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

Cyclicity of different orders is evaluated particularly in Lower Devonian c arbonate sediments. The high-frequency cyclicity, expressed in bedding coup lets, shows in selected sections 410-450 couplets in the Lochkovian, 350-38 0 couplets in the Pragian, around 150 (?) couplets (or bundles) in the Zlic hovian (partly overprinted by discyclic events) and 250-380 couplets in the Dalejan. They point to a comparable duration of the Lochkovian, Pragian an d Dalejan, though differences exist, and data on Zlichovian are not conclus ive. The duration of individual bedding couplets is estimated at 17000-2100 0 years (possibly corresponding to Milankovie's earth's axis precessional p eriods). The megacycles are of two distinct orders: the shorter ones corres pond to individual formations, roughly equivalent to Lower Devonian stages established in the Barrandian, and comparable with the T-R cycles of some o ther areas. The longer megacycles are expressed by the periodical recurrenc e 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.