THE EARLY MID-DEVONIAN CHOTEČ EVENT: DO PALYNOMORPHS HAVE THE POTENTIAL FOR LONG-DISTANCE CORRELATIONS?

The Basal Choteč Event in its type area, the Prague Basin, is generally characterized by the occurrence of the Choteč Limestone (Choteč Formation, Eifelian, costatus Zone) and its equivalents, just above the Lower-Middle Devonian boundary. Standard conodont preparation of Choteč Limestone samples at the Choteč type locality (Na Škrábku quarry at Choteč village) has yielded masses of three-dimensionally preserved palynomorphs. Successive microscopic analyses indicate that the overall majority of these well-preserved palynomorphs can be classified as prasinophyceaen algae. In addition, a few mazuelloids and scolecodonts have been observed, whereas acritarchs, spores and chitinozoans appear to be absent.

At this position, light-gray bioturbated skeletal wackestones/packstones of the Třebotov Limestone change to an alternation of dark-gray crinoidal and peloidal grainstones with dark-gray laminated lime-mudstones and slightly bioturbated middle-gray to dark-gray wackestones of the Choteč Limestone. The prasinophyceaen algae occur in a dark peloidal grainstone with abundant micritized echinoderm ossicles, 20 cm above the base of the Choteč Limestone. To date, there is no published record of palynomorphs from this interval in Bohemia or elsewhere. Here, we present the discovery of assemblages of prasinophytes (Tasmanites/Leiosphaeridia) which occur in great abundance just above the base of Choteč Limestone. These most probably record a phytoplankton bloom during the basal Choteč Event. Potential causes for the accumulation of this nearly monospecific assemblage will be examined and the broader implications of this event will be discussed.

At present, this event is regarded as minor, although it is globally documented by distinct changes in facies, fauna, and inferred sea-level. New, ongoing study of several Appalachian Basin sections should provide critical new perspective as to its global scale.