The Chignik Area

The Chignik Formation

For an area that has been studied for so long inevitably the stratigraphic nomenclature of the Alaska Peninsula has undergone considerable change. The most recent in depth treatment of the geology of the Alaska Peninsula is that of Detterman et al., (1996).

The Chignik Formation is composed of marine and terrestrial (coal-bearing) shale, sandstone, and conglomerate.  It was originally defined by Atwood (1911) although the coal beds had previously been briefly described by Dall (1896) and Stone (1905) who pointed out that they contained marine fossiliferous Upper Cretaceous beds.  Stanton and Martin (1905) published brief descriptions of the Upper Cretaceous at Chignik Bay and included a list of fossil plants identified by Knowlton, and marine fossils determined by Stanton who correlated them with the Upper Cretaceous of other regions.  The positions of the Chignik area localities are shown in the Maps and Localities page.

The Chignik Formation at the type section unconformably overlies the Upper Jurassic Naknek Formation and is in turn overlain by Tertiary rocks. The Cretaceous-Tertiary contact is described as conformable by Atwood (1911) but Martin (in Hollick, 1930) believed there was strong evidence for a marked unconformity (although the evidence was not presented).  The base of the Chignik in Herendeen Bay unconformably overlies the Naknek Formation and Lower Cretaceous Herendeen Limestone.  Martin (in Hollick, 1930) used Atwood's three divisions of the Chignik (lower, middle, and upper members) to plot the distribution of fossil forms throughout the formation; the lower member yielding only marine molluscs, the middle member from the type section yielding no fossils (but those rocks assignable to the middle member from Herendeen Bay yielding invertebrates and plants), and the upper member yielding both plants and invertebrate marine fossils.  However, Martin pointed out that the stratigraphic position of the Herendeen Bay localities was not known with any certainty.

The marine fossils are not the same species in the three members.  Only one species extends from one member to another (Inoceramus undulatoplicatus Roemer).  Stanton (in Atwood, 1911; and Stanton and Martin, 1905) considered the lower member of the Chignik Formation to be correlative with a horizon in the Chico Formation in California and the Nanaimo of Vancouver Island and concluded that all the Chignik was Late Cretaceous in age and no older than early Senonian. Although the plant fossil evidence is inconclusive it is " *** not out of harmony with Stanton's assignment of the fauna of the lower member to a horizon in the Chico" (Hollick, 1930), (p. 17).

The fauna of the upper member is quite distinct from the lower member and according to Stanton (in Atwood, 1911) is Late Cretaceous in age.  Again the plant fossils are of little use for age determination in Hollick's (1930) opinion because they included a large number of then unknown species.  The species that had been previously described were known elsewhere from rocks assigned to ages ranging throughout the whole of the Late Cretaceous.

Burk (1965) divided the Chignik Formation into two units, naming the lower unit the Coal Valley Member.  Both units were dated as Campanian on the presence of pelecypods.  Burk states that the Chignik is often difficult to distinguish from overlying Tertiary rocks and that many of the plant fossils reportedly from the Chignik were in fact collected from Tertiary strata.  Subsequently detailed studies of the Chignik Formation have revealed that the formation represents a cyclic nearshore marine, tidal flat, and nonmarine flood plain and fluvial depositional environment (Fairchild, 1977; Detterman, 1978). Nonmarine sediments occur throughout the formation leading Detterman et al. (1996) to conclude that the Coal Valley Member is not the basal part of the Chignik Formation as previously thought. Although the Coal Valley Member forms the basal part of the Chignik Formation between Port Moller and Herendeen Bay, elsewhere nonmarine coal-bearing rocks are found at different stratigraphic levels within the Chignik Formation. More recently the rocks of the Chignik area have been remapped (Detterman et al, 1982; Detterman et al. 1996) and according to Detterman et al. (1996) the Chignik Formation has its greatest thickness (500 to 600 m) in the area between Port Moller and Chignik Bay where it contains tongues of nonmarine coalbearing strata mapped by Burk (1965) as his Coal Valley Member. Northeast and southwest from this area the unit thins and becomes entirely marine. Massive units of pebble-cobble conglomerate and coarsegrained sandstone interbedded with carbonaceous shale, coal, and siltstone between Port Moller and Herendeen Bay are evidence that the area was part of a fluvial deltaic system adjacent to a source area northwest of the peninsula.

Atwood (1911, p. 41) designated the type section of the Chignik Formation to be on Whalers Creek, one-half mile from the shore of Chignik Lagoon. However Detterman et al., (1996) designated a slightly thicker and more accessible section along the shore of Chignik Lagoon as a reference section (their section 14) for the unit. The reference section, some 490 m thick and consisting of nearshore sandstone and siltstone with two intervals of floodplain and fluvial deposits including coal and carbonaceous shale, is located along the northwest shore of Chignik Lagoon. Marine fossils, mainly the bivalves lnoceramus balticus var. kunimiensis and I. schmidti and the ammonite Canadoceras newberryanum, indicate a late Campanian to early Maastrichtian (Late Cretaceous) age for the Chignik Formation (Detterman et al., 1996).

The Chignik commonly overlies the Naknek Formation, but it may locally overlie either the Herendeen or Staniukovich Formation. The lower contact of the Chignik Formation is a major unconfomity that represents a considerable pan of Cretaceous time. However it is likely that the Alaska Peninsula was fairly stable tectonically during the 45 to 75 m.y. represented by the missing strata because at the 490-m-thick reference section of the Chignik Formation, although incomplete, the Chignik Formation is in fault contact with the underlying Naknek Formation and the is only a minor structural discordance between the Chignik and underlying rocks; bedding attitudes generally differ by no more than a few degrees. The upper contact of the Chignik is also an unconformity in most localities, where the Tolstoi Formation of late Paleocene to early middle Eocene age overlies the Chignik Formation with only minor discordance. Locally the Chignik is in contact with the Hoodoo Formation, which is, in part, the age equivalent of the Chignik and is generally considered to be a deep-water lateral facies equivalent of the Chignik rather than an overlying unit (Detterman et al., 1996).

Detterman et al., (1996) consider the volcaniclastic succession exposed at Coal Bluff, on the east shore of Herendeen Bay, to be unassjgned Tertiary strata rather than the Coal Valley Member of the Chignik Formation as mapped by Burk (1965). This is because it is richer in volcaniclastics (Fairchild, 1977) than exposures of the Chignik Formation a few kilometers distant and more similar to nearby volcanic-rich Paleogene sedimentary rocks.

Above: Lithological log of the Chignik Formation type section at Chignik Lagoon modified from Detterman et al. (1996), their section 14.

Fossil Localities:

M6974 Bivalves Anomia sp., Calva sp..

M6975 Bivalves Inoceramus baltica var. kunimiensis (Nagao and Matsumoto), Inoceramus schmidti (Michael).

M6976 Ammonite Canadoceras sp.

M6977 Ammonite Canadoceras sp., Bivalve Inoceramus sp..

M7084 Ammonite Canadoceras newberryanum (Meek), Bivalve Inoceramus schmidti (Michael).