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Reference
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| Title: |
Characteristic Lower Cretaceous megafossils from northern Alaska
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1961
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Abstract.--The megafossils from the Lower Cretaceous rocks of northern Alaska prove that only the Berriasian, Valanginian, and Albian states are represented by strata. They prove that the lower Berriasian coincides with an erosional unconformity that locally truncates rocks of Jurassic, Triassic, and late Paleozoic age and that Hauterivian, Barremian, and Aptian stages coincide with an angular unconformity involving mountain building, overthrusting, and considerable erosion. They date the Okpikruak formation as being of Berriasian and Valanginian ages and show the earliest Cretaceous sea advanced over a hilly terrane, of which some high spots were not covered until the middle Valanginian. They date the Fortress Mountain formation, the Torok formation, and the overlying formations to the base of the Cenomanian Ninuluk formation as of Albian age only. This is rather astonishing considering that the sedimentary rocks involved, excluding the Fortress Mountain formation, range in thickness from about 9,000 to 11,000 feet. The abrupt termination of the megafossils species at the top of the Albian strata indicates that some major geologic event occurred at the end of Albian time, either in Alaska or in the Boreal region. The Okpikruak formation of Berriasian to Valanginian age in many places contains an abundance of the pelecypod Aucella, but otherwise contains very few fossils. The Aucellas belong to species that are common in the Boreal region and along the west coast of North America. They premit dividing the Okpikruak formation into three faunal zones based on the stratigraphic distibution of the species. The thick shells of the Aucellas and their common occurrence in coarse detrital sedimentary beds suggest that they lived in the shallowest part of the neritic zone or even partly in the littoral zone. The beds of Albian age in northern Alaska include more genera and species of marine megafossils than have ever been described previously from Albian beds in lands bordering the Arctic Ocean. Most of the fossils are from beds that are dated as middle Albian. A scarcity of megafossils in the lower Albian beds coincides with a scarcity of microfossils and is ascribed to exceedingly rapid sedimentation. A scarcity of marine megafossils in the latest Albian beds is explained by the fact that part of these beds are continental and much of the remainder are littoral in origin. The Albian sedimentary rocks are divided on the basis of megafossils into 5 faunal zone, of which 3 are of early Albian age, 1 is of middle Albian age, and 1 is probably of late Albian age. The general composition of the megafossil assemblage living in, or on the bottom of the Albian sea in northern Alaska greatly resembles assemblages living today in shallow sea bottoms of temperate latitudes. |
| Report by: |
Ralph W. Imlay
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| | Age: | Albian |
| Formation: | Kukpowruk Formation |
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