Cottonwood Bay and Chilikradrotna Greenstones

Unit symbol: Trcb
Age range Late Triassic, Norian? (247.2 to 201.3 Ma)
Lithology: Igneous - Volcanic
Group name: Massive basalt and greenstone
Cottonwood Bay Greenstone is largely dark-gray to dark-green, porphyritic to amygdaloidal basaltic flows altered to greenstone (Detterman and Reed, 1980). Unit occurs either as roof pendants of or east of Alaska-Aleutian Range batholith in Iliamna and Mount Katmai quadrangles and, as a result, is commonly contact metamorphosed. Unit is associated with the Kamishak Formation (unit ^cnk here); the basal beds of Bruin Limestone Member of Kamishak Formation contain volcanic rocks similar to Cottonwood Bay Greenstone; using this evidence, Detterman and Reed (1980) assigned a Late Triassic age to unit. Because the Kamishak Formation is Norian age, Detterman and Reed (1980) stated that the Cottonwood Bay Greenstone is probably older than Norian. Together, the Cottonwood Bay Greenstone and Kamishak Formation constitute a package very similar to that defined for the Wrangellia terrane. Because of metamorphism, it can be difficult to distinguish the Cottonwood Bay Greenstone from the Talkeetna Formation (Detterman and Reed, 1980). Chilikradrotna Greenstone (Bundtzen and others,1979), largely metabasalt, locally includes andesite, chert, limestone, and tuffaceous sedimentary rock, all weakly metamorphosed and exposed to the west of Alaska-Aleutian Range batholith in the Lake Clark quadrangle. Also associated with this unit is highly fractured dark-gray limestone and chert (Eakins and others, 1978), here assigned to the Kamishak Formation. Eakins and others (1978) also mapped gray shale and gray volcaniclastic sandstone in close proximity to outcrops of the Chilikadrotna Greenstone. These units were originally thought to be of Silurian age on the basis of an erroneous fossil determination in the associated limestone; it has since been revised (R.B. Blodgett, 2004, written commun.)

Source map information

Source map Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p., 1 sheet, scale 1:250,000.
Symbol Trc
Unit name Cottonwood Bay Greenstone
Description Dark-gray to dark-green porphyritic to amygdaloidal basaltic flows altered to greenstone. In general, slightly more metamorphosed than Nikolai Greenstone.
Lithology Metamorphic

Correlated geologic units

Label Trcb
Description Cottonwood Bay and Chilikadrotna Greenstones
Geologic age Triassic
Geologic setting Extrusive
Lithology Form Importance
Greenstone < Metaigneous < Metamorphic Greenschist Major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Chert < Chemical < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Tuffaceous Indeterminate, major