Palaeoenvironmental response to the Early Cretaceous volcanic ash fall: Evidence from palynology and palynofacies of the tonstein-bearing coal seam of Kharanor Deposit, Transbaikalia, Russia

Abstract

Volcanic ash falls, being one of the consequences of eruptive palaeoevents, are reflected in the geological record of coal-bearing sediments as tonsteins, thin clayey interlayers. Information on the palaeontological characteristics of tonsteins and assessments of the influence of ash on the palaeoenvironment are still fragmentary. This paper presents new palynological and palynofacies data obtained for the tonstein-bearing Kharanor coal deposit located in Transbaikalia, Russia. The spore and pollen assemblages indicate the early Albian (Cretaceous) age of the sediments. The tonstein palynomorph composition differs markedly from the host coal one, which is expressed in an increased number of fern spores and a decreased content of gymnosperm pollen. The palynofacies composition of the Kharanor tonstein is characterised by a high content of phytoclasts of the cutinite subgroup and an increased content of palynomorphs and amorphous organic matter, whereas the lower and upper coals are dominated by vitrinite and inertinite phytoclasts. The results indicate peat-forming conditions that existed in a swampy area where a predominantly gymnosperm-fern forest grew in a wet and warm-temperate climate. At one point the area was covered by a relatively thick layer of volcanic ash, resulting in the burial of the lower forest layers, possibly during their spore production period. Rapid rates of sedimentation and subsequent compaction resulted in the conservation of organic matter, including unstable soft plant tissues. However, the impact of the volcanic event on vegetation and palaeoenvironment had a noticeable but short-term effect. No fundamental changes in flora or sedimentation conditions followed in the long term.