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The drowned Oman Exotics – Upper Jurassic to Cretaceous pelagic and hardground successions
Stephan Wohlwend

Building: Muséum d'Histoire Naturelle de Genève
Room: Lobby /Hall/
Date: 2018-12-05 12:09 PM – 12:12 PM
Last modified: 2018-11-24

Abstract


The time between the Late Jurassic and the Early Cretaceous experienced dramatic changes in plate configuration, paleoceanography and global carbon cycle. The Jurassic-Cretaceous transition remains best documented in the Western Tethys and North Atlantic Oceans. Available data from the eastern part of the Tethys and the Pacific Ocean are not very abundant and additional information from these regions is of importance for better understanding how these regions were affected by oceanographic and tectonic changes. As shown by Celestino et al. (2017), the Oman Mountains preserve Mesozoic successions, which were deposited along the Arabian continent in the eastern Tethys. In addition, the seamounts of the “Oman Exotics” (e.g., Bernoulli & Weissert, 1987), preserve pelagic successions of Late Jurassic and Cretaceous age in the Kawr Group. These sediments serve as archive of equatorial ocean history at the Jurassic-Cretaceous transition. Sediments of the Kwar Group provide data for the reconstruction of the impact of climatic and/or environmental changes in a region, which can be described as a window towards the Indo-Pacific Ocean.

Stable carbon and oxygen isotope geochemistry was performed on bulk carbonate samples from the sedimentary successions of the Kawr Group (Upper Jurassic to Lower Cretaceous) formed on two isolated seamounts (Kawr and Hamrat al Hasan). In addition, calpionellid assemblages were defined, allowing the definition of biostratigraphic tie points. These exotics form part of the Hawasina Nappe pile containing continental slope, deep marginal basin and the seamount units. The highly deformed sediments of the Hawasina Nappes are outcropping today in the Central Oman Mountains.

The calpionellid biostratigraphy and the measured δ13C values confirm the Late Jurassic to Early Cretaceous time frame. Condensed red crinoidal limestones (Fatah Fm) of latest Jurassic age mark the beginning of continuous marine sedimentation after a long time of condensed sedimentation and hardground formation and therefore, indicates a major change in paleoceanographic conditions. The successions are overlain by pelagic chert-nodule bearing nannofossil-limestones (Nadan Fm) of earliest Cretaceous age. The top of the Nadan Fm on the Hamrat al Hasan seamount is of Late Valanginian age, which can be shown by the onset of the prominent perturbation of the carbon cycle, interpreted as the Valanginian C-isotope Event (CIE). The change towards more positive δ13C values is accompanied by an increase in silicified limestone beds and chert layers. This succession is in contrast with the succession of the Kawr seamount. There, the top of the Nadan Formation seems to be eroded probably due to strong currents affecting the exposed seamount surfaces during the late Early to mid-Cretaceous. The eroded top of the Nadan Fm on the Kawr seamount is again overlain by the Cenomanian to Late Turonian Safil Fm (Wohlwend, 2015).

The equatorial position of the Hawasina Basin with the isolated drowned seamounts provides information on a peculiar oceanographic situation. The Hamrat al Hasan seamount preserves a complete succession of the Nadan Fm containing the Valanginian CIE, while the Kawr seamount provides evidence of an enhanced circumequatorial current system, which may have become active in the Late Valanginian causing erosion on parts of the Kawr seamount.