Geologic Map of Alaska: Nikolai and Goon Dip Greenstones and equivalent rocks

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	Nokleberg, W.J., Aleinikoff, J.N., Lange, I.M., Silva, S.R., Miyaoka, R.T., Schwab, C.E., and Zehner, R.E., with contribution for selected areas from Bond, G.C., Richter, D.H., Smith, T.E., and Stout, J.H., 1992, Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open File Report 92-594, scale 1:250,000 [Received as CAD coverage produced in 1996].
Symbol	Trn
Unit name	Nikolai Greenstone
Description	Chiefly massive dark-gray-green, dark-gray-brown, reddish-brown, and maroon-gray subaerial, amygdaloidal basalt flows, separated locally by thin beds of reddish-brown nonmarine volcaniclastic rocks. Unit pervasively metamorphosed to lower greenschist facies. Disconformably overlies Eagle Creek Formation (unit Pe), overlain by Upper Triassic limestone (unit Trl). In southeastern Mount Hayes quadrangle, unit locally composed of Middle Triassic shale, limestone, and chert.
Lithology	Metamorphic

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