The Alaskan Peninsula is of special geological interest since it represents the continental margin of southern Alaska as well as a part of the Aleutian island arc. The structural configuration of the Alaska Peninsula, based on recent field data, is shown in a new geologic map (1:250,000), and the regional geology of the area is presented on a new tectonic map (1:1,000,000). These maps, together with new stratigraphic data, provide a basis for interpreting the structural evolution of the Alaska Peninsula area.
The start of an Alaska Peninsula was marked by the Early Jurassic intrusion of granitic plutons into a widespread sequence of volcanic and marine sedimentary rocks of Permian to Early Jurassic age. These plutons were up-lifted almost immediately along faults at their southeastern edge, and the cover of older rocks was eroded and redeposited as Middle Jurassic sediment. The granite itself contributed essentially all the debris comprising the thick Upper Jurassic and Lower Cretaceous arkosic sequence of the area. Minor deformation and uplift occurred throughout most of the Cretaceous, and a thin Campanian and Maestrichtian sequence was deposited disconformably on Kimmeridgian to Valanginian rocks.
A thick flysch sequence containing fine volcanic debris from unknown sources, and probably representing most of the Cretaceous, was deposited on an unknown basement along the southeast coast of the Alaska Peninsula. These turbidites were intruded during the Paleocene by granodioritic plutons and uplifted to form the present Pacific continental shelf.
Marine and nonmarine volcanic material and sediment accumulated to great thicknesses throughout the Early Tertiary, resting with only gentle disconformity on older rocks in the Alaska Peninsula. Paleocene and Eocene deposits were widespread, and volcanism seems to have been ubiquitous. Oligocene strata accumulated to greater thicknesses in the outer Alaska Peninsula than to the northeast. These Lower Tertiary rocks were intruded along the present Pacific coast by a series of quartz diorite plutons, but mid-Tertiary deformation of this area was not severe.
Subsidence continued throughout Miocene time, and debris from older rocks, as well as new volcanic material, accumulated to great thicknesses along the northwest coast and outer parts of the Alaska Peninsula. Volcanism was restricted generally to the present Pacific coastal area. Pliocene deposits are represented by thin patches of volcanic material in the mountains and by isolated marine sediments near the present coast, resting discordantly in both instances on all older rocks. All the prominent structural features of the Alaska Peninsula were formed by post-Miocene deformation. A general uplift of the area is continuing.
Of the five periods of deformation affecting the Alaska Peninsula, three were associated with plutonic intrusion (early Jurassic, early Tertiary, and mid-Tertiary) accompanied by only minor structural warping. The mid-Cretaceous deformation is represented by a major hiatus on the Alaska Peninsula and probably by thick, flysch accumulation at the continental margin. Pliocene deformation was severe and produced essentially all the structural details now exposed in this area. The history of the Alaska Peninsula is characterized largely by differential vertical movement following restricted periods of plutonic intrusion.
The outer part of the Alaska Peninsula is deformed into three faulted anti-clinal complexes, probably reflecting greater sediment thickness in this area than to the northeast, where broader folds would suggest less severe compression. The fundamental structure in both areas may be deep faults bounding wedges of uplifted crustal rocks. The tightly deformed flysch of the continental shelf is also broken by many longitudinal faults. The regional dip of these rocks is toward the northwest, and the strike is generally parallel to the edge of the continental shelf. On Sanak Island, however, the flysch sequence strikes toward the edge of the Bering Sea Shelf, suggesting that both shelves may form a single structural feature. The Shumagin-Kodiak Shelf formed at the close of the Mesozoic; the Aleutian Trench is no older than Tertiary, and it may be much younger.
The plutons were restricted areally as well as temporally and are now exposed at regular alternate intervals along two longitudinal lines, the older one in the continental shelf and the younger one on the Alaska Peninsula. Volcanism seems to have moved gradually northwestward since the Oligocene.
Approximately 30,000 feet of Mesozoic sedimentary rocks accumulated on the Alaska Peninsula. Possibly an equal amount of flysch was deposited in the area of the present continental shelf, perhaps in a late Mesozoic oceanic trench. The same amount of Tertiary sediments accumulated in the outer parts of the Alaska Peninsula. All these deposits accumulated at an average rate of about 12 cm per thousand years.
The Alaska Peninsula and Aleutian Islands contain most features considered to be characteristic of other island arcs and of many continental margins, and the geologic history of this area should be representative of similar areas throughout much of the world.