Chauna Region

The Chauna Group, in the northern part of the OChSR, consists of a large volume of ignimbrites and andesitic basalts and tuffs (see below right). These rocks contain abundant plant fossils comprising the Chauna Flora (sometimes referred to as the Chauna Floral Assemblage, Tchaunia-Kolymella Flora, or Palyavaam Flora) which has no direct equivalents within Northeastern Russia. Its age and correlation with fossil floras from other subregions of Northeastern Russia and Alaska has traditionally, therefore, represented a great enigma in the regional phytostratigraphy.

The Chauna Group consists of five formations (Belyi, 1977):

(1) Alkakvun Formation, 1000 – 1200 m thick, made up of ignimbrites and rhyolitic tuffs with lenses of tuffaceous conglomerates and sandstones.

(2) Kalenmuvaam Formation, 800 – 900 m thick, consisting of andesite-dacite ignimbrites and lavas, and tuffs.

(3) Pykarvaam Formation, 50 – 1000 m thick, composed of rhyolitic ignimbrites.

(4) Voron’ina Formation, 50 – 550 m thick, made up of ignimbrites of dacite and andesite-dacite, acid and moderately-acid tuffs, tuffaceous sandstones, siltstones, mudstones, shales, carbonaceous mudstones and coals.

(5) Koekvun Formation, 550 m thick, consisting of andesite-basalts, andesites, rare basalts and dacites, together with reworked volcaniclastics.

The Chauna Group overlies Triassic and Valanginian deposits of marine origin and coal-bearing deposits of supposedly Aptian age, and is overlain by volcanigenic rocks (Ergyvaam, Emuneret and Enmyvaam Formations) that contain taphofloras of Ust-Emuneret and Ola ( = Arkagala) types (Belyi, 1977; Lebedev, 1987). In the Chineyveem River (left bank of the Anadyr River) conglomerates that overlie ignimbrites probably belonging to the Ergyvaam Formation contain the inoceramid and ammonite remains Inoceramus sp. cf. I. hobetsensis var. nonsulcatus Nagao et Matsumoto, I. sp. cf. I. patootensis Sokolov, Parallelodon sp. cf. P. sachalinensis Schmidt, Gaudryceras sp., Scaphites sp. indet., Hypophylloceras sp. indet., and Neopuzosia sp. indet. (Belyi, 1977; Lebedev, 1987) of late Turonian - early Senonian (up to early Campanian) age.

The five formations of the Chauna Group cannot be distinguished on the basis of their individual floral assemblages which collectively represent a single floral type (Samylina, 1974; Fillipova, 1975; Yefimova in Belyi, 1977; Shczepetov, 1991; Belyi, 1997). Nevertheless, all workers emphasised significant peculiarities, specifically the presence of endemic ferns (Tchaunia, Kolymella, etc.), large-leaved cycadophytes (Ctenis, Heilungia), czekanowskialeans (Phoenicopsis), numerous advanced ‘Cenophytic’ conifers, and an extreme rarity of angiosperms. The same flora persisted throughout the period of deposition of the Chauna Group and relatively advanced angiosperms (such as Quereuxia and Trochodendroides) occur even in the lower formations implying that the Chauna sediments were deposited quite quickly in geological terms.

Samylina (1974, 1988) suggested that the Chauna flora reflects vegetation that existed on the volcanic plateau during a period of intensive volcanic activity and surface uplift, and that it represents a type of Cretaceous vegetation not normally preserved. Exceptional preservation was due to rapid burial in volcaniclastics. While we agree with this interpretation we also suggest that disturbance, edaphic factors, and climate all contributed to the floral peculiarities.

Map of the area yielding the Chauna flora showing positions of areas where the taphofloras have been recovered.

Stratigraphy and principal lithologies of the Chauna Group, N.E. Russia, (Kelley et al., 1999).

The age of this flora has stirred considerable debate. Samylina (1974, 1988) supposed that it was late Albian based on the presence of both Early (Tchaunia, Kolymella, Cladophlebis grandis Samylina, C. tchaunensis Samylina, C. tchuktchorum Philippova, Sphenobaiera, Ctenis, Heilungia, Phoenicopsis, Elatocladus zheltovskii Philippova, Araucarites subacutens Philippova etc.) and Late Cretaceous forms (Picea, Sequoia cf. reichenbachii, Menispermites, Zizyphoides etc.). Belyi (1977, 1997) correlated the Chauna flora with floral Zone II of the Chandler River sequence in northern Alaska (Smiley, 1969a) suggesting a late Albian age for both, while Filippova (1975) considered the Chauna flora to be Late Cretaceous and drew attention to the presence of relatively advanced conifers and angiosperms. Shczepetov (1991) initially implied a Cenomanian and early Turonian age but later (Shczepetov, 1995) defined the age of the Chauna flora as Early-Late Cretaceous.

Rb-Sr dating of the volcanigenic rocks of the Chauna Group provided internally consistent data:

(1) Alkakvun Formation: 100 ± 4.4 Ma (lower part, Belyi, pers. comm. to A.B. Herman, 1994), 95 ± 6 Ma (6 point isochron, 87Rb/86Sr range 1.2 – 5.6, initial 87Sr/ 86Sr=0.70398 ± 0.00019, Milov et al., (1987)

(2) Pykarvaam and Voron’ina formations: 94 ± 7 Ma (7 point isochron, 87Rb/86Sr range 0.15 – 4.9, initial 87Sr/86Sr=0.70525 ± 0.00044, Milov et al., (1987)

(3) Koekvun Formation: 87 ± 6 Ma (6 point isochron, 87Rb/86Sr range 0.29 – 4.9, initial 87Sr/ 86Sr=0.70580 ± 0.00039, Milov et al., (1987)

but these ages were at variance with the paleontological evidence, particularly the presence of angiosperms and some advanced conifers known only from the mid Late Cretaceous elsewhere.

More recently Kelley et al. (1999) using laser ablation single crystal 40Ar/39Ar analaysis of amphiboles and plagioclases returned ages of 87.0 ± 2.0, 87.2 ± 2.4, 87.9 ± 1.0 and 88.7 ± 1.0 Ma over a 300 - 350 m interval representing the upper three formations of the Chauna Group. The close clustering of ages suggests the eruption of a large volume of material over a short interval of time around 88 Ma. Although they did not date the lower parts of the Chauna Group the similarity of the flora suggests no great differences in age and overall the Chauna Flora should be considered as Turonian - Campanian and not Albian - Cenomanian as previously thought.