Nikolai and Goon Dip Greenstones and equivalent rocks

Unit symbol: Trn
Age range Late and Middle Triassic (247.2 to 201.3 Ma)
Lithology: Igneous - Volcanic
Group name: Massive basalt and greenstone
Massive, dark-gray-green, dark-gray-brown, and maroon-gray, subaerial and submarine basalt flows and minor interbedded volcaniclastic sedimentary rocks, aquagene and epiclastic tuff, breccia, argillite, and radiolarian chert (Nokleberg and others, 1992a), commonly metamorphosed to lower greenschist facies. Widely distributed and several thousands of meters thick. Includes unnamed Triassic greenstone units in Talkeetna Mountains quadrangle (Csejtey and others, 1978). Commonly associated with Late Triassic carbonate and cherty carbonate rocks. Together with Chitistone and Nizina Limestones, this is one of the diagnostic units of the Wrangellia terrane (Jones and others, 1977). Plafker and others (1976) and Jones and others (1977) correlated the Goon Dip Greenstone of southeast Alaska with the Nikolai Greenstone based on similar lithology and stratigraphic position relative to the overlying Whitestripe Marble, which they correlated with the Upper Triassic Chitistone Limestone. Goon Dip Greenstone is dominantly massive greenstone and minor greenschist and marble. The greenstone also commonly contains sparsely distributed copper-bearing sulfides; the Nikolai was the host rock for the Kennecott group of mines. Similar and possibly correlative units, the Cottonwood Bay and Chilikradrotna Greenstones of the Alaska Peninsula, are described below as unit Trcb

Source map information

Source map Smith, T.E., 1981, Geology of the Clearwater Mountains, south-central Alaska: Alaska Division of Geological and Geophysical Surveys Geologic Report 60, 73 p., 1 sheet, scale 1:63,360.
Symbol Trab
Unit name Amphitheatre group
Description Grayish-green and grayish-red basalt and basaltic andesite. Weakly metamorphosed. Flow thickness varies from inches to about 100 feet. Chemically similar to island-arc tholeiites. Mainly subaerial and submarine. Numerous pillowed horizons. Northern part of unit abundantly amygdaloidal. Amygdule minerals are quartz, chlorite, calcite, epidote, prehnite, and pumpellyite. Unit locally includes dikes and sills of similar composition NOTE: This unit label is used twice.
Lithology Igneous

Correlated geologic units

Label Trn
Description Nikolai Greenstone, Late and/or Middle Triassic
Geologic age Anisian to Norian
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Major
Basalt < Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Incidental
Conglomerate < Clastic < Sedimentary Bed Incidental
Sandstone < Clastic < Sedimentary Bed Incidental
Siltstone < Clastic < Sedimentary Bed Incidental