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 Jtt
Unit name Tuxedni Group, Twist Creek Siltstone
Description Soft, poorly consolidated, thin-bedded to massive siltstone and silty shale as much 125-m-thick. The siltstone is dark-gray, weathering to dark-rusty brown and contains many thin beds of volcanic ash that weather a bright orange color. Small limestone concretions commonly fossiliferous and are common throughout the unit
Lithology Sedimentary

Correlated geologic units

Label Jtt
Description Tuxedni Group, Twist Creek Siltstone
Geologic age Bajocian
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Siltstone < Clastic < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major