Magnitude of sea-level changes in the Ordovician of Baltoscandia

strate deepening of the basin starting after regression at the base of the Latorp sequence. The Volkhovian deposits are the most widespread and the total area of marine red beds in the Volkhovian exceeds the area they cover in the Latorpian and Kundan (Mannil 1966). A rapid change in the depositional environment, from tide-dominated to storm-dominated at the base of the Latorp sequence, as well as an invasion of new groups of fauna, are also attributed to a rise of the sea level. At that time epicontinental sea covered almost all of the Russian platform, providing connections between the Urals, Moscow basin and Baltoscandia. The lower boundary of the Volkhov sequence is interpreted as a 2 nd -type sequence boundary (Dronov & Holmer 2002) with a long period of stillstand and non-deposition. The magnitude of the sea-level lowering probably did not exceed 10–20 m. The overlying Kunda sequence is very similar to the Volkhov sequence in its lithology. The sea-level drop at the Volkhov–Kunda boundary was larger than that at the Latorp–Volkhov boundary (30–40 m). The Tallinn sequence is represented by shallower-water deposits than the underlying Kunda and Volkhov sequences. The shallowing of the basin was not a result of forced regression but rather a consequence of an increasing sediment input. In the Tallinn sequence, the marine red beds in the central parts of the basin were replaced by grey deposits. The organic-rich kukersite-bearing strata demonstrate progradational stacking patterns and form the highstand systems tract of the sequence. The Kegel sequence is comparable in lithology with the underlying Tallinn sequence. The unconformity at the base of the Kegel sequence is well developed only in north-eastern Estonia and north-western Russia, where shallow-water kukersite-bearing facies are well developed. The sea-level drop probably did not exceed 10 m. The Kegel sequence is remarkable for its transition from cool-water temperate to warm-water carbonate sedimentation and the rapid growth of reefs (Dronov 2002). The unconformity at the base of the Wesenberg sequence is one of the most notable conformities in the entire Ordovician The two main factors controlling the general shape of any reconstructed sea-level curve are: (1) time scale, e.g., relative duration of the time intervals corresponding to stratigraphic units and gaps in the succession under consideration; (2) magnitude of relative sea-level changes. The latter factor is most difficult to calculate. The sea-level drops caused by forced regressions are usually manifested by erosional unconformities and gaps in shallow-water environments. To estimate the magnitude of a sealevel drop, one should take into consideration the distribution area of the erosional surface and significance of the erosion of the underlying rocks. Precise estimation of the sea-level rises is even more difficult as they are almost not expressed in deep-water settings. In shallow-water environments one can take into account the area covered by sediments of the corresponding depositional sequence and facial expression of the transgressive systems tract deposits. The Ordovician succession of Baltoscandia has been subdivided into ten major depositional sequences. All these sequences represent third-order cycles of relative sea-level changes (in the sense of Vail et al. 1977) and have an average duration of 1.5–3.0 to 8–9 m.y. To ease reference and identification, an individual name has been given to each depositional sequence (from base to top): (1) Pakerort, (2) Latorp, (3) Volkhov, (4) Kunda, (5) Tallinn, (6) Kegel, (7) Wesenberg, (8) Fjacka, (9) Jonstorp and (10) Tommarp (Dronov & Holmer 1999). The most prominent unconformities with extensive erosion of the underlying beds coincide with the base and top of the Ordovician succession as well as with the base of the Latorp and Wesenberg sequences. The strong erosion and development of regional unconformities can be regarded as evidence for forced regressions and sea-level drops of great magnitude comparable to modern glacial regressions (about 100 m). Unconformities at the same stratigraphic levels can be recognised in the Siberia (Kanygin 2001), Gondwana (Carr 2002) and Laurentia (Ross & Ross 1992, 1995)