Albian–Palaeocene Flora of the North Pacific: Phytostratigraphy and Palaeofloristics
The stratigraphic settings, age, systematic composition, and periodisation of development of the Albian–Palaeocene floras in the Anadyr–Koryak (AKSR) and northern Alaska (NASR) subregions of the North Pacific are discussed. The high-resolution Upper Albian–Palaeocene phytostratigraphic schemes of these subregions are based on perceived phases of their floral evolution. In the AKSR, the scheme includes seven subdivisions of subregional extent: the Early Ginter (upper Albian), Grebenka (upper Albian–Cenomanian–lower Turonian), Penzhina (upper Turonian), Kaivayam (Coniacian), Barykov (Santonian–lower to middle Campanian), Gornorechenian (upper Campanian–lower Maastrichtian), and Koryak (lower to upper Maastrichtian–Danian) phytostratigraphic horizons. The phytostratigraphic scheme of the NASR includes three subregional phytostratigraphic horizons and five plant-bearing beds. These are the Kukpowruk (lower to middle–upper Albian), Niakogon (upper Albian–Cenomanian), and Kaolak (Turonian) horizons, and beds containing the Tuluvak (Coniacian), Early Kogosukruk (upper Santonian–Campanian), Late Kogosukruk (Campanian–Maastrichtian), Early Sagwon (Danian–Selandian), and Late Sagwon (Selandian–Thanetian) floras. A comparative analysis of coeval floras distinguished in the AKSR and NASR shows that they are either similar to each other (floras of the early Ginter and Kukpowruk, Grebenka and Niakogon, Penzhina and Kaolak, Koryak and Early Sagwon phases) or different in composition (floras of the Kaivayam and Tuluvak, Gornorechenian and Late Kogosukruk phases). Similarities between the floras imply that plant assemblages of the two subregions evolved under comparable climatic conditions and freely intercommunicated via the Bering Land Bridge during the Albian–Turonian and terminal Maastrichtian–Palaeocene. Floras of the AKSR and NASR, which are of different composition, existed during particular intervals of geological history when trans-Beringian plant migrations were limited or even ceased because of palaeoclimatic differences between the subregions. Floras of the AKSR and NASR survived the crisis at the Cretaceous–Paleogene boundary without essential evolutionary consequence. Phytostratigraphy and floral evolution in the Albian–Palaeocene of the North Pacific are discussed. A primary driver of Albian–Late Cretaceous florogenesis was the gradual invasion by novel angiosperm-rich plant communities into the Asiatic continental interiors and the replacement of pre-existing vegetation dominated by ancient ferns and gymnosperms.