|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
The Okhotsk-Chukotka Subregion
The Okhotsk-Chukotka Subregion included volcanic uplands with variably differentiated relief, which were formed in the middle–late Albian. Volcanogenic and terrigenous deposits of the OCSR reflect depositional environments of intermontane valleys and volcanic plateaux. Development of the late Albian–Late Cretaceous floras in this subregion was inherited from the Buor-Kemus phase.
In the OCSR, the following fossil floras are known): Buor-Kemus (early–middle Albian and part of the late Albian), Dukchanda (late Albian–early Turonian), Amka (presumably late Albian–early Turonian), Chingandzha (Turonian–Coniacian), Arman’ (Turonian–Coniacian), Chauna (Turonian–Coniacian), Ola (Santonian–middle Campanian), and Ust’-Emuneret (Santonian–middle Campanian).
Taphofloras from the base of volcanogenic deposits of the OCSR resting on molasses of the Verkhoyansk–Chukotka Mesozoides (Belyi, 1994) are comparable in taxonomic composition with the Buor-Kemus regional flora). They mark the initiation of volcanic activity that resulted in the origin of the volcanogenic belt in this subregion.
Interactive map showing the main paleobotanical-paleogeographic subregions in Northeastern Russia.
The Dukchanda Flora includes a series of floristic assemblages from the Ul’ya Trough, Penzhina–Anadyr Sector of the Okhotsk–Chukotka volcanogenic belt, and northern Okhotsk area. Lebedev (1987) regarded them as belonging to the individual Arinda and Dukchanda floral evolutionary phases (stratofloras) of the OCSR. Angiosperms predominating in the Dukchanda Flora are associated with abundant ferns and conifers. Among the conifers there are relatively ancient plants typical of the Early Cretaceous as well as more advanced Late Cretaceous genera. The Dukchanda Flora is comparable with the AKSR floras, being closest in taxonomic composition to the late Albian–early Turonian Grebenka flora.
The Amka Flora first described by Lebedev (1987) from the Ul’ya Trough is dominated by diverse conifers. Unlike the Dukchanda Flora, angiosperms are subordinate components, less abundant and diverse, but are represented by the same genera (and species coeval with the Dukchanda Flora and corresponds in age to the end of the Albian–beginning of the Turonian). However, new U-Pb SHRIMP dating of the Amka Formation (Akinin and Miller, 2011) suggests a younger age of the plant-bearing deposits, which could be Turonian – Santonian. Obviously a revision of the taxonomic composition and age of the Amka Flora is needed.
The Chingandzha Flora includes floristic assemblages from the northern Okhotsk area (Samylina, 1984a, 1984b; Shchepetov, 1988, 1991a, 1995), the Volchinsk taphoflora from the Anadyr River basin (Devyatilova et al., 1980; Nevretdinov, 1980), the Tunum assemblage from the Dzhugdzhur Foredeep (Lebedev, 1987), and probably a taphoflora from the Lower Subformation of the Emuneret Formation of the Anadyr River basin (Nevretdinov, 1980). The predominance of diverse angiosperms, large-leaved platanoids and Trochodendroides, is a characteristic feature of the Chingandzha flora. The Chingandzha Flora shows great similarity to the Penzhina and Kaivayam floras of the AKSR and to the Kaolak flora of the NASR, being especially close to the Penzhina. Judging from these data, the Chingandzha Flora is likely to be of Turonian–Coniacian age.
The Turonian–Coniacian Arman’ Flora (Herman, 2011a, 2011b; Akinin and Hourigan, 2002; Hourigan and Akinin, 2004; Akinin and Miller, 2011) shows a peculiar combination of Early and Late Cretaceous plants (Samylina, 1974, 1976, 1988). It is also remarkable that typical Early Cretaceous ferns and gymnosperms dominate the Arman’ floral assemblages together with taxa widespread in the Late Cretaceous, including a large group of angiosperms, among which there are the large-leaved platanoids. With such a composition, the Arman’ Flora is similar to the Penzhina and Kaivayam floras of the AKSR and to the Chingandzha flora of the OCSR. However, the Arman’ flora exhibiting a combination of relict and advanced plants, is less close to the floras of AKSR and NASR than the Chingandzha flora.
The Chauna Flora is known from volcanogenic accumulations of the Chauna Group in the northern part of the OCSR (Belyi, 1977, 1994; Shchepetov, 1991b; Filippova, 2001). This flora has a distinctive taxonomic composition and consists of diverse ferns, among which are taxa endemic or rare in other floras (Tchaunia, Kolymella, and some species of the genus Cladophlebis), abundant large-leaved cycadophytes such as Heilungia and Ctenis, ginkgoaleans, czekanowskialeans, conifers, including the endemic species Elatocladus zheltovskii and angiosperms. Remains of relictual (ancient) plant forms co-occur in this flora with relatively advanced genera of conifers and extremely rare angiosperms belonging, however, to Late Cretaceous genera. The stratigraphic range of the Chauna Group and its synonymous flora corresponds to the Turonian–Coniacian interval (Kelley et al., 1999; Akinin and Miller, 2011).
The Ola Flora, widespread in the OCSR, is completely represented by taphofloras from the Ola, Pervomaisk and Mygdykit formations of the Arman’ and Ola river basins northward of the Okhotsk Sea (Belyi and Samylina, 1987; Samylina, 1988; Filippova, 2007). Its characteristic features are a predominance of conifers, frequent occurrences of Phoenicopsis species, and an extreme rarity (excepting leaves of aquatic plant Quereuxia angulata) of angiosperms that are largely represented by small-leaved forms. The general “Mesophytic” aspect of this flora is combined with the presence of conifer and angiosperm genera (Herman and Shchepetov, 1997) which are widespread in floras of the latest Cretaceous and Cenozoic. The age of the Ola flora is determined by its taxonomic composition, palynological and paleomagnetic data, and isotopic dates. According to all these approaches, this flora is of Senonian age and probably spans the stratigraphic interval from the Santonian to at least the middle Campanian, inclusively.
The Ust’-Emuneret Flora is represented by a series of taphofloras from the Central Chukotka and Penzhina–Anadyr sectors of the Okhotsk–Chukotka volcanogenic belt, middle courses of the Anadyr River, and Ul’ya Trough (Filatova and Lebedev, 1982; Lebedev, 1987). The taphofloras are dominated by abundant and diverse leaf impressions of angiosperms, which as a rule are of small size. Ferns and ginkgoaleans are rare. Among conifers, there are advanced “Cenophytic” forms. Relict Phoenicopsis and Sphenobaiera coexist in taphofloras with the Senonian angiosperms 'Quercus' tchucotica, Hollickia quercifolia, and Macclintockia species. Large-leaved platanoids occur as single specimens. Based on these angiosperm taxa, the Ust’-Emuneret flora can be correlated with the reliably dated Barykov flora of the AKSR. It is most likely Santonian–early Campanian in age.
From the preceeding it would appear that in the OCSR floras of two different types coexisted in the late Albian – Campanian time. The Dukchanda, Chingandzha, Arman’ and Ust’-Emuneret floras, dominated by angiosperms, are similar to co-eval floras of the AKSR and NASR. The Amka, Chauna and Ola floras are quite different, being characterized by a combination of relict Early Cretaceous ferns and gymnosperms together with rare advanced conifers and angiosperms, typical of the Late Cretaceous.