Relative sea-level changes in Baltoscandia in the Cambrian and early Ordovician:: the predominance of tectonic factors and the absence of large scale eustalic fluctuations

Fluctuations in sea depth within a magnitude 20-100 m and a duration of 1-10 m.y. are often explained by rapid eustatic changes - so called 'third-order eustatic events'. Considerable influence of regional tectonics on relative sea-level changes has been demonstrated by many authors, but because of uncertainties in the timing of short events in widely separated regions, the problem of separating tectonic and eustatic factors still remains unsolved. In this paper, a new and simple approach is used to reveal the presence or absence of eustatic events. We consider the St. Petersburg area and North Estonia in the north-eastern region of the East Baltic. From the late Early Cambrian until the middle of the Tremadoc (early Ordovician), deposition was extremely slow and the sea bed remained for a long time in a well defined peritidal zone in a water depth less than or equal to 10 m. Under such environmental conditions, a sea-level rise of greater than or equal to 10 m would result in marked changes in the character of faunas and sedimentation. In the time interval considered here, significant sea deepening in the north-eastern Baltic region occurred only twice, and its magnitude did not exceed 10-20 m. A fall of sea level by greater than or equal to 10 m would result in complete regression in the peritidal zone. This situation also occurred in region. However, the preservation of a sequence of unconsolidated sands, which is only a few tens of metres thick and includes all the main stratigraphic subdivisions on a regional scale, indicates that the crustal surface reached a very low altitude less than or equal to 10-20 m above sea level. These data show that in the late Late Cambrian to the middle of the Tremadoc, over a period of similar to 40 m.y. long, eustatic sea-level changes did not exceed +/-10-20 m. This limits the magnitude of several third-order cycles - eustatic events with duration of a few million years, which have been proposed previously for the epoch of the transition from the Cambrian to the Ordovician. In the late Early Cambrian to the Late Cambrian, transgressions and regressions with a magnitude of similar to 50-150 m took place in southern Sweden and Lithuania. Since these phenomena occurred when there were no comparable eustatic sea-level changes, they must be associated with regional tectonic movements. Some were rapid and could be easily misinterpreted as indications of third-order eustatic changes. It is probable that some of the other eustatic events that have been proposed for the Phanerozoic were actually not of eustatic but of tectonic origin. Such rapid tectonic movements with magnitude of similar to 50-100 m in cratonic areas can be caused by changes in the forces in the lithospheric layer with a laterally variable thickness, and by phase transitions in the mafic lower crust. Depending on the spatial distribution of vertical crustal movements, both these mechanisms could have been operating in the East Baltic and southern Sweden in the Cambrian. (C) 2000 Elsevier Science B.V. All rights reserved.