Nome Complex, undivided

Unit symbol: PzPxnc
Age range early Paleozoic to Proterozoic (1000 to 358.9 Ma)
Lithology: Metamorphic
Group name: Nome Complex, undivided
A wide variety of largely metasedimentary rocks and some metaigneous rocks was renamed the Nome Complex from the earlier named Nome Group because the unit does not represent related lithostratigraphic units, but rather a structurally deformed assemblage on the Seward Peninsula (Till and others, 2011). In addition to common metamorphic features, the units of the Nome Complex also display common structural features (Till and others, 2011). Till and others (2011) recognized three main components in the Nome Complex; the dominant one they called the “layered sequence,” which consistts of mappable lithologic units that occur in a consistent, layer-cake structural relation to one another over much of central Seward Peninsula. The layered sequence is represented here by units |ncs, DOnx, Ocs, Onim, and <ngn. A subunit of granitic orthogneiss in the layered sequence is included in unit Dogn and a subunit of felsic schist is included in unit Das because of their clear correlation with those other map units in northern Alaska. Unit |np of the layered sequence is separated here into what Till and others (2011) called the “low-grade layered sequence” because it is of significantly lower metamorphic grade than other rocks of the layered sequence. A second component, called the “scattered metacarbonate rocks” (Till and others, 2011) consists of volumetrically minor dolostone and marble that are widely distributed on the Peninsula and are included here in the undivided Nome Complex. The final component, named “metaturbidites” (Till and others, 2011) is exposed only on the north and southeast coast of the Seward Peninsula and is here included in unit D_mt. The descriptions here are largely abstracted from Till and others (2011), but some units described by Till and others (2011) have been grouped here, and, as mentioned above, some have been included with correlative rocks that outcrop elsewhere in northern Alaska. Analysis of available data by Till and others (2011) for the Nome Complex and for correlative rocks in the Brooks Range and Ruby geanticline suggests that the blueschist metamorphic event that affected the Nome Complex occurred during the Late Jurassic. Subsequent lower-pressure greenschist metamorphic-facies assemblages overprint the blueschist assemblages; the best estimate for the timing of the greenschist episode is during the middle Cretaceous (A.B. Till, oral commun., 2012). In this undivided Nome Complex unit, we have included Till and others’ (2011) units Pznm, Pzm, Ddm, Sd, Od, Cd, and PzPxm. Their map units lumped together here, all carbonates of different ages, are typically light- to medium-gray, fine-grained marble and dolostone and coarsely crystalline marble, and also include medium- to dark-gray-weathering, dark-gray to black dolostone, metalimestone and marble and minor associated chert. The unit is widely distributed on the Seward Peninsula and in the western parts of the adjacent Candle and Norton Bay quadrangles. Fossils ranging in age from Early Cambrian to Late Devonian have been collected from these carbonate rocks; one conodont collection in the eastern Bendeleben quadrangle may be Early Mississippian in age (Till and others, 2011). Available data suggest that rocks of Devonian age form the dominant part of unit, but a number of the subunits defined by Till and others (2011) lack specific age control and are presumed to be Paleozoic in age. Well-defined conodont collections and megafauna definitively indicate the presence of Devonian, Silurian, and Ordovician rock units, and all suggest shallow-water warm-water biofacies, although at least one collection, of middle to late Early Ordovician age, indicates a cooler and (or) deeper water setting (Till and others, 2011). The limited exposures of Cambrian dolostone and more widely exposed black marble (in unit D_mt) in the Nome Complex “contain lapworthellids, a phosphatic microfossil indicative of Early (to possible early Middle) Cambrian age and a shallow-water depositional environment” (Till and others, 2011) Subdivided into three broad categories described below: a low-grade layered sequence, a layered sequence, and metaturbidites

Source map information

Source map Till, A.B., Dumoulin, J.A., Gamble, B.M., Kaufman, D.S., and Carroll, P.I., 1986, Preliminary geologic map and fossil data, Solomon, Bendeleben, and southern Kotzebue quadrangles, Seward Peninsula, Alaska: U.S. Geological Survey Open-File Report 86-276, scale 1:250,000.
Symbol Ddm
Unit name Dolostone and marble
Description Black to dark gray dolostone and marble with relict sedimentary structures and megafossils
Lithology Metamorphic

Correlated geologic units

Label Dnd
Description Nome Complex dolostone, metalimestone, and marble
Geologic age Devonian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Calcareous Major
Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major