Talkeetna Formation

Unit symbol: Jtk
Age range Early Jurassic (201.3 to 182.7 Ma)
Lithology: Igneous - Volcanic
Group name: Talkeetna Formation
Volcanic flows, breccia, tuff, and agglomerate that is locally interbedded minor sandstone and shale, all typically somewhat altered or metamorphosed (Detterman and Hartsock, 1966; Detterman and Reed, 1980), widely distributed in the Talkeetna Mountains and west of Cook Inlet. First described in Talkeetna Mountains of south-central Alaska by Paige and Knopf (1907); the name Talkeetna Formation was introduced by Martin (1926) and now is recognized as a widespread and important marker horizon in southern Alaska. The unit consists of andesitic flows, flow breccia, tuff, and agglomerate and includes subordinate interbeds of sandstone, siltstone, and limestone in a dominantly shallow marine sequence 1,000 to 2,000 m thick (Csejtey and others, 1978). Detterman and Hartsock (1966) geographically extended unit into Cook Inlet area and formally divided unit into three members: Horn Mountain Tuff, Marsh Creek Breccia, and Portage Creek Agglomerate. Detterman and others (1987a, 1996) further geographically extended unit to include a section composed primarily of gray-green, coarse-grained tuffaceous sandstone with lesser amounts of green to red, massive coarse-grained tuff and minor brownish-gray siltstone and gray to gray-brown limestone. This section is 405 m thick and is exposed on the northeast shore of Puale Bay; it was originally considered part of the Bidarka Formation (Kellum, 1945). At Puale Bay, rocks of the formation record an inner-neritic to sublittorial environment; to the northeast, the formation is primarily volcanic rocks (Detterman and Hartsock, 1966). Stratigraphically and lithologically equivalent rocks have been encountered in drillholes as far southwest as the Cathedral River on the southwest part of Alaska Peninsula. An Early Jurassic age (Hettangian and early Sinemurian) is based on an abundant megafauna; this megafauna is present in great abundance in only a few horizons and may represent mass kills as a result of volcanic eruptions (Detterman and others, 1996). At Puale Bay, contact of Talkeetna with underlying calcareous Kamishak Formation (unit Trcnk) is conformable and gradational; it is arbitrarily placed where clastic sedimentary rocks replace limestone as major constituents of rock sequence. Contact of Talkeetna with overlying Kialagvik Formation (included in unit Jt, here) is structurally conformable, but it is considered a disconformity because rocks of late Sinemurian, Pliensbachian, and most of Toarcian Stages are missing. In the Talkeetna Mountains, fossils (Weyla) in the upper part of Talkeetna Formation, which is considered correlative to the rocks of Horn Mountain Tuff Member, indicate a late Pliensbachian and Toarcian (Early Jurassic) age (Arthur Grantz, oral commun., 1963, cited in Detterman and Hartsock, 1966). No lower contact is known from the Talkeetna Mountains

Source map information

Source map Winkler, G.R., compiler, 1992, Geologic map and summary geochronology of the Anchorage 1° x 3° quadrangle, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2283, scale 1:250,000.
Symbol JTrt
Unit name Talkeetna Formation
Description Andesitic, dacitic, and basaltic flows, flow breccia, tuff, shallow sills, and agglomerate. In many places contains subordinate interbedded volcaniclastic sandstone, conglomerate, and fossiliferous marine siltstone and shale. Limestone and marble bodies are mapped separately as Trl. Estimated to be 1000 to 2000 m thick but bottom contact is not exposed.
Lithology Igneous

Correlated geologic units

Label Jtk
Description Talkeetna Formation Unit varies in character, essentially volcanic in Cook Inlet region, sedimentary on Alaska Peninsula. Was originally called Bidarka Formation on the Alaska Peninsula at Puale (Cold) and Wide Bay (1945)
Geologic age Early-Jurassic to Pliensbachian
Geologic setting Extrusive
Lithology Form Importance
Volcanic < Igneous Flow Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major