Intra-Turonian stratigraphic reorganisation on the Arabian Plate

Abstract

Shallow-marine carbonate deposition was widespread across the Arabian Plate during much of the Late Permian - Mesozoic but was significantly interrupted by a major tectonically-driven stratigraphic reorganisation that took place within the Turonian. Published literature terms this the “Wasia-Aruma Break”, “the mid-Turonian Unconformity”, or “K150 Sequence Boundary (SB)”. It is widely understood that this event relates to subduction in a closing Neotethys Ocean and obduction onto the continental margin, especially in the south-eastern part of the plate. As well as a change in facies, a hiatus of variable duration is present, associated with erosion of the underlying stratigraphy. Despite the event being known for several decades, precision on its timing has been lacking due to the limitations of biostratigraphic data and because few other chronostratigraphic proxies have been employed. In order to rectify this, all relevant biostratigraphic data from across the Arabian Plate has been synthesised and evaluated in the context of a review of chronostratigraphic calibration of standard biozones from multiple fossil groups. This shows that the youngest preserved rocks beneath K150 SB can be assigned to the middle Turonian helvetica planktonic foraminifera zone or woollgari ammonite zone. The oldest rocks above K150 SB can be assigned to the same biozones, supporting the assertions that it is a very short duration event. Whilst obduction may have initiated earlier (as supported by the recognition of precursor events), crustal loading was sufficient by 91.5 Ma to develop a forebulge in the eastern part of the plate with associated uplift and erosion. A review of relative sea-level change during the Turonian has been carried out to investigate eustatic signals during this time. In multiple sections around the world, a sea-level fall in the upper part of the woollgari Zone of European usage can be detected. This appears to coincide with an episode of climate cooling. However, despite there being correspondence between K150 SB and a significant eustatic event, this seems mostly coincidental as the tectonic drivers at this time across Arabia were much more dramatic. This is especially true in south-eastern Arabia. In north-western Arabia, tectonic quiescence may have allowed eustasy to dominate. In any case, a eustatic fall would have amplified the effects of tectonics across the entire region.