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 Blodgett, R.B., LePain, D.L., Clough, J.G., and Smith, T.N., 2000, Generalized geologic map, Holitna area, Alaska in, LePain, D.L., Blodgett, R.B., Clough, J.G., Ryherd, T.J., and Smith, T.N., 2000, Generalized stratigraphy and petroleum potential of the Holitna region, southwest Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2000-1, 34 p., 1 sheet, scale 1:250,000.
Symbol Jrsh
Unit name Silty shale, siltstone
Description Silty shale, siltstone. Minor thin beds of fine-grained sandstone turbidites, and rare thin yellow-weathering volcanic ash bed. Early Jurassic (Pliensbachian) age based on radiolarians fauna
Lithology Sedimentary

Correlated geologic units

Label Jsh
Description Early Jurassic silty shale, siltstone, sandstone turbidites
Geologic age Pliensbachian
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Minor