Modifications des paléoenvironnements et des associations biologiques autour de la limite Crétacé-Tertiaire dans les montagnes d'Oman

Two new sites, revealing a record of the events at the K/T boundary, have been recently discovered in the Oman Mountains at the eastern end of the Arabian plate. In the Buraymi Basin, located at the northwestern flank of the chain, the K/T boundary is intersected by a basinal facies succession, whilst in the Sur area, the transition is illustrated within a confined carbonate platform sequence. This period exhibits important palaeoenvironmental and biotic changes which originated from the conjunction of multiple factors. These came together over differing intervals of time, i.e., long period of time, short time scale and instantaneous event. At the scale of the long period of time (4 Ma) stretching from late Maastrichtian to the Danian (P1c), the Oman Mountains recorded profound modifications in terms of their palaeogeographic context, undoubtedly linked to plate reorganisation. This was initially shown by the emersion of the rudist platforms and the flooding of the margins in the late, but not terminal, Maastrichtian. This first tectonic event introduced an hemipelagic and a turbiditic sedimentation. As a consequence, this episode created, at the southern limb of the chain, the confined Murka sub-basin characterised by a carbonate platform sedimentation. Because the transition terminal Maastrichtian-earliest Danian correspond to a period of tectonic quiescence, the sedimentation persisted through the K/T boundary without any notable modification. A second tectonic episode in the Danian P1b/P1c interval, accentuated the flooding of the plate margins where basin deposits were accumulating. The renewal of planktic foraminifera took place in stages suggesting a gradation of palaeoecological conditions spread over a short time scale (1 to 2 Ma). This gradation is marked by the succession of three waves of extinction which took place from the late Maastrichtian to the K/T boundary. Diversity of the benthic foraminifera then increased progressively from subzone P1b onwards, showing the re-establishment of the ecosystem in P1c. The iridium anomaly detected at the K/T boundary at both sites would tend to reinforce the hypothesis of a meteorite impact, the effects of which would have added to these events that unfolded over a longer time scale.