Sylur Polski jako wskaźnik zróżnicowania szelfu Fennosarmacji i rozwoju oceanu Prototetydy

Abstract

POLISH SILURIAN AS INDEX OF DIFFERENTIATION OF THE FENNOSARMATIAN SHELF AND DEVELOPMENT OF THE PROTO-TETHYS Summary The Caledonian geosyncline, that is Proto-Tethyan Ocean, gradually originated between landmasses of the north-east European Platform and Gondwana presumably since the Early Cambrian, fairly wide and the deep in the Ordovician. Tectonic lands (microcontinents) could have appeared in that Ocean as early as the Arenigian. They played an important role in the history of the Caledonian geosyncline, acting as either alimentary areas for rapidly subsiding zones of buffers in subduction. The Taconian movements from the end of the Ordovician affected the Fennosarmatian shelf and resulted in structural remodelling of the Proto-Atlantic and Proto-Tethyan Oceans. At that time the Ardennes Brabant and East-Elbe massifs were pushed toward the East-European Platform, resulting in compression of fairly thick Ordovician series from southern slope of that Platform. The formation of uplifted paleostructures began in the western part of the Peri-Baltic syneclize in Landovery time. The structures also comprised eastern part of the Podlasie depression and the Volhyn and Podole areas. The sedimentation began in these areas in different times but generally after the Llandoverian of not before Wenlockian. A phase of marked disquiet in the Proto-Tethys may be traced in the Upper Wenlockian and especially at the Wenlockian/Ludlowian boundary. In some places it is accentuated by incrased subsidence or lithofacial changes and increased volcanic activity as e.g. in the Barrandian. Essential changes took place in the Proto-Tethys in the Ludlowian in connection with the Caledonian movements of the Cracow phase. That phase clearly coincides with final stage of closing of the Proto-Atlantic (Iapetus) Ocean which broke deposition of clays with graptolites and initiated formation of carbonates with benthic fauna typical of the Fennosarmatian shelf in the Wales. The ceasing of the deep-water deposition in the Proto-Atlantic and the affinity to the same shelf zone is evidenced by identical benthic fauna recorded in the Stonehouse Formation in the New Scotland in the Appaleches in North America. At these times ochreous limestones originated in the Saxo-Thuringian zone. These limestones indicate that the Ocean was moderately deep, most probably in result of uplift of oceanic floor. The zone of distribution of these limestones may delineate direction close to that of axial part of the contemporaneous geosyncline (Fig. 2). In Poland, characteristic siltstones and claystones of a remarkable thickness (Siedlce series) were formed along the Platform shelf. Coarser deposits were originating in the northern parts of the Holy Cross Mts, greywackes and arcoses south of these Mountains and conglomerates in the Cracow area. This facies precluded development of complete cycle of graptolites Formosograptus in the Barrandian and the equivalents of some Podlasie graptolite biozones in the Přidoli deposits. The extent of the Proto-Tethys diminishes which is evidenced by very shallow-water Upper Silurian deposits from the Montagne Noire, Spain and Brittany and lagoonal Downtonian deposits from the Great Britain. Marine sedimentation gradually retreated from Estonia, Latvia and Lithuania and thereafter from Gotland and Peri-Baltic syneclize where appeared marly-carbonate equivalents of Beyrichienkalk facies. The shelf zone retreated to Central Poland at the beginning of the Gedinnian. It was very closely connected with Rhineland, Ardennes, Brittany and Spain on the west and the Podole region on the east. A deeper zone still existed in the Sudety Mts, Thuringien, Frankenwald, Sardinia and parts of Barrandien. The closing of the Proto-Tethyan geosyncline presumably took place in the Siegenian and before the Emsian. There was no continental collision of the Norwegian-Scottish type between Gondwana ond Fennosarmatian landmasses presumably because of the buffer effect of microcontinents (Figs. 1, 2, 3). A better knowledge of history of these microcontinenst may markedly contribute to accuracy of reconstruction on independences of Gondwana and Fennosarmatian continents in the terms of plate tectonics.