Facies development, depositional settings and sequence stratigraphy across the Ordovician–Silurian boundary: a new perspective from the Barrandian area of the Czech Republic

Abstract

AbstractThe Hirnantian and Llandovery sedimentary succession of the Barrandian area has been assigned to middle and outer clastic‐shelf depositional settings, respectively. Deposition was influenced by the remote Gondwanan glaciation and subsequent, long‐persisting, post‐glacial anoxia triggered by a current‐driven upwelling system. High‐resolution graptolite stratigraphy, based upon 19 formally defined biozones—largely interval zones—and five subzones, enabled a detailed correlation between 42 surface sections and boreholes, and enabled linking of the sedimentary record, graptoloid fauna dynamics, organic‐content fluctuations and spectral gamma‐ray curves. The Hirnantian and Llandovery succession has been subdivided into four biostratigraphically dated third‐order sequences (units 1–4). Time–spatial facies distribution recorded early and late Hirnantian glacio‐eustatic sea‐level lowstands separated by a remarkable mid‐Hirnantian rise in sea‐level. A major part of the post‐glacial sea‐level rise took place within the late Hirnantian. The highstand of Unit 2 is apparently at the base of the Silurian succession. Short‐term relative sea‐level drawdown and a third‐order sequence boundary followed in the early Rhuddanian upper acuminatus Zone. Early Aeronian and late Telychian sea‐level highstands and late Aeronian drawdown of likely eustatic origin belong to units 3 and 4. Sea‐level rise culminated in the late Telychian, which may also be considered as a highstand episode of a second‐order Hirnantian–early Silurian cycle. Facies and sequence‐stratigraphic analysis supports recent interpretations on nappe structures in the core part of the Ordovician–Middle Devonian Prague Synform of the Barrandian. Copyright © 2006 John Wiley & Sons, Ltd.