|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
|Phytostratigraphy||Images of fossils characteristic of the Type Taphoflora|
The Early Kogosukruk Phase of Floral Evolution
Characteristic of the Early Kogosukruk phase of floral evolution in the NASR are taphofloras from the Barrow Trail Member of the Schrader Bluff Formation and from the Kogosukruk Tongue equivalent to the lower part of the Prince Creek Formation in the Umiat-Chandler area. Plant-bearing deposits of the phase are termed the Beds with the early Kogosukruk flora; the type flora is known from that part of the Kogosukruk Tongue exposed in the lower reaches of the Chandler River near its confluence with the Colville River (Smiley, 1969a). The marine Barrow Trail Member contains the inoceramids Inoceramus patootensis de Loriol and I. steenstrupi de Loriol and is regarded to be late Santonian–early Campanian in age (Jones and Gryc, 1960; Detterman et al., 1963; Pergament, 1978).
The lower part of the Kogosukruk Tongue grades along strike into marine deposits of the Sentinel Hill Member of the Schrader Bluff Formation (Smiley, 1969a) corresponding in range to the late Santonian (?)–Campanian, or most likely to the Campanian only (Detterman et al., 1963; Frederiksen and McIntyre, 2000). Therefore the flora of the early Kogosukruk phase is late Santonian–Campanian in age. The approximate duration of the phase is 8 – 9 m.y.
Plant taxa characteristic of the flora are listed in the panel to the right. The Early Kogosukruk flora is not diverse in taxonomic aspect (20 species), and the diversity decrease relative to the previous Kaolak phase is especially obvious among the angiosperms. Of note is that the early Kogosukruk floras contain frequent occurrences of the aquatic angiosperm Quereuxia angulata. Ferns of the flora belong to the genera Gleichenites, Cladophlebis, and Kolymella; conifers to Elatocladus, Sequoia, Parataxodium, and Cupressinocladus; angiosperms to Monocotylophyllum, Quereuxia, Paraprotophyllum, Trochodendroides, and Dicotylophyllum (? Sapindophyllum). Prevailing in some assemblages are impressions of Equisetites sp., Kolymella aff. raevskii and Sequoia fastigiata.
It is remarkable as well that leaf impressions of terrestrial (non-aquatic) angiosperms, including platanoids and Trochodendroides, have been found in only two localities in the lower part of the Kogosukruk Tongue and in the Barrow Trail Member. In contrast to the older Tuluvak flora, flora of the Early Kogosukruk phase is only half as diverse, containing relatively rare angiosperms, terrestrial taxa which occur sporadically, and in comparison to angiosperms, more abundant conifers. Although in age the Early Kogosukruk flora is in part the analog of the Barykov flora in the Anadyr–Koryak Subregion, they are dissimilar in terms of taxonomic composition.