Casadepaga Schist

Unit symbol: Ocs
Age range Ordovician (485.4 to 443.4 Ma)
Lithology: Metamorphic
Group name: Layered sequence (Nome Complex)
Light green, silvery green and greenish-brown mafic, feldspathic, and calcareous schist interpreted to have a largely igneous protolith. “Tors of metabasite, abundant plagioclase porphyroblasts in dark-green, chlorite-rich schist, and the quartz-poor nature of the rocks are characteristic of this unit” (Till and others, 2011). Smith (1910) provides illustrations that demonstrate characteristics that strongly suggest an intrusive igneous character of this unit. Dark-green-weathering schist is interpreted to be metamorphosed mafic rock and is rich in chlorite, epidote, actinolite, and plagioclase (Till and others, 2011). Medium- to pale-grayish-green-weathering pelitic schist is common; plagioclase, chlorite, white mica, and quartz are the dominant minerals, present in roughly equal amounts, and epidote, carbonate, and glaucophane (or pseudomorphs of chlorite and plagioclase after glaucophane) are also typical minerals of many of these schists (Till and others, 2011). Based on major-element chemistry, the protoliths of these pelitic schists were shale and graywacke (Werdon and others, 2005). Carbonate-rich schist layers are typically buff- or pale-brown-weathering and tend to be more recessive in outcrop than other lithologies. Pure carbonate layers are rare and thin and weather pale brown, black, or gray (Till and others, 2011). Greenish-black boudins, lenses, and layers of fine- to coarse-grained, massive metabasite in tors within the unit are composed of glaucophane, actinolite, chlorite, epidote, garnet, albite, white mica, titanite, and locally quartz, Fe-carbonate, pyroxene, and barroisite. No direct evidence exists for the age of this unit (Till and others, 2011). Seven detrital zircon samples, collected from widely distributed parts of the unit, contain very similar grain populations; most grains fall into the range of 700–600 Ma; several samples contain small populations of Ordovician or Cambrian grains (Amato and others, 2003; Till and others, 2006b; 2008b, 2011). No fossils have been found in the Casadepaga Schist, but its age may be somewhat constrained by its inclusion in the layered sequence with units DOnx and Onim, both of which contain Ordovician conodonts

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 Oc
Unit name Casadepaga schist
Description Light green and greenish-brown mafic- and calcareous-rich schist Appears as unit Ocs in source BN002.
Lithology Metamorphic

Correlated geologic units

Label Ocs
Description Casadepaga Schist
Geologic age Ordovician
Geologic setting Metamorphic, undivided
Lithology Form Importance
Calc-silicate-schist < Schist < Metamorphic Major