Upper Ordovician sequences of western Estonia

摘要

The Upper Ordovician (uppermost Caradoc-Ashgill) section of western Estonia consists of a series of seven open-shelf carbonate sequences. Depositional facies grade laterally through a series of shelf-to-basin facies belts: grain-supported facies (shallow shelf), mixed facies (middle shelf), mud-supported facies (deep shelf and slope) and black shale facies (basin). Locally, a stromatactis mud mound occurs in a middle-to-deep shelf position. Shallow-to-deep shelf facies occur widely across the Estonian Shelf and grade laterally through a transitional (slope) belt into the basinal deposits of the Livonian Basin. Each sequence consists of a shallowing-upward, prograding facies succession. Sequences 1 (Upper Nabala Stage) and 2 (Vormsi Stage) record step-wise drowning of underlying shelf units (lower Nabala) that culminated in the deposition of the most basinal facies (Fj?cka Shale) in the Livonian Basin. Sequences 3–6 comprise the overlying Pirgu Stage and record the gradual expansion of shallow and middle-shelf facies across the Estonian Shelf. The Porkuni Stage (sequence 7) is bracketed by erosional surfaces and contains the shallowest-water facies of the preserved strata. The uppermost part of the section (Normalograptus persculptus biozone) is restricted to the Livonian Basin, and includes redeposited carbonate and siliciclastic grains; it is the lowstand systems tract of the lowest Silurian sequence 8. Sequence 7 and the overlying basinal redeposited material (i.e., the lowstand of sequence 8) correspond to the latest Ordovician (Hirnantian) glacial interval, and the bracketing unconformities are interpreted as the widely recognized early and late Hirnantian glacial maximums. The sequences appear correlative to Upper Ordovician sequences in Laurentia. Graptolite biozones indicated that the Estonian sequences are equivalent to carbonate ramp sequences in the western United States (Great Basin) and mixed carbonate-siliciclastic sequences in the eastern United States (Appalachian Basin–Cincinnati Arch region). These correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins.