COMPARISON OF MEGAFLORAL CHANGES ACROSS THE BARBORA AND ENNA MARINE ZONES, OSTRAVA FORMATION (SERPUKHOVIAN [NAMURIAN A]), UPPER SILESIAN BASIN, CZECH REPUBLIC

The Late Mississippian Ostrava Formation in the Silesian Basin consists of cyclothemic coal-bearing continental and nearshore-to-offshore marine deposits with abundant and well preserved plants. Two thick marine intervals are known in the Upper Namurian A (Serpukhovian), each of which exhibit a different megafloral dynamic. The Enna Horizon, the youngest deposit in the Hrušov Fm., is the boundary between the Hrušov and Jaklovec Mbrs., and is characterized by the loss of Culm-type megaflora at its base. It's thickness is between 100-200 m, with overthickening due to tectonic deformation. In the Těšín and Frenštát districts, a reduced thickness of 30-50 m occurs. The Barbora Horizon is the uppermost faunal interval of the Jaklovec Member and is the boundary between the Jaklovec and Poruba Mbrs. Its' thickness varies from 50-90 m, with erosional loss and reduced thickness in the Těšín and Frenštát districts.

Megafloral data come from 38 and 23 boreholes from the Enna and Barbora horizons, respectively, and supplement a previously compiled large data set for the Ostrava Formation. A reduced number of taxa are found in the selected boreholes for each marine interval when compared with floral elements found above and below these horizons. Both horizons are dominated mainly by lyginopterid pteridosperms, although medullosans also are present. Unassigned pteridophyllous taxa (e.g., Sphenopteris, Rhodeopteridium, Sphenocyclopteridium) are common, with occassional true fern taxa. Overall, lycopsids and sphenopsids are rare indicating that plant-part contribution to marine deposits originated from riparian elements. Both the Barbora and Enna horizons are used biostratigraphically due to the changes in the Namurian flora across their boundaries.

Biostratigraphic boundaries were based on the dramatic floral changes in the Late Mississippian of the northern hemisphere. Gamma-log examination of both marine intervals indicates that several condensed sections occur therein, representing cryptic glacial-eustatic cycles. The megafloral replacement across each boundary probably is a reflection of rapid floral change in response to global perturbation that is responsible for an extended period of Highstand deposition in this and other coeval basins.