Classification of the Harjuan (Upper Ordovician) Rocks of the Mainland of Sweden

Abstract

The available data about the litho-stratigraphy of the Harju Series or Upper Ordovician of the mainland of Sweden are summarized and discussed in some detail. A consistent terminology based on geographical names is proposed for all Harjuan litho-stratigraphic divisions below the Dalmanitina beds and outside the graptolitic succession that previously have lacked such names. For the Black Tretaspis Shale the name Fjäcka Shale is introduced. The sequence which is currently subdivided into Green and Red Tretaspis beds is regarded to constitute one formation, called the Jonstorp Formation. In areas where a distinctive limestone unit (on Kinnekulle previously termed the Tretaspis Limestone) is developed within the formation, three members are distinguished: Lower Jonstorp Member, Öglunda Limestone, and Upper Jonstorp Member. In areas where the Öglunda Limestone is missing, a subdivision of the formation according to the colour of the rock is not found to be meaningful. The grey beds between the Jonstorp Formation and the Dalmanitina beds of the Siljan district, Östergötland, and Kinnekulle are termed Nittsjö Formation, and for the dark mudstones in the same stratigraphic position in the Billingen-Falbygden district the term Ulunda Formation is introduced. In Scania the term Jerrestad Formation is proposed for the sequence of mudstones between the zone of Pleurograplus linearis and the Dalmanitina beds (corresponding to the Tretaspis beds sensu Olin).

The chrono-stratigraphic classification of the predominantly pelitic sequence of the series on the mainland of Sweden, on Bornholm, and in Poland is discussed. This sequence is subdivided into three stages for which new terms are proposed. The Vasagaard Stage corresponds to the zone of Pleurograplus linearis, the Jerrestad Stage comprises the sequence between the zone of P. linearis and the base of the Dalmanitina beds, and the term Tosterup Stage is introduced for the sequence currently named the Dalmanitina beds. For the beds on top of the Jerrestad Stage that contain a mixed Dalmanitina, Tretaspis, and Staurocephalus assemblage (Staurocephalus beds sensu Linnarsson) the term Alleberg beds is introduced.

Some palaeogeographical aspects of the Harju Series in Sweden and certain adjoining areas are briefly considered. The evidence is summarized which indicates that south of the Siljan district and of Gotska Sandön beds corresponding to at least part of the Slandrom Limestone are missing, and that there the base of the series is marked by a conspicuous break of varying extent. Southwards, possibly already in SE. Scania and on Bornholm, the break becomes filled by graptolitic shales. In Sweden the sediments deposited during the Jerrestadian age seem to be arranged into belts of different lithofacies that extend, in the main, in a meridional direction. The westernmost belt consists predominantly of mudstones and can be imagined to extend from the easternmost nappes of Jämtland via Västergötland, Scania, and Bornholm to central Poland. The middle belt is formed mainly by finely nodular limestones and marls and can be followed from the Siljan district via Östergötland to the bottom of Baltic E. of Öland. In the easternmost belt calcilutites predominate (Östersjö Limestone) along a zone from the South Bothnian district to Gotland. The contemporaneous beds of northern Estonia are formed mainly by calcarenites which seem to extend southwards to Lithuania.