Tuxedni Group

Unit symbol: Jt
Age range Middle Jurassic (170.3 to 166.1 Ma)
Lithology: Sedimentary
Group name: Shelikof and Chinitna Formations and Tuxedni Group
Fossiliferous, light- to dark-gray and green marine graywacke, conglomerate, siltstone, and shale (Detterman and Hartsock, 1966). Graywacke ranges from feldspathic to lithic to laumontitic conglomerate composed mainly of volcanic clasts in a graywacke matrix. Unconformably overlies Talkeetna Formation and is disconformably overlain by Chinitna Formation. Unit locally subdivided into, in ascending stratigraphic order: the Red Glacier Formation, Gaikema Sandstone, Fitz Creek Siltstone, Cynthia Falls Sandstone, Twist Creek Siltstone, and Bowser Formation. Red Glacier Formation is about 40 percent thin-bedded to massive, red-brown weathering dark-gray to moderate olive-gray siltstone concentrated in the upper part of the unit; underlying this is light-tan to buff arkosic sandstone that consititutes about 25 percent of the unit and a thick black, silty to arenaceous, very fissile shale that constitutes the rest of the unit. Gaikema Sandstone is resistant, cliff-forming, massive to thin-bedded graywacke sandstone and cobble conglomerate that has graded bedding 150 to 260 m thick. Fitz Creek Siltstone is a thick sequence (up to 400 m thick) of massive, bluish dark-gray, arenaceous coarse- to fine-grained siltstone that commonly weathers rusty orange and contains many small limestone concretions. Cynthia Falls Sandstone is massive to thick-bedded graywacke sandstone and pebble conglomerate about 200 m thick. Twist Creek Siltstone is soft, poorly consolidated, thin-bedded to massive siltstone and silty shale as much 125 m thick. Bowser Formation is a heterogeneous assemblage of sandstone, conglomerate, shale, and siltstone characterized by rapid facies changes. Age of Tuxedni Group was revised to lower Bathonian to Aalenian by Detterman and Westermann (1992). In Talkeetna Mountains quadrangle, this unit is included in an undivided Tuxedni Group–Chinitna Formation unit (Csejtey and others, 1978). In the Anchorage quadrangle (Winkler, 1992), it is mapped as undivided Tuxedni Group. See Detterman and Hartsock (1966) and Detterman and others (1996) for a fuller description of each of the included formations

Source map information

Source map Detterman, R.L., and Hartsock, J.K., 1966, Geology of the Iniskin-Tuxedni region, Alaska: U.S. Geological Survey Professional Paper 512, 78 p., 6 plates, map, scale 1:63,360.
Symbol Jtg
Unit name Tuxedni Group, Gaikema Sandstone
Description Resistant, cliff-forming, massive to thin-bedded graywacke sandstone and cobble conglomerate showing graded bedding. Conglomerate is confined to the Iniskin Peninsula, clasts in it consist of red and green felsic volcanic rocks, aphanitic igneous rocks and minor metasedimentary rocks, all thought to be derived from the Talkeetna Formation. Rare granitic clasts are the first appearance in the Middle Jurassic of rocks presumably derived from the Alaska-Aleutian Range batholith. Locally, thin-bedded to massive, generally coarse grained to sandy, gray to olive-gray, brownish to rusty brown weathering siltstone is as much as 40 percent of the formation, however, it constitutes generally less than 10 percent of the unit, occurring mainly as thin interbeds in sandstone. Siltstone apparently does not occur in close proximity to the conglomeratic parts of the formation
Lithology Sedimentary

Correlated geologic units

Label Jtg
Description Tuxedni Group, Gaikema Sandstone
Geologic age Bajocian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major