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Ediacaran and Cambrian Stratigraphy in Estonia: an Updated Review

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Ediacaran and Cambrian Stratigraphy in Estonia: an Updated Review Estonian Journal of Earth Sciences, 2017, 66, 3, 152–160 https://doi.org/10.3176/earth.2017.12 Ediacaran and Cambrian stratigraphy in Estonia: an updated review Tõnu Meidla Department of Geology, Institute of Ecology and Earth Sciences, Faculty of Science and Technology, University of Tartu, Ravila 14a, 50411 Tartu, Estonia; [email protected] Received 18 December 2015, accepted 18 May 2017, available online 6 July 2017 Abstract. Previous late Precambrian and Cambrian correlation charts of Estonia, summarizing the regional stratigraphic nomenclature of the 20th century, date back to 1997. The main aim of this review is updating these charts based on recent advances in the global Precambrian and Cambrian stratigraphy and new data from regions adjacent to Estonia. The term ‘Ediacaran’ is introduced for the latest Precambrian succession in Estonia to replace the formerly used ‘Vendian’. Correlation with the dated sections in adjacent areas suggests that only the latest 7–10 Ma of the Ediacaran is represented in the Estonian succession. The gap between the Ediacaran and Cambrian may be rather substantial. The global fourfold subdivision of the Cambrian System is introduced for Estonia. The lower boundary of Series 2 is drawn at the base of the Sõru Formation and the base of Series 3 slightly above the former lower boundary of the ‘Middle Cambrian’ in the Baltic region, marked by a gap in the Estonian succession. The base of the Furongian is located near the base of the Petseri Formation. Key words: Ediacaran, Cambrian, correlation chart, biozonation, regional stratigraphy, Estonia, East European Craton. INTRODUCTION The latest stratigraphic chart of the Cambrian System in Estonia (Mens & Pirrus 1997b, p. 39, table 6) is The main features of the latest Precambrian and drawn mainly in accordance with the stratigraphic chart Cambrian stratigraphy in Estonia were summarized in of the Cambrian System ratified in 1983 and published the book Geology and Mineral Resources of Estonia three years later (Reshenie… 1986). Mens & Pirrus (Raukas & Teedumäe 1997). The description of the latest (1997b, p. 39) write: ‘As the global standard is still in Precambrian strata in this volume (Mens & Pirrus 1997a) preparation, the boundaries between the Lower/Middle is based on the stratigraphic chart of the Vendian and Middle/Upper Cambrian are not strictly formal.’ Complex, accepted in 1976 at the Baltic Stratigraphic After the formal definition of the lower boundary of the Conference (Resheniya… 1978). The term ‘Vendian’ Cambrian System (Brasier et al. 1994) and the lower was applied to the latest Precambrian of Estonia in this boundary of the Ordovician System (Cooper et al. 2001), paper, apparently because the International Commission a fourfold subdivision of the Cambrian System into on Stratigraphy ratified a geochronometric subdivision series was proposed at the 9th International Field of the Proterozoic but left the terminal Proterozoic period Conference of the Cambrian Stage Subdivision Working ‘for later definition and characterization’ (Knoll et al. Group (Babcock et al. 2005). 2006). More than ten years later, the terminal Proterozoic The aim of this paper is to update the regional period called ‘Ediacaran’ was introduced by a vote in stratigraphic charts and apply new global units and the Subcommission of Precambrian Stratigraphy and correlations to the Estonian Ediacaran and Cambrian defined in accordance with the concept proposed already succession. The revised correlation and related problems by Cloud & Glaessner (1982). The proposal was officially are discussed and the charts are drawn on a timescale. ratified by the International Union of Geological Sciences in 2004 (Knoll et al. 2006) and the lower boundary was dated as 635 Ma. These changes in the global Precambrian THE EDIACARAN SYSTEM stratigraphy call for a more detailed discussion of the stratigraphic position and age of the late Precambrian Sokolov (1952) proposed the name ‘Vendian’ for a system strata of Estonia. and period comprising the uppermost/latest Precambrian. © 2017 Author. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0). 152 T. Meidla: Ediacaran and Cambrian stratigraphy in Estonia A year later, this name was introduced for the western official age of the lower boundary of the Vendian and part of the East European Craton (Sokolov 1953). the Vilchan Group in Belarus (600 Ma – Kruchek et al. The relevant part of the stratigraphic succession was 2010) disagrees with several other dates. For example, distinguished in its present extent in the correlation the Gorbashev Formation (the basal formation of the chart approved by the Baltic Stratigraphic Conference Volynian Group) is usually dated at ca 630 Ma but (Resheniya… 1978) and a commented version of the occasional dates are reaching 1040–1120 Ma (Zapolnov same chart was published two years later (Mens & 1993). At the same time, the proposed age for the base Pirrus 1980). An English version (Mens & Pirrus 1997a) of the Volynian Group is 660 ± 20 Ma (Zapolnov 1993), summarizes the present subdivision of the Vendian i.e. the late or latest Cryogenian. System in Estonia. Although Pirrus (1992) suggested a very high sedi- The term ‘Vendian System’ is used in Russia and mentation rate for the clay deposits of the Kotlin Belarus for a unit comprising a succession from the Formation (5 m/10 000 yrs), the possible age implications upper Cryogenian to the lowermost Cambrian and having for the whole of the Ediacaran succession in Estonia are a threefold (e.g. Chumakov 2007; Kruchek et al. 2010) not considered in former publications. No radiometric or fourfold (e.g. Sokolov 2011) subdivision. The Vendian dates are available for the Ediacaran strata in Estonia Complex of the Baltic States is subdivided into two but the age limits of this succession can still be tentatively regional stages (Resheniya... 1978; Puura et al. 1982). specified based on the data from Russia and Belarus. The lower, Redkino Regional Stage is characterized by Sokolov (2011) roughly estimated the age of the lower a soft-bodied biota and the upper, Kotlin Regional Stage boundary of the Kotlin Regional Stage to be 570–580 Ma, is characterized by acritarchs and algal remains (Pirrus but other dates suggest that this may be far too old. 1992). In some papers, the oldest part of the Vendian Grazhdankin et al. (2011) demonstrate an indirect but Complex is distinguished as the Drevlyan Regional Stage well-argued correlation of the lower boundary of the (Jankauskas 1993; Paškevičius 1997; Kruchek et al. Kotlin Regional Stage on the Lublin Slope of the craton 2010). This unit is unfossiliferous, although its suggested and constrain the age of this boundary between 551 and equivalent in Belarus, the Lyozno Regional Stage, contains 548 Ma, based on U–Pb dates from zircons. Considering an acritarch assemblage that differs considerably from the these dates, the correlation of Ediacaran strata in Estonia assemblages in the overlying strata (Kruchek et al. 2010). can be preliminarily drawn on a time scale (Fig. 1). This As the term ‘Ediacaran System’ was formally adopted shows that the Ediacaran in Estonia represents the in 2004, the strata formerly termed ‘the Vendian youngest part of the system and likely formed within the Complex’ in Estonia, Latvia and Lithuania should be last 7–10 Ma of the period. attributed to the Ediacaran System. The Ediacaran strata in Estonia are correlated with the Kotlin Regional Stage (Resheniya… 1978; Mens & Pirrus 1980, 1997b). THE CAMBRIAN SYSTEM IN ESTONIA The differences between the lower boundaries of the Ediacaran System and the Vendian Complex are well The Cambrian System has been traditionally subdivided known (see, e.g., Chumakov 2007; Meert et al. 2011) into three series in Estonia. The strata in the northern and do not complicate the use of the term ‘Ediacara’ for Estonian outcrop area were attributed to the Lower Estonia, where the system is represented by its upper Cambrian already by Bekker (1923) and Öpik (1925) part only. The Gudenieki and Zūra strata comprise and a threefold subdivision of the Cambrian System was the oldest part of the ‘Vendian’ in western Latvia. adopted for Estonia in the same papers. The lower The Ediacaran age of these units is proved by the boundary of the Cambrian System in the western part of radiometric dating of the Gudenieki alkali basaltic lavas the East European Craton is drawn at the base of the (595 ± 25 Ma) and the fossil content of the Zūra clastic Baltic Group (Mens et al. 1987) and the level is marked beds (Laminarites-type algal laminae, Leiosphaeridia sp.) by a regional unconformity in Estonia. The appearance suggesting the Redkino or Kotlin age of these strata of the first skeletal fossils and a change among ichno- (Lukševičs et al. 2012; see also Šliaupa & Hoth 2011). and phytofossils are recorded at this level (Mens & The lowermost ‘Vendian’ in Lithuania, the Merkys Pirrus 1997b). Formation, is attributed to the upper Drevlyan Regional The presence of the Upper Cambrian in Estonia Stage (Paškevičius 1997) and is of Ediacaran age by was demonstrated by Volkova (1982) and Kaljo et al. Lukševičs et al. (2012). (1986). The occurrence of the Middle Cambrian was In Belarus, the lower boundary of the Ediacaran may clearly argued in the 1970s (Resheniya… 1978; Kala tentatively be drawn within the strata attributed to the et al. 1984). The lower boundary of the Middle Cambrian Vendian in the most recent stratigraphic chart (Kruchek on the East European Craton was traditionally drawn near et al. 2010). The information is contradictory, as the the appearance level of the acritarchs Baltisphaeridium 153 Estonian Journal of Earth Sciences, 2017, 66, 3, 152–160 Fig. 1. Ediacaran chronology and stratigraphy in Estonia, based on Mens & Pirrus (1997a). latviense, B. pseudofaveolatum, Liepaina plana, The boundary of the Upper Cambrian was traditionally Lophosphaeridium variabile and rare specimens of the drawn at the base of the Agnostus pisiformis Zone genera Cristalinium and Eliasum (Mens et al.
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