he Lower Tertiary Kootznahoo Formation, a clastic unit, crops out in scattered areas in the Admiralty Trough, southeastern Alaska. The formation consists mainly of arkosic sandstone, conglomerate, and lesser amounts of coal and shale. Samples were collected and outcrops described from three main outcrop localities in an area beginning in the Kootznahoo Inlet area on the west-central side of Admiralty Island and extending southward to the Keku Strait area between Kuiu and Kupreanof Islands. X-ray diffraction, optical mineralogy, scanning electron micrographs, and stable isotope and other chemical analyses were utilized during sample studies. Three stagtts of diagenesis were recognized. Stage I occurred under the conditions of nonmarine fluvial and paludal deposition and was characterized by development of sideritic concretions and cement. Stage II was characterized by local uplift of the formation and the entrance of oxygenated meteoric ground water, which resulted primarily in kaolinization. Stage Ill was characterized by the deposition of calcite and dolomite apparently in response to subsidence and the invasion of the unit by ground water of marine origin. Uranium mineralization associated with carbonaceous material has been found only where diagenesis to stage II has occurred. Scattered carbonized wood fragments enriched to as much ·as 0.2% uranium were found in a sandstone bed in the Kootznahoo Formation near Kadake Bay on Kuiu Island. The source of the uranium appears to be a heavy-mineral zone within the sandstone. Parts of the Kootznahoo where stage II diagenesis has occurred are favorable uranium exploration targets, but exposures are poor and the probability of finding commercial size deposits is low.