Cenomanian–turonian platform evolution and records of OAE 2 on the drowned northern arabian carbonate platform (se Turkey): Integration of biostratigraphy, sequence stratigraphy, sedimentology, and stable isotopes


Thesis Type: Doctorate

Institution Of The Thesis: Middle East Technical University, Turkey

Approval Date: 2020

Student: Oğuz Mülayim

Consultant: İSMAİL ÖMER YILMAZ

Abstract:

The evolution of the Cenomanian–Turonian (C–T) carbonate platform (northern Arabian Platform) in southeastern Turkey has been embraced with special emphasis on the sedimentology, biostratigraphy, lithostratigraphy, sequence stratigraphy, and stable–isotope geochemistry. The interplay of different sedimentologic features on the ramp type platform is deciphered on the basis of field and subsurface observations, detailed stratigraphic sections, macrofacies, and microfacies analyses, stable δ 13C and δ 18O isotope ratios, and fossil distribution patterns. A new multibiostratigraphic framework is based on planktonic foraminifera, rudists, and calcareous nannofossils supplemented by benthic foraminifera, pithonellids, and roveacrinids. The sequence stratigraphic framework of the Cenomanian–Turonian boundary was constructed and recorded in the transition of the transgressive systems tract (TST) to highstand systems tract (HST), coinciding with a maximum flooding surface. There are three sedimentary sequences that are separated by the C–T sequence boundary. The studied sections indicate a change from TST to HST in shallow to deeper marine deposits. Furthermore, comparisons between the ramp type platform successions and sequence patterns of SE Turkey and those from neighbouring areas allow us to differentiate local, regional, and global controlling factors of platform evolution within the study area. The microfacies analyses, multibiostratigraphic datings, and paleoenvironmental interpretations suggest that the platform was drowned near the Cenomanian–Turonian Boundary Event (CTBE) into shallow–water environments.as a result of changing environmental conditions. A filament event is also recorded around the boundary. On a global scale, the filament event beds illustrate sea–level rise related to eustacy and/or climatic change. The abundance of filaments close to the C–T boundary is a biological marker of high organic productivity resulting in a climatic change to warmer conditions. The prevailing higher sea–level conditions were then favourable to the accumulation and preservation of organic–rich facies, characteristic of the Oceanic Anoxic Event 2 (OAE 2). Regional/local subsidence and a coeval sea–level rise during the late Cenomanian to early Turonian interval were the cause of the drowning of the platform, including regional anoxia at the northern Arabian Platform linked to the OAE 2. Carbon–isotope stratigraphy of these successions in shallow–water environments revealed a positive shift δ13C (V–PDB) values that reached between 0.21‰ and 2.15‰ and represent the CTB interval excursion pointing to the presence of OAE 2. These results spotlight the potential use of carbon isotopes as a dating and high–resolution correlative tool in shallow–water carbonate rocks and help to elucidate the timing of oceanographic events affected the area that we studied. In particular, it is suggested that the highest rate of a relative, possibly tectono–eustatic sea–level rise took place during the latest Cenomanian, that was followed by the global oceanic anoxic event (OAE 2) around the CTB interval, and that peak transgression or maximum flooding was achieved during the early Turonian in this region. The time difference between the end of the OAE and the establishment of a shallow platform is attributed to the Turonian sea–level rise and tectonic effect.