Pekul'nei Ridge (Western Slope)

Lithology and Stratigraphy

Towards the south of the Tylpegyrgynai Mountains and in the Afon’kina River basin, on the western slopes of the northern Pekul'nei Range, Upper Cretaceous deposits belonging to the non-marine plant-bearing deposits of the Tylpegyrgynai Formation are widely exposed. These form the lowermost part of the Upper Cretaceous succession in this area. At the base of the Tylpegyrgenai Formation is an erosional unconformable contact with the underlying marine Pekulneyveem Formation which includes volcanics of Volgian (approximately equivalent to Tithonian) to Valanginian age (see panel below right). The Tylpegyrgynai Formation is 1000 - 1100 m thick and is divided into three subformations (Terekhova and Filippova, 1984).

The Lower Subformation, 300 - 400 m thick, is largely composed of sandstones interbedded with siltstones, mudstones and tuffs. The Middle Subformation, 400 - 600 m thick, is mainly composed of siltstones interbedded with sandstones, later deposits being characterised by the presence of white tuffs.

The Upper Subformation, 100 - 300 m thick, consists of interbedded volcanic lavas, breccias and tuffs, with subordinate layers of tuffaceous sandstones, siltstones and mudstones. All three subformations contain plant fossils, but foliar remains are particularly abundant in the Middle Subformation. The overall succession is indicative of a floodplain environment subject to increasing volcanic influence.

Overlying the Tylpegyrgynai Formation is the Yanranay Formation, which has an overall thickness of 700 - 800 m, and is divided into two subformations (Terekhova and Filippova, 1984). The Lower Subformation is 250 - 300 m thick and is made up of tuffaceous sandstones with lenses of tuff and inter-bedded layers of siltstones and mudstones.The Upper Subformation, 400 - 500 m thick, is primarily composed of interbedded tuffaceous sandstones, siltstones and tuffs.

Among the marine molluscs found in the Yanranay Formation are Inoceramus naumanni Yok., Parallelodon sachalinensis Schmidt, Variamussium sp., Limatula sp., Terebratulina sp., Neopuzosia ? cf. ishikawai (Jimbo), together with remains of echinoids, crinoids and crabs. This fossil fauna has been correlated with the Inoceramus yokoyamai zone, indicative of a Late Coniacian to Early Santonian age (Terekhova and Filippova, 1984).

The upper age limit of the Tylpegyrgynai Formation is constrained by the overlying inoceramid-bearing marine deposits of the Yanranay Formation (Late Coniacian to Early Santonian) and the age of the contained flora is considered to be Early Coniacian. Because of the erosional contact at the base and the unconformable relationship with the underlying Pekulneyveem Formation of Valanginian age the lower age limit on the Tylpegyrgynai Formation is poorly constrained.

Map showing positions of the Tylpegyrgynai floral localities.

Lithology and stratigraphy of rocks yielding the Typegyrgynai Flora

The Tylpegyrgynai Flora

Plant fossil remains are found in all three sub-formations of the Tylpegyrgynai Formation. The Tylpegyrgynai Flora is dominated in terms of species diversity by angiosperms (40%) followed by conifers and ferns. Other plant groups such as cycadophytes, ginkgoales, sphenophytes, lycopods and probable liverworts, are less diverse and less abundant.

Large-leaved platanoids such as Arthollia, Pseudoprotophyllum and Paraprotophyllum are the most abundant of the angiosperms. Smaller-leaved Trochodendroides and Zizyphus are also well represented. Other flowering plant genera include Menispermites, Araliaephyllum, Leguminosites, Dalbergites, Celastrinites, 'Viburnum' (Viburniphyllum), Grewiopsis and Terechovia.

Ferns are found throughout the formation but are not particularly abundant. They typically include Coniopteris, Osmunda sp., Ochotopteris sp. and the large-leaved Hausmannia bipartita Samyl. et Shczep. The cycadophyte Nilssonia occurs in all subformations, but Ctenis is absent. Ginkgoales are only represented by Ginkgo ex gr. adiantoides. Czekanowskiales are present and, despite the fact that only isolated linear leaves of the form genus Desmiophyllum have been found, Filippova suggested that these might belong to the genus Phoenicopsis (Terekhova and Filippova, 1984). The most abundant conifer remains are Cephalotaxopsis and Sequoia, with Thuja cretacea, Elatocladus, Pityophyllum and Metasequoia being less common.

The Yanranay Flora

Although marine, the Yanranay Formation has yielded a small number of fossil plants in association with the molluscs. These include Gingko ex gr. adiantoides (Unger) Heer, Nilssonia sp., Cephalotaxopsis intermedia Hollick, Sequoia reichenbachii (Gein.) Heer, Sequoia cf. fastigiata (Sternb.) Heer, Thuja cretacea (Heer) Newb., Elatocladus sp., Quereuxia angulata (Newberry) Krysht. and Dicotylophyllum sp.