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The upper Mesozoic fossils from the Nabesna district in the eastern Alaska Range are listed below from top to bottom according to the stratigraphic scheme shown on page 3 of the transmittal statement. These collections were examined by me instead of David Jones because they happened to be sent here directly from Alaska, but will all be sent to Dave at Menlo Park for further examination. The collections there already include the Lower Cretaceous Buchias collected by Moffit and others (see Imlay and Reeside, 1954, GSA Bull. v. 65, p. 235). The Jurassic Buchia collections from the eastern Alaska Range (Imlay, 1952, GSA Bull. v. 63, p. 981) are here in Washington, but will also be sent to Menlo Park, because some are from your map area and some may actually be of Cretaceous age. It was a pleasure to examine these collections. |
| Comment: | The Valanginian stage of the Lower Cretaceous is represented by Buchia sublaevis Imlay (non Keyserling) at four localities and by Buchia pacifica Jeletzky at one locality. Your descriptions show that three occurrences of B. sublaevis are in the upper half of about 5000 feet of argillite mudstone, the occurrence of B. sublaevis is many thousands of feet lower. The last occurrence is shown on your map near a fault and at a considerable distance from all the other occurrences. It is possibly, therefore, not as low in the sequence as indicated. The age range of Buchia pacifica Jeletzky in California, Oregon and western British Columbia, based on studies of associated ammonites recently completed by David Jones and me, is late early to early middle Valanginian. The age range of Buchia sublaevis of Imlay is probably late middle to late Valanginian. I described the species from northern Alaska (Prof. Paper 335, p. 49, pl. 8, figs. 1-15) as B. sublaevis (Keyserling) and assigned it a probable early Valanginian age on the basis of its supposed occurrence below B. crassicollis (Keyserling). Subsequently the species was recorded by Jeletzky from northeastern British Columbia and Arctic Canada as B. cf. B. <>pacifica Jeletzky (1965, Canada Geol. Survey Bol. 103, p. 48, also fig. 2 opposite p. 2) and as B. n. sp. aff. inflata (Toula) (Jeletzky, 1964, Canada Geol. Survey Paper 64-11, pl. 11, figs. 4, 8, 9, pl. 12, figs. 3, 5, 7, 9, pl. 13, figs. 1, 2, 4, 5). Jeletzky recorded it within beds characterized by the late Valanginian ammonite Homolsomites quatsinoensis (Whiteaves) as well as in older beds of middle Valanginian age. (Jeletkzy, 1964, Bol. 103, table 1), but did not find it in the same sequence as B. pacifica and had no evidence that it was as old as that species. The record of B. sublaevis of Imlay in the same sequence with B. pacifica in the Nabesna area is important, therefore, as it is the first such record and establishes the stratigraphic succession of the two species. Furthermore, it is probably associated with B. crassicollis solida (Lahusen) at locality 68ARh284, which species in California and Oregon occurs at the very top of the Valanginian. |