Marble, graphitic rocks, and schist

Unit symbol: DOnx
Age range Devonian to Ordovician? (485.4 to 358.9 Ma)
Lithology: Metamorphic
Group name: Layered sequence (Nome Complex)
Interlayered pure and impure marble, graphitic metasiliceous rock, pelitic schist, calc-schist, and mafic schist, unit DOx of Till and others (2011) is defined, in part, by a position structurally below the Casadepaga Schist (unit Ocs, here). The most common rock types are gray- and orange-weathering, pale-gray to white, coarse crystalline marble forming rounded ridges that may extend along strike for several kilometers, and generally homogeneous, dark gray-black-weathering graphitic metasiliceous rock forming rounded hills that can be recognized from great distances (Till and others, 2011). Good exposures are rare; minor lithologies generally do not crop out. Lithologic units vary in thickness along strike on a scale of kilometers, a characteristic that may be depositional as well as structural. “In the western Solomon quadrangle and Nome quadrangle, there is a consistent general stacking pattern of lithologies within DOx. The structurally upper part of the unit is composed of mixed schist and marble, including pelitic schist, gray marble, orange-weathering impure marble, black schistose marble, and black metasiliceous rock * * * interlayered on scale of meters and decameters. The uppermost lithology is commonly an orange-weathering chlorite marble * * *. The structurally lower parts of the unit are dominated by gray marble or black metasiliceous rock. Where the gray marble is dominant, it reaches thicknesses of 1–2 km and contains minor thin (less than 50 meters) layers of metaquartzite, pelitic schist, and chlorite-albite schist. Where the black metasiliceous rock is dominant, it reaches thicknesses of around 500 meters. It is underlain by 10–30 meters of gray marble interlayered with thin bands of pelitic schist” (Till and others, 2011). Metabasite that consists of boudins or layers of glaucophane-epidote-garnet bearing metabasite, or chlorite-albite-actinolite-bearing metabasite, similar to that found in the Casadepaga Schist (unit Ocs), is found in both the mixed schist and marble package and within the gray marble and black metasiliceous rock. “The age range of DOx is not strictly known. Conodonts of Ordovician age were obtained from relatively pure marble in the Solomon quadrangle; marble in the Nome quadrangle produced conodonts of early Paleozoic age. * * * Recrystallized radiolarians collected in the northern Darby Mountains in banded calcite-bearing graphitic metasiliceous rock are of probable pre-Devonian age (B.K. Holdsworth, written commun., 1985)” (Till and others, 2011). In the Teller quadrangle “* * * conodonts of late Silurian-Devonian age have been recovered from two localities; a third locality produced a fauna of Silurian (late Llandovery-Ludlow) age. Sedimentary structures and conodont biofacies suggest a warm, shallow-water depositional setting. Faunal and lithofacies data indicate that these rocks may correlate, at least in part, with unit Sd [unit |ncm, here] in the Nome Complex * * *. Shallow-water Silurian rocks also occur widely in the Brooks Range” (Till and others, 2011). Three detrital zircon samples have been collected from the unit on the southern Seward Peninsula. One yielded largely Neoproterozoic zircons, another yielded zircon populations as young as Silurian, and the third yielded a robust population of Middle and Early Devonian zircons (J.M. Amato, Univ. of New Mexico, written commun., 2008); apparently part of this unit must be Devonian or younger (Till and others, 2011)

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 OCx
Unit name Mixed unit
Description Interlayered pure and impure marble, quartz-graphite schist, and lesser pelite, calc-schist, and mafic schist
Lithology Metamorphic

Correlated geologic units

Label DOnx
Description Nome Complex, mixed marble, graphitic metaquartzite, and schist
Geologic age Ordovician to Devonian
Geologic setting Sedimentary, undivided
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Quartz-feldspar-schist < Schist < Metamorphic Indeterminate, major
Amphibole-schist < Schist < Metamorphic Minor
Calc-silicate-schist < Schist < Metamorphic Minor