|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
|Phytostratigraphy||Images of fossils characteristic of the Type Taphoflora|
The Kaolak Phase of Floral Evolution
Attributed to the Kaolak phase are taphofloras from lithologic Unit 7 of the Kuk-Kaolak section at the Kaolak River and an impoverished taphoflora from the Seabee Formation exposed along the Chandler River in the Umiat-Chandler area. The taphoflora from lithologic Unit 7 is designated the type flora of the phase that is named after the river where the type flora originates and the stratotype of the Kaolak phytostratigraphic horizon is established.
The boundary between the Niakogon and Kaolak phases can be determined in sections of the Kuk-Kaolak and Umiat-Chandler areas. The taphoflora from Unit 7 of the Kuk-Kaolak section is of Turonian age judging from a considerable similarity between it and the Turonian Penzhina flora of the Anadyr–Koryak subregion (Herman, 2004a, 2011a). In the Kuk-Kaolak area, there is no evidence of a hiatus between plant-bearing deposits of lithological Units 6 (containing plant fossils of the Niakogon phase) and 7 (containing plant fossils of the Kaolak phase), as is done in the scheme by Smiley (1966). Although geological mapping, examination of sections, and paleobotanical data do not suggest a stratigraphic hiatus here, a small (beyond the "resolution ability" of paleobotanical method) unconformity between lithological Units 6 and 7 might exist. It is most likely to occur somewhere within the upper Cenomanian and corresponds to a stratigraphic and angular unconformity observed in the Umiat-Chandler area (Smiley, 1969a). Consequently, the boundary between the Niakogon and Kaolak phases is between lithologic Units 6 and 7 of the possibly continuous Kuk-Kaolak section, i.e., approximately at the level of the Cenomanian–Turonian boundary.
In the Umiat-Chandler area, a stratigraphic and angular unconformity is observable between the Ninuluk and Chandler formations (the latter with taphofloral assemblages of the Niakogon phase in its upper part) and the Seabee Formation containing a taphoflora of the Kaolak phase (Detterman et al., 1963; Smiley, 1972а, 1972b). In the opinion of Smiley (1969a), this hiatus corresponds to the upper Cenomanian, although it would be more correct to suggest that it spans just a part of this stratigraphic interval. The Seabee Formation age range corresponds to the entire Turonian stage (probably including the latest Cenomanian), and the Seabee taphoflora is, consequently, of Turonian age. Thus, the Kaolak phase lower boundary in the Umiat-Chandler area is also close to the the Cenomanian–Turonian boundary. The time span of the Kaolak phase with the currently available Turonian taphofloras is estimated to be about 4.5 m.y.
The flora of the Kaolak phase is 5 to 6 times less diverse than the preceeding flora of the Niakogon phase. Plant taxa of the Kaolak flora are listed in the panel to the right. To some extent (maybe to a considerable one), this dissimilarity can be explained by the fewer number of the Kaolak flora assemblages compared to those of the Niakogon flora. This is largely due to the paucity of non-marine rocks of Turonian age preserved in the NASR due a regional transgression during that time. It can yet be discounted, however, that the Kaolak flora may actually be less diverse than the Niakogon.
Ferns of the Kaolak flora are represented by two species only, with Birisia ochotica being the most frequent. Conifers are also not diverse with only four species being present. Among these Cephalotaxopsis intermedia is the most widespread. Much more diverse are the angiosperms represented by at least 15 species belonging to 12 genera. Dominant in this group are the platanoids, frequently large-leaved, represented by genera Ettingshausenia, Arthollia, Credneria, Pseudoprotophyllum, and two or three species of the genus Trochodendroides, which commonly occur in association with representatives of the genus Menispermites. Leaf impressions of other angiosperms are rare, and only one entire-margined leaf with a coriaceous texture belonging to Leguminosites has been found. The Kaolak flora lacks ginkgoaleans and cycadophytes, the characteristic components of the older Niakogon flora. In general, its distinctive features are as follows: the much lower diversity overall, insignificant occurrences of ferns and conifers, a prevalence of angiosperms in terms of numbers of taxa (two thirds of all the plants) and the abundance of leaf impressions, a nearly complete absence of typical Early Cretaceous plants except for Podozamites (?) sp., a predominance of platanoids and Trochodendroides leaves among the remains of dicotyledonous plants, and, to-date, a sole representative of entire-margined angiosperm leaves.