Upper Cretaceous (Cenomanian–lower Campanian) nannofossils and sequence stratigraphy of southwestern Crimea

Abstract

The first high-resolution study of the Cenomanian to lower Campanian calcareous nannofossils of two most continuous but poor in macrofossils sections of southwestern Crimea allowed the detailed division of these successions on the basis of standard nannofossil zonations and the correlation with Tethyan and Boreal Upper Cretaceous. After initial Cenomanian sea-level fall, nannofossil assemblages increase their abundance and taxonomic diversity toward the end of Cenomanian. The most of the Cenomanian bioevents are recognised in the sedimentary record. The Cenomanian/Turonian transition is the most stratigraphically intricate interval in the Late Cretaceous Crimean basin. The lowest occurrences (LOs) of Cylindralithus biarcus, Quadrum intermedium, Eprolithus octopetalus, Ahmuellerella octoradiata are identified in the upper Cenomanian. The highest occurrences (HOs) of some stratigraphically important for this time span taxa like Corollithion kennedyi, Cretarhabdus striatus and Axopodorhabdus albianus are found significantly above the levels defined as relevant subzonal boundaries (UC3e, UC4b and UC5b, respectively). The TOC-rich sediments corresponding to Bonarelli Level (OAE2) is intercalated in the upper part of the upper Cenomanian limestones (undivided interval of UC5c-UC6a subzone/zone). Helenea chiastia is scarce in the studied succession and its HO cannot be used for definition of zonal boundary (UC6) and the base of Turonian. The most reliable bioevent closest to the Cenomanian/Turonian boundary in this basin is the LO of Eprolithus moratus (base of UC6b subzone). The middle Turonian to lower Campanian nannofossil assemblages display the succession of the main bioevents similar to those in Tethyan basin, but show earlier LOs of Micula and Reinhardtites in the middle Turonian.

Nannofossil biostratigraphy revealed the stratigraphic range of hiatuses in this sedimentary record. Being combined with the levels of the major facies changes, these levels enabled the recognition of five Cenomanian and four Turonian sequence boundaries consistent with the sequence stratigraphy established for Europe. Two sharp erosional surfaces occurred in the upper part of the studied succession correspond to the Coniacian KCo1 and Santonian KSa2 sequences.