Sedimentary rocks, undivided

Unit symbol: Tsu
Age range Tertiary (66 to 1.806 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks, undivided
Widely distributed around Alaska, unit typically consists of nonmarine, moderately to poorly consolidated deposits of variable composition that range from conglomerate to sandy gravel, gravelly sand, sand, and pebbly mud. Locally, in northern Alaska (Reiser and others, 1971), may include some marine beds and in southeast Alaska includes marine calcareous sandstone and siltstone (Gehrels and Berg, 1992). Unit is lithologically similar to unit Tcb below except coal is not reported. In the Healy quadrangle, consists of poorly consolidated fluvial dark-gray shale, yellowish-gray sandstone, siltstone, and pebble conglomerate of possible Eocene to Miocene age (Csejtey and others, 1992). In the Mount Hayes quadrangle, (Nokleberg and others, 1992a), unit consists of brown sandstone and graywacke and interbedded conglomerate and argillite of possible Oligocene to Pliocene age, and light-colored, fine-grained, poorly sorted sandstone, of Eocene to Miocene age, which locally contains interbedded siltstone, pebbly sandstone, pebble to cobble conglomerate, and sparse, thin coal layers, as well as poorly sorted, crudely bedded to massive, polymictic conglomerate and subordinate sandstone. In the Circle quadrangle (Foster and others, 1983), unit consists of gray or tan conglomerate that grades into gray, tan, or iron-oxide-stained sandstone. In the Big Delta quadrangle (Weber and others, 1978), unit consists of light-gray, poorly consolidated, poorly bedded, fine to very coarse conglomerate, olive-gray, brown, or orange-brown coarse- to fine-grained sandstone and olive-gray siltstone. In the Talkeetna Mountains quadrangle (Csejtey and others, 1978), unit consists of fluvial conglomerate, sandstone, and claystone greater than 160 m thick, which contains a few interbeds of lignitic coal. These rocks lithologically resemble Chickaloon Formation (included in unit Ttk here), which outcrops to the south, but lack fossil evidence for definitive correlation (Csejtey and others, 1978). In the McGrath quadrangle (Bundtzen and others, 1997a), consists of thick- to thin-bedded moderately indurated sandstone interbedded with poorly indurated, laminated, fissile, carbonaceous shale and fine-grained sandstone as well as limestone conglomerate. Age range from unit inferred to be Paleocene to Miocene, but is largely Eocene

Source map information

Source map Carter, L.D., and Galloway, J.P., 1985 (revised 2005), Engineering-geologic map of the Harrison Bay quadrangle, Alaska: U.S. Geological Survey Open-File Report 2005-1194, 32 p., 2 sheets, scale 1:250,000.
Symbol Tgs
Unit name Gravelly sand
Description Gravelly sand to sandy gravel. Clasts predominantly pebble-sized, well rounded, and composed of chert and quartz. Very poorly exposed
Lithology Unconsolidated

Correlated geologic units

Label Tsu
Description Sedimentary rocks, undivided
Geologic age Paleocene to Pliocene
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Minor
Lignite < Coal < Sedimentary Bed Incidental