Tatina River volcanics of Bundtzen and others (1997a) and similar mafic volcanic rocks

Unit symbol: JTrmv
Age range Jurassic and Triassic (252.2 to 152.1 Ma)
Lithology: Sedimentary
Group name: Tatina River volcanics of Bundtzen and others (1997a) (Mystic structural complex)
The informally defined Tatina River volcanics of Bundtzen and others (1997a) consists of three members: a lower volcanic member, a phosphatic shale and volcaniclastic sandstone member, and a conglomerate and volcanic sandstone member. The lower volcanic member consists of dark-greenish-gray elongate bodies of pillow basalt that include interbeds and lenses of mudstone, shale, and siltstone. Locally, gabbro bodies, interpreted by Bundtzen and others (1997a) as feeders to the pillow basalt flows, are included in the unit. In the western Talkeetna quadrangle, includes basaltic rocks mapped by Reed and Nelson (1980) as units KJb and Pzbs. Also includes a thick submarine sequence of several hundred—and perhaps several thousand meters—of basalt flows, mostly pillowed, and associated sills and dikes of diabase and gabbro and subordinate dark-gray to dark-grayish-green, fine-grained sandstone, siltstone, and argillite, some having abundant angular tuffaceous material. Mapped by Csejtey in the Healy and Mount McKinley quadrangles (Csejtey and others, 1992; Bela Csejtey, Jr. written commun., 1993). Each of these units has similar reported major element oxide contents and also shows similar copper content, between 200 and 500 ppm. Age control is lacking; assignment is based on association with the sedimentary members. Laterally gradational with the volcanic member is a volcaniclastic member (unit Trs of Bundtzen and others, 1997a), which consists of tan to greenish-gray, buff- to orange-weathered, pebble rich, immature conglomerate, coarse volcaniclastic sandstone, distinctly brown silty shale, and light-gray, green, and black chert. Bundtzen and others (1997a) reported the occurrence of Monotis subcircularis, and Halobia cf. H. fallax, which indicates a Late Triassic (Norian) age. The three uppermost members of the unit are medium- to very dark-gray, bluish-white bleached phosphatic shale: green, medium-grained, concretion-rich, volcaniclastic sandstone; and minor tan chert-cobble pebble conglomerate. Bundtzen and others (1997a) report Entolium sp. and Eopecten(?) sp. of Jurassic age. Includes Lower Jurassic brachiopod, pelecypod, and ammonite-bearing, reddish-brown-weathering sandstone and dark-gray shale in the western Talkeetna quadrangle (Reed and Nelson, 1980), as well as Early Jurassic silty shale and siltstone in the western Lime Hills quadrangle (Blodgett and others, 2000a). Unit occurs on both sides of the Denali Fault System

Source map information

Source map Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic map of the eastern half of the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14a, 34 p., 1 sheet, scale 1:125,000.
Symbol lJs
Unit name Tatina River Volcanics; phosphatic shale and green volcaniclastic sandstone
Description Medium- to very-dark-gray, distinctly bleached bluish-white, phosphatic shale; green, medium grained, concretion-rich, volcaniclastic sandstone; and minor tan chert-cobble pebble conglomerate. Farewell Composite Terrane
Lithology Sedimentary

Correlated geologic units

Label JTrps
Description Tatina River volcanics, Upper member, phosphatic shale and volcaniclastic sandstone
Geologic age Jurassic to Early-Jurassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Phosphatic-shale < Shale < Mudstone < Clastic < Sedimentary Bed Major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental
Chert < Chemical < Sedimentary Bed Incidental