Lower Devonian platform carbonates from Kasaan Island, southeastern Alaska, Alexander Terrane

Abstract

Facies representing shallow subtidal conditions, an open lagoon or shelf, offshore biostromal banks, and a restricted lagoon or shelf form the Lower Devonian (Emsian) sequence of rocks exposed on Kasaan Island in southeastern Alaska. This complex of carbonate environments developed in an island arc, which is now preserved within the accretionary Alexander Terrane. The lower part of the platform sequence represents the restricted shallow subtidal zone and consists of rhyolitic tuffs and lime mud-stones and wackestones containing low-diversity assemblages of leperditiid ostracodes and loxonematid-like gastropods. Following the end of volcanic activity, dendritic corals and branching stromatoporoids colonized the outer, deeper portions of the subtidal zone. Higher in the sequence, boundstones consisting of massive stromatoporoids are associated with brachiopod packstones that together represent the growth of biostromal colonies at the seaward edge of the platform and the diversification of normal marine biota within a lagoon or shelf. Restricted circulation eventually developed across vast portions of the lagoon or shelf, which resulted in the expansive growth of Amphipora and the decline in all other invertebrate populations.The sequence preserved in the rocks on Kasaan Island records a period of waning volcanic activity within the arc, which is marked by the relatively thick cover of lime sediments that accumulated on an evolving carbonate platform. The apparent lack of coeval limestones that exhibit identical stratigraphic successions in other parts of the Alexander Terrane probably reflects the evolution of isolated carbonate platforms adjacent to submarine or subaerial topographic highs that developed locally within the arc during or before the Early Devonian.