278
L. Ainsaar, T. Martma, T. Meidla, M. Rubel and N. SidaraviCiene
This biotic event coincided approximately with a regressive episode (K6rts et
al. 1991; Ainsaar et al. 1996) and changes in carbonate sedimentation pattern of
the region (ManniI1966; Hints et al.1989; Nestor 1990). These events, associated
with suggested changes in climate (Jaanusson 1973; Webby 1984; Hints et al.
1989), led to a remarkable facies differentiation in the late Viruan-Harjuan
(ManniI1966; Nestor 1990). According to Kaljo et al. (1996), the late Keilan extinction and the Oanduan crisis of acritarch and chitinozoan communities in
northern Estonia may be related to a global drop in diversity of different invertebrate groups, described from the late Caradoc by Sepkoski (1995b).
Recent studies by Ainsaar et al. (1995) on stable isotopic composition of carbonate rocks in South Estonian core sections reported a significant positive shift
in carbon isotopic composition (expressed as c l3 C) in the upper part of the Keila
Stage. The isotopic shift, which is supposed to reflect a change in ocean water
chemistry, is stratigraphically close to faunal changes in the studied sections
(Fig. 3). For the interpretation of such a coincidence, the events need to be dated
to reveal their order or contemporaneity. The existing descriptions of faunal dynamics, based both on the regional stages (Hints et al. 1989) or detailed logs of
selected single section (Kaljo et al. 1995, 1996) are too rough or too local, respectively, for reliable dating of the events. In this case, the necessary reliability for
dating the short-time isotopic shift has been achieved by the use of ostracode
data, organized by means of a quantitative stratigraphical method (Rubel and
Pak 1989).
4
Ostracode Data
Ostracodes have been chosen for the construction of the regional scale for the
East Baltic area. The choice was mainly based on the suitability of microfauna
for characterizing the subsurface area and on the large data set available. The
distribution data of 255 ostracode species from the 48 Ordovician core sections of Lithuania (see Fig. 1) range from the Volkhov to the Porkuni Stage
(SidaraviCiene 1992, 1996). The construction of the ostracode scale was based
on a data set, which includes local ranges of species collected in the Lithuanian
and Central Baltoscandian confacies belts of Lithuania (see Fig. l). The scale is
applied for dating of particular levels in the Keila, Oandu and Rakvere stages
in two South Estonian sections, Ristikiila (Ainsaar et al. 1996) and Tartu. Ostracodes from these sections are included into the construction of the scale.
The same intervals of the sections were analyzed for stable isotope composition. Ristikiila and Tartu boreholes are situated near the transition between
the North Estonian and Central Baltoscandian confacies belts (see Fig. 1).
They represent a nearly complete succession of the studied interval, on the
whole corresponding to major regressions (Ainsaar et al. 1996).
L. Ainsaar, T. Martma, T. Meidla, M. Rubel and N. SidaraviCiene
This biotic event coincided approximately with a regressive episode (K6rts et
al. 1991; Ainsaar et al. 1996) and changes in carbonate sedimentation pattern of
the region (ManniI1966; Hints et al.1989; Nestor 1990). These events, associated
with suggested changes in climate (Jaanusson 1973; Webby 1984; Hints et al.
1989), led to a remarkable facies differentiation in the late Viruan-Harjuan
(ManniI1966; Nestor 1990). According to Kaljo et al. (1996), the late Keilan extinction and the Oanduan crisis of acritarch and chitinozoan communities in
northern Estonia may be related to a global drop in diversity of different invertebrate groups, described from the late Caradoc by Sepkoski (1995b).
Recent studies by Ainsaar et al. (1995) on stable isotopic composition of carbonate rocks in South Estonian core sections reported a significant positive shift
in carbon isotopic composition (expressed as c l3 C) in the upper part of the Keila
Stage. The isotopic shift, which is supposed to reflect a change in ocean water
chemistry, is stratigraphically close to faunal changes in the studied sections
(Fig. 3). For the interpretation of such a coincidence, the events need to be dated
to reveal their order or contemporaneity. The existing descriptions of faunal dynamics, based both on the regional stages (Hints et al. 1989) or detailed logs of
selected single section (Kaljo et al. 1995, 1996) are too rough or too local, respectively, for reliable dating of the events. In this case, the necessary reliability for
dating the short-time isotopic shift has been achieved by the use of ostracode
data, organized by means of a quantitative stratigraphical method (Rubel and
Pak 1989).
4
Ostracode Data
Ostracodes have been chosen for the construction of the regional scale for the
East Baltic area. The choice was mainly based on the suitability of microfauna
for characterizing the subsurface area and on the large data set available. The
distribution data of 255 ostracode species from the 48 Ordovician core sections of Lithuania (see Fig. 1) range from the Volkhov to the Porkuni Stage
(SidaraviCiene 1992, 1996). The construction of the ostracode scale was based
on a data set, which includes local ranges of species collected in the Lithuanian
and Central Baltoscandian confacies belts of Lithuania (see Fig. l). The scale is
applied for dating of particular levels in the Keila, Oandu and Rakvere stages
in two South Estonian sections, Ristikiila (Ainsaar et al. 1996) and Tartu. Ostracodes from these sections are included into the construction of the scale.
The same intervals of the sections were analyzed for stable isotope composition. Ristikiila and Tartu boreholes are situated near the transition between
the North Estonian and Central Baltoscandian confacies belts (see Fig. 1).
They represent a nearly complete succession of the studied interval, on the
whole corresponding to major regressions (Ainsaar et al. 1996).