Chemostratigraphy of the Upper Albian to mid-Turonian Natih Formation (Oman) - how authigenic carbonate changes a global pattern

The Oman Mountains preserve a Cretaceous continental margin transect with the proximal Arabian carbonate shelf and the adjacent deep Hawasina Basin. Today, the sediments from the Arabian Platform outcrop in the Oman Mountains (Jabal Akdhar and Saih Hatat) and in the Adam Foothills. The western part of the Adam Foothills provides insight into the evolution of a Late Cretaceous intra-platform basin with organic-rich sediments in the central part of this basin. The aims of this study are (a) to establish a biostratigraphy and chemostratigraphy of the Natih Formation and (b) to reconstruct depositional conditions of organic-rich sediments in an intra-platform basin during Cenomanian-Turonian times. The hypothesis that local black shale formation is an expression of global perturbations of the global carbon cycle will be tested. Reconstruction of the depositional history of the Arabian Platform and its intra-platform basin within a global palaeoclimatic framework requires an accurate time frame. The Upper Albian to mid-Turonian biostratigraphy of the Natih Formation has resulted in controversial age models that will be integrated into a solid chemostratigraphic framework with additional biostratigraphic data. A major positive C-13 excursion (+46 parts per thousand) has been identified as of Middle Cenomanian age, which is confirmed by an ammonite datum. A second positive C-13 excursion (+45 parts per thousand) following a major negative excursion (-10 parts per thousand) confirms the existence of the Cenomanian/Turonian Boundary Event. The accurate chemostratigraphy and biostratigraphy confirms that major source rocks in the Mishrif-Natih Basin precede OAE2. Low C-13 values measured in the sediments of the Natih B member are considered a consequence of diagenetic alteration. Elevated organic carbon contents and argillaceous sediments alternating with limestones resulted in diagenetic conditions favouring formation of authigenic calcite depleted in C-13.