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

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.采自新泽西海域外陆坡中部深海钻探计划（Deep Sea Drilling Project, DSDP）612站位的始新世下部钙质超微化石灰岩（calcareous nannofossil limestone），几乎完整保存了生物地层序列（biostratigraphic sequence）；仅最低生物带（biozone）CP9a；NP10*未被取获。该地层厚度达198米，超微化石保存状态良好，且无长期沉积间断（hiatus），足以将该剖面作为西北大西洋边缘始新世下部的标准参考剖面。 目前广泛应用且相似度极高的Martini（NP分带）与Bukry-Okada（CP分带）超微化石分带方案（nannofossil zonations）被稍加修订，使其始新世下部生物带实现同期对比；此外，本次研究新建5个亚带（subzones），对CP10与CPU（对应NP12与NP13）分带进行细分。原有生物带名称因概念改动极小得以保留，但字母数字编码体系（alphanumeric code systems）略有调整：通过添加星号（*）以标识经过修订的生物带。 CP10*（NP12*）被划分为两个亚带：新生盖石囊藻（Lophodolithus nascens）亚带（CP10*a；NP12*a）与半盘球石藻（Helicosphaera seminulum）亚带（CP10*b；NP12*b）。CPU*（NP13*）则被划分为三个亚带：冠盖球石藻（Helicosphaera lophota）亚带（CP11*a；NP13*a）、假伽马环囊石藻（Cyclicargolithus pseudogammation）亚带（CP11*b；NP13*b）以及细弱杆球石藻（Rhabdosphaera tenuis）亚带（CP11*c；NP13*c）。 在612站位，基于既往研究中定年的超微化石首现基准面（first-occurrence datums）建立的时间-深度曲线显示，沉积速率（sediment accumulation rate）呈平稳下降趋势：从CP10*（NP12*）带的4.9厘米/10³年降至CP12*（NP14*）带的2.8厘米/10³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-深度曲线估算，结果如下：半盘球石藻（Helicosphaera seminulum）为55.0 Ma；冠盖球石藻（Helicosphaera lophota）为54.5 Ma；假伽马环囊石藻（Cyclicargolithus pseudogammation）为53.7 Ma；细弱杆球石藻（Rhabdosphaera tenuis）为52.6 Ma；以及膨胀杆球石藻（Rhabdosphaera inflata）为50.2 Ma。 位于邻近陆隆上部的613站位，其厚度139米的同期地层中存在一处对应CP11*b（NP13*b）亚带的不整合面（unconformity），对应沉积间断时长约110万年。该剖面下部的沉积速率达9.7厘米/10³年，为612站位同期地层沉积速率的两倍。613站位的沉积间断与偏高的沉积速率，或代表早始新世陆坡与陆隆环境中周期性块体搬运作用（mass wasting）的影响。