Structure of Upper Cenozoic Strata Beneath Kodiak Shelf, Alaska

Abstract The structure of upper Cenozoic strata beneath the Kodiak Shelf has been determined from an interpretation of 3,500 km of 24-fold seismic reflection data. The shelf southeast of Kodiak Island is divided into three segments that are defined by depth to a regional unconformity at the base of upper Miocene or Pliocene strata. Two of the segments are basins, the third is a high-standing area between them. Albatross basin, under the southwestern end of the shelf, contains as much as 5 km of undeformed to moderately deformed fill. The seaward limit of the basin is Albatross Bank, which began to grow in the late Miocene or Pliocene. The Dangerous Cape high makes up the central shelf segment and separates Albatross basin from Stevenson basin, which is on the northeast flank of the high. The basal late Miocene or Pliocene unconformity is generally at shallow depth (1 to 2 km) in the area of the high and is nearly flat, except in an area midway between Kodiak Island and the shelf break where numerous reverse faults disrupt the unconformity. Stevenson basin is the northeasternmost shelf segment. Portlock anticline strikes northwest and divides Stevenson basin into a northeast and a southwest subbasin. The northeast subbasin is larger and contains as much as 5 to 7 km of undeformed to gently deformed fill. The southwest subbasin contains as much as 3.5 km of fill. A large submarine channel once cut into the fill in the southwest subbasin. Growth of structures that form the local shelf break dammed the channel, causing it to fill. Strata that overlie a regional unconformity may be of late Miocene and younger age. A lens of strata in Albatross basin may be older than late Miocene. The unconformity at the base of upper Miocene or Pliocene strata may be a subaerial or shallow submarine erosion surface. Prior to the late Miocene, when the present shelf began to submerge, it was at least partly emergent. Local uplift of large shelf-break structures began in the late Miocene or Pliocene. At Albatross Bank the uplift occurred at a minimum rate of 300 m/m.y.