Б.Марковский и Д.Наливкин Задонские и Елецкие слои

Archiaci Mu r c h . , CyrtiaMurchisoniana К о n. and related forms and the absence of Atrypa and Pentamerus. These pecularities are very conspicuous in the beds of Zadonsk and Eletz. It gives the possibility of comparing these beds with the Famennian horizon of West Europe. C o m p a r i s o n w i t h t h e s e c t i o n s of t h e N o r t h - W e s t e r n c o u n t r y shows, that in this country the horizons corresponding to the beds of Zadonsk, Eletz and Dankovo-Lebedian are represen­ ted by continental sediments — red sandstones. C o m p a r i s o n w i t h t h e s e c t i o n s of t he U r a l s a n d t h e S o u t h T i m a n is the most interesting and important. It shows that in the Urals facies apparently identical to the limestones and dolomites of the beds of Eletz are deve­ loped. They are represented by yellowish-gray, fairly thick, thick-bedded lime­ stones, at places dolomitised. They include a «poor fauna not yet completely studied, characterized by an abundance of Spirifer Archiaci Mu r c h . that cannot be distinguished from that of Eletz. The conditions of the bedding of these limestones at the river Awe are very interesting. They are underlain by dark limestones with Spirifer Anossofi Ve r n . and Spirifer Katavensis Nal . and are overlain by siliceous limestones with Cheiloceras. Limestones with Cheiloceras contain a rich and various goniatite fauna, closely related to the fauna of the limestone Nehden in Germany. Above them lie shaly sandstones deprived of fos­ sils, siliceous and argillaceous shales, analogous to the Fossley and Matagne beds of West Europe. At the South Timan sandstones with Spirifer Archiaci Mu r c h . corresponding to similar limestones of the Urals are developed. They also overlie the beds with Spirifer Anossofi Ve r n . , but are covered by the Middle Carboni­ ferous, occuring transgressively. P a l e o g e o g r a p h y The Zadonsk period is characterized by short uplifts of the central and western part of the Middle-Russian platform. As the results of the uplifts of the platform the shore-line of the upper Devonian sea is sharply shifted to the West and is marked by arenaceo-argil- laceous rock-mass of Livny and Russky Brod. The elevated parts of the ground form numerous banks. The nearness of the shore, shallow depth andthe great distance from the open sea brings some changes of normal sea conditions in the Zadonsk bay. During the process of adaptation to the modified conditions a change of general charac­ ter of the fauna takes place; to the change between the groupes composing it, corresponds an energetic formation of species within individual groups and the appearance of new ones. In the second part of the Zadonsk period some gradual subsidence of the central part of the platform is observed; a new change of the shore line occurs, which is again removed to the East towards Orel. Towards the close of the Zadonsk period the process of the depression of the platform and of the transgression of the sea are more violent and in the beginning of the period of Eletz the sea covers a more considerable area, still keeping the character of a shallow sea bay. At the end of the Eletz period the platform is slowly uplifted, the Eletz bay gets shallower and by the Lebedian period becomes practically isolated from the open sea and is gradually closed into a lagoon. 3*