About This Item
Share This Item
In 1946 several specimens of the plant genus Annularia [cf. A. stellata (Schlotheim) Wood], a primitive horsetail of Pennsylvanian or Permian age, were found by Robert R. Coats of the U.S. Geological Survey in tuffaceous sandstone exposed near Andrew Lake, northern Adak Island, Alaska. Because of the geographic position of these upper Paleozoic fossils near the middle of the Aleutian insular chain, it has been a challenging if not a vexing problem to fit the enclosing rocks into a structural scheme for the development of the Aleutian ridge. In view of this, the enigmatic upper Paleozoic section was restudied during July 1968 to enlarge the fossil collection and to determine its depositional environment and stratigraphic setting.
The Annularia-bearing beds are associated with a sequence of sedimentary rocks that is more than 850 m thick, and that consists mainly of northwest dipping tuffaceous sandstone, siltstone, and shale, and siliceous and calcareous siltstone and shale interbedded with basaltic flows and/or penecontemporaneous sills a few tens of meters thick. Specimens of Annularia have been found only within the basal 5-10 m of this section, which lies with apparent depositional contact on the massive and intensely altered andesitic and basaltic flows and pyroclastic rocks of the Finger Bay Volcanics that form most of Atak Island. The top of the section is not exposed. The section is uncomplicated structurally and dips homoclinally northwestward between 40° and 65°; it is exposed across an are of about 2.5 km2 (approximately 1 sq mi).
Mollusks, Foraminifera, sponge spicules, fish scales, and skeletal remains were found by the writers in the lower 350 m of the section just above the basal Annularia-bearing beds. Included in this fauna is the mud pecten Propeamossium [cf. P. stanfordensis (Arnold)], indicating a probable Eocene age; the associated foraminiferal fauna is of definite Eocene (most likely late Eocene) age, and the fish scales are similar to those found in the Refugian and Narizian (Eocene-Oligocene) of California. The microfossils imply a paleobathymetry of 1000 meters or deeper. In consideration of these new findings, the rock matrix surrounding specimens of Annularia was searched for microfossils. A substantial dinoflagellate flora was found--establishing that the Annularia-bearing beds are themselves arine units of early Tertiary age. The paleontologic and stratigraphic significance of Annularia is now being reevaluated.
The Adak findings clearly establish that at least a portion of the structural framework of the Aleutian Ridge was in existence by early Tertiary time.
End_of_Article - Last_Page 459------------