ON THE CHAUN PALAEOFLORA FROM NON-MARINE CRERACEOUS OF CHUKOTKA

The Cretaceous fossil floras from the formations of the Okhotsk-Chukotka volcanogenic belt differ significantly by its systematic composition from the same-age fossil floras from the coastal lowland sedimentations. The peculiarities of fossil floras from the volcanogenic disposals are the most obvious in Chaun Flora firstly found in the volcanogenic formations of Pegtymelski Arch in Central Chukotka. L.B. Golovneva presented the results of the long-term study of this flora in the monograph published in 2018. On the base of these data and using our own experience in the stratigraphy of the Okhotsk-Chukotka volcanogenic belt and the study of the vegetation cover dynamics in the areas of modern volcanism of Kamchatka we showed that most likely Chaun Flora formed on juvenile substrata in the central parts of the vast volcanic fields being isolated form the sources of diasporas. The Chaun Flora has not clear affinity in neither lateral nor stratigraphic localization, because the fossil remains of characteristic plant species occur in different-age formations on all extent of the volcanogenic belt. The species characteristic for Chaun Flora are indicators of the certain environmental conditions and reflect the peculiarities of the processes of fossilization in the areas of terrestrial volcanism from Turon until Campanian inclusive.

New data on the volcanism of the Central Chukotka segment of Okhotsk-Chukotka volcanogenic belt

The article presents new geochronological and geochemical data on the age and composition of the undifferentiated Cretaceous volcanics of the basin of the Palyavaam river Central Chukotka region. These data indicate the heterogeneity of the rocks of etchikun suites, which are divided into two groups. The andesites of the first group (actually etchikun suites ) are of early Cretaceous age and represent a magmatic phase preceding the formation of the Okhotsk-Chukotka volcanogenic belt (OCHVB). The andesites of the second group correlate well in age and composition to the volcanic rocks of OCHVB. They lie at the base section of Chaunskaya series of OCHVB and indicate gomodromos the evolution of volcanism in the Central-Chukchi sector OCHVB.

Ginkgophytes of the Ul’ya flora (the Ul’ya depresiion, the Okhotsk-Chukotka volcanogenic belt)

The Ul’ya flora comes from the Coniacian volcanogenic deposits of the Amka Formation (the Ul'ya depression, southern part of the Okhotsk-Chukotka volcanogenic belt). Ginkgoaleans are diverse in this flora and represented by three genera: Ginkgo, Sphenobaiera and Baiera. All specimens have no cuticle and were assigned to morphotaxa. Genus Ginkgo includes two species: G. ex gr. adiantoides (Ung.) Heer with entire leaves and G. ex gr. sibirica Heer with dissected leaves. Genus Sphenobaiera also consists of two species: S. ex gr. longifolia (Pom.) Florin with 4–8 leaf lobes and S. ex gr. biloba Prynada with two leaf lobes. Genus Baiera is represented by new species B. lebedevii Golovn., sp. nov.Leaves of this species are 25–30 cm long and 13–16 cm wide, narrowly wedge-shaped with flat slender petiole, dichotomously dissected 4–5 times into linear segments 3–6 mm wide with 6–12 veins. The length of ultimate segments is equal to about a half of leaf length. Leaves attached spirally to ovoid short shoots about 2 cm long. Among the Late Cretaceous floras similar diversity of ginkgoaleans was recorded only in the Turonian-Coniacian Arman flora from middle part of the Okhotsk-Chukotka volcanogenic belt (Herman et al., 2016). Four species of ginkgoaleans from the Ul’ya flora (except G. ex gr. adiantoides) are considered as the Early Cretaceous relicts.

The Gedan floristic assemblage from the Late Cretaceous deposits of the Kholchan Formation of the Okhotsk-Chukotka volcanogenic belt

The Gedan floristic assemblage occurs from upper layers of the Kholchan Formation of the Okchotsk-Chukotka volcanogenic belt (OCVB). The locality is situated at the Gedan River in the middle part of the Arman River basin. The Gedan assemblage is composed of 6 taxa: Cladophlebis sp., Sphenobaiera sp., Ginkgo ex gr. adiantoides (Ung.) Heer, Taxodium amguemensis (Efimova) Golovn., Metasequoia sp., Pagiophyllum sp. The similarity of the Gedan floristic assemblage with the Karamken and the Khirumki floristic assemblages from the Kholchan Formation of the Okhotsk sector of the OCVB allows us to join them in the Kholchan flora. This flora is distinct from more ancient Arman flora, which dated as the Turonian-Coniacian and from younger Ola flora, which dated as the Santonian-early Campanian. The age of the Kholchan flora is estimated as the Coniacian on the basis of stratigraphic position, presence of Podozamites, Metasequoia and Quereuxia and also isotopic data. This flora is equivalent with the Chaun flora of Central Chukotka, with the Aleeki flora from the Villigha and Toomahni Rivers interfluve and with the Ulya flora from the southern part of the Okhotsk-Chukotka volcanogenic belt.

The Envymaam floristic assemblage from the Upper Cretaceous deposits of the Okhotsk-Chukotka volcanogenic belt

The Envymaam floristic assemblage occurs from basaltic rocks of the Envymaam Formation in the Envymaam River basin, Central Chukotka segment of the Okhotsk-Chukotka volcanogenic belt. This assemblage consists of 18 taxa: Hepaticites sp., Coniopteris tschuktschorum (Krysht.) Samyl., Phoenicopsis ex gr. angustifolia Heer, Cupressinocladus enmyvaamensis Golovn., sp. nov., Araliaephyllum philippoviae Golovn., sp. nov. Among them, two species are fi rstly described. The Envymaam assemblage is characterized by presence of the Early Cretaceous relicts (Phoenicopsis), by rarity of angiosperms and by small sizes of their leaves. Age of the Envymaam floristic assemblage is determined as the late Campanian on the basis of systematic composition, palynological, paleomagnetic and isotopic data. Direct continuity between the Envymaam and the Ust’-Emuneret floristic assemblages from the subjacent Emuneret Formation it isn’t observed that is caused probably by the poor composition of the Envymaam floristic assemblage. Palynological assemblages from the Envymaam, Ust’-Emuneret, Ola and Arkagala Formations are very similar.