|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
|Phytostratigraphy||Images of fossils characteristic of the Type Taphoflora|
The Niakogon Phase of Floral Evolution
Belonging to this phase are the Niakogon taphoflora of the Umiat-Chandler area, taphofloras from lithological units 5 and 6 of the Kuk-Kaolak area, and the Corwin taphoflora of the Utukok-Corwin area. The Niakogon taphoflora is more abundant and diverse than the others and is designated as the type flora of the phase named after the mostly non-marine Niakogon Tongue of the Nanushuk Formation. The other fossil floras of the phase are known from the uppermost lower and upper parts of the Killik Tongue, also part of the Nanushuk Formation. Therefore the stratotype of the Niakogon phytostratigraphic horizon extends from the lower part of the Killik Tongue to the Niakogon Tongue, inclusively.
The boundary between the Kukpowruk and Niakogon horizons (and between respective phases in flora evolution) is detectable in sections in the Umiat-Chandler, Kuk-Kaolak and Utukok-Corwin areas. In the Umiat-Chandler region, the boundary is defined in a continuous succession of the lower part of the Killik Tongue. Here the Lower Killik taphoflora belonging to the Kukpowruk phase is confined to the lower part of the Killik Tongue. Towards the top of the lower part of the Killik Tongue the Niakogon taphoflora (the type taphoflora of the Niakogon evolutionary floral phase) first occurs and subsequently extends through the upper part of the Killik Tongue and throughout the Niakogon Tongue (Spicer, 1983, 1987).
In the Utukok-Corwin area, the Kukpowruk/Niakogon phase boundary is traceable in a continuous section of the upper part of the Corwin Formation. Fossil plants of the Kukpowruk taphoflora are known from lower stratigraphic levels of the Kukpowruk and Corwin formations; the Corwin taphoflora belonging to the Niakogon phase comes from the upper beds of the latter (Spicer and Herman, 2001; Herman and Spicer, 2002). In the Kuk-Kaolak area, the boundary between Kukpowruk and Niakogon phases is within the unexposed or poorly exposed interval between lithological units 4 and 5, which is about 200 m thick (Smiley, 1966).
The Niakogon phase corresponds in age to the late Albian–Cenomanian based on the following observations. First, the Niakogon flora occurs in deposits conformably overlying plant-bearing beds with plant remains of the Kukpowruk phase of the middle to, presumably, late Albian (excluding the latest Albian). Second, fossil plants of the Niakogon phase have been found in association with Cenomanian inoceramids (Inoceramus dunveganensis MacLearn) in the Ninuluk Formation of the Umiat-Chandler area, which is the stratigraphic equivalent of the plant-bearing beds. Third, the Seabee Formation basal beds of marine origin, attributed to the uppermost Cenomanian and lower Turonian, overlie, in the Umiat-Chandler area, deposits that contain plant remains of the Niakogon phase. Finally, the flora of the Niakogon phase is very close to the late Albian–Cenomanian Grebenka flora of the Anadyr River basin and the middle Cenomanian Middle Ginter taphoflora from the Ugol'naya Bay, which are confidently dated in the Anadyr–Koryak subregion. The Niakogon phase lasted approximately 5 – 6 m.y.
Remains presumably belonging to liverworts are represented by scarce impressions of Thallites sp. and Marchantites ex gr. jimboi. Single impressions of lycopod shoots have been attributed by Smiley to the genus Lycopodium. Equisetites sp. exemplifies horsetails. Abundant and diverse ferns represent indispensable components of the Niakogon flora. The identified fern genera are Osmunda (?), Gleichenia, Gleichenites, Birisia, Coniopteris (Birisia ?), Arctopteris, Dennstaedtia, Kolymella, Onychiopsis, Asplenium, Cladophlebis, Ochotopteris, and Sphenopteris. The genera Gleichenites (five species), Birisia (four to five species), and Cladophlebis (three to four species) are most diverse in this group. Among these species the most abundant impressions are those of Gleichenia crenata, Gleichenites ex gr. zippei, Birisia jelisejevii, Onychiopsis psilotoides, Cladophlebis frigida, and Arctopteris penzhinensis. The species Gleichenites asiatica, Asplenium dicksonianum, Cladophlebis virginensis, and Ochotopteris ochotensis also occur frequently in some assemblages. Of special interest are Gleichenites cf. asiatica and Arctopteris penzhinensis that are unknown in older floras. Remains of other ferns are less abundant, and some occur as single specimens.
Cycadophytes, less abundant and diverse than ferns, are represented by two or three Nilssonia species and by the genera Taeniopteris and Ctenis; the Nilssonia forms occur more or less regularly. Of the ginkgoaleans Ginkgo is the most diverse and is represented by four species in the Niakogon flora, the most frequent impressions being Ginkgo ex gr. adiantoides. Representatives of the genus Sphenobaiera are not numerous. Czekanowskialeans are exemplified by rare impressions of Phoenicopsis ex gr. angustifolia. Remains of narrow linear leaves with parallel venation determined as Desmiophyllum possibly belong to ginkgoaleans (Sphenobaiera) or czekanowskialeans (Phoenicopsis). Single impressions of poorly preserved leaves have been tentatively attributed to Sagenopteris, the only genus of caytonialeans in the flora.
Conifers of the Niakogon flora are second after ferns in abundance and diversity. Their remains belong to the genera Elatocladus, Athrotaxopsis, Podozamites, Pagiophyllum, Araucarites, Pityophyllum, Pityostrobus, Cephalotaxopsis, Sequoia, Cryptomeria, Glyptostrobus, Torreya, Parataxodium, and Thuja. Especially diverse are genera Podozamites (several species) and Sequoia (five shoot species and cones). In terms of abundance, prevailing taxa of conifers are Cephalotaxopsis intermedia (dominant or subdominant taxon practically in all the assemblages), Podozamites ex gr. lanceolatus, and Podozamites sp. (isolated leaves); Sequoia remains are also frequent in some assemblages. Other conifers are rare, occurring only as single occasional specimens. Several reproductive structures of unclear taxonomic affinity are determined in this work as Stenorachis striolatus and Carpolithes.
Angiosperms represent the third group of plants characterising the Niakogon flora. In contrast to the older Kukpowruk phase containing scarce impressions of three or four angiosperm taxa, no less than 35 – 40 species of flowering plant remains have been identified in the Niakogon flora. Being abundant in all taphofloras of the phase, angiosperms belong to diverse genera such as “Magnolia”, Magnoliaephyllum (?), Menispermites, Nelumbites (?), “Smilax”, Diospiros, Ettingshausenia, Protophyllum, Pseudoprotophyllum, Credneria (?), Arthollia, Populites, Leguminosites (?), “Hedera”, Araliaephyllum, Dalbergites, Scheffleraephyllum, Celastrophyllum, Cissites, “Zizyphus”, Rhamnites, Viburniphyllum, Trochodendroides, Grebenkia, Dalembia, and Dicotylophyllum. In many assemblages, Menispermites sibiricus, M. septentrionalis, Pseudoprotophyllum boreale, and Scheffleraephyllum venustum make up the most abundant impressions. Characteristic components of the Niakogon flora are the entire-margined genera Magnoliaephyllum, Scheffleraephyllum, Celastrophyllum, Araliaephyllum, Leguminosites (?), and Dalbergites, together with compound forms (Leguminosites (?), Scheffleraephyllum venustum, Dalembia vachrameevii, and Dalbergites cf. simplex), and lobed (Menispermites sibiricus, M. septentrionalis, Dalembia vachrameevii, representatives of genera Araliaephyllum and Cissites) leaves and leaflets. Leaves of platanoids, also typical of the Niakogon phase taphofloras, are sometimes very large (genera Ettingshausenia, Protophyllum, Pseudoprotophyllum, Credneria (?) and Arthollia). There are also three or four species of the genus Menispermites.
This diversity means that the Niakogon phase is most diverse among all the Cretaceous floras of the NASR. In contrast to the Kukpowruk flora it includes an insignificant number of forms typical of the Early Cretaceous. Furthermore, its dominant plant taxa, primarily angiosperms that represent nearly one third of the Niakogon floral composition, are widespread in the Late Cretaceous floras of the North Pacific.