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Climatic disaster at theTriassic–Jurassic bound ary – a clay minerals and major elements record from the Polish Basin

Brański P.

Geological Quarterly

2014

Vol. 58, No. 2

291--310

The Triassic-Jurassic boundary interval (ca. 201 Ma) was a time of sudden global environmental changes triggered by Pangea breakup and Central Atlantic Magmatic Province development. The bulk-rock mineralogy, clay mineralogy and major element geochemistry of 87 continental mudrock samples collected from four borehole cores yield information on Rhaetian-earliest Hettangian palaeoclimatic changes in the Polish Basin. During the Rhaetian, smectite preponderance was replaced by kaolinite and illite domination. This fundamental shift in clay mineral assemblages (supported by major element data) indicates very significant change in climate humidity. Moreover, some beds in the Zagaje Formation (Upper Rhaetian-Lower Hettangian) are particularly rich in kaolinite indicating extreme chemical weathering in humid-subtropical to tropical climate episodes in the aftermath of powerful warming and abundant rainfall. Importantly, the first distinct kaolinite enrichment appears already in the Lower Rhaetian Wielichowo Beds. In addition, abrupt and episodic shifts in the kaolinite-illite ratio and in values of weathering indices point to profound climate destabilisation and a sequence of frequent, catastrophic climatic reversals in the Late Rhaetian and at the Triassic-Jurassic boundary. These results are generally consistent with carbon isotope records in sections worldwide.