|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
|Phytostratigraphy||Images of fossils characteristic of the Type Taphoflora|
The Kukpowruk Phase of Floral Evolution
The phase is exemplified by the Kukpowruk and Kokolik taphofloras, floral assemblages from western localities of the NASR (the Corwin Bluff, Pitmegea River, and Punak Creek outcrops in the Utukok-Corwin area), taphofloras from lithological Units 1 – 4 of the Kuk-Kaolak section, and by the Lower Killik taphoflora from the Umiat-Chandler area. The type flora of the phase is that from the Kukpowruk River basin (Spicer and Herman, 2001; Herman and Spicer, 2002).
The Lower boundary of the Kukpowruk phase is positioned based on relationships between plant-bearing beds and marine deposits in three areas of the NASR. In the Utukok-Corwin area the middle Albian Paragastroplites spiekeri McLearn found in the upper part of the Kukpowruk Formation (Chapman and Sable, 1960), suggests that the lower age limit of the Kukpowruk taphoflora corresponds to the early(?) – middle Albian. In the Kuk-Kaolak area, Entolium sp., found in the upper part of lithologic Unit 1, points to a middle Albian age of host deposits (Imlay, 1961, cit. after Smiley, 1966), and consequently the taphoflora from this unit is not older than the middle Albian. In the Umiat-Chandler area marine beds of the Tuktu Formation conformably underlying plant-bearing deposits contain Gastroplites, Cleoniceras, Inoceramus anglicus Woods and Inoceramus cf. I. cadottensis McLearn (Detterman et al., 1963; Smiley, 1969a) and correspond, therefore, to the basal middle Albian. Consequently, the lower age limit of the Kukpowruk phase flora is middle (maybe lower in one case) Albian in all three localities according to its stratigraphic position relative to beds with marine fauna.
The upper age limit of the Kukpowruk phase corresponds most likely to the late Albian, the second half of the substage exclusively, because plant-bearing deposits of the phase are conformably overlain everywhere by beds containing plant remains of the Niakogon phase, and the Niakogan floral phase is very close in composition to the late Albian–Cenomanian–early Turonian Grebenka flora confidently dated in the Anadyr–Koryak subregion. The duration of the Kukpowruk phase is approximately 8 – 9 m.y.
The flora of the Kukpowruk phase is very diverse, consisting of more than a hundred species (Spicer and Herman, 2001; Herman and Spicer, 2002) of liverworts (?), horsetails, ferns, cycadophytes, czekanowskialeans, caytonialeans (?), conifers, gymnosperm incertae sedis, angiosperms, and plant remains of unclear taxonomic affinity. Plant taxa characteristic of the flora are listed in the panel to the right.
Remains, presumably belonging to liverworts, are represented by the genera Thallites, Mirella, and Marchantites (?), which are not abundant and occur relatively rarely. Horsetails belong to frequently occurring representatives of Equisetites ex gr. burejensis, which are widespread in the Jurassic and Cretaceous floras of North Asia, and to impressions of Equisetites sp. unidentifiable at the species level. Abundant and diverse ferns represent one of the dominant groups of the flora. Ferns include the genera Osmunda, Gleichenites (?), Birisia, Coniopteris, Arctopteris, Onychiopsis, Asplenium, Ruffordia, Tchaunia, Cladophlebis, and Sphenopteris. Representatives of Birisia and Arctopteris are most frequent, and their dominant species are Birisia alata and Arctopteris rarinervis. Similarly abundant are species of the genera Coniopteris (especially C. yukonensis), Cladophlebis (primarily C. virginensis) and Onychiopsis. Other ferns are represented in the Kukpowruk flora by less abundant impressions.
Cycadophytes of the Kukpowruk flora are less abundant than ferns, although still quite diverse. The identified genera of cycadophytes are Pseudocycas, Nilssonia (several species), Heilungia, Butefia, Taeniopteris, Ctenis, Jacutiella (?), and Anomozamites. The most abundant species in this group belong to the genus Nilssonia (first of all N. magnifolia, N. decursiva, and N. menneri) and Taeniopteris lundgreni. Ginkgoaleans are represented by genera Ginkgo, Baiera, and Sphenobaiera, impressions of which are commonly not frequent, except for Ginkgo ex gr. adiantoides and Sphenobaiera ex gr. czekanowskiana. Among czekanowskialeans there are two Phoenicopsis species, Czekanowskia ex gr. rigida, and Leptostrobus marginatus. A single problematic impression of Sagenopteris representing caytonialeans is poorly preserved and cannot be confidently assigned.
Conifers of the Kukpowruk flora are diverse and quite abundant in many localities. Genera identified in this group are Podozamites (several species), Pagiophyllum, Elatides, Elatocladus, Araucarites, Florinia (?), Pityophyllum (two or three species), Pityostrobus, presumable Pityolepis, Abietites, Cephalotaxopsis, Sequoia, Taiwania, Athrotaxopsis, and Parataxodium. Abundant isolated leaves and leafy twigs of Podozamites occur practically in all the localities; Pityophyllum and sometimes Pagiophyllum impressions are also fairly abundant, whereas remains of other conifers are considerably less frequent. Large elongated, commonly linear leaves with parallel venation are classed with Desmiophyllum, the form genus of gymnosperms of unclear taxonomic affinity. Judging from their morphology, they belong mostly to species D. magnum, while the others likely represent another species and are assigned to Desmiophyllum sp.
Single leaf impressions of angiosperms are known from all distribution areas of the Kukpowruk taphoflora. Besides their rarity, it is remarkable that these angiosperms with small- to medium-sized leaves are represented by different taxa in separate areas: by Nelumbites in the Kuk-Kaolak area, Araliaephyllum in the Umiat-Chandler area, and by Vitiphyllum multifidum Fontaine and Dicotylophyllum sp. in the Utukok-Corwin area. Collections of Smiley, Spicer and Herman also include specimens of reproductive structures of unclear taxonomic affinity. In this work, they are classed with Kenella filatovii, Stenorachis striolatus, genus Carpolithes, and three taxa lacking formal names.