Mineralogical evidence for multiple dust sources in an early Triassic loessite

Abstract

Loessite present in a borehole into the Smith Bank Formation (early Triassic age, Central North Sea) differentiates five coeval source terranes for aerosol dust, three long distance sources and two local sources. All were active immediately following the end Permian mass extinction. Long distance sources are sedimentary, basic magmatic and acid–intermediate volcanic. Although predominantly silt‐sized and dominated by quartz with subordinate feldspars, muscovite and illite, evidence of basic and acid–intermediate magmatic/volcanic sources are pervasive. Baddeleyite is diagnostic of basic magmatism, an origin supported by enrichment of plagioclase relative to potassium feldspar. Deduction of acid–intermediate volcanism comes from the collective occurrence of irregular geometry quartz, volcanic shards, Ti‐mineralization, euhedral biotite, sanidine, the co‐occurrence of apatite and zircon, and the common occurrence of a tosuditic clay mineral. The tosuditic phase occurs as an unusual diagenetic dioctahedral chlorite/smectite formed at low temperature (<45°C), during very shallow burial by the decomposition of unstable rhyo‐dacitic and andesitic grains in alkaline pore water from an adjacent lake that yielded pore fluids with a high Al:Si ratio. The Siberian Traps large igneous province is the likely source terrane for the magmatic and volcanic silt. Locally sourced clay pellets and kaolinite booklets formed from aeolian erosion of an adjacent, periodically desiccated lake‐floor and a kaolinitic regolith, respectively. Inference of a prolonged harsh, arid climate leaves no evidence of any periods of sustained humidity or climatic fluctuation, such as pedogenesis. The association between the end Permian mass extinction, emplacement and aeolian erosion of the Siberian Traps large igneous province, and location of the Smith Bank Formation in a large lacustrine endorheic basin, combine to preserve a record of prolonged harsh climate in the early Triassic.