(Table 1) Dry bulk density and porosity of ODP Hole 165-1002C sediments, Cariaco Basin

Two geographically close hemipelagic deposits are compared: (1) the Pleistocene-Holocene part of the Cariaco Basin succession off Venezuela drilled in Hole 1002C and (2) the Upper Cretaceous (Cenomanian to Santonian) La Luna-Querecual Formation, which crops out in northern Venezuela from the Sierra de Perija, west of the Gulf of Maracaïbo, to the Gulf of Paria eastward. This analysis is based on thin-section descriptions, porosity data, and examination of core photographs. Despite their very different ages, these formations are similar in facies, structure, and diagenetic behavior. Their facies are calcareous black shales rich in organic matter and planktonic remains such as foraminifers, radiolarians, and diatoms. Their structures result in a high-frequency cyclic fabric with superimposed units ranging from obliquity cycles to annual varves, as well as several types of solar and pluriannual El Niño-type cycles. In the Cariaco succession, the frequencies of pluriannual cycles have been determined by a statistical study on deposits with standardized compaction and sedimentation rates to convert the thickness of cycles to a scale proportional to their duration. Their diagenetic behavior is signaled by (1) early carbonate precipitation under bacterial control leading to beige micritic layers, (2) carbonate and siliceous concretions before major compaction, (3) recrystallization, (4) deformation from compaction, and (5) dissolution.
The main observed lithologic differences between the Cariaco Basin and the La Luna-Querecual successions result from the weak diagenetic evolution for the Cariaco series. This evolution reached an ultimate state for the La Luna-Querecual Formation because of tectonic overburial, leading to recrystallization and precipitation of large carbonate and siliceous nodules, and to cementation. Thus, it is appropriate to consider the Cariaco series as an illustration of the initial state of the La Luna Formation. Major common characteristics relative to the genesis of the two deposits signify the similar anoxic depositional environment (900 and 500 m deep for the Cariaco and the La Luna-Querecual Formations, respectively), similar paleogeographic and latitude (2°-15°N) settings on the Guyanese Craton Margin, and similar climate and current controls leading to varves and lower order depositional cycles. Cycles are manifested as an alternation of light layers enriched with planktonic microfossils deposited during dry periods and of dark layers composed of clay-organic complexes with a maximum of detrital quartz, deposited during wet periods.
The major discrepancy in the sedimentation rates (128 m/m.y. for the Cariaco series, 13 m/m.y. for the La Luna Formation) of equally compacted deposits fits principally with different tectonic and sea-level histories in the two series. Indeed, the sedimentary fluxes were low during the Late Cretaceous, before the Laramide orogeny. They were much more important in the Pleistocene-Holocene because of enhanced land erosion due to block faulting in the Venezuelan Andes and the Guyanese Craton and to a lower sea level.

本研究对比了两处地理邻近的半远洋沉积（hemipelagic deposits）：其一为委内瑞拉外海卡里科盆地（Cariaco Basin）1002C钻孔（Hole 1002C）所揭示的更新世-全新世（Pleistocene-Holocene）沉积序列；其二为委内瑞拉北部出露的上白垩统（塞诺曼阶至桑托阶）拉卢纳-奎雷夸尔组（La Luna-Querecual Formation），其分布范围西起马拉开波湾（Gulf of Maracaïbo）西侧的佩里哈山脉（Sierra de Perija），向东延伸至帕里亚湾（Gulf of Paria）。本项分析基于薄片鉴定描述、孔隙度数据及岩芯照片检视开展。尽管二者形成时代差异悬殊，但在岩相、构造及成岩作用特征上具有相似性。其岩相均为富含有机质与浮游生物残骸（包括有孔虫（foraminifers）、放射虫（radiolarians）及硅藻（diatoms））的钙质黑色页岩；构造上均呈现高频旋回结构，叠加单元涵盖黄赤交角旋回至年纹层（annual varves），并包含多种太阳周期与多年际厄尔尼诺型旋回。在卡里科盆地沉积序列中，研究人员通过对标准化压实与沉积速率的沉积层开展统计研究，确定了多年际旋回的频率，将旋回厚度转换为与其持续时间成正比的标尺。二者的成岩作用特征表现为：① 细菌作用控制下的早期碳酸盐沉淀，形成米色微晶岩层；② 强压实作用前形成的碳酸盐与硅质结核；③ 重结晶作用；④ 压实变形；⑤ 溶解作用。

卡里科盆地序列与拉卢纳-奎雷夸尔组之间的主要岩性差异，源于卡里科序列较弱的成岩演化程度。拉卢纳-奎雷夸尔组因构造深埋（tectonic overburial）达到成岩演化的终极阶段，出现重结晶作用、大型碳酸盐与硅质结核沉淀及胶结作用（cementation），因此可将卡里科序列视为拉卢纳组初始成岩状态的典型实例。二者共有的主要成因特征表明，两者的沉积环境均为缺氧沉积环境（卡里科盆地与拉卢纳-奎雷夸尔组的沉积深度分别为900米与500米），均处于圭亚那克拉通边缘（Guyanese Craton Margin）的古地理与纬度背景（北纬2°-15°），且受相似的气候与洋流控制，形成纹层与低级别的沉积旋回。旋回表现为干旱期沉积的富浮游微体化石浅色层，与湿润期沉积的以碎屑石英（detrital quartz）为主的粘土-有机质复合体暗色层互层。

两处同等压实程度的沉积层的沉积速率存在显著差异（卡里科序列为128米/百万年，拉卢纳组为13米/百万年），这一差异主要源于两个序列不同的构造与海平面演化历史。晚白垩世拉拉米造山运动（Laramide orogeny）前，沉积通量（sedimentary fluxes）较低；而在更新世-全新世时期，因委内瑞拉安第斯山脉与圭亚那克拉通的块断作用（block faulting）加剧了陆地侵蚀，且海平面较低，沉积通量大幅提升。