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Calcareous Nannofossils from the Niobrara Formation(Upper Cretaceous)and Pareoenvironments of the North American W estern Interior Basin Part II:Pareoenvironment Study
- LIU Huai-bao1， D.K.Watkins2
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2004, 10(1):
26-38.
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The Cretaceous western Interior Basin was a huge foreland basin east of the Rocky Mountains in North America.It was flooded from the Aptian to the Maastrichtian and formed a broad interior seaway.To study pareoenviromental variations within the basin,four field sections of the Niobrara Formation located in western Kansas and eastern South Dakota are selected.The Niobrara Formation is significantly different between western Kansas and eastern South Dakota in lithology and sedimentary thickness.In western Kansas,the Niobrara Formation consists of pure chalk and chalky limestone,thickness totared 205.9 m.In eastern South Dakota,the Niobrara Formation only correlates to the upper part,consisting of interbedded chalk,calcarenite,chalky share,and a large amount of share.The thickness of the Niobrara Formation in eastern South Dakota is less than 25％ as it in western Kansas.Considering pure chalk consists largely of coccoliths and coccolithophorid debris,the different nannoplankton productivity in the two regions should be one ofthe most important factors contro1ling the lithology and sedimentary thickness of the Niobrara Formation.The results from the Shannon's species diversity analysis also show different trends through the Niobraradeposition time between western Kansas and eastern South Dakota.Watkins and others(1993) recognized the high positive correlation between nannofossil species diversity and sediment carbonate content,as well as the surface water condition in the Western Interior Basin.Therefore,one can expect that the different species diversity trends in the study area imply surface water condition variance.Based on the species diversity results, the water mass around western Kansas should be more favorable to the nanoplankton community during the early deposition time of the Niobrara Formation.In Cretaceous,abundant Watznaueria barnesae usually related to wamer waters.Relative abundance of this species in western Kansas and Miner,South Dakota is counted and calculated .In western Kansas,the percentage of W.barnesae shows a continued decline through the Niobrara deposition time(Fig. II-4).Result from Miner section does not show this trend(Fig.II-5).Other important nannofossil taxa used as warm water indicators during the Niobrara deposition period include Ceratolithoides verbeekiand Nannoconus.Although the two taxa were common in the southern part of the interior basin,such as Texas.C.verbeeki is very rare and Nannoconus does not occur in both western Kansas and eastern South Dakota.Two suggestions can be made from the warm water species distributions.First,during the Niobrara deposition period the water temperature in the seaway was not the same,but was warm in the south,cold in the north.Second,the water temperature,at least around Kansas,experienced graduallycooling through the Niobrara deposition time.The water temperature changed through time is also evidenced by the distribution of some cold water nannofossil indicators.For the late Cretaceous,the most famous high latitude nannofossil species include Seribiscutum primitivum,Biscutum notaculum,and Repagalumparvidentatum ,R.parvidentatum do not occur in the study areas.Before this study,S.primitivum and B.notaculum were only reported from localities with latitude higher than 40°.In late Cretaceous,these two species were especially restricted within southern hemisphere with latitude around 60°or higher (Figs.II-6&7).Both S.primitivum and B.notaculum occur in the study areas.but their first occurrences in the Niobrara Formation are different.In eastern South Dakota sections,they first occur at the base of CC16a subzone,while they do not appear until CC18a subzone in western Kansas(Fig.II-8).Further south in the basin,these two speciesnever been found.The different occurrences of the two typical cold water species indicate environmental, especially water temperature, variations within the basin, Their occurrences in the study areas also indicate that the Western Interior Basin had a direct connection with the boreal ocean,and the gradually coling of water in the basin should be resulted from the polar water southward invasion in late Cretaceous (Fig.II-9).The slack immigration of these polar nannofossil species,Middle Santonian in South Dakota and Early Campanian in western Kansas,suggests the cold water invasion was gradual and slow,and the polar water mass seemed never arrive southern part of the basin.
