The composition and age of the Ul’ya flora (Okhotsk-Chukotka volcanic belt, North-East of Russia): paleobotanical and geochronological constraints

Abstract

The Ul’ya flora comes from the Amka Formation of the Ul’ya Depression, located in the Okhotsk–Chukotka volcanic belt (North-East Russia). This flora includes ~50 species, among which conifers predominate. Ferns and angiosperms are also diverse. The Ul’ya flora is characterized by high endemism and by the presence of numerous Early Cretaceous relicts (Hausmannia, Podozamites, Phoenicopsis, Baiera, Sphenobaiera). Four new endemic species of conifers from the Ul’ya flora are described: Elatocladus amkensis Golovneva, sp. nov., Araucarites sheikashoviae Golovneva, sp. nov., Elatocladus gyrbykensis Golovneva, sp. nov. and Pagiophyllum umitbaevii Golovneva, sp. nov. Two-lobed leaves of Sphenobaiera are assigned to S. biloba Prynada based on their epidermal structure. Because of its systematic composition the Ul’ya flora is correlated with the Coniacian Chaun flora of Central Chukotka, with the Coniacian Aliki flora from the Viliga–Tumany interfluve area, and with the Coniacian Kholchan flora of the Magadan Region. The U-Pb age of zircon (ID-TIMS method) from plant-bearing tuffites within the Amka Formation at the Uenma River is 86.1 ± 0.3 Ma. Thus, Coniacian age (most likely the end of the Coniacian, near the Coniacian/Santonian boundary) is assigned to the Ul’ya flora and plant-bearing pyroclastic deposits of the Amka Formation on the basis of paleobotanical and isotopic data.

Keywords

• late cretaceous flora
• u-pb geochronology
• okhotsk–chukotka volcanic belt