Paleozoic carbonate strata deposited in shallow platform to off-platform settings occur across the Seward Peninsula and range from unmetamorphosed Ordovician–Devonian(?) rocks of the York succession in the west to highly deformed and metamorphosed Cambrian–Devonian units of the Nome Complex in the east. Faunal and lithologic correlations indicate that early Paleozoic strata in the two areas formed as part of a single carbonate platform.
The York succession makes up part of the York terrane and consists of Ordovician, lesser Silurian, and limited, possibly Devonian rocks. Shallow-water facies predominate, but subordinate graptolitic shale and calcareous turbidites accumulated in deeper water, intraplatform basin environments, chiefly during the Middle Ordovician. Lower Ordovician strata are mainly lime mudstone and peloid-intraclast grainstone deposited in a deepening upward regime; noncarbonate detritus is abundant in lower parts of the section. Upper Ordovician and Silurian rocks include carbonate mudstone, skeletal wackestone, and coral-stromatoporoid biostromes that are commonly dolomitic and accumulated in warm, shallow to very shallow settings with locally restricted circulation.
The rest of the York terrane is mainly Ordovician and older, variously deformed and metamorphosed carbonate and siliciclastic rocks intruded by early Cambrian (and younger?) metagabbros. Older (Neoproterozoic–Cambrian) parts of these units are chiefly turbidites and may have been basement for the carbonate platform facies of the York succession; younger, shallow- and deep-water strata likely represent previously unrecognized parts of the York succession and its offshore equivalents. Intensely deformed and altered Mississippian carbonate strata crop out in a small area at the western edge of the terrane.
Metacarbonate rocks form all or part of several units within the blueschistand greenschist-facies Nome Complex. The Layered sequence includes mafic metaigneous rocks and associated calcareous metaturbidites of Ordovician age as well as shallow-water Silurian dolostones. Scattered metacarbonate rocks are chiefly Cambrian, Ordovician, Silurian, and Devonian dolostones that formed in shallow, warmwater settings with locally restricted circulation and marbles of less constrained Paleozoic age. Carbonate metaturbidites occur on the northeast and southeast coasts and yield mainly Silurian and lesser Ordovician and Devonian conodonts; the northern succession also includes debris flows with meter-scale clasts and an argillite interval with Late Ordovician graptolites and lenses of radiolarian chert. Mafic igneous rocks at least partly of Early Devonian age are common in the southern succession.
Carbonate rocks on Seward Peninsula experienced a range of deformational and thermal histories equivalent to those documented in the Brooks Range. Conodont color alteration indices (CAIs) from Seward Peninsula, like those from the Brooks Range, define distinct thermal provinces that likely reflect structural burial. Penetratively deformed high-pressure metamorphic rocks of the Nome Complex (CAIs ≥5) correspond to rocks of the Schist belt in the southern Brooks Range; both record subduction during early stages of the Jurassic–Cretaceous Brooks Range orogeny. Weakly metamorphosed to unmetamorphosed strata of the York terrane (CAIs mainly 2–5), like Brooks Range rocks in the Central belt and structural allochthons to the north, experienced moderate to shallow burial during the main phase of the Brooks Range orogeny. The nature of the contact between the York terrane and the Nome Complex is uncertain; it may be a thrust fault, an extensional surface, or a thrust fault later reactivated as an extensional fault.
Lithofacies and biofacies data indicate that, in spite of their divergent Mesozoic histories, rocks of the York terrane and protoliths of the Nome Complex formed as part of the same lower Paleozoic carbonate platform. Stratigraphies in both areas feature Lower Ordovician and mid-Silurian shallow-water deposits with some deeper water facies of late Early to Middle Ordovician age. Most significantly, Ordovician conodont faunas in both successions contain a characteristic, distinctive mixture of Laurentian and Siberian-Alaskan endemic forms.
Lithologic and faunal resemblances also link Seward Peninsula platform strata with coeval successions in the Brooks Range and in interior Alaska (Farewell and White Mountains terranes) and imply that all of these rocks were once part of a single carbonate platform situated between Laurentia, Siberia, and Baltica. Little is known about the basement on which Alaskan platform strata formed, and correlations between Cambrian and older rocks in these areas remain tentative. Similarities between strata and fossils in northern and interior Alaska are strongest during the Ordovician, and diminish by Middle Devonian; correlations between Seward Peninsula and Brooks Range rocks, however, extend into the Carboniferous. Ordovician mafic volcanism in the Nome Complex and the White Mountains terrane could reflect a rifting episode that began to separate platform rocks of the interior from those of Arctic Alaska.
Lower Paleozoic off-platform successions on Seward Peninsula also correlate well with equivalent sections in northern and interior Alaska, and have some similarities with strata in southeast Alaska (Alexander terrane). Silurian (mainly Wenlock–Ludlow) mass flow deposits derived at least in part from a carbonate source overlie condensed graptolitic shales in most of these successions; this coeval influx of calcareous detritus suggests a common tectonic cause.
