The Heceta Formation of southeastern Alaska (Alexander terrane) comprises a 3000-m-thick limestone-siliciclastic deposit of Early–Late Silurian age. The limestones record the first widespread evidence of carbonate platform development in this ancient island arc. Interbedded polymictic conglomerates represent interruption in platform evolution during onset of the Klakas orogeny, an arc-continent collisional event that occurred in the Late Silurian–Early Devonian. Conglomerates grade upward into finer-grained siliciclastics capped by shallow-marine limestones in sequences that are 200–300 m thick. Clasts range in diameter from 2 to 30 cm, are subangular to well rounded, poorly to moderately sorted, and densely packed in disorganized, poorly stratified beds. Most of the clasts are volcanic (basaltic-andesitic), but limestone clasts predominate in some sections; rare fragments of volcaniclastic, plutonic, and indeterminate rocks also occur. Clast compositions match the lithology of rocks in the underlying Heceta and Descon formations, and sedimentary attributes indicate redeposition of recycled material by debris flows and rivers in a coastal alluvial fan complex. This evidence—together with affinities of marine fossils, paleomagnetic and detrital zircon data, associated Old Red Sandstone-like facies, and coincidence in timing of tectonism—suggests the Klakas orogeny was a Caledonide event that is manifest in Alaska's Alexander terrane.