Quantitative Stratigraphy of Sedimentary Sequences:
A Case Study of the Middle Ordovician Event
1. Ainsaar, T. Martma, T. Meidla, M. Rubel and N. SidaraviCiene
1
Introduction
The irreversibility of evolution is one of the most reliable tools to date geological
events, which makes the palaeontological record valuable in reconstruction of
the history of sedimentary basins. The resolution of palaeontological time scales
can be improved by applying methods of quantitative stratigraphy, due to more
rigorous and/or more complete approach by using the distribution of taxa as a
reliable tool for correlation, i.e. a time scale (Shaw 1964; Guex 1987; Agterberg
1990). Moreover, the ordinal scales of taxa which are independent of the stratigraphical framework can be used to describe the dynamics in the diversity of
taxa and, to some extent, the evolution of the corresponding groups. Such an opportunity was noted by Paul (1985), but mostly ignored in studies on radiation
and extinction of taxa, in spite of the actuality of the problem (Sepkoski 1995a;
Holland 1989).
In this chapter, an algorithm for the construction of the palaeontological time
scale (Rubel and Pak 1984) is used to process distributional data of ostracodes
from several Ordovician core sections in the East Baltic. The changes in origin
and extinction rates of ostracodes during the Ordovician period are discussed,
based on the approach of quantitative stratigraphy. The age of a remarkable
middle Ordovician biotic event in Baltoscandia is compared with the age of a geochemical event - deviation in carbon isotopic composition of seawater - using
a regional ostracode succession constructed by a deterministic algorithm as a
time scale. The relationship of the faunal changes with the oceanographic event
is demonstrated and the contemporaneity of geochemical signals in different
sections is proved.
2
The Baltoscandian Palaeobasin: Geological Setting
In the Ordovician, the Baltoscandian Palaeobasin represented an epicontinental
sea on the Baltica palaeocontinent situated in Southern Hemisphere. With the
northward drift of Baltica, the palaeolatitude of the central part of the basin
changed from approximately 50° to 20° during the Ordovician period (Torsvik
A Case Study of the Middle Ordovician Event
1. Ainsaar, T. Martma, T. Meidla, M. Rubel and N. SidaraviCiene
1
Introduction
The irreversibility of evolution is one of the most reliable tools to date geological
events, which makes the palaeontological record valuable in reconstruction of
the history of sedimentary basins. The resolution of palaeontological time scales
can be improved by applying methods of quantitative stratigraphy, due to more
rigorous and/or more complete approach by using the distribution of taxa as a
reliable tool for correlation, i.e. a time scale (Shaw 1964; Guex 1987; Agterberg
1990). Moreover, the ordinal scales of taxa which are independent of the stratigraphical framework can be used to describe the dynamics in the diversity of
taxa and, to some extent, the evolution of the corresponding groups. Such an opportunity was noted by Paul (1985), but mostly ignored in studies on radiation
and extinction of taxa, in spite of the actuality of the problem (Sepkoski 1995a;
Holland 1989).
In this chapter, an algorithm for the construction of the palaeontological time
scale (Rubel and Pak 1984) is used to process distributional data of ostracodes
from several Ordovician core sections in the East Baltic. The changes in origin
and extinction rates of ostracodes during the Ordovician period are discussed,
based on the approach of quantitative stratigraphy. The age of a remarkable
middle Ordovician biotic event in Baltoscandia is compared with the age of a geochemical event - deviation in carbon isotopic composition of seawater - using
a regional ostracode succession constructed by a deterministic algorithm as a
time scale. The relationship of the faunal changes with the oceanographic event
is demonstrated and the contemporaneity of geochemical signals in different
sections is proved.
2
The Baltoscandian Palaeobasin: Geological Setting
In the Ordovician, the Baltoscandian Palaeobasin represented an epicontinental
sea on the Baltica palaeocontinent situated in Southern Hemisphere. With the
northward drift of Baltica, the palaeolatitude of the central part of the basin
changed from approximately 50° to 20° during the Ordovician period (Torsvik