Abstract
The eastern periphery of the Russian Platform houses an exceptional record of the Jurassic/Cretaceous transition, which is represented by very thin sandstone beds. The presence of glaucony grains, phosphorite concretions and shark teeth indicates that the transitional sediments constitute a condensed succession, although the allochtonous origin of the glaucony grains in itself is not a reliable indicator. The combination with thin ammonite zones and a strongly diminished sedimentation rate, as low as ~0.05 cm/ka are, however, convincing evidence. The Jurassic/Cretaceous transitional deposits accumulated in a basin the depth of which decreased simultaneously with a global eustatic sea-level fall. This coincidence suggests that condensed successions may form in shallowing environments, which contradicts the sequence-stratigraphic concept. Considering the character of the sediments under study, it appears that both stratigraphic and taphonomic condensation patterns occur in this part of the eastern Russian Platform.
Highlights
Condensed successions result from an exceptionally low sedimentation rate
The most comprehensive characteristics of this phenomenon were provided by GÓMEZ & FERNÁNDEZ-LÓPEZ (1994) and FERNÁNDEZ-LÓPEZ (2000), who distinguished between sedimentary, stratigraphic, and taphonomic condensation and who emphasized the possible occurrence of condensed successions in a variety of sedimentary environments
KITAMURA (1998) and AMOROSI (2003) showed that the presence of glaucony grains is not always a reliable indicator of condensed successions, which is in disagreement with the sequence-stratigraphic concept (LOUTIT et al 1988)
Summary
Condensed successions result from an exceptionally low sedimentation rate. HEIM (1934) was the first to study this phenomenon, its present-day concept is rooted in sequence stratigraphy (LOUTIT et al 1988). That at the Jurassic/Cretaceous transition on the eastern Russian Platform (Fig. 1). The transitional Jurassic/Cretaceous strata are represented in the north-eastern part of the Uljanovsk–Saratov Basin by centration of several ammonite zones within thin siliciclastic beds of this succession, as well as a general basin shallowing.
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