Paper No. 2
Presentation Time: 9:00 AM-6:30 PM


ROHR, David1, KARL, Susan M.2, BLODGETT, Robert B.3, BAICHTAL, James F.4 and MEASURES, Elizabeth1, (1)Earth and Physical Sciences, Sul Ross State Univ, Alpine, TX 79832, (2)USGS, 4210 University Dr, Anchorage, AK 99508-4626, (3)Geological Consultant, 2821 Kingfisher Drive, Anchorage, AK 99502, (4)U.S. Forest Service, Tongass National Forest, Thorne Bay Ranger District, P.O. Box 19001, Thorne Bay, AK 99919,

Paleozoic rocks on Prince of Wales Island make up part of the Alexander terrane , one of the major tectonic elements of southeastern Alaska. Silurian rocks in the Alexander Terrane include a very thick section of shallow-water shelfal limestone and basinal clastics from Prince of Wales Island north to Glacier Bay. The underlying Ordovician rocks of the Descon Formation, however, contain only rare exposures of limestone.

The Breccia of Luck Creek was informally proposed by Eberlein and others in 1983 for andesitic breccia that intertongues with the Descon Formation. The Luck Creek is considered to be Upper Ordovician to early Silurian in age, based on a previous K-Ar hornblende age of 440 Ma, and fossils from small limestone lenses that are Late Ordovician (Edenian-Richmondian) in age.

Near Thorne Bay a debris flow within Descon argillite contains interbedded limestone, dark gray mudstone, and intraformational conglomerate. The matrix of the conglomerate contains the coral Reushia previously identified by W. Oliver as probably late Middle or Late Ordovician. Reushia is only known from here, the Seward Peninsula and the Altai Mountains in Asia. In addition, a clast in the conglomerate contains a coiled cephalopod, a trimerellid brachiopod, and a single specimen of the gastropod Daidia, identified by us. Daidia occurs in ancestral North America, but this and the Seward Peninsula are the only know occurrences outside Laurentia.

The post-Ordovician Karheen Formation contains uncommon limestone clasts. One large limestone clast from north of Thorne Bay contained a shallow-water fauna of abundant “sphinctozoan” sponges, tabulate corals, stromatoporoids, gastropods (operculum of Maclurites and Tropidodiscus), and the brachiopod Catazyga which indicate an Ordovician age for that clast. Other skeletal limestone clasts at the locality did not yield diagnostic fossils.

Although Ordovician limestone clasts were reworked from a shallow-water source, we find no evidence of a larger carbonate platform in southeast Alaska. We conclude that the Ordovician limestones were only present as patch reefs around small volcanic islands in the southeast Alaska part of the Alexander Terrane.