Cenomanian and Cenomanian-Turonian boundary in the southern part of the Bohemian Cretaceous Basin, Czech Republic
Cech Stanislav; Hradecka Lenka; Svobodova Marcela; Svabenicka Lilian
Bulletin of Geosciences (Praha) 80(4): 321-354
Initial transgressive Cretaceous deposits are described from boreholes in the southern part of the Bohemian Cretaceous Basin, i.e. siliciclastic sediments of Cenomanian age (Peruc-Korycany Formation), and hemipelagic marlstones and limestones of the Turonian age (Bila Hora Formation). Transgressive successions include fluvial, supratidal marsh, estuarine tidal flat and channel, estuarine and mouth sand, inner shelf and open marine facies assemblages interpreted on the basis of sedimentological and paleontological features. Fluvial-estuarine facies filled an incised valley that formed a tributary of the main paleovalley in the central part of the basin. Fluvial facies are characterized by either the prevalence of spores and the presence of freshwater green algae (swampy conditions), or the prevalence of angiosperm pollen grains (alluvial plain assemblage). Marsh and estuarine facies are characterized by the presence of marine microplankton tolerant to changing salinity conditions, acritarchs, prasinophycean algae, agglutinated foraminifers, thick-walled spores, and halophyte and taxodiaceous pollen. Inner shelf facies exhibit rare sporomorphs (often thick-walled), increase of gonyaulacean dinocyst, marine macrofauna, foraminifers with non-keeled planktonic foraminifera, and sparse calcareous nannofossils. Open shelf facies are characterized by hemipelagic sediments containing keeled planktonic foraminifers and diverse calcareous nannofossils. Concerning calcareous nannofossils, the base of the Turonian is marked by the first occurrence of Eprolithus octopetalus (within foraminiferal Whiteinella archaeocretacea Interval and Partial range Zone) just above prominent erosion surface at the base of the Bila Hora Formation. The first appearance of nannofossil species Eprolithus moratus coincides with first occurrence of foraminiferal planktonic species Helvetoglobotruncana helvetica in hemipelagic sediments of the Bila Hora Formation.