北美西部内陆海盆Niobrara组的钙质超微化石及其环境意义——Ⅰ：钙质超微化石及其分带

北美西部内陆海盆上白垩统Niobrara组中的钙质超微化石十分丰富,且多保存良好,呈现典型的晚白垩世远洋钙质超微化石组合面貌,经系统鉴定,计有60属100余种和亚种。为适应不同的环境,Kansas西部和South Dakota东部的钙质超微化石组合面貌稍有不同。经与白垩纪颗石藻化石带对比,可将Kansas西部的Niobrara组划分为6个化石带(CC13～CC18)和8个亚带,其中,根据本文研究地区的化石序列,CC15和CC16带被进一步划分。根据与同一剖面所建立的无脊椎动物化石带对比,钙质超微化石CC17带的时代被重新厘定,即该带应始于中Santonian的晚期,结束于晚Santonian的早期,并据此将Santoniatr／Campanian的界线划在CC18带之内。

Calcareous Nannofossils from the Niobrara Formation (Upper Cretaceous) and Paleoenvironments of the North American Western Interior Basin Part I:Nannofossil Assemblage Characteristics and Zonation

Calcareous nannofossils from the Upper Cretaceous Niobrara Formation in western Kansas and eastern South Dakota are systematically studied (Fig. I-1). All the samples collected from four sections contain abundant and mostly well-preserved nannofossils. More than 100 species belonging to 60 genera are identified by this study. The Niobrara Formation in western Kansas is 205.9m, mainly consists of pure chalk and chalky carbonate rocks. The Niobrara Formation in eastern South Dakota is different from western Kansas in both lithology and thickness. Lithologically, in stead of pure chalk, the Niobrara Formation in eastern South Dakota contains large amount of terrigenous materials, and is significantly condensed (Fig. I-3). Because the principal components of the Niobrara chalk and calcium carbonate are of coccoliths and coccolithophorid debris, different nannoplankton productivity in the Cretaceous interior basin could be one of the important factors affecting lithological and sedimentary thickness variations within the study area. The Niobrara nannofossil assemblage shows typical Late Cretaceous pelagic assemblage feature, but with variations of dominate species in western Kansas and eastern South Dakota. One of the nannofossil assemblage characteristics is the extremely abundance of Marthasterites furcatus at certain horizons of the Niobrara Formation in eastern South Dakota. According to the co-occurrence of some species with delicate structures, the " M. furcatus blooming event" should be resulted from special environmental conditions which were favorite to this species, but not from diagenesis.By correlation, 6 Cretaceous Coccolith zones (CC zones) with 8 subzones are recognized from the Niobrara Formation (Fig. I-5). According to the throughout occurrence of M. furcatus , the Niobrara Formation in western Kansas deposited from subzone CC13b (late Coniacian) through subzone CC18c (early Campanian). Biostratigraphically, the Niobrara Formation in eastern South Dakota is correlated to only part of the interval. Based on the nannofossil sequence in the study area, the CC15 and CC16 zones are subdivided. The subzone CC15a started from the first occurrence (FO) of Reinhardtites anthophorus and ended at FO Lithastrinus grillii . The age of CC15a is assigned to middle Early Santonian. The subzone CC15b started at FO L. grillii , ended at FO Lucianorhabdus cayeuxii , and is aged as late Early Santonian. The subzone CC16a, started at FO L. cayeuxii and ended at the last occurrence (LO) of Lithastrinus septenarius , belongs to early Middle Santonian. The subzone CC16b, started at LO L. septenarius and ended at FO Calculites obscurus , is aged as late Middle Santonian. The Calculites obscurus zone (CC17) has been differently aged by several authors. In western Kansas section, the CC17 zone spans from 102.5 m to 119.1m, which is close to the occurrence of ammonites Clioscaphites vermiformis and C. choteauensis , 99.9 m to 119.5 m in the same section (Hattin, 1982). Cobban (1993) aged the C. vermiformis zone as Middle Santonian, and the C. choteauensis zone as early Late Santonian. By correlation, the CC17 zone should start in late Middle Santonian and end in early Late Santonian. In this case, the boundary between Santonian and Campanian should be within the Aspidolithus parcus (CC18) zone (Fig. I-6).