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The δ 13 C isotope data from the Tartu-453 core section in southern Estonia enabled creation of a continuous Ordovician carbon isotope record, ranging from the Floian to the end of the Hirnantian. Five positive δ 13 C excursions and one negative δ 13 C excursion were recognized. Most of the positive excursions correspond to those defined on the Baltoscandian carbon isotope chemostratigraphic scheme, except for the Paroveja and Rakvere ICEs which are not well expressed in the Tartu-453 curve. Besides the positive δ 13 C excursions, a distinctive negative δ 13 C excursion, the Lower Sandbian NICE, was recorded in the Tartu-453 succession. The well-expressed HICE extends about 10 m higher than the traditional Ordovician-Silurian boundary (between the Porkuni and Juuru regional stages) into the Juuru Regional Stage, suggesting that the Ordovician-Silurian boundary is positioned significally higher than traditionally suggested for the East Baltic area.

Located in the interior of the East European Craton (EEC), the Baltic Ordovician-Silurian Basin hosts an elongated tonguelike deepmarine depression, the Livonian Tongue (LT), which extends from Sweden across Latvia and separates the Estonian and Lithuanian shallowmarine shelves. The tectonic origin of the LT has been suggested already since its discovery in the early 1960s. However, the nature of tectonic forces and mechanisms behind the evolution of this narrow intracratonic subsidence zone in the Ordovician-Silurian of the Baltic Basin has remained poorly understood. The origin of the LT can be related to an extensive intracratonic dislocation zone known as the Leba Ridge-Riga-Pskov Fault Zone (LeRPFZ) that coincides largely with the axis of the LT. The LeRPFZ reveals some heavily uplifted basement blocks and has, therefore, been considered as an upwarped anticline type structure. Recent studies show that the LT has developed in highly complex and changing stress field conditions during the Caledonian orogeny. The subsidence and widening phase of the LT in the Ordovician and early Silurian coincides with, and was possibly governed by, the Avalonia collision with Baltica from the SW when high shear stress forced LeRPFZ blocks to move obliquely towards the NE. As Laurentia was approaching Baltica and finally collided with it in the midSilurian, the shear stress became progressively mingled with compression from the NW and the subsidence of the LeRPFZ became reversed, triggering LT withdrawal to the SW. Thus, being once the deepwater centre of the Baltic Ordovician-Silurian Basin, the LT became the most uplifted and intensely eroded EEC interior zone by the Devonian.

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