Marine sedimentary rocks of the Wrangell Mountains, undivided

Unit symbol: Js
Age range Jurassic, Kimmeridgian to Pliensbachian (190.8 to 152.1 Ma)
Lithology: Sedimentary
Group name: Staniukovich and Naknek Formations, Kotsina Conglomerate, and similar rocks of southern Alaska
Consists of three units defined by MacKevett (1969) in the McCarthy quadrangle. Uppermost is the Root Glacier Formation, which is mainly mudstone and siltstone, and less abundant graywacke, arenite, shale, and well-indurated pebble and cobble conglomerate. It has an estimated thickness of 400 to 1,200 m and is Late Jurassic (late Oxfordian and Kimmeridgian) in age. The Root Glacier Formation disconformably overlies the Nizina Mountain Formation, which is mainly dark-greenish-gray and weathers to reddish brown, fine- to very fine-grained graywacke that has shaly partings and a few limy lenses and concretions. Thickness is about 410 m at type locality and is Middle and Late Jurassic in age. It disconformably overlies the Lubbe Creek Formation, which consists of medium-gray weathering to medium-brown, impure spiculite and minor coquina and chert lenses; about 90-m-thick; age is late Early Jurassic, Pliensbachian and Toarcian (MacKevett, 1969)

Source map information

Source map MacKevett, Jr., E.M., 1978, Geologic map of the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1032, 1 sheet, scale 1:250,000.
Symbol Js
Unit name Undivided Jurassic marine sedimentary rocks
Description Lower parts of the Matanuska-Wrangell terrane; subdivided into Lubbe Creek Formation: spiculite and minor coquina; Nizina Mountain Formation: graywacke; Root Glacier Formation: mudstone, siltstone, and subordinate sandstone, shale, and conglomerate. Broadly correlate with Jurassic rocks in Valdez Quad and Talkeetna Mtns
Lithology Sedimentary

Correlated geologic units

Label Jsu
Description Jurassic sedimentary rocks, marine, undivided
Geologic age Jurassic
Geologic setting Undivided
Lithology Form Importance
Sedimentary Bed Major