Extent of the northern Baltic Sea during the Early Palaeozoic Era - new evidence from Ostrobothnia, western Finland

Abstract

This study reports new evidence of the extent of the northern Baltic Sea during the Cambrian and Ordovician periods. A drillcore (DC304) from the Lappajärvi impact crater, western Finland, and erratics from the surrounding area were studied for acritarchs. The acritarchs from the drillcore were reworked by the explosion but indicate, however, that the Baltic Sea extended beyond the western coast of central Finland during the Lower Cambrian Vergale and the Middle Cambrian Kibartai regional stages as well as during Middle Ordovician transitions from Aseri to Lasnamägi and from Idavere to Jöhvi regional stages. The provenance of the erratics was the Bothnian Bay, further north than the Lower Palaeozoic deposits previously known in the Bothnian Sea. The age could be determinated by their acritarch composition which shows that the sea also covered Central Ostrobothnia during the Lower Ordovician Billingen and early Middle Ordovician Lasnamägi regional stages. The drillcore samples contained also unknown spores. They suggest that there have been sediments and/or a terrestrial flora later than Ordovician in the Lappajärvi area.

Early Cambrian small carbonaceous fossils (SCFs) from an impact crater in western Finland

“…Probable Terreneuvian strata are known from the extreme north of Finland (Systra & Jensen ), but how these relate to contemporaneous deposits in southern Baltoscandia is still unclear. Cambrian sediments are also known to the west of the Lappajärvi crater, from the Söderfjärden impact crater in western Finland (Tynni , ; Hagenfeldt ,c; Uutela ), from a number of scattered erratics and fissures around Finland (Simonen & Kouvo ; Martinsson ; Simonen ; Tynni ; Uutela ), from fissure‐fills in Åland (Martinsson ; Bergman ; Tynni ) and from sediments underlying the Bothnian Sea (Hagenfeldt ,c; Slater et al . ).…”

“…B, map of Lake Lappajärvi region showing position of Vimpeli‐302 core (red dot and core symbol) in relation to the impact region. Position of core Kärnänsaari‐304 studied by Uutela () also indicated ( DC ‐304). Numbers between dashed lines indicate zones of deformation and rock exposure associated with the impact crater (after Pipping ); 1 = outer rim, mainly Palaeoproterozoic pegmatite granite and mica schist; 2 = siliciclastic deposits (silts and sands intersected in Vimpeli‐302 core); 3 = suevite and breccias formed during the Late Cretaceous impact; 4 = melt sheet formed during Late Cretaceous impact.…”

“…The Cambrian acritarchs found in this fissure have been ascribed to a mixture of ‘Vergale’ and ‘Kybartai’ regional ages, approximately corresponding to the lower part of global Cambrian Series 2 stage 4, and the stage 4–5 transition, respectively (see Meidla for details of regional stratigraphical schemes). These microfossil assemblages have been interpreted as being derived from Cambrian and Ordovician sediments which existed prior to the Lappajärvi crater‐forming impact, and which were subsequently reworked during the impact to fill a fissure within the suevite (Uutela ). Based on this, the proposed extent of the epeiric Cambrian sea in this region was enlarged to encompass Lappajärvi at its eastern extent during Cambrian stages 4–5 (Uutela ; see also Hagenfeldt ).…”

“…These microfossil assemblages have been interpreted as being derived from Cambrian and Ordovician sediments which existed prior to the Lappajärvi crater‐forming impact, and which were subsequently reworked during the impact to fill a fissure within the suevite (Uutela ). Based on this, the proposed extent of the epeiric Cambrian sea in this region was enlarged to encompass Lappajärvi at its eastern extent during Cambrian stages 4–5 (Uutela ; see also Hagenfeldt ). Sediments towards the outer rim of the Lappajärvi crater, like those of the Vimpeli‐302 core, however, have thus far remained assigned to a Mesoproterozoic age (Vaarma & Pipping ; Schmieder & Jourdan ; Elminen et al .…”

“…One such crater is the Lappajärvi impact structure in western Finland (Svensson 1968;Lehtinen 1970Lehtinen , 1976Pipping & Lehtinen 1992;Schmieder & Jourdan 2013;Kenny et al 2019). Pre-impact sediments (i.e., those which existed prior to the crater formation) in the central parts of the crater have largely been heavily altered or destroyed via the impact-generated melt sheet (Schmieder & Jourdan 2013), although some reworked remnants survive in fissures (Uutela 1998). Near to the outer rim of the Lappajärvi crater, however, sediments are preserved in more or less cohesive packages within slumped or displaced megablocks (Pipping & Lehtinen 1992).…”