Carbonate Platform Evolution in a Silurian Oceanic Island: A Case Study from Alaska's Alexander Terrane

Abstract

ABSTRACT Silurian limestones exposed in the Alexander terrane of Alaska are the oldest carbonates of wide distribution in the region and represent the evolution of a shallow-marine platform in an island-arc setting during waning volcanism and the onset of orogenesis. Four main stages in carbonate platform development are recognized, beginning with calcareous turbidites that accumulated on a volcaniclastic ramp in a slope environment. Overlying deposits record the formation of a metazoan-dominated fringing reef and of backreef sites that were affected by cyclic changes in water depth and sedimentation patterns. Following the earliest effects of orogenesis and widespread progradation of conglomerates, small bioherms and biostromes grew in shallow subtidal settings on the rejuvenated platform. Co temporaneously, a consortium of microbial organisms associated with accessory metazoans, principally sphinctozoans, created a rimmed carbonate shelf with the construction of stromatolite 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 deep-marine slope. Carbonate sedimentation was terminated within the arc as a result of uplift, shoaling, and progradation of a clastic wedge during culminating phases of the Klakas progeny in the Late Silurian-Early Devonian. This island-arc suite is characterized by extraordinarily thick platform and periplatform carbonates, sequential evolution of fringing and barrier reefs, and patterns of faunal t rnover that differentiate these deposits from coeval carbonates that formed under different 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 oceanic settings.