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 Jtb
Unit name Tuxedni Group, Bowser Formation
Description Heterogeneous assemblage of sandstone, conglomerate, shale and siltstone characterized by rapid facies changes. Massive light- to dark-gray sandstone and conglomerate are the dominant lithologic types on the Iniskin Peninsula. The sandstone and the conglomerate matrix are coarse-grained and composed of angular fragments of feldspar and quartz, having biotite, augite, and magnetite as common accessory minerals. The light-gray sandstones are interbedded with the dark-gray sandstone and are commonly calcareous and contain numerous coquina beds composed almost entirely of the pelecypods Inoceramus and Trigonia. Clasts in the conglomerate are dominantly felsic volcanic rocks and basalt, but include about 10 percent granitic rocks. Siltstone as much as 100-m-thick occurs with the conglomerate and sandstone and north of Chinitna Bay occurs in units as much as 250-m-thick, where it forms the dominant unit in the Bowser Formation. The siltstone is massive to thin-bedded, medium- to coarse-grained, dark-brownish gray, weathering to light-brown. Lenticular limestone concretions containing ammonites are common north of Chinitna Bay. The Formation ranges in overall thickness from 380 to 560 m
Lithology Sedimentary

Correlated geologic units

Label Jtb
Description Tuxedni Group, Bowser Formation
Geologic age Bathonian to Callovian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Minor