The Karheen Formation of the Alexander Terrane of Southeast Alaska: Its Age Reinterpretation (Late Silurian) and Faunal Alliance with Other Equivalent Red Bed Successions in Northeast Asia (Kolyma Region)

Blodgett, Robert B.; Baranov, Valeryi V.; Boucot, A.J.; and Rohr, David M.
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The Karheen Formation was established by Eberlein and Churkin (1970) for a succession up to 1,800 m of sandstones, conglomerates, and shale of red-bed affinity, which they assigned to the late Silurian to Early Devonian. It is one of the key marker units of the Alexander terrane, representing a prominent clastic wedge sequence which resulted from the Klakas orogeny. The formation (or its equivalents) can be traced from near the south end of Prince of Wales Island northward to Glacier Bay. A marked decrease in clast size is noted along the western side of Prince of Wales Island, clast size decreasing northward from its source area in the south, with much finer grained beds recognized to the north on Kosciusko Island where shallow-marine, intertidal deposits with a restricted, low-diversity fauna of brachiopods, gastropods, and leperditiid ostracodes are present (Ovenshine, 1975). The more coarse-grained conglomeratic phase exposed to the south suggests subaerial deposition. The formation is equivalent to the late Silurian age “sandstone and graywacke” unit of Buddington and Chapin (1929). One of the primary reasons Eberlein and Churkin included it in the Early Devonian was the recognition of Pragian conodonts and brachiopods in a limestone exposure thought to be within the Karheen Formation on northeast Heceta Island.

However, our field study of the Heceta Island locality indicates that the Pragian limestones there represent a structurally discordant, downfaulted block of the overlying Devonian Wadleigh Limestone, and not an interbedded limestone succession within the Karheen Formation. The brachiopods from this locality also occur in limestones on Ham Island (USGS Silurian-Devonian locality 2038-SD), mapped as Wadleigh Limestone (Eberlein and Churkin, 1970). A few other putative Early Devonian fossil ages from the Karheen (i.e., Noyes Island) are from exposures which do not have clastic wedge affinity (in some instances consisting of even graptolitic shales), and should be assigned to as yet unnamed superjacent units. The faunal affinities of late Silurian and Early Devonian Alexander terrane brachiopods are closest with those of Northeast Asia (Kolyma region) and suggest close proximity at this time. It has been earlier postulated that the Alexander terrane was derived from the Uralian Seaway (near Baltica) based on faunal similarity and detrital zircon ages. A better match is indicated with Northeast Asia due to its much greater faunal resemblance (sharing many of the same co-occurring late Silurian and Early Devonian brachiopod species, most of which are unknown in the Urals), similar coeval redbed sequences in Northeast Asia such as the Mirninskaya and Artykhyuryakhskaya Suites, as well as new unreported detrital zircon ages for the upper Silurian Mirninskaya Suite closely matching those of the Karheen Formation.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013