Diverse depositional and geochemical signatures of the Frasnian-Famennian global event in western Thailand reveal palaeotethyan vs. Western Australian geotectonic affinities

Abstract

Two studied sections of the Frasnian-Famennian (F-F) boundary beds in western Thailand differ significantly in their depositional and geochemical characteristics. The highly condensed, monotonous Mae Sariang (MS) limestone succession generally corresponds to the event-chemostratigraphic pattern of the F-F biocrisis based primarily on German sections, and brief anoxic episodes are identifiable in trace-metal signatures (but neither organic-rich intercalations nor distinct volcanic signals). In the case of the Thong Pha Phum (TPP) site, two specific features are especially notable: (1) a δ13C positive excursion, comparable only with the peculiar Western Australian biogeochemical signature, and (2) elemental proxies that indicate exclusively oxic and probably largely oligotrophic conditions during both Kellwasser intervals. Therefore, the demonstrated biogeochemical and environmental differentiation confirms the totally different paleogeographic settings of the studied successions, as indicated previously by the Nd isotope composition of Late Devonian seawater. The distinctiveness of the TPP section clearly indicates its affinity with the Western Australian shelf successions, which are characterized by a well-known “atypical” biogeochemical and ecological signature. In terms of provenance, the siliciclastic fraction in the MS succession corresponds to continental margin. The Paleotethyan region was sourced by felsic-volcanic or granite-gneissic massifs. In the case of the TPP section, the continental margin, as well as the partly the continental island arc, are recorded in the detritus, predominantly derived from older, continental sedimentary-metasedimentary terrains. Therefore, the assumed Western Australian geotectonic assignment of TPP corresponds to the variable, partly recycled mate