SEQUENCE STRATIGRAPHY AND BASIN EVOLUTION OF THE UPPER ORDOVICIAN (UPPER CARADOC-ASHGILL) OF ESTONIA

Upper Ordovician strata of Estonia accumulated on the Estonian Shelf, and span the transition from the Baltic Shield to the north and the Livonian Basin to the south. The section is predominately carbonate with variable siliciclastic content, and numerous boreholes provide good-to-excellent coverage of facies patterns. Baltic regional stages are commonly used and their correlation to international stages is relatively well established.

The upper Nabala-Porkuni Stages (equivalent to the upper Caradoc-Ashgill Stages) consist of seven complete stratigraphic sequences and the lowstand deposits of an eighth sequence. The sequences are represented in shelf sections by shallowing-upward, prograding carbonate shelf successions. Slope sections are predominately mudstones that are locally truncated by submarine erosional surfaces. Basinal deposits are also muddy with greater siliciclastic content.

In the lowest sequence (upper Nabala Stage), low-energy facies extend across most of the study area. In sequence 2 (Vormsi Stage), facies belts are more differentiated and siliciclastic muds are most abundant, perhaps reflecting tectonic influences. Sequences 3-7 (Pirgu and Porkuni Stages) record the step-wise progradation of shelf sediments that produced a relatively broad Estonian Shelf. The lowstand unit that caps the sucession (uppermost Porkuni Stage) corresponds to the widely documented, latest Hirnantian glacial sea-level drop.

The Estonian sequences appear correlative to those defined for Laurentia, suggesting a eustatic sea-level control on sequence timing. However, the facies changes between sequence 1 and the overlying sequences (sequences 2-7) probably reflect tectonic changes within the Baltic Basin. This suggests that the controls on facies patterns are a mixture of eustatic and tectonic influences