Geologic Map of Alaska: Metaturbidite marble and calcareous schist

Metaturbidite marble and calcareous schist

Unit symbol:	DCmt
Age range	Devonian to Cambrian (541 to 358.9 Ma)
Group name:	Metaturbidite marble and calcareous schist (Nome Complex)

Consists of three units of Till and others (2011). First, their unit DCbm is dark-gray to black marble and subordinate impure fissile marble, calcareous schist, and mafic schist that is best exposed in sea cliffs along Norton Bay and in rubble-covered hills inland. Marble is in layers 1 to 20 cm thick and has rhythmically alternating purer, coarse-crystalline and more impure, fine-crystalline layers. Green-weathering, fine-grained chlorite, actinolite, albite, and white mica assemblages that Till and others (2011) interpreted to be mafic dikes, sills, and plugs intruding the carbonate rocks are common. Glaucophane inclusions are found in the albite. Till and others (2011) report that contact metamorphic effects are preserved, including bleached carbonate rocks and skarn assemblages. Mafic minerals also commonly form layers or are disseminated in the fine crystalline carbonate rocks, suggesting that some mafic volcanism accompanied deposition of the carbonate strata. Seven conodont faunas were obtained from six localities in this subunit. Two faunas are middle to early late Silurian, one is middle Early Devonian, and one is late Silurian to Early Devonian. Two faunas from westernmost exposures are considerably older: middle Early through Late Ordovician and Early Cambrian. Thus, there is a gap of over 100 million years between the oldest and youngest faunas; Till and others (2011) infer a number of scenarios to explain the gap, possibly involving limited collection and therefore missing intervals, hiatus, and (or) reworking of older units. The second of Till and others’ units (DObm) that we include in this unit consists of dark-gray to black metalimestone and marble and subordinate dolostone exposed in sea cliffs on Kotzebue Sound. “A 15- to 20-m-thick interval of dominantly matrix-supported carbonate breccia, with rounded and angular clasts as much as 5 m in diameter, occurs in the section, as well as thinner ( 1 m thick) intervals of carbonate-clast breccia. Local solution collapse features occur, and dedolomitization textures were seen in thin sections. Subordinate argillite, phyllite, and radiolarian chert are found about 2.4 km west of Cape Deceit; quartz-graphite schist and impure marble (containing as much as 20% graphite, quartz, albite, and white mica) are abundant in the western exposures” (Till and others, 2011). This subunit “has yielded tightly dated fossil collections of Middle Ordovician through late Silurian age and some longer ranging collections that could be as young as Devonian (Ryherd and Paris, 1987). The argillite-dominated interval west of Cape Deceit contains abundant Middle and Late Ordovician graptolite assemblages, as well as Ordovician conodonts (Ryherd and Paris, 1987; Harris and others, 1995; Ryherd and others, 1995; Dumoulin and others, 2002). Higher in the unit, a continuous section of allodapic carbonate rocks, at least several hundred meters thick, produced a middle to late Silurian (Wenlock to Ludlow) conodont succession (Dumoulin and others, 2002). Several conodont collections from eastern exposures of the unit could be as young as Devonian” (Till and others, 2011). Subunit three (Till and others’ [2011] unit D_ks) is a dark-brownish-gray, rust-spotted, well-foliated, medium-grained schist composed predominantly of quartz, calcite, white mica, chlorite, plagioclase, and graphite. It locally shows millimeter-scale dark to light layering. It is interlayered on a meter to kilometer scale with the other subunits upon which its age was assigned

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	DCbm
Unit name	Black marble, Nome Group
Description	Dark gray to black marble and subordinate fissile impure marble, calcareous schist, and mafic schist that crops out in the eastern Solomon and western Norton Bay quadrangles. The unit is best exposed in sea cliffs along Norton Bay and forms rubble covered hills inland. Marble occurs in layers 1 to 20 cm thick, with rhythmic alternations of purer, coarse-crystalline and more impure, fine-crystalline layers. Common green-weathering mafic dikes, sills, and plugs, 0.5 to 2 m across, intrude the carbonate strata; preserved contact metamorphic effects include bleached carbonate rocks and albite-, chlorite-, epidote- and tourmaline-bearing skarn assemblages. Mafic rocks consist of fine-grained chlorite, actinolite, albite, and white mica; glaucophane inclusions are found in the albite. Commonly, mafic minerals form layers and are disseminated in the fine crystalline carbonate rock, suggesting that some mafic volcanism accompanied deposition of the carbonate strata. All of the mafic volcanic rocks were affected by regional metamorphism, as were the carbonate rocks. Marble protoliths were likely carbonate turbidites and periplatform ooze. Seven conodont faunas were obtained from six localities in DCbm (Table A-1). Two faunas are middle to early late Silurian (Wenlock-Ludlow), one is middle Early Devonian (Siegenian), and one is late Silurian to Early Devonian. Two faunas from westernmost exposures of DCbm are considerably older: one is middle Early through Late Ordovician and the other is Early Cambrian. There is thus a gap of over 100 million years between the oldest and youngest faunas obtained from this unit. At least three explanations can be invoked for this faunal distribution. If all conodont collections represent depositional ages, the unit may represent two episodes of deposition of lithologically similar material-one of Early Cambrian age, one of Late Ordovician-Early Devonian age, separated by a long interval of non-deposition. Or, deposition may have extended more or less continuously from Cambrian into Devonian time-present fossil collections from DCbm are few and future collections may eliminate the apparent hiatus. Alternately, some or all of the conodont collections could be reworked, and the depositional age of the entire unit could be late Silurian-Early Devonian or younger. In this case, the western faunas would represent material eroded from an older (Cambrian and Ordovician) carbonate platform source; the eastern faunas could represent erosion from a younger source or faunas contemporaneous with deposition. DCbm may be a more deformed equivalent of DObm, and it is intercalated with, and possibly a facies equivalent of, DCks. It is lithologically similar to, and may be partly coeval with, Silurian carbonate and siliciclastic turbidites in the west-central Brooks Range (map unit "Spl" of Till and others, 2008b). The easternmost exposures of DCbm include some outcrops of Ddm that are too small to be mapped separately. Equivalent to "DCbm" of Till and others (1986), and partly equivalent to "Dld" of Miller and others (1972) and "Pzcs", "PzZus", and "PzZss" of Patton and others (2005)
Lithology	Metamorphic

Correlated geologic units

Label	DCbm
Description	Black marble, Nome Complex
Geologic age	Cambrian to Devonian
Geologic setting	Sedimentary, carbonate

Lithology	Form	Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic		Major