Island Arc Carbonates from the Silurian Heceta Formation of Southeastern Alaska (Alexander Terrane)

Abstract

ABSTRACT The Heceta Formation of Early to Late Silurian age forms part of a well-preserved Paleozoic island arc sequence that is exposed within the Alexander terrane of southeastern Alaska. Limestones and limestone breccias predominate within the formation and are interbedded with polymictic conglomerates and volcaniclastic sandstones. These deposits conformably overlie the Lower Ordovician-Lower Silurian Descon Formation, which is characterized by volcaniclastic wackes and mudstones that accumulated as turbidites in a deep-water setting. Interbedded with the turbidite deposits are basalt-andesite pillow flows and volcanic breccias that formed during incipient stages of island arc construction. The limestones within the Heceta Formation record the earliest occurrence of widespread carbonate deposition within the arc. Although previously described as having only a shallow marine origin, the limestones can be differentiated into shallow-water platform, platform margin, and slope facies. Shallow-water deposits comprise a diversity of skeletal grains that are preserved as micritized fragments associated with cyanobacterial encrustations. They are also characterized by low-diversity brachiopod communities, Amphipora, and oncolites that have a predominance of dasycladacean alga nuclei. These units reflect restricted, quiet areas of deposition that were affected by intermittent turbulence on the platform. Stromatolitic buildups and stromatoporoid reefs constructed at the pla form margin generally are not preserved in situ. They occur predominantly as slumps that initiated carbonate debris flows and deformed the underlying slope deposits during periodic collapse of the platform margin. Graded skeletal grainstones containing shelly debris of shallow-water origin and finely laminated lime mudstones represent turbidites that were affected by synsedimentary deformation in a slope environment, as indicated by the convoluted bedding, slump folds, microfaults, and truncation surfaces. Rapid facies changes, shallowing- and deepening-upwards sequences, and thick polymictic conglomerates record phases of tectonism within the arc that periodically disrupted carbonate deposition. The predominance of cyanobacteria in the limestones suggests that these organisms were largely responsible for stabilizing the substratum by constructing stromatolitic buildups and encrusting skeletal debris during early stages in carbonate platform development and following periods of tectonic disturbance within the arc.