CARBONATE PLATFORM EVOLUTION IN A SILURIAN OCEANIC ISLAND - A CASE-STUDY FROM ALASKA ALEXANDER TERRANE

Silurian limestones exposed in the Alexander terrane of Alaska are the oldest carbonates of wide distribution in the region and represent th e evolution of a shallow-marine platform in an island-are setting duri ng waning volcanism and the onset of orogenesis. Four main stages in c arbonate platform development are recognized, beginning with calcareou s turbidites that accumulated on a volcaniclastic ramp in a slope envi ronment. Overlying deposits record the formation of a metazoan-dominat ed fringing reef and of backreef sites that were affected by cyclic ch anges in water depth and sedimentation patterns. Following the earlies t effects of orogenesis and widespread progradation of conglomerates, small bioherms and biostromes grew in shallow subtidal settings on the rejuvenated platform. Contemporaneously, a consortium of microbial or ganisms associated with accessory metazoans, principally sphinctozoans , created a rimmed carbonate shelf with the construction of stromatoli te reefs at the seaward edge of the platform. During platform drowning , downslope transport from the platform margin resulted in significant accumulations of reefal material as debris flows and slumps along a d eep-marine slope. Carbonate sedimentation was terminated within the ar e as a result of uplift, shoaling, and progradation of a elastic wedge during culminating phases of the Klakas orogeny in the Late Silurian- Early Devonian. This island-are suite is characterized by extraordinar ily thick platform and periplatform carbonates, sequential evolution o f fringing and barrier reefs, and patterns of faunal turnover that dif ferentiate these deposits from coeval carbonates that formed under dif ferent tectonic conditions. A model for carbonate deposition in island arcs, involving high rates of sediment accumulation, steep submarine slopes, crustal instability, and biogeographic isolation, contributes new data about carbonate platform development in tectonically active o ceanic settings.