Abstract
The paper discusses the reliability of different biozones in terms of their synchroneity when crossing facies boundaries within a sedimentary basin. Graptolite biozones are the most trusted ones, but also biozones based on conodonts, chitinozoans and less 'authoritative' groups like ostracodes and vertebrates are used. The integration of bio- and chemostratigraphy aids the understanding of the pattern and timing of fossil distribution. Despite different environments most of the analysed biozones are in general synchronous units and in the majority of cases their time signals do not contradict each other. The much discussed Ozarkodina crispa (and its two morphs) has a stratigraphical range in the East Baltic that is longer than commonly recognized elsewhere. Its first occurrences in Estonia have been reported from shallow-water facies below the Mid-Ludfordian Carbon Isotope Excursion. The geographical distribution of Oz. crispa is much wider in the upper Ludfordian open shelf rocks and a few occurrences are also known in the lowest Přidoli in the East Baltic. Analogous ecostratigraphical trends are characteristic of some other fossil species. The analysis demonstrates that, when properly studied, all of the fossil groups considered can provide useful biostratigraphical information. The subdivision of the Přidoli Series into two stages is discussed. Bio-chemostratigraphical data confirm the late Ludfordian age of the Kuressaare Formation and its correlatives. The bio- and chemostratigraphical testing of biozonal indices suggests some tentative correlations with other areas, in particular the type Ludlow area, and enables identification of the Silurian-Devonian boundary in the East Baltic.
Highlights
Ecostratigraphy was officially inaugurated as an IGCP project in 1973 and became popular within the two decades
The analysis demonstrates that, when properly studied, all of the fossil groups considered can provide useful biostratigraphical information
In the text above we evaluated the occurrences of FADs of all upper Silurian biozonal indices established in three Baltic core sections
Summary
Ecostratigraphy was officially inaugurated as an IGCP project in 1973 and became popular within the two decades. The Torgu, Kuressaare, Kaugatuma, Ohesaare, Dubysa, Pagegiai (Mituva and Engure), Ventspils, Minija and Jura (Targale) formations (below Fm. or fms) have been established (Fig. 2) and mostly subdivided into beds or members (not shown here) Such a terminology reflects changes in three main lithofacies belts, from the shoreface towards the deeper shelf and basin, as well as sea level fluctuations and a general regressive trend through the late Silurian. Plotting the FADs of the most valuable biozonal index species against the noted benchmarks according to the actual depths of occurrences along the sections (Fig. 3), we try to understand how stable the consecutive order of the FADs is and whether these appearances are synchronous in sections of different facies origin For this purpose the FADs of five graptolite biozonal species and nine ostracode indices in the Latvian sections are shown according to data from Gailite et al (1987) and Ohesaare ostracode data from Sarv (1971). Those of seven chitinozoan biozones are presented according to Nestor (2009, 2011, 2012), seven conodont ones from Viira & Aldridge (1998) and Viira (1999, 2000) and seven vertebrate indices according to Märss (1986, 1997) and Märss & Männik (2013)
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