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., Werdon, M.B., and Bleick, H.A., 2010, Preliminary bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2009-1254, 1 pamphlet, 57 p., 2 plates, scale 1:500,000, and database.
Symbol DOx
Unit name Mixed marble, graphitic metaquartzite, and schist, Nome Group
Description Interlayered pure and impure marble, graphitic metasiliceous rock, pelitic schist, calc-schist, and mafic schist. Gray and orange weathering marble and dark gray-black weathering graphitic metasiliceous rock are the most common lithologies in the unit, which is dominated locally by one or the other. Gray-weathering pure marble forms rounded ridgelines which may stretch along strike for several kilometers, and rounded hills of slabby, black graphitic metasiliceous rock can be recognized from great distances. The unit is defined by its position structurally below the Casadepaga schist (unit Ocs). Good exposures are rare; minor lithologies generally do not crop out. Lithologies thicken and thin along strike on a scale of kilometers, a feature which 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. Everywhere in those areas, 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; these lithologies are interlayered on scale of meters and decameters. The uppermost lithology is commonly an orange-weathering chlorite marble that forms stepped outcrops. The total thickness of the mixed schist and marble package varies from 250 meters to over 2 kilometers. 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. Metabasites are found in both the mixed schist and marble package and within the thick, unit-dominating gray marble and black metasiliceous rock. They are boudins or layers of glaucophane-, epidote-, and garnet-bearing metabasite, or chlorite-, albite-, actinolite-bearing metabasite similar to those found in the Casadepaga schist. They occur in greatest volume in DOx south of Salmon Lake near the boundary between the Nome and Solomon quadrangles. The gray-weathering pure marble is pale gray to white on the fresh surface, and composed of coarse crystalline calcite. The graphitic metasiliceous rock is generally homogeneous, dark-gray or black, and compositionally limited to quartz, graphite, and very small amounts of white mica, albite, and chlorite. Graphite may be present in sufficient quantities to rub off on the hands. Locally a centimeter-thick banding of dark gray-black quartz-graphite schist and gray-black quartz-graphite-calcite schist are found. In the Nome quadrangle, DOx contains several small rubble patches of carbonate conglomerate, as well as rubble and outcrops of light gray dolostone, locally mottled with orange, pink, or light brown, that retains relict sedimentary features. Conglomerate is matrix supported and contains rounded to angular clasts, as much as 8 cm long, of pale-orange to medium-gray dolostone; at one locality, subordinate clasts of marble and quartz-white mica schist also occur. The matrix is beige to orange pink or light brown dolomite with lesser quartz and white mica. Relict textures, seen in conglomerate clasts and in mottled dolostone rubble, include coated grains, crinoid ossicles, and possible brachiopod fragments. In the northwestern Solomon quadrangle, DOx contains an interval of dolostone-clast conglomerate as much as 100 to 200 m thick and 480 m in lateral extent. The conglomerate ranges from matrix-supported (1 to 10 percent clasts) to clast-supported (>80 percent clasts) and contains interlayers and (or) lenses of clast-free schist and dolostone. Clasts are rounded to irregular; most have a flattened, ovoidal shape but some are rod-like. Sorting is poor; clasts range from a few millimeters to 70 cm in maximum dimension. Some clasts are finely laminated. Most clasts consist of light-gray- to rusty-weathering, very light gray, fine-grained ferric dolostone with minor amounts of quartz, white mica, and calcite. About 1 to 5 percent of clasts are medium light gray marble. The conglomerate matrix is quartz schist that also contains white mica, dolomite, chlorite, epidote, and chloritoid. A less extensive (several meters thick by 10 m long) but otherwise similar lens of dolostone-clast conglomerate occurs in Ocs in the Nome quadrangle. The dolostone clasts at both localities resemble, and may have been derived from, dolostone equivalent to that in unit _d. 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 (Table A-1, T. Carr and T. Hudson, written commun., 1982, 1984). 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). The part of the unit in the Teller quadrangle shown with the diagonal line overlay is distinct in age and lithology. Dolostone, dolomitic marble, and marble form an elongate belt, extending for almost 40 km, in the westernmost part of DOx in the eastern Teller quadrangle; only the southern half of the belt has been examined. The unit forms rubble-covered hills with rare outcrops. Dolostone is medium gray to dark gray and weathers light to medium gray, very pale orange, or dark yellow brown. Sedimentary structures include millimeter- to centimeter-scale parallel lamination and lesser color mottling that likely reflects bioturbation. Intraclasts (maximum 1 cm in diameter) and millimeter-scale burrows occur locally. In thin section, dolostone is mostly finely crystalline and non-ferroan; some samples contain minor amounts of fine-grained quartz and white mica, or rare clasts (bioclasts?). Marble is white to grayish black and has few relict sedimentary features other than locally well-developed parallel lamination. Conodonts of late Silurian-Devonian age have been recovered from two localities; a third locality produced a fauna of Silurian (late Llandovery-Ludlow) age (Table A-1). 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 in the Nome Complex. Correlation with units Ddm and SOdl and SOul (York terrane) is also possible, although these units contain more abundant megafossils than DOx (overlay). Shallow-water Silurian rocks also occur widely in the Brooks Range. Units that contain such strata and could correlate, at least in part, with DOx (overlay) include "DOb" (Baird Group), "DOc", and "DSc" of Till and others (2008b); Silurian lithofacies in "DSc" are an especially good match. DOx with overlay is equivalent to parts of "Pzm" and "pCn" of Sainsbury (1972). Three detrital zircon samples have been collected from DOx in 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 (Amato, written commun., 2008). Apparently, at least part of DOx must be Devonian or younger. On the eastern edge of the map area, on the boundary of the Norton Bay and Candle quadrangles, exposures of metasedimentary rocks include calcareous, pelitic, and quartz-rich lithologies. These rocks have not been studied in detail; some may ultimately prove to have histories separate from the Nome Complex. These rocks are equivalent to units "PzZus" and "MzPzq" of Patton and others (2005). DOx is between 0.8-1.5 km thick, and best exposed in the eastern Solomon D-5 quadrangle on the ridge to the northwest of upper Birch and Auburn Creeks, and in east-central Solomon D-6 quadrangle on the ridge top just south of Nelson Creek. The unit includes parts of "pCs" and "slate of the York region" of Sainsbury (1974)
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