Restricted Gemuk Group

Unit symbol: KTrs
Age range Lower Cretaceous to Triassic? (252.2 to 100.5 Ma)
Lithology: Sedimentary
Group name: Volcanic and sedimentary rocks of southwest Alaska
Unit consists of marine volcaniclastic, gray to gray-green argillite, cherty argillite, calcareous argillic graywacke, and minor mudstone, tuffaceous sandstone, limestone, and chert. Unit description is largely derived from the field notes and maps of J.M. Hoare, W.H. Condon (1969–1970), and W.M. Cady (1943–1944), as well as from fieldwork and mapping of Miller and others (2007). Primarily occurs in the northwest Taylor Mountains and southwest Sleetmute quadrangles. Unit was originally considered part of the Gemuk Group (now Togiak-Tikchik Complex) and is distinguished from the Kuskokwim Group because of the more common presence of silty limestone, calcareous graywacke, cherty argillite, and chert and limited fossil data (Hoare and Coonrad, 1959a). Map unit also includes limited areas of multi-colored chert and gray to gray-green argillite, graywacke and mudstone. The presence of sparse tuff or tuffaceous sedimentary rocks and the absence of other volcanic rocks distinguish this unit from similarly aged rocks to the south in the Dillingham quadrangle. Miller and others (2007) reexamined and mapped the area where the Gemuk Group was originally defined by Cady and others (1955) in the northwestern Taylor Mountains and southwestern Sleetmute quadrangles. Following abandonment of the name “Gemuk Group,” (Wilson and Coonrad, 2005), Miller and others (2007) newly defined these rocks as the Restricted Gemuk Group. Miller and others (2007) estimate that the redefined Restricted Gemuk Group is at least 2,250 m thick and consists of Upper Triassic pillow lavas, siltstone, chert, and limestone overlain by Jurassic siltstone and chert, in turn overlain by Lower Cretaceous greywacke. Locally, may include areas of Kuskokwim Group (map unit Kk) where data are insufficient to distinguish these units. Cady and others (1955) reported a single collection of Buchia [Aucella] crassicollis as well as nondiagnostic Cretaceous Inoceramus from this unit in the Sleetmute quadrangle, and Box and others (1993) reported poorly preserved radiolarians of Early Cretaceous and Jurassic age from rocks correlated with this unit in the Bethel quadrangle. Cady and others (1955), Sainsbury and MacKevett (1965), and Miller and others (2007) reported twelve new fossil determinations that range in age from Late Triassic to Early Cretaceous, two U/Pb zircon ages (~140 Ma) from tuff, and 110 U/Pb detrital zircon age determinations. Radiolaria from siltstone and chert yielded Cretaceous to Triassic ages, whereas graywacke and tuffaceous rocks yielded detrital zircons as young as 130 Ma (Early Cretaceous) (Miller and others, 2007). The Triassic (Norian and Carnian) fossils are from a volcanic and sedimentary sequence in the northwestern Taylor Mountains quadrangle originally defined as part of the Gemuk Group of Cady and others (1955); those rocks are included in unit Trvsw here

Source map information

Source map Miller, M.L., Bradley, D.C., Bundtzen, T.K., Blodgett, R.B., Pessagno, E.A., Jr., Tucker, R.D., and Harris, A.G., 2007, The restricted Gemuk Group: A Triassic to Lower Cretaceous succession in southwestern Alaska, in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O'Neill, J.M., eds., Tectonic growth of a collisional continental margin: Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p. 273-305.
Symbol KTrg
Unit name Gemuk Group
Description Chiefly dark mudstone, less abundant maroon and green mudstone, chert, cherty tuff, tuff, and turbiditic sandstone and rare limestone
Lithology Sedimentary

Correlated geologic units

Label KTrs
Description Marine volcaniclastic sandstone, conglomerate, and argillite
Geologic age Triassic to Early-Cretaceous
Geologic setting Undivided
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major