13 New Discoveries in Resolving the Terrane Assembly of Western North American and Northeast Asia

Tuesday, 12 May 2015
1:30 PM - 5:30 PM
Lecture Hall

AN OVERLOOKED PERMIAN UNIT IN THE CRAIG QUADRANGLE, ALEXANDER TERRANE, SE ALASKA

Paper number: 13-5
Program: Theme Sessions
Time: 2:35 PM – 2:55 PM

Authors

Abstract

Buddington and Chapin, (1929) noted that, “In the Ketchikan district the known Permian is exposed only on the small narrow peninsula at the extreme eastern side of Suemez Island. These beds belong to the upper division of the Permian and are described by Chapin as consisting of about 300 feet of closely folded blue and gray limestone, which appears to have unconformable relations to the older formations”. Girty identified 20 taxa from a collection of fossils made by Chapin from Suemez Island. In 1961, W.H. Condon mapped these Permian rocks as the only Permian strata in the Craig Quadrangle, but in 1970, Eberlein and Churkin omitted the location from their geologic map.

In our reconnaissance of the coastal Permian section, we found that the unit strikes approximately parallel to the shoreline and dips steeply to the southwest. It is dominated by rhythmically bedded carbonates that contain lenses of skeletal debris often deposited in cross-bedding. We observed large, abundant brachiopods in one heavily silicified shell bed which included Horridonia, Waagenoconcha, Linoproductus, Meekella, Rhynchonopora, Cleiothyridina, Neospirifer, Timaniella (or Fasciculatia), Spiriferellina, and Dielasma. Together these brachiopods suggest a Middle Permian age. A bryozoan, Tabulipora aff. T. colvillensis indicates a Wordian (Middle Permian) age. Samples processed for conodonts were barren and yielded only a few dermal scutes. Our limited collection appears to be a different fauna than the fossiliferous Permian beds (Halleck and Pybus Formations) on Kuiu Island. This unnamed unit is lithologically distinct from the other Permian formations in the Alexander Terrane.

Southeastern Alaska is not known as an oil-producing area, but the upper Paleozoic conodonts indicate low thermal alteration, and the limestones produce an oil slick when etched in acid. Oil slicks are also noted from the acidization of Middle Devonian limestones in the San Alberto Islands north of Craig.