Mid‐Cenomanian Event Controls the Bioproductivity and Organic Preservation of Bituminous Limestones From North Jordan

ABSTRACT The Mid‐Cenomanian event has been recognised in a sedimentary outcrop exposed in north Jordan. A section belonging to the Cenomanian was logged in the Ajloun area to investigate calcareous nannofossil biostratigraphy, microfacies, the carbon and oxygen isotope composition and elemental compositions. The Ajloun section is assigned to the UC‐2 biozone (early Cenomanian) based on the occurrence of the calcareous nannofossil marker species Gartnerago segmentatum and Cylindralithus sculptus . The first occurrence of Lithraphidites acutus in Sample 18 marks the base of the UC‐3 biozone (middle Cenomanian). The Ajloun bituminous limestone comprises four microfacies, namely wackestone, packstone, pack‐wackestone and mudstone. The X‐ray fluorescence (XRF) results indicate that the pre‐existing sediments underwent episodic erosion and weathering processes during the early‐middle Cenomanian. Moreover, TiO 2 exhibited the same trend as P 2 O 3 , SiO 2 and Al 2 O 3 . The organic carbon isotope (δ 13 C) signatures reveal a trend towards the negative excursion that marked the onset of the anoxic event during the middle Cenomanian, indicating an increased abundance of light carbon within the Cenomanian seas. The following rapid positive shift of δ 13 C values in the middle part suggests that marine organic carbon accumulation rates were high as a result of the increasing ocean organic productivity. The relatively high TiO 2 , P 2 O 5 , SiO 2 and Al 2 O 3 contents confirm this observation since productivity increased significantly, followed by the extreme burial of organic carbon. The oxygen isotope (δ 18 O) data also trend towards negative values in samples deposited during the high productivity interval, reflecting an increase in temperatures associated with the enhancement of the hydrologic cycle in such a restricted and occasionally subaerially exposed setting, leading to a sea‐level rise that could have been the main factor controlling the terrigenous input.