Cyclicity and duration of Lower Devonian stages:: Observations from the Barrandian area, Czech Republic

Cyclicity of different orders is evaluated particularly in Lower Devonian carbonate sediments. The high-frequency cyclicity, expressed in bedding couplets, shows in selected sections 410-450 couplets in the Lochkovian, 350-380 couplets in the Pragian, around 150 (?) couplets (or bundles) in the Zlichovian (partly overprinted by discyclic events) and 250-380 couplets in the Dalejan. They point to a comparable duration of the Lochkovian, Pragian and Dalejan, though differences exist, and data on Zlichovian are not conclusive. The duration of individual bedding couplets is estimated at 17000-21000 years (possibly corresponding to Milankovie's earth's axis precessional periods). The megacycles are of two distinct orders: the shorter ones correspond to individual formations, roughly equivalent to Lower Devonian stages established in the Barrandian, and comparable with the T-R cycles of some other areas. The longer megacycles are expressed by the periodical recurrence of formation couplets with characteristical facies groups. The limits of megacycles are marked by events of mostly global character, which show some periodicity but not equal character and effects.