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Reference
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| Title: |
Report on Referred Fossils
,
1970
(12/09)
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Collections upon which this report is based are from a 2,300-foot measured section of the Tugidak Formation. The base of the measured section is at the extreme southwest end of the island; the top of the section is near the middle of the northwest coast of the island. This material supplements three earlier collections from Tugidak made by George Moore during 1962 and 1963. These collections were identified by F.S. MacNeil (A-62-8M and A-63-44M). Moore's stratigraphically lowest collection (M1493) is equivalent to M4393 of this report, his intermediate collection (M1895) is equivalent to M4401, and his stratigraphically highest collection is from the (M1494) stratigraphical interval between M4402 and M4403 of this report. |
| Report by: |
Warren O. Addicott
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| Referred by: |
George Plafker
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| | Age: | Pliocene |
| Formation: | Tugidak Formation |
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| Comment: | Comments: The new collections from Tugidak Island indicate that Chlamys tugidakensis extends from the base of the exposed section almost to the uppermost strata. Previously it was known to occur only in the uppermost one-third of the Tugidak Formation. The biozone of C. tugidakensis has been regarded as indicative of a pre-Quaternary, late Pliocene age in the Gulf of Alaska (MacNeil, 1967, p. 32; Hopkins, 1967, p. 60). Although most of the species in the present collections are still living in the North Pacific and the Arctic there are several extinct mollusks in the material collected so far from the Tugidak. The occurrence of a significant element of the extinct mollusks can be taken as further evidence for a pre-Quaternary age. Among the extinct taxa are species of Chlamys, Neptunea n. sp. (or n. spp.), Volutopsius aff. V. stefanssoni, and Astarte hemicymata (the latter two species are represented only in the earlier collections from Tugidak). There is a notable degree of faunal similarity between the collections from Tugidak and those from Middleton Island. Approximately half of the taxa of this report are also represented in recent collections from Middleton (A-70-22M). The Middleton Island fauna differs in having a much greater diversity of species of Astarte, Chlamys, and Neptunea. There are more species of the pelecypod Macoma in the Tugidak collections as well as a far greater diversity of minute gastropods, particularly in the older collections. The later may be a reflection of more favorable conditions of preservation or possibly differences in sampling techniques. According to the representation of presumably extinct species in both faunal sequences, it would appear that the two are approximately equivalent in age. Representatives of the Chlamys islandica group of MacNeil first appear in the upper parts of both stratigraphic sections and the extinct species C (Leochlamys) tugidakensis ranges from the base to near the top of both sections. Mollusks in the Tugidak fauna that are still living in northern seas are characteristic of the sublittoral or neritic zone. Most of these have been dredged from the Arctic coast of Alaska in water depths of from about 20 to 80 fathoms. |
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