Late Cretaceous Inoceramid Bivalves of the Kuskokwim Basin, Southwestern Alaska, and Their Implications for Basin Evolution

Abstract

Upper Cretaceous rocks of the Kuskokwim Group are exposed in a large region of southwestern Alaska and are mainly composed of deformed turbidite deposits that contain few fossils other than inoceramid bivalves. This paper documents the taxonomy of the inoceramids in the Kuskokwim Group, develops an inoceramid biostratigraphy based on known ranges in other regions, and analyzes biogeographic patterns, paleoecology, and depositional history of the Kuskokwim Group.Most of the inoceramid bivalves present in the Kuskokwim Group are of Cenomanian and Turonian age, and an assemblage of species typical of late Turonian age rocks is particularly well developed. Only two localities appear to be as young as Santonian age. The following 16 species or subspecies are discussed and illustrated in detail:Birostrina tamuraiMatsumoto and Noda,Inoceramus virgatusSchlüter,I. pennatulusPergament,I. pictus minusMatsumoto,I.cf.I. yabeiNagao and Matsumoto,I.? sp. aff.I. costatusNagao and Matsumoto,I. hobetsensisNagao and Matsumoto,I. longealatusTröger,I. frechiFlegel,I. waltersdorfensis waltersdorfensisAndert,I.cf.I. waltersdorfensis hannovrensisHeinz,I. kuskokwimensisn. sp.,Mytiloidescf.M. opalensis(Böse),M. teraokai(Matsumoto and Noda),M.cf.M. incertus(Jimbo), andSphenoceramus naumanni(Yokoyama). In addition, a specimen with affinities toMytiloides striatoconcentricus carpathicus(Simionescu) and a specimen that may belong to theI. (Cremnoceramus?) rotundatus–I.(C.)erectuslineage are illustrated.Most of the taxa present in the Kuskokwim region are found in other regions of the North Pacific, particularly Japan and eastern Siberia, or are found throughout the Northern Hemisphere. Only one species,I. kuskokwimensisn. sp., is new and may be endemic. North Pacific taxa are predominant in the Kuskokwim region, but intervals near the Cenomanian–Turonian Stage boundary and in the upper Turonian contain taxa characteristic of Europe and the Western Interior basin of North America; some of these taxa have not been recorded previously in the North Pacific region. Turonian heteromorph ammonite assemblages associated with inoceramids in the finer grained facies of the Kuskokwim region are similar to those found in coeval rocks of Japan and Germany.The depositional area of the Kuskokwim Group can be broken into two northeast-trending subbasins, the Kuskokwim River subbasin to the northwest and the Mulchatna River subbasin to the southeast, connected by the Nushagak Hills corridor. Within the Kuskokwim River subbasin, deposition apparently started earlier in the north (middle Cenomanian) than in the south (late Cenomanian to early Turonian), and prograding deltaic sedimentation along the western margin also appears to have started earlier in the north. No marine fossils younger than latest Turonian to earliest Coniacian are known from the Kuskokwim River subbasin. The youngest fossils identified are Santonian in age and are from deep-water deposits in the Nushagak Hills corridor. Few fossils are known from the Mulchatna River subbasin and age control is limited.