Paleosols from the Kazanian Red Beds of the Sentyak Section, Russia

Abstract

This article summarizes the results of a comprehensive study of paleosol profiles in the Lower Kama Regionfrom the Kazanian reference section near Sentyak and from the reference section of the boundary deposits of the Ufimian and Kazanian in Elabuga. The paleosol profiles in the Kazanian deposits were identified, and their features were analyzed to reconstruct the paleoclimatic and paleolandscape conditions. The following three types of paleosols were singled out, differing from one another in their degree of development and pedogenic features: Protosols and moderately and strongly developed Calcisols. The latter proved to be characteristic only of the Lower Kazanian deposits. Based on the obtained sedimentological and paleontological characteristics of the host rocks, the fluviolacustrine depositional settings were reconstructed. The pedofeatures of the paleosols (slickensides, carbonate nodules) indicate a seasonally warm climate. The mean annual precipitation inferred from the geochemical indices of the paleosol profiles is 539 and 476 mm/yr for the Lower and Upper Kazanian paleosols, respectively, which, along with the pedofeatures, is indicative of a subhumid/semiarid climate. The oxygen isotopic composition of the carbonate nodules from the paleosols reflects some aridization of the climate in the Early Kazanian (29.7‰ SMOW δ 18 O) compared to the Ufimian (18.7‰ SMOW δ 18 O) and Late Kazanian (22.3‰ SMOW δ 18 O) times. The lack of gley features argues for the automorphic character of the paleosols, while their small thickness, truncated upper horizons, and weak development in the Upper Kazanian deposits suggest that the rate of sedimentation exceeded the rate of soil formation.