Annales de la Société Géologique de Belgique

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Abstract

The succession across the Devonian-Carboniferous boundary in the key section at Kozhim, Polar Urals consists of siliceous, carbonate and argillaceous deposits with ammonoid, conodont, ostracod and foraminiferal faunas. Devonian palmatolepid assemblages are replaced by siphonodellids and Protognathodus. The basal part of bed 58 is characterized by the appearance of Siphonodella praesulcata, S. sulcata and transitional forms. Since the differentiation of S. praesulcata and S. sulcata was difficult and subjective, we used a biometric approach based on the measurements utilised by Flajs and Feist (1988) for the specimens of Siphonodella from the La Serre section. Comparing the curvature of our specimens with those from La Serre, we identified Siphonodella sulcata as low as sample H-6a, that is at the base of bed 58. Thus, S. praesulcata and S. sulcata appear at the same level in the Kozhim section. Ostracod faunas also change at the base of bed 58. Conodonts, ammonoids and ostracods of the underlying deposits (sample H-5) are undoubtedly of late Famennian age. Thus, the Devonian-Carboniferous boundary is identified between the Zigansky and "Humerovsky" horizons, or between samples H-5 and H-6a of the Kozhim section.

The Kozhim section is important because conodont, ostracod, ammonoid and foraminiferal sequences are recorded across the Devonian-Carboniferous boundary. Additional detailed investigations will help to clarify the position of the Devonian-Carboniferous boundary.

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