Sandbian chitinozan biostratigraphy - tracking the North Estonian Ordovician regional stages across the Paleobaltic basin

TY - ABST

T1 - Sandbian chitinozan biostratigraphy - tracking the North Estonian Ordovician regional stages across the Paleobaltic basin

AU - Stouge, Svend

AU - Bauert, Heikki

AU - Bauert, Garmen

PY - 2017/9/12

Y1 - 2017/9/12

N2 - Chitinozoa are a group of organic-walled microfossils with uncertain origin that appeared in the Early Ordovician and became extinct in the Devonian. Their key characteristic features- rather simple morphologies, rapid diversification in the Ordovician, a short life span for a number of key species, a planktonic/ nectonic life style, and usually abundant presence in various lithologies of marine origin - have made the chitinozoans an important biostratigraphic correlation tool for the Lower Palaeozoic rocks in the last decades. The Sandbian succession of the Palaeobaltic basin embraces the Kukruse, Haljala and partly the Keila regional stages, with the Kukruse and Keila stages being introduced already over 130 years ago and the Haljala Stage erected by V. Jaanusson in 1995 by merging the historical Idavere and Johvi stages. These regional stages were based on lithological composition combined with finds of certain macrofossils in the outcrops ofNorth Estonia, whereas the International Commission on Stratigraphy recommends establishing the stage boundary stratotypes in sections known to represent the maximum complete sedimentation record and to be based on unequivocally determinable key fossil species that can be recorded in great numbers in drill cores as well. Commonly, conodonts and graptolites are used to determine the Ordovician global stages, but distinct chitinozoan key species have a very high potential for recording the Upper Ordovician regional stages in the Palaeobaltic basin. At present, zonal key chitinozoans are used to recognize the bases of the Upper Ordovician regional stages: the Nabala Stage (FAD of Armoricochitina reticulifera), the Pirgu Stage (LAD of Acanthochitina barbata), and the Porkuni Stage (FAD of Spinachitina taugourdeaui), but so far microfossil-based boundaries are not defined for any North Estonian regional stages of Sandbian age. The purpose of this contribution is to provide precise boundary definitions for the Kukruse and Haljala regional stages, based on chitinozoan key species and related to conodont zones (Fig. 1 ). The proposed horizons (1) allow for basinwide correlations across the Palaeobaltic basin and (2) permit correlation with other Ordovician basins. High-resolution Sandbian chitinozoan studies from a dozen cores (derived from Estonia, Sweden, Lithuania, Poland and NW Ukraine), have shown the highest correlational potential of these easily and unambiguously determinable zonal key species for tracking the base boundaries of the Kukruse and Haljala stages: -base of the Kukruse Stage: FAD of Laufeldochitina stentor, appearing in the uppermost Uhaku strata of present sense and reported from all studied Kukruse age sections in the Palaeobaltic basin. By adjusting the base of the Kukruse Stage to the FAD of L. stentor, this species would be strictly spanning the Kukruse age, often occurring in a continuous range. This conspicuously large chitinozoan (600 - > 1000 IJ.m) is preceded by its direct ancestor Laufeldochitina strita, known already since the Aseri times. The proposed stage boundary marks the transition from the Pygodus anserinus to the Amorphognathus tvaerensis conodont zones. -base of the Haljala Stage: FAD of Lagenochitina dalbyensis, appearing in the lowermost Haljala strata and recorded all over the Palaeobaltic basin, where rocks of early Haljala age exist. While being common in the Palaeobaltic basin sections, this species has also been reported in the northern Gondwana domain as well (southern Morocco, central Portugal and the Arrnorican Massif of western France). Other chitinozoans that can be used to track the Kukruse/Haljala boundary beds interval are Cyathochitina giraffa and the very short-ranging species Armoricochitina granulifera, Angochitina curvata and Conochitina tigrina. Besides chitinozoans, the short-ranging Baltoniodus gerdae conodont species straddles the Kukruse/Haljala boundary interval. Baltoniodus gerdae is a widely spread taxon, useful for the correlation of Upper Ordovician sections of Baltica and Laurentia. The FAD of L. stentor at the depth of340.83 m and the FAD of L. dalbyensis at the depth of 326.07 m in the Tartu-453 core (drilled in the southern outskirts ofTartu town, southern Estonia, coordinates 58.342086, 26.685527, and stored at the University of Tartu) will be respectively proposed as the stage boundary stratotype for the Kukruse and Haljala regional stages (Fig. 1).

AB - Chitinozoa are a group of organic-walled microfossils with uncertain origin that appeared in the Early Ordovician and became extinct in the Devonian. Their key characteristic features- rather simple morphologies, rapid diversification in the Ordovician, a short life span for a number of key species, a planktonic/ nectonic life style, and usually abundant presence in various lithologies of marine origin - have made the chitinozoans an important biostratigraphic correlation tool for the Lower Palaeozoic rocks in the last decades. The Sandbian succession of the Palaeobaltic basin embraces the Kukruse, Haljala and partly the Keila regional stages, with the Kukruse and Keila stages being introduced already over 130 years ago and the Haljala Stage erected by V. Jaanusson in 1995 by merging the historical Idavere and Johvi stages. These regional stages were based on lithological composition combined with finds of certain macrofossils in the outcrops ofNorth Estonia, whereas the International Commission on Stratigraphy recommends establishing the stage boundary stratotypes in sections known to represent the maximum complete sedimentation record and to be based on unequivocally determinable key fossil species that can be recorded in great numbers in drill cores as well. Commonly, conodonts and graptolites are used to determine the Ordovician global stages, but distinct chitinozoan key species have a very high potential for recording the Upper Ordovician regional stages in the Palaeobaltic basin. At present, zonal key chitinozoans are used to recognize the bases of the Upper Ordovician regional stages: the Nabala Stage (FAD of Armoricochitina reticulifera), the Pirgu Stage (LAD of Acanthochitina barbata), and the Porkuni Stage (FAD of Spinachitina taugourdeaui), but so far microfossil-based boundaries are not defined for any North Estonian regional stages of Sandbian age. The purpose of this contribution is to provide precise boundary definitions for the Kukruse and Haljala regional stages, based on chitinozoan key species and related to conodont zones (Fig. 1 ). The proposed horizons (1) allow for basinwide correlations across the Palaeobaltic basin and (2) permit correlation with other Ordovician basins. High-resolution Sandbian chitinozoan studies from a dozen cores (derived from Estonia, Sweden, Lithuania, Poland and NW Ukraine), have shown the highest correlational potential of these easily and unambiguously determinable zonal key species for tracking the base boundaries of the Kukruse and Haljala stages: -base of the Kukruse Stage: FAD of Laufeldochitina stentor, appearing in the uppermost Uhaku strata of present sense and reported from all studied Kukruse age sections in the Palaeobaltic basin. By adjusting the base of the Kukruse Stage to the FAD of L. stentor, this species would be strictly spanning the Kukruse age, often occurring in a continuous range. This conspicuously large chitinozoan (600 - > 1000 IJ.m) is preceded by its direct ancestor Laufeldochitina strita, known already since the Aseri times. The proposed stage boundary marks the transition from the Pygodus anserinus to the Amorphognathus tvaerensis conodont zones. -base of the Haljala Stage: FAD of Lagenochitina dalbyensis, appearing in the lowermost Haljala strata and recorded all over the Palaeobaltic basin, where rocks of early Haljala age exist. While being common in the Palaeobaltic basin sections, this species has also been reported in the northern Gondwana domain as well (southern Morocco, central Portugal and the Arrnorican Massif of western France). Other chitinozoans that can be used to track the Kukruse/Haljala boundary beds interval are Cyathochitina giraffa and the very short-ranging species Armoricochitina granulifera, Angochitina curvata and Conochitina tigrina. Besides chitinozoans, the short-ranging Baltoniodus gerdae conodont species straddles the Kukruse/Haljala boundary interval. Baltoniodus gerdae is a widely spread taxon, useful for the correlation of Upper Ordovician sections of Baltica and Laurentia. The FAD of L. stentor at the depth of340.83 m and the FAD of L. dalbyensis at the depth of 326.07 m in the Tartu-453 core (drilled in the southern outskirts ofTartu town, southern Estonia, coordinates 58.342086, 26.685527, and stored at the University of Tartu) will be respectively proposed as the stage boundary stratotype for the Kukruse and Haljala regional stages (Fig. 1).

KW - Faculty of Science

KW - Biostratigraphy

KW - Chitinozoans

KW - Ordovician

KW - Estonia

KW - Peribaltic basin

M3 - Conference abstract for conference

SP - 15

EP - 16

T2 - 10th Baltic Stratigraphic Conference

Y2 - 12 September 2017 through 14 September 2017

ER -