Hoodoo and Kaguyak Formations

Unit symbol: Khk
Age range Upper Cretaceous (83.6 to 66 Ma)
Lithology: Sedimentary
Group name: Nonmarine to shelf sedimentary rocks
These two similar Alaska Peninsula units are typically dark-gray to black or brown, thin-layered and rhythmically bedded, splintery to pencil-fracturing shale, siltstone, and fine sandstone. Hoodoo Formation was named by Burk (1965; see also, Detterman and others, 1981a) for exposures on the southern Alaska Peninsula. In the vicinity of the type section, the Hoodoo Formation contains ammonite-bearing channel conglomerate composed of clasts of plutonic and volcanic rocks, chert, and quartz. In general, sandstone beds are 0.3 to 1 m thick, and siltstone and shale beds are 1 to 2 m thick, although individual layers are as thin as 1 cm (Detterman and others, 1981). Depositional environment for most of unit is characteristic of lower slope of a submarine fan; structures imply submarine slumping and turbidity current flow. Locally, thick sandstone and conglomerate in upper part of unit imply an upper fan environment. Detterman and others (1996) indicate that the Hoodoo may be considerably thicker than where it was measured at a 630-m-thick reference section, but complex folding and faulting, including well-developed olistotromes, and absence of good marker beds make it difficult, if not impossible, to determine true thickness of formation. Hoodoo is, in part, age equivalent to Chignik Formation (Kcs), which can be considered a shallow-water-facies equivalent of the Hoodoo (Mancini and others, 1978; Molenaar, 1980; Detterman and others, 1996). However, where the two units are in contact, the Hoodoo conformably overlies the Chignik Formation, which indicates a generally transgressive sequence. Sparse megafauna indicate an age of late Campanian to early Maastrichtian for Hoodoo (J.W. Miller, written commun., 1983-85). Kaguyak Formation has a measured thickness of more than 1,200 m (Detterman and others, 1996). Named by Keller and Reiser (1959), unit is mapped on south side of Kamishak Bay at the northern end of the Alaska Peninsula. Similar to the Hoodoo Formation, the proportion of sandstone increases up section. Load and flute casts are common; in upper part of unit, graywacke is graded with numerous rip-up clasts. Overall depositional environment was midfan within multichanneled system, but uppermost part of unit may have been deposited in upper-fan regime (Detterman and others, 1996). In general, fossils are sparse, but they are locally abundant in lower part of unit. Ammonites are most common and range in size to as much as 1 m across. Fossils indicate latest Campanian and early Maastrichtian age

Source map information

Source map Riehle, J.R., Detterman, R.L., Yount, M.E., and Miller, J.W., 1993, Geologic map of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2204, 1 sheet, scale 1:250,000.
Symbol Kk
Unit name Kaguyak Formation
Description Lower part consists of thin-bedded, medium- to dark-gray siltstone and local thin limestone beds; contains abundant ammonites. Upper part consists of medium- to dark-gray or pale-brown, graded graywacke sandstone interbedded with siltstone; flame structures occur in siltstone and sandstone is thin-bedded to massive and has rip-up clasts, and load and flutes casts. Overall depositional environment of formation is near mid-fan within multi-channeled system; however, uppermost part of unit may have been deposited in upper-fan regime (Detterman and others, 1996).
Lithology Sedimentary

Correlated geologic units

Label Kkg
Description Kaguyak Formation
Geologic age Maastrichtian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Siltstone < Clastic < Sedimentary Bed Major