Palaeoenvironments of the Middle–Late Mississippian Moscow Basin (Russia) from multiproxy study of palaeosols and palaeokarsts
Alekseeva et al
Numerous pedogenically altered subaerial exposure surfaces within Middle–Late Mississippian (Viséan–Serpukhovian) shallow carbonates in the Moscow Basin attest to sea-level fluctuations and climate change. In total, fifteen palaeosols at seven stratigraphical levels from five open pits were described and studied in detail. They show both stratigraphic and lateral soil type variability. The factors of major influence are substrate type, time of exposure (tectonics and relief), and climate. All palaeosols are polycyclic and consist of at least two parts. The lower unit (substrate), consisting mainly of limestone, with siliciclastic deltaic and palustrine beds, was altered to different degrees by weathering/pedogenesis. Depending on climate, the limestone was transformed into calcrete or caliche. The substrate is capped by a terrigeneous topclay which represents an individual palaeosol developed prior to subsequent marine flooding. Indications of gley conditions, as well as the presence of coal, sphaerosiderite and framboidal pyrite, indicate a wet peat–swamp environment that was, however, periodically dry. The calculated mean annual precipitation (MAP) based on a CIA-K proxy of siliciclastic units shows that the climate was characterized by alternating periods of aridity and humidity, with the most humid conditions (~ 1250 mm) during the middle Visean. The same proxy applied to Late Mississippian sediments of the Appalachian Basin shows a similar MAP range. High-amplitude sea-level fluctuations at the Mikhailovian–Venevian boundary followed by prolonged exposure resulted in the development of a laterally extensive deep karst capped by a pedocomplex and then by a palustrine bed. The structure and scale of this unconformity are comparable to those of the Florida Everglades environment, which represents a modern marsh landscape. The majority of topclays are smectitic, except for the most humid and most arid environments, which are dominated by kaolinite and palygorskite, respectively. Besides mineralogy, we also provide data regarding geochemistry, carbon isotopic composition of carbonate, organic matter characteristics including solid state 13C NMR and δ13C data, and evidence of vegetation types based on field observations and SEM images.