Climato-environmental controls on clay mineralogy of the Hettangian–Bajocian successions of the Mecsek Mountains, Hungary: An evidence for extreme continental weathering during the early Toarcian oceanic anoxic event

Abstract

Clay mineralogy has been successfully used in palaeoclimatic interpretation of Jurassic rocks. In the Mecsek zone (Hungary), the clay mineral assemblages of the Hettangian–Sinemurian fluvial and shallow marine coal-bearing siliciclastic rocks (Gresten facies, Mecsek Coal and Vasas Marl Formations) comprise predominantly illite/smectite (I/S) mixed-layer minerals, illite (ill), and kaolinite (kao) with the presence of berthierine and/or chlorite. These clay mineral suites suggest a humid-subtropical climate with short-term climate changes of high and low rainfall and a high supply of terrigenous clastics to the basin. In the Pliensbachian part of the sequence studied, which is composed of predominantly hemipelagic mudstones (Allgäu facies) with intercalations of redeposited sandstone and limestone bodies, the Hosszúhetény Calcareous Marl, Mecseknádasd Sandstone and Kecskehát Limestone Formations are predominantly made up of illite and very little kaolinite and I/S mixed-layer minerals (kao/ill < 1). Based on these results, the Pliensbachian time interval is interpreted to represent warm and seasonally humid (monsoon-like) climatic conditions and a moderate input of terrigenous clastics relative to the underlying formations. Towards the Toarcian, kaolinite becomes the dominant clay mineral (Óbánya Siltstone Formation; kao/ill > 1) suggesting a humid climate and intense continental weathering possibly related to the oceanic anoxic event. In contrast, rocks of the Bajocian Komló Calcareous Marl Formation contain high proportions of I/S mixed-layer mineral and illite with sparse occurrence of kaolinite (kao/ill = 0 or kao/ill < 1). This clay mineral assemblage reflects a warm climate with a seasonal (monsoon-like) contrast in humidity and a minor input of terrigenous clastics from a relatively distant source area.