Paleosols in Late Moscovian (Carboniferous) Marine Carbonates of the East European Craton Revealing "Great Calcimagnesian Plain" Paleolandscapes

Abstract

Abstract Late Paleozoic strata commonly include lithologic cycles (“cyclothems”), driven by sea-level fluctuations that repeatedly flooded and exposed broad expanses of epeiric shelves. Such successions, commonly with multiple unconformities, are well documented in Permo-Pennsylvanian sedimentary successions, but remain less understood in the East European Craton (EEC), part of the Euramerican continental shelf rim. This study documents unconformities and paleosols from the Podolskian to Myachkovian (middle Pennsylvanian) epeiric-carbonate succession of the EEC and explores the non-actualistic soil types, landscapes, and the character and density of vegetation in this Pennsylvanian calcimagnesian dryland environment. Thin (mostly < 1 m) paleosols developed at seven successive unconformities consist of an upper terrestrial clay layer or “topclay”, a crust of beta calcrete beneath the topclay, and a weakly karsted substrate limestone with rare rhizocretions. Solution pits and small (< 3 m deep) karst sinkholes are rare. Nontruncated topclays commonly consist of two layers: in situ clayey epipedons and resedimented terrestrial clays. The latter are interpreted as playa sediments. The Myachkovian unconformities of the northernmost localities studied, near Arkhangelsk, lack beta calcrete and root traces, and the limestone beneath these unconformities is sepiolitized. In Podolskian paleosols of the southern Moscow Basin, the topclays are predominantly palygorskitic. The Myachkovian paleosol clays are smectitic–illitic with occasional chlorite admixture. The majority of late Moscovian paleosols surveyed formally conform to lithic (rendollic) haplocalcids within the aridisol class or rendzic calcic leptosols. A detailed study of a representative Podolskian paleo-pedon reveals development of shallow soil carbonate, low alumina/bases and Ba/Sr ratios, enhanced Mn and Sr, presence of soil gypsum and opal, and a characteristic peak in magnetic susceptibility, all suggesting a semiarid to arid pedogenic environment. The palygorskite clay of this paleo-pedon retains 1.1–1.5% of connate organic matter in the form of covalently bound organomineral complexes, and is fulvate-dominated resembling organic matter from extant dryland soils. The palygorskitic composition of Podolskian topclays and shallow (< 30 cm) pedogenic carbonates is interpreted to reflect hot, well-drained semidesert conditions with precipitation less than 300 mm/yr. The transition from palygorskitic to smectitic soil clays across the Podolskian–Myachkovian boundary, apparently deeper soil carbonate, and an increase in the degree of karstification record a general increase in humidity such that annual precipitation may have exceeded 300–400 mm. The available data reconstruct the “great calcimagnesian plain” landscapes formed during low sea level, supposedly glacial phases, across tens of thousands of square kilometers of the EEC. This non-actualistic system apparently lacked a fluvial network and was covered by uniform soils and playas. The primary productivity of this early (310–305 Ma old) dryland ecosystem inferred from high TOC and rhizogenic beta calcretes was not markedly different from present-day steppe or savanna grasslands and semideserts. In the northernmost outcrops surveyed, near Arkhangelsk, drier conditions probably limited colonization by higher plants. The available data open a “hidden chapter” in the Paleozoic evolution of terrestrial ecosystems which is not recorded by paleobotany and urges modifications in the concept of low-productivity character of pre-Miocene dry landscapes.