ORIGIN OF DEVONIAN REEF-RELATED DEPOSITS ON "HARLEQUIN DUCK ROCK," ALEXANDER TERRANE, SOUTHEASTERN ALASKA.

BROGENSKI, Colleen B., and SOJA, Constance M., Geol. Dept., Colgate Univ., Hamilton, NY 13346.
cbrogenski@center.colgate.edu

Lower-Middle Devonian deposits belonging to the Wadleigh Formation are exposed in southeastern Alaska (Alexander terrane) on a small island informally named "Harlequin Duck Rock" (HDR). Carbonates on HDR were previously interpreted as an in situ reef based on reconnaissance work by the U.S.G.S. However, recent study shows that two major lithologic units are exposed on the island, neither of which represents an in situ reef. The lower 5 meters of section are thin to medium bedded lime mudstones interbedded with skeletal wackestones/packstones with an abundance of ang-ular clasts and low-diversity fossil assemblages. These limestones comprise fine- grained fossil hash, including ostracods, gastropods, and radiolaria with rare trilobite debris, crinoids, corals, and scattered brachiopods. This abund-ance of muddy sediment and plankton is typical of foreslope environments.

The overlying 13.5 meters constitute thick, unbedded skeletal packstones and boundstones with massive stromatoporoids, corals, crinoids, Renalcis, other faunas, abundant "peloids," and clasts. Comparisons with other Devonian reefs indicate that the upper section is composed of reefal biotas. However, while the fossil assemblages may have formed a reef, there is no evidence of a wave-resistant framework. Thus this upper unit represents the mass slumping and downslope transport of a reef from the shelf margin to the foreslope environ-ment. Lack of exposure of lateral and vertical facies precludes determining if the disturbance that caused the mass slumping was a storm, earthquake, or gravitational collapse related to oversteepening of the platform margin.

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