Wrench tectonics and major environmental changes across the Permo-Triassic boundary recorded in a coal-bearing basin, South Mongolia

Abstract

Syn-tectonic deposition of the Late Permian coal-bearing Yamaan Us Formation of the Tsagaan Tolgoi sub-basin, South Mongolia, was abruptly terminated due to the most severe mass extinction in Earth’s history. As the principal dying out events have occurred simultaneously with tectono-magmatic upheavals characterizing major geological time boundaries, the Permo-Triassic sedimentary fill of the sub-basin is a characteristic example of Earth’s pulse-like history. It is considered axiomatic that the pulsing globe is triggered by internal outgassing, with related mass reorganization, episodic dynamic implications, and inertia-based moderate wrench tectonics. In this development scheme, the moderate tectonically deformed sub-basin, c. 1,000 m thick, records a dramatic transition from humid fluvio-paralic coal-forming environments characterized by abrupt facies changes during the Late Permian, to arid barren terrestrial conditions during the Early Triassic. Coal ash composition analysis shows a marked increase in average SO3 content from 2.09% in the thick basal seam to 6.88% in the topmost, which might be coupled to pulses of toxic gas exhalation - in this case perhaps mainly from the contemporary Siberian Traps. Very rare fire-scarred fossil wood fragments, discovered in the topmost part of the coal measures, probably resulted from wildfire- caused by outgassed methane ignited by lightning. Preliminary analysis of fossil wood growth rings shows significant internal variation in annual ring thickness and overall decrease upwards towards the End Permian Mass Extinction, indicating critical stress in plant ecosystems linked to complex toxic haze, acid rain and hypoxia. However, the sedimentary record shows no spatial-temporal diminution of coal seams prior to the End Permian Mass Extension indicating abrupt termination of peat mire deposition.