Climatically-driven cessation of coal formation in tropical Pangea around the Pennsylvanian-Permian boundary; an example from alluvial-lacustrine succession of the Líně Formation (Czechia)

Abstract

Increasing climatic seasonality within tropical Pangea during the Pennsylvanian–Permian transition had a significant impact on landscape habitat and terrestrial biota evolution. One of the important consequences was cessation of peat formation in tropical Pangea – an underexplored issue because successions of this age in major European coalfields are either not preserved or are stratigraphically poorly constrained. The about 1 km thick Líně Formation (upper Gzhelian–lower Asselian) of Bohemian basins dominated by fluvial and alluvial red-beds interbedded by grayish lacustrine deposits records ∼3.5 Myr long tectono-sedimentary, climatic and biotic history already calibrated by TIMS ages to the Pennsylvanian–Permian transition. By integrating outcrop- and basins-scale data on sedimentary facies as well as on terrestrial biota, this paper evaluates the effects of climate and of post-orogenic tectonic processes on resulting depositional and biotic records and their effect on peat accretion. The current data indicate that the climate oscillated in about 450 kyr scale between less and more seasonal. During the Late Pennsylvanian, minimum seasonality (max. humidity) supported presence of ‘wet fluvial systems’ and meromictic lake formation lined with rheotrophic peat swamps, whereas during the driest climate (max. seasonality), well-drained fluvial landscape dominated the basin lowland. During the Early Permian, well-drained fluvial/small shallow lake habitats (‘dry fluvial systems’) prevailed due to long-term aridification trend in tropical Pangea, and wetland – dryland biomes coexisted over most of the time in a preservation window of the basin lowland and/or adjacent upland habitats. Their proportion, however, varied significantly in accord with climatic oscillations. The Líně Fm. was deposited in a half-graben basin system formed in response to post-Variscan intra-plate extension. The basin filling reflected three ‘rift phases’: (1) initiation, with localized subsidence and opening of initial sub-basins; (2) climax, with more distributed, maximum subsidence and interconnecting of sub-basins to form extensive basin system; (3) waning, likely dominated by thermal subsidence.

Formation of major, basinwide Klobuky Lake was facilitated by wet period ocurring during the rift climax stage with the most rapid creation of the accommodation space. Peat swamps formed and the peat accreted only during the late Gzhelian up to TIMS calibrated CP boundary. Increasing aridification, however, constrained peat swamps from floodplain to shallowed lakes, the water table of which suffered less from seasonal oscillations comparing to floodplain areas. Purely rheotrophic, usually high-ash peat swamps formed. The peat-swamp development was often interrupted by temporary rise of lake level and influx of clastic material. Resulting coal beds are usually few tens of centimeters thick, and if thicker, they commonly contain clastic partings or high-ash coal intervals. Peat-swamp vegetation is dominated by free-sporing plants, mainly ferns and sphenophytes, however, present are also lepidodendrid lycopsids. Since the beginning of the Permian, the aridification trend already did not support long-term and widespread peat accretion and the absence of economic coal beds is in agreement with replacement of meromictic lakes by shallow well‑oxygenated small lakes, increased proportion of red-beds and apparent reduction of wetland flora. Instead, drier habitats supported dryland biome dominated by walchian conifers and probably also by pteridosperms of peltaspermalean affinity.