Eocene calcareous nannofossils in DSDP Holes 95-612 and 95-613@en

Lower Eocene calcareous nannofossil limestone cored at DSDP Site 612 on the middle slope off New Jersey represents an almost complete biostratigraphic sequence; only the lowest biozone (CP9a; NP10*) was not recovered. The thickness of the strata (198 m), the good preservation of the nannofossils, and the lack of long hiatuses justify the acceptance of this section as a lower Eocene reference for the western North Atlantic margin. The widely recognized and very similar nannofossil zonations of Martini (NP zones) and Bukry-Okada (CP zones) are emended slightly to make their lower Eocene biozones coeval; in addition, five new subzones are erected that subdivide zones CP10 and CPU (NP12 and NP13). Established biozone names are retained as they are altered little in concept, but alphanumeric code systems are changed somewhat by appending an asterisk (*) to identify zones that are emended. Zone CP10* (NP12*) is divided into two parts, the Lophodolithus nascens Subzone (CP10*a; NP12*a) and the Helicosphaera seminulum Subzone (CP10*b; NP12*b). Zone CPU* (NP13*) is divided into three parts, the Helicosphaera lophota Subzone (CP11*a; NP13*a), the Cyclicargolithuspseudogammation Subzone (CP11*b; NP13*b), and the Rhabdosphaera tenuis Subzone (CP11*c; NP13*c). At Site 612, a time-depth curve based on nannofossil datums dated in previous studies reveals a smoothly declining sediment accumulation rate, from 4.9 cm/10**3yr in CP10* (NP12*) to 2.8 cm/103 yr. in CP12* (NP14*). The ages of first-occurrence datums not previously dated are approximated by projection onto this timedepth curve and are as follows: Helicosphaera seminulum, 55.0 Ma; Helicosphaera lophota, 54.5 Ma; Cyclicargolithus pseudogammation, 53.7 Ma; Rhabdosphaera tenuis, 52.6 Ma; and Rhabdosphaera inflata, 50.2 Ma. At nearby Site 613 on the upper rise, strata of similar age, 139 m thick, contain an unconformity representing Subzone CPll*b (NP13*b) and a hiatus of approximately 1.1 m.y. duration. The sediment accumulation rate in the lower part of this section (9.7 cm/10**3yr.) is twice that observed for equivalent strata at Site 612. The hiatus and the heightened sediment accumulation rate at Site 613 probably represent the effects of episodic mass wasting on the early Eocene continental slope and rise.新泽西外海中等坡度海域的深海钻探计划（Deep Sea Drilling Program, DSDP）612号钻孔所取心的下始新统钙质超微化石灰岩，保存了近乎完整的生物地层序列，仅最底部的生物带（CP9a；NP10*）未被获取。该地层厚度达198米、超微化石保存状态良好且不存在长期沉积间断，使其足以作为北大西洋西部边缘下始新统的标准剖面。 广为认可且相似度极高的Martini（NP生物带）与Bukry-Okada（CP生物带）超微化石分带方案被小幅修订，以确保其下始新统生物带的时代对应关系一致；此外，本次研究新建5个亚带，将CP10与CP11（原文笔误为CPU）（NP12与NP13）生物带进一步细分。已确立的生物带名称因概念改动极小而得以保留，但字母数字编码体系略有调整：通过添加星号（*）以标识经过修订的生物带。 CP10*（NP12*）生物带可划分为两个亚带：新生拟石球藻（*Lophodolithus nascens*）亚带（CP10*a；NP12*a）与半粒盘星石（*Helicosphaera seminulum*）亚带（CP10*b；NP12*b）。CP11*（原文笔误为CPU*；NP13*）生物带则划分为三个亚带：顶棘盘星石（*Helicosphaera lophota*）亚带（CP11*a；NP13*a）、假伽马环石（*Cyclicargolithus pseudogammation*）亚带（CP11*b；NP13*b）以及纤细棒石球藻（*Rhabdosphaera tenuis*）亚带（CP11*c；NP13*c）。 在612号钻孔，基于前人研究中定年的超微化石基准面所构建的时间-深度曲线显示，沉积物堆积速率呈平稳下降趋势：从CP10*（NP12*）带的4.9厘米/千年降至CP12*（NP14*）带的2.8厘米/千年。此前未完成定年的首现基准面年龄可通过投影至该时间-深度曲线进行估算，结果如下：半粒盘星石（*Helicosphaera seminulum*）为55.0 Ma；顶棘盘星石（*Helicosphaera lophota*）为54.5 Ma；假伽马环石（*Cyclicargolithus pseudogammation*）为53.7 Ma；纤细棒石球藻（*Rhabdosphaera tenuis*）为52.6 Ma；以及膨大棒石球藻（*Rhabdosphaera inflata*）为50.2 Ma。 在邻近的上隆区613号钻孔中，厚度139米的同期地层存在一处代表CP11*b（NP13*b）亚带的不整合面，以及一段持续约110万年的沉积间断。该剖面下部的沉积物堆积速率为9.7厘米/千年，是612号钻孔同期地层堆积速率的两倍。613号钻孔的沉积间断与升高的沉积物堆积速率，可能反映了早始新世大陆坡与隆区发生的间歇性块体搬运作用影响。