Abstract
AbstractThe term “re-flooding window” was recently proposed as a time-interval connected with the transgressive stage of present day peri-reefal development. In the analysis presented here, a fossil record of a re-flooding window has been recognized. Nine Late Silurian carbonate sections exposed on the banks of the Dnister River in Podolia (Ukraine) have been correlated base on bed-by-bed microfacies analysis and spectral gamma ray (SGR) measurements. Correlated were sections representing settings ranging from the inner part of a shallow-water carbonate platform to its slope, through an organic buildup. The reconstructed depositional scenario has been divided into six development stages, with the first three representing a regressive interval and the latter three a transgressive interval of the basin’s history. The re-flooding window has been identified at the beginning of a transgressive part of the succession. Surprisingly, it is characterized by an extremely fast growth of a shallow, tide-dominated platform and by deposition of calciturbiditic layers in a more basinal area. The interpreted succession is a small-scale model illustrating the reaction of carbonate depositional sub-environments to sea level changes and determining the facies position of the stromatoporoid buildups within the facies pattern on a Silurian shelf. The use of SGR analyses in shallow water, partly high-energy, carbonate facies, both for correlation purposes and for identifying depositional systems, is a relatively new method, and thus can serve as a reference for other studies of similar facies assortment.
Highlights
In the Silurian the SW margin of the Baltica continent was occupied by a carbonate ramp, with the deposition governed by eustatic sea level changes
The phases distinguished of the above-described regressive-transgressive cycle have been interpreted in terms of sequence stratigraphy
Facies studies of the Upper Silurian deposits exposed along the Dnister River, combined with field spectral gamma ray (SGR) measurements made throughout the sections, allow presentation of the following general conclusions: 1. In spite of its small thickness range, the studied interval is characterized by distinct and conspicuous facies variability
Summary
In the Silurian the SW margin of the Baltica continent was occupied by a carbonate ramp, with the deposition governed by eustatic sea level changes. In spite of good exposures, the Gotland sections do not enable study of the anatomy of a carbonate platform in detail and for a selected time horizon, and give no opportunity to reconstruct lateral facies changes. This results from a substantial tectonic tilting of the layers, which is combined with relatively shallow erosional cuts and with a general lack of sections perpendicular to the facies zones. The reconstructions of facies patterns in the area are based mainly on seismic profiles analyses (e.g., Flodén et al 2001; Bjerkéus and Eriksson 2001), and the resulting models are effects of their interpretations
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