The early search for uranium in the continental Tertiary sediments in Alaska raised the question of whether the necessary processes to produce epigenetic uranium deposits ever occurred under the paleoenvironmental conditions that existed this far north. Recent discoveries, however, have demonstrated that epigenetic uranium enrichment occurred in these Tertiary sediments.

At present, the most intensely explored and best known uranium occurrences in Tertiary sediments are found in the Healy Creek basin. The richest occurrence found in this area is in discontinuous beds of sideritic nodules near the base of the Oligocene and Miocene Healy Creek Formation where it overlies the Mississippian(?) Totatlanika Schist. The Healy Creek Formation consists of a clay-rich, gray to light-reddish-brown conglomeratic sandstone containing carbonized plant debris. The nodules are 2-5 cm in diameter. They have a reddish-brown, outer goethitic rind, a yellowish-brown, inner sideritic core, and scattered gray areas containing uraninite and manganite. A composite sample of the sideritic nodules contained 717 ppm uranium and 2,000 ppm manganese. A low-grade roll-front deposi has also been reported from this area. The roll front is in the Miocene Suntrana Formation, which consists of coal-bearing pebbly quartz sandstone.

Slight uranium enrichment (12 ppm) had been reported from a thin carbonaceous sideritic bed in a conglomeratic part of the Tertiary Kenai Formation near Camp Creek in the Susitna Lowlands. Additional collections yielded a sample containing 72 ppm.

A uranium occurrence has been reported from the Tertiary Kootznahoo Formation on Kuiu Island in southeastern Alaska. The Kootznahoo consists of arkosic dolomitic sandstone containing carbonized wood fragments. As much as 0.23% gamma eU (0.13% beta eU) was found in the carbonized wood fragments. Some of the samples also contained apatite and siderite.

Siderite and carbonized plant material are common to all the known uranium deposits in Tertiary sediment in Alaska. The siderite suggests alkaline conditions and the carbonaceous material indicates reducing conditions. Tertiary ash beds and other volcanic sediments were the apparent sources of uranium for the Healy Creek and the Kuiu Island deposits. Cretaceous and Tertiary granite and quartz monzonite were the apparent sources of uranium in the Susitna Lowlands occurrence.

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