the dark shales attributed to the Koporye Formation in the St. Petersburg area (Kheraskova et al. 2005; Fig. 3).

In contrast to the Baltic areas, the Moscow Basin received increased clastic material during the course of the Furongian-Tremadocian. The clastics were derived from the northeast by erosion of the Cadomian and Salair orogenic structures and formed the Nikolskoye Formation. Transgressions followed from the same northeastern direction resulting in the deposition of the following Bugino Formation (Kheraskova et al. 2005).

During the Furongian, the Moscow Basin also appears to have been subjected to tectonic movements caused by the remobilization of old fault zones, and during the Ordovician, a counterclockwise rotation of Baltica (Kheraskova et al. 2001, 2003; Torsvik & Clark 2017).

Poland

In northern Poland, correlatives to the ASF are the Slowinski and Piasnica Formations (Mens et al. 1990; Modliński & Szymański 1997; Fig. 2). The Polish formations consist of dark, bitumen-rich shales, which, in the west straddle the Cambrian-Ordovician boundary without any lithological changes. The transition is indicated only by the shift from Furongian to Tremadocian faunal components (Modliński & Szymański 1997). The Piasnica Formation is in turn covered by Floian glauconitic shales and limestones of the Slochowo Formation (Fig. 2).

In northeastern Poland, the Mlynary Formation represents a condensed succession equivalent to the Slowinski Formation, and correlated to the Olenus and the Parabolina spinulosa zones (Mens et al. 1990). The Sepopol Formation, which overlies the Mlynary Formation, occurs as downfaulted patchy beds (Modliński & Szymański 1997) and contains thin layers of dark shales probably coeval with the Furongian ASF. The Sepopol Formation is, in turn, overlain by the Pieszkowo Formation of Floian age (Fig. 2), and contains glauconitic limestones (Modlińsky & Szymański 1997).

In summary, both the offshore and onshore areas of northern Poland contain equivalents to the Miaolingian and Furongian ASF such as sandstones and thin layers of dark shale. In general, the ASF, in its upper part, grades laterally and vertically into sandstones in northeastern Poland and the east Baltic area, while a continuous deposition of dark shales take place in the northwestern Poland and the offshore part of the Polish economic zone (Heinsalu & Bednarcyk 1997). The onset of dark shale sedimentation in northern Poland (Slowinski Formation; Fig. 3) commences during the Agnostus pisiformis Zone which is coeval with the onset of Miaolingian ASF deposition on Öland and Gotland (Ahlberg 1989; Artyushkov et al. 2000; Fig. 2).

Kaliningrad area

In the Kaliningrad area, the Ladushkino Formation (see Mens et al. 1990 for a summary; Fig. 2) contains organic-rich limestones coeval with the Kakeled Member of the Furongian ASF (sensu Nielsen & Schovsbo 2007) as developed in the provinces of Västergötland, Östergötland and Närke in south-central Sweden (Lashkov & Jankauskas 1993; Fig. 1). The Ladushkino Formation has a thickness of 0.2 to 1.65 m and contains quartz sandstone and dark-coloured, well-consolidated shales which are kerogen-bearing (Mens et al. 1990). The formation contains Acrotreta sp., Homagnostus sp. and Orusia lenticularis Wahlenberg 1818, which indicate the presence of the Agnostus pisiformis Zone and the Olenus and Parabolina Superzones (Fig. 2). It should be noted that the Ladushkino Formation is younger than the lowermost ASF on Gotland, Öland and in northern Poland.

The Nivenskaya Formation lies discordantly on the Furongian Ladushkino Formation and consists of light-grey sandstone with abundant shells belonging to the Family Obolidae, which indicate a Tremadocian age (Lukyanova et al. 2011). In the offshore part of this area, this formation is expressed as a coarse-grained, brown-grey sandstone containing pebbles of silt and Cambrian sandstone (Lukyanova et al. 2011; Fig. 2). The thickness, reported by Lukyanova et al. (2011) of the Nivenskaya Formation, is 30–50 m, which is significantly more compared with surrounding areas. However, Zytner & Fenin (2010) are of the opinion that the maximum thickness of the Nivenskaya Formation does not exceed 2 m and only occurs as down-faulted patchy beds. The Gudkovskaya Formation, containing glauconitic sandstones of Floian age, overlies the Nivenskaya Formation in the onshore region of the Kaliningrad area (Lukyanova et al. 2011; Fig. 2).

Lithuania

In Lithuania, the Furongian and the Tremadocian is represented by loosely consolidated sandstones of the Kallavere Formation in the north and the contemporaneous Salantai Formation in the central part of Lithuania (Luksevicius et al. 2012; Meidla et al. 2022). Both formations are less than 2 m thick (Fig. 3; Jankauskas 2002; Luksevics et al. 2012). The units occur as downfaulted patchy beds, especially in the southwest along the Kaliningrad-Gusev dislocation fault zones (Jankauskas & Lendzion 1992; Modliński et al. 1999, fig. 3). Inarticulate brachiopods of the species Ungula cf. convexa Pander 1830 indicate a Late Furongian age (Lashkov et al. 1993). However, acritarchs also indicate the presence of the Tremadocian (Jankauskas 2002).

Latvia

In Latvia, there are contemporaneous beds to the ones discussed previously. The Salantai Formation is correlateable with the ASF and dated by acritarch to the Late Furongian and Tremadocian (Fridrichsone & Zabels 1995; Jankauskas 2002). A 0.3 m thick glauconitic sandstone overlies these layers (internal report, Svenska Petroleum Exploration AB), which is probably contemporaneous with the Zebre Formation (Luksevicius et al. 2012; Meidla et al. 2022), found in the E6 borehole just off the Coast of Latvia (Fig. 6). Furthermore, onshore occurrences in boreholes of the same strata are also reported from the coastal area of Latvia (Heinsalu 1986, fig. 1; Dronov et al. 2011, fig. 1). The sandstone is similar to the Leetse Formation in northern Estonia (Viira et al. 2006). These sandstones may also be contemporaneous to the glauconitic conglomerate and sandstones of the Grötlingbo-1 borehole on Gotland (Fig. 4) which are dated to the Late Tremadoc based on acritarchs (Hagenfeldt & Bjerkéus 1991).

The area to the east of Poland and Lithuania constitutes of the Mazury-Belarus High (Poprawa et al. 1999). This major horst structure is believed to make up the Furongian-Tremadocian shoreline, with submarine areas to the west and north of the horst.

Discussion

The presence of downfaulted patchy beds of the ASF indicates an initially more extensive distribution of the formation than hitherto reported. Due to the scarcity of drill cores, especially offshore ones, it has not been possible to determine the exact former extent of the ASF, but there is enough data to at least establish a sketch of the original distribution. On Gotland, some cores, i.e., File Haidar-1, Grötlingbo-1, När-1, Viklau-1, Visby-1 and Hamnudden-1 on the Gotska Sandön have penetrated the sedimentary sequence with full recovery. These cores form the base for the development of the Cambrian (Miaolingian, Furongian) and the Lower Ordovician (Tremadocian) in the Baltic Basin. In addition, there are drill cores for oil prospecting. However, the samples available from these latter wells were those stratigraphic horizons of interest to the oil companies, which gives a more uncertain picture since these data are based only on the core logs. The general distribution of the ASF based on this new data is presented in Fig. 4.

In general, the map presented by Buchardt et al. (1997, fig. 7) corresponds to ours, but since their map is based on fewer data, provided by Svenska Petroleum Exploration AB, it does not show as much details as the map presented here (Fig. 4) which is the result of extensive literature studies. Furthermore, the downfaulted patchy beds of the ASF on Gotland have not been reported earlier. Together with the contemporaneous beds of the same type in the Sopopol and Nivenskaya Formations of the South Baltic Basin, and the Furongian-Tremadocian Kallavere and Salantai sandstones (Lashkov et al. 1993), a new pattern is shown where downfaulted patchy beds surround the main cover of dark shales as the ASF (Fig. 7).

Furthermore, downfaulted patchy occurrences of the ASF in the northern Gotland area (Figs. 4 and 5) raise the question whether they were the result of tectonic influence creating differences in the palaeotopography where down-faulted areas were preserved from erosion. A similar process can also have been active in northeastern Poland where Tremadocian siliciclastics of the Sopopol Formation have been preserved as downfaulted patchy layers (Modliński et al. 1999; Fig. 4). Furthermore, a pattern of partially raised bedrock is also reported from northern Estonia and Lithuania (Heinsalu & Bednarczyk 1997).

The patchy occurrences of the ASF and its equivalents were probably created by epeirogenic and tectonic movements in the investigated area during the Late Furongian to Early Tremadocian, before the onset of the CTE(Nielsen & Schovsbo 2015; Artyushkov et al. 2000). During the Late Furongian to Early Tremadocian erosion affected elevated parts of the ASF, while down-faulted blocks of the formation remained undisturbed. The erosion is believed to have taken place along the periphery of the main distribution of ASF; one example of this is the Rute high on northern Gotland where tectonic movements reduced the thickness. The same process is believed to have taken place all along the edge of the ASF and its coeval deposits – the Kallavere, Salantai, Sopopol and the Nivenskaya Formations in the southern part of the Baltic Basin – creating downfaulted beds along the border of the formation (Modliński & Szymański 1997; Zytner & Fenin 2010). The thickness of the ASF is <5 m in the Gotland area. This suggests an estimated erosion of at least 5 m or more for the formation in areas where it is absent on Gotland.

There are two main events that are believed to have influenced the distribution of the Miaolingian–Furongian ASF on Gotland and elsewhere on the East European Platform: the transgression during Agnostus Pisiformis Trilobite Zone (Ahlberg et al. 2009) and the ARE (Erdtmann 1986). During the first event, transgression in the Gotland, Poland and Kaliningrad areas led to the deposition of dark shales. On Gotland, three zones of the ASF have been identified: the Agnostus pisiformis Zone of the Guzhangian Stage of the Miaolingian Series, the Ctenopyge flagellifera Zone of the Furongian Series, and the Tremadocian part of the ASF which is a continuation of dark shale, into the Ordovician (Andersson et al. 1985; Ahlberg 1989). Apart from a depleted fauna (Ahlberg et al. 2009), elevated values of vanadium and uranium in this area indicate the presence of Furongian ASF belonging to the Peltura Superzone (see also Andersson et al. 1985; Nielsen et al. 2018). The same pattern is seen in the B-7 core south of Gotland (internal report, Svenska Petroleum Exploration AB). In some of the Gotland wells with a full coring, various parts of the ASF are preserved. However, in most cases, the ASF is absent, probably as a result of extensive erosion. This could have taken place during the Furongian ARE, sensu Erdtmann (1986). This event also influenced several parts of the eastern part of Baltica, creating subaerial denudation surfaces in many places (Teve & Lindström 1988; Ahlberg et al. 2018). There are also indications of subaerial denudation surfaces south of Gotland in the B-7 core, probably of the same age as those developed on Gotland. Later, during the Tremadocian, the CTE (Erdtmann 1986) marked the end of dark shale deposition and initiated the onset of glauconitic shales, sandstones and limestones of a Late Tremadocian-Early Floian age on Gotland (Hagenfeldt & Bjerkéus 1991), as well as on the rest of the East European Platform. This pattern is also seen in the strata overlying the ASF in mainland Sweden (Andersson et al. 1985).

The development of the Cambrian in the east Baltic Basin, the northwestern part of the Moscow Basin and the St. Petersburg area, is in part similar to the development in the central Baltic Basin area. This fact suggests a continuation of the ASF from the northern Estonian area and St. Petersburg areas to Gotland, and to the south Baltic Basin (see also fig. 1 in Nielsen et al. 2018). There was also a possible continuation via the Åland Islands, with its sandstone fissures dated to the Furongian, to the South Bothnian Basin (Männil 1966, fig. 4; Nielsen & Schovsbo 2015).

The nearshore facies of the South Bothnian Basin area and south-central Sweden are believed to have had a lateral continuation through east-central Sweden based on the existence of a peneplain with fissure fillings of probable Cambrian age (Hagenfeldt 1995; Söderberg & Hagenfeldt 1995). Furthermore, the Estonian Kallavere and Türisalu formations, together with the Skåningstorp Sandstone beds of the ASF, are believed to have been connected via deposits stretching from northern Estonia to the Gotland area, but have since eroded (Nielsen & Schovsbo 2007).

Three cores (Viklau-1 and Grötlingbo-1 on Gotland, and B-7 south of Gotland) indicate the presence of the Tremadocian based on occurrences of Dictyonema sp. and high values of vanadium (1867 ppm; Andersson et al. 1985; unpublished material, Svenska Petroleum Exploration AB) and acritarchs (Hagenfeldt & Bjerkéus 1991). The Tremadocian ASF, intercalated with cross-bedded sandstone in all three cores, suggests an extension of the Tremadocian to the Gotland area from the contemporaneous Kallavere and Türisalu Formations in the northern Estonia area, and the Tosna and Koporye Formations in the northwest and central parts of the Moscow Basin. A further extension to the southern Baltic Basin with the B-7 core is also proposed herein. Eastwards, from the B-7 core, the presence of a 0.3 m glauconitic sandstone facies in the E6-1 drill core in the offshore Latvian area (Fig. 7), together with occurrences in onshore drill cores, suggests the sandstone to be contemporaneous with the Leetse Formation (unpublished material, Svenska Petroleum Exploration AB).

The ASF is lacking in the submarine area between Gotland and the Baltic States, as well as onshore in the latter area (Fig. 5a; internal reports, Svenska Petroleum Exploration AB; Buchardt et al. 1997, fig. 7; Nielsen et al. 2018, fig. 1). Instead, these areas contain glauconitic conglomerates, sandstones, limestones and shales of a Late Tremadocian-Early Floian age which covers the Terreneuvian, Series 2 and Miaolingian siliciclastics of the Cambrian (unpublished material, Svenska Petroleum Exploration AB). It is here proposed that the deposition of the ASF never reached these areas and did not extend very far eastwards from its present distribution (Fig. 5a; Buchard et al. 1997, fig. 7; Nielsen et al. 2018, fig. 1). The easternmost extension is indicated by thin streaks of Tremadocian dark shale that are found along the western edge of the Moscow Basin limited by the Ladoga-Porkhov line in the east (Mens et al. 1990), and the western parts of the Baltic States (Heinsalu & Bednarczyk 1997). However, apart from findings of the Koporye Formation in the vicinity of Lake Onega and the western limits of the Moscow Basin, 19 m of the Koporye Formation is also present in the Pestovo-1 core in the central Moscow Basin (Alikhova 1971; Zoritcheva 1978). This indicates a wider extension and thickness than earlier recorded. Also, the presence of presumed equivalent of the Miaolingian ASF in the Tolbukhino Formation has to be considered in the reconstruction of the development of the Cambrian.

The main focus of this publication was to investigate why the ASF formed the observed downfaulted beds. It can be concluded that the ASF, the Kallavere and Türisalu formations, as well as the Piasnica and Sopopol formations are preserved under the cover of sedimentary beds laid down during the Late Tremadocian to Early Floian CTE. Where the ASF deposits are thin, they were probably almost completely eroded; for example, at least 5 m were eroded on parts of the Gotland area. The absence of the ASF may be due to the fact that in elevated areas the ASF was completely eroded. Areas where the ASF being protected where downfaulted, as a result of tectonics at the Cambrian-Ordovician boundary, or from being deposited on an uneven surface (Modliński et al. 1999). Outcrops were also preserved in downfaulted positions at the periphery of the area of the major cover of the ASF. This seems to have been the case in northeastern Poland, the Kaliningrad area, southeastern Lithuania (pers. comm. Dr. Jolanta Ceroizie 2016), western Latvia and in the Gotland area.

The onset of the CTE (Erdtmann 1986) may have started as early as the Late Tremadocian in the Gotland area (Hagenfeldt & Bjerkéus 1991) and moved further eastwards. As with the previous transgression under Guzhangian, starting with the deposition of dark shales containing Agnostus pisiformis, an asynchronous sedimentary sequence developed with the oldest layers in the west, youngig eastwards. Furthermore, on Gotland, three drill cores have yielded dateable fossils. In these cores, the Agnostus pisiformis (Ahlberg 1989) and Ctenopyge flagellifera Zones (Andersson et al. 1985), belonging to the Peltura Superzone (Terfelt et al. 2008; Nielsen et al. 2014) have been found. It is also herein proposed that the high values of uranium in this part of the Gotland ASF succession are indicative of the Upper Furongian Peltura Superzone. An uranium peak is found in the B-7 core south of Gotland indicating the Peltura Superzone to be present here, too (internal report, Svenska Petroleum Exploration AB; see also Nielsen et al. 2018).

In summary, the onset of the ASF, during the time of the Agnostus pisiformis Zone, affected a large part of the East European Platform. The transgression was asynchronous with its onset beginning in the western part of Baltica. Later on, during the Furongian, regressions such as the ARE (Erdtmann 1986), parts of the ASF were eroded.

During the Tremadocian, a transgression resumed deposition of the Tremadocian dark shales. Shallow conditions occurred at the edge of the basin indicated by beach sands with cross-bedding and eroded fragments of phosphatic inarticulate brachiopods. Later in the Tremadocian, regressive events eroded at least 5 m of the ASF in some parts of the basin. Lastly, the remaining ASF deposits were covered by glauconitic limestones during the CTE evolving from the west on the East European Platform and going eastwards. This ended the conditions forming the ASF. Instead, deposition of Upper Tremadocian-Floian glauconitic limestone commenced.

Conclusions

1. The present extent east of the ASF in the Gotland area does not differ to any significant extent from the original distribution.

2. The eastern part of the Baltic Basin contains equivalents to the Kallavere Formation, which is contemporaneous with the Furongian ASF, but these are now eroded and only remain preserved as downfaulted patchy beds.

3. Finds of dark shales in the Tolbukhino Formation, situated in the Moscow Basin, are assumed to be equivalents of the Miaolingian ASF. On the East European Platform, a possible connection between the Baltic Basin and the Moscow Basin may have existed during the Miaolingian, but physical evidence of this is lacking, perhaps due to erosion.

4. On Gotland, the main phases in the development of the ASF were: (i) the onset of sedimentation during the Agnostus pisiformis Zone in the Miaolingian; and (ii) two major erosional events during the Furongian and the Tremadocian, the ARE and the PRE, respectively. More than 5 m were probably eroded from parts of the ASF in the Gotland area before the beginning of sedimentation of the overlying glauconitic limestone and shales during the CTE of the Late Tremadocian-Floian. The glauconitic limestone, contemporaneous with the Leetse Formation in North Estonia, generally occurs in all the areas on the East European Platform.

5. In the Baltic Basin, the Tremadocian deposits are presumed to extend from the area of the Polish Economic Zone to the Swedish one.

6. Equivalents to the Kallavere Formation are suggested to occur on the Åland Islands, the South Bothnian Basin, the provinces of Dalarna and Östergötland, the Gotland area, south of Gotland (B-7 core), parts of the Baltic States and northeastern Poland.

7. In the Moscow Basin, within the Ukhra Formation, Tremadocian dark shales, which are here tentatively attributed to the Koporye Formation, reach greater thickness (19 m) compared to other areas on the eastern part of the East European Platform.

8. Tectonics are believed to have created block movements that elevated parts of the ASF and contemporaneous siliciclastics and dark shales, resulting in total or partial erosion of the sedimentary rocks due to epeirogenic events.

Acknowledgements

Svenska Petroleum Exploration AB is thanked for giving access to unpublished reports. The Geological Survey of Sweden (SGU) is also very much thanked for letting us have access to the Grötlingbo-1 and B-7 cores. Determination of the material and the completion of the paper was undertaken by SH. All the authors contributed to editing and refinement of the paper. We thank two anonymous reviewers for their work in providing constructive critism and suggestions to improve the article. We also thank Marina Amantova for the help of correcting the figures. Finally, we thank Chris Mays for linguistic corrections of the manuscript. The project was supported by Svenska Petroleum Exploration AB, Sweden.

Disclosure statement

No potential conflict of interest was reported by the authors.