Silurian rocks in Alaska have been identified in 12 accreted terranes and in the Tatonduk-Nation River area of east-central Alaska, which represents part of autochthonous North America. Most of the terranes are in situ or structurally imbricated portions of the North American (or Siberian) continental margin. An exception is the Alexander terrane of southeastern Alaska, which originated as an offshore island arc. Discontinuously exposed and (or) highly altered sequences have precluded detailed investigations of Silurian rocks in most parts of Alaska, but reconnaissance-level studies reveal that graptolitic shales of turbidite or hemipelagic origin record deep-water or “shale out” conditions west or north of the ancient continental margin of North America. Platform carbonates are also exposed in many areas and are particularly well represented in southwestern (Nixon Fork subterrane of Farewell terrane) and southeastern (Alexander terrane) Alaska, indicating that much of Alaska resided close to the paleoequator in the Silurian. Subtidal stromatolite reefs in southwestern and southeastern Alaska that are similar to those in Salair and the Ural Mountains, Russia, indicate a paleobiogeographic connection between these two parts of Alaska, Siberia, and eastern Baltica via the Uralian Seaway in the Late Silurian. Deposition of vast accumulations of red beds and other siliciclastic rocks beginning in the Late Silurian suggests that parts of Alaska may have been affected by late stages in Cale-donian orogenesis and (or) early stages in the Ellesmere orogeny during formation of the Laurussian landmass.

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.