Lithostratigraphy, graptolites and brachiopods communities of the Ludlow (Silurian) of the Eastern slope of the Baltic Syneclise

Abstract

Milaičiai-103 well is located on the East slope of the Baltic Sineclise. Dubysa, Mituva and Ventspils formations of the Ludlow Stage were defined within the slope. The Šešupė and Nova beds are distinguished in the Dubysa Formation. The Šešupė Bed is composed of ten sedimentation cycles of limestone and marl rhythmical interbedding. The lower boundary of the Šešupė Bed is unclear in the Milaičiai-103 well because the borehole was drilled without core sampling starting from the depth of 1 301.0 m. The Nova Bed is composed of two cycles composed of limestone in the lower part and marlstone in the upper part. The Mituva Formation is composed of four cycles of limestone and marlstone rhythmical interbedding. The Ventspils Formation is composed of three limestone layers showing different features. The Ludlovian biostratigraphy is based on graptolite fauna. There are distinguished progenitor-scanicus, incipiens graptolite biozones of the Gorstian and praecornutus, cornutus, bohemicus tenuis, balticus and valleculosus biozones of the Ludfordian. The description of zonal species Monograptus valleculosus Tsegelnuk is given like Pristiograptus dubius cf. frequens Jaekel. There are distinguished Lissatrypa obovata–Jonsea grayi, Isorthis amplificata–Lissatrypa obovata, Gypidula–Atrypoidea, Atrypa reticularis–Shaleria ornatella and Isorthis parvulus – Atrypoidea brachiopod communities in the Ludlow section of the well Milaičiai-103. According to the lithological composition and described graptolites and brachiopods the Milaičiai-103 well contains different ecostratigraphic levels.