PELAGIC , TURBIDITIC , AND CONTOURITIC SEQUENTIAL DEPOSITS ON THE CAPE VERDE PLATEAU ( LEG 108 , SITE 659 , NORTHWEST AFRICA ) : SEDIMENT RECORD DURING NEOGENE TIME 1
PELAGIC , TURBIDITIC , AND CONTOURITIC SEQUENTIAL DEPOSITS ON THE CAPE VERDE PLATEAU ( LEG 108 , SITE 659 , NORTHWEST AFRICA ) : SEDIMENT RECORD DURING NEOGENE TIME 1
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佛得角高原上的远洋、浊流和沿岸层序沉积（西北非 659 号段 108 段）：新近纪期间的沉积记录 1
DOI：
10.2973/odp.proc.sr.108.138.1989
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
C. Latouche
中科院分区：
文献类型：
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作者：
J. Faugères;P. Legigan;N. Maillet;C. Latouche
在佛得角高原，新近纪沉积物由主要的远洋和半远洋沉积物组成。这些沉积物显示了由硅质碎屑和碳酸盐含量不同的两个岩性组成的气候序列。在这些矿床中夹有几个浊积层序和浊积层序。浊积层序为细粒薄层状，频率很低(新近纪约有12个层序)。它们由富含石英的硅质碎屑或火山碎屑沉积物组成。富石英浊积岩起源于塞内加尔边缘。它们在上新世早期略高的频率表明，较强的浑浊洋流，可能是最丰富的大陆输入，发生在那个时期。火山碎屑浊积岩仅存在于早中新世(约17 Ma)和早更新世(1 Ma)。它们是从邻近的佛得角群岛起飞的，反映了该地区两次强烈的火山活动。等深岩层序由生物成因的砂质粉砂组成，占沉积堆积的不到5%，似主要沉积于晚更新世。这些不同的层序显示了粘土矿物在整个新近纪的变化。高岭石在中新世和上新世下部沉积物中占主导地位，此后这种矿物减少，最上部的上新世和更新世沉积物中的伊利石有增加的趋势，表明撒哈拉大陆沉积物来源的变化约为2.6 Ma。引进地点659位于佛得角高原的最上面，分布在佛得角群岛和塞内加尔边缘之间(图1)。为了获得几乎完整的地层层序，对三个伴生孔进行了连续取芯(659A和659B)或点取心(659C)。在地震记录中，新近系沉积看起来非常规则；透明层中规则距离的反射层覆盖在较强的基底地震层上。在这里发现的新近系沉积剖面(0-250 MBsf；图2)由两个主要的岩性单位组成，具有良好的生物地层和磁性地层时间控制(Ruddiman，Sarnthein等人，1988年)。下中新世到上中新世的沉积物由浅灰色的纳米化石渗出和黄褐色或蓝色的纳米化石粉质粘土组成的远洋旋回(单位IIB；24-7 Ma)。我们观察到底部的蓝色渗出(IIB亚基)和覆盖的棕色渗出(IIA亚基)之间有明显的颜色变化。由于严重的溶解条件，单元II的碳酸盐含量变化很大。上中新世至全新世沉积(单元I；7-0 Ma)由白色和浅灰色有孔虫-超微化石组成的远洋旋回组成，含有少量的粉砂和粘土，碳酸盐含量高于单元II。两个单元之间的逐渐变化，沉积速率的强烈修改和4.6 Ma时CaC03的溶解，是深水海洋学的一个重大事件。此外，在单元II中记录了两个事件：一个是由约11 Ma的显著颜色变化记录的，另一个是由约18.5 Ma至23.5 Ma的早期中新世间断的生物地层学证据记录的(Ruddiman，Sarnthein等人，1988)。1 Ruddiman，W.，Sarnthein，M.等人，1989。程序ODPSci.结果，108：大学站，德克萨斯州(海洋钻探计划)。地质学实验室-海洋学，波尔多第一大学，学院大道，33405 Cedex，法国。除了两个主要的岩性变化外，远洋和半远洋沉积物在这两个单元中看起来非常均匀。但在新近系沉积物中夹杂着由粗粒有孔虫渗流和少量硅屑粘土-砂质粉砂组成的深色厘米级粉砂层。在659A或659B孔(图2)中，大约有12个这样的暗层被编号；它们由主要的陆源或火山物质组成。由于其地理位置，佛得角高原可以获得来自塞内加尔大陆边缘(富含石英的硅质碎屑)和佛得角群岛(火山物质)的沉积物。由于其形态状况，现代沉积物将具有主要的远洋或半远洋特征(Diester-Haass，1975；Moyes等人，1979)，浊积岩沉积应该是罕见的。然而，这些最后的沉积物在古近纪和中新世沉积中很常见(Dean等人，1978年)。此外，有几项研究报告说，最近的沉积物受到与北大西洋或南极底层水有关的底流控制(Hobart等人，1975；Sarnthein等人，1982；Lonsdale，1982；Jacobi和Hayes，1982；Meinert，1985；Sarnthein和Meinert，1986)。微地貌研究显示了沉积波场等大量证据。但这些波浪沉积是细粒的，没有典型的冲积特征(Sarnthein和Meinert，1986)；因此，等值线水流过程在相沉积中并不总是被很好地记录下来。然而，我们必须考虑到这些过程及其变化，因为在新近纪(Kennett，1982)期间发生了强烈的地转环流变化，并且在659站点的中新世沉积物中发生了三个重大事件。方法(1)分析大洋旋回沉积、硅屑暗层和富有孔虫渗层三种沉积物的岩性特征；(2)确定每种类型的
On the Cape Verde Plateau, Neogene deposits are composed of major pelagic and hemipelagic sediments. These sediments show climatic sequences composed of two lithologic terms that differ in their siliciclastic and carbonate contents. Several turbiditic and contouritic sequences are interbedded in these deposits. Turbidite sequences are fine grained and thin bedded with a very low frequency (about 12 sequences during the Neogene). They are composed of quartz-rich siliciclastic or volcaniclastic sediments. Quartz-rich turbidites originated from the Senegalese margin. Their slightly higher frequency during the early Pliocene indicates that the stronger turbidity currents, and probably the most abundant continental inputs, occur at that period. Volcaniclastic turbidites are only present in the early Miocene (about 17 Ma) and the early Pleistocene (1 Ma). They have flown from adjacent Cape Verde Islands and reflect two episodes of high volcanic activity in this area. Contourite sequences, composed of biogenic sandy silts, represent less than 5% of the sediment pile and seem to have been mainly deposited during the late Pleistocene. These different sequences show clay mineral variations throughout Neogene time. Kaolinite is predominant in the Miocene and lower Pliocene deposits; this mineral decreases thereafter, with an increased trend of illite in the uppermost Pliocene and Pleistocene sediments, suggesting a change in sediment sources on the Saharan continent at about 2.6 Ma. INTRODUCTION Site 659 is located on the uppermost part of the Cape Verde Plateau, which spreads between the Cape Verde Islands and the Senegal margin (Fig. 1). Three companion holes were cored continuously (659A and 659B) or spot cored (659C) in order to obtain a nearly complete stratigraphic sequence. On seismic records, Neogene sedimentation seems very regular; regularly distanced reflectors in a transparent layer overlie a stronger basal seismic layer. The Neogene sediment section recovered here (0-250 mbsf; Fig. 2) comprises two major lithologic units that have good biostratigraphic and magneto stratigraphic time control (Ruddiman, Sarnthein, et al., 1988). Lower to upper Miocene sediments consist of pelagic cycles of light gray nannofossil ooze and yellowish brown or bluish nannofossil silty clay (Unit IIB; 24-7 Ma). We observed a sharp color change between the basal bluish ooze (Subunit IIB) and the overlying brownish ooze (Subunit IIA). Due to severe dissolution conditions, Unit II shows strong variations of carbonate contents. Upper Miocene to Holocene sediments (Unit I; 7-0 Ma) consist of pelagic cycles of white and light gray foraminifer-nannofossil ooze with minor amounts of silts and clays and with higher carbonate contents than in Unit II. The gradual change between the two units, with strong modifications in sedimentation rates and CaC0 3 dissolution at 4.6 Ma, underlies a major event of deep-water oceanography. In addition, two events are recorded in Unit II: one is documented by the striking color change, about 11 Ma, the other by biostratigraphic evidences of an early Miocene hiatus spanning from around 18.5 to 23.5 Ma (Ruddiman, Sarnthein, et al., 1988). 1 Ruddiman, W., Sarnthein, M., et al., 1989. Proc. ODP, Sci. Results, 108: College Station, TX (Ocean Drilling Program). Laboratoire de Geologie-Oceanographie, Universite de Bordeaux I, Ave nue des Facultes, Talence 33405 Cedex, France. Except for two major lithologic changes, pelagic and hemipelagic sediments appear very homogeneous throughout the two units. However, some levels composed of coarse grained foraminifer ooze and a few dark centimetric layers of siliciclastic clayey-sandy silts are interbedded in the Neogene sediments. About 12 of these dark layers have been numbered in Holes 659A or 659B (Fig. 2); they are composed of major terrigenous or volcanic material. Because of its geographic situation, the Cape Verde Pla teau could be supplied with sediments originating from the Senegalese continental margin (quartz-rich, siliciclastic mate rial) and from the Cape Verde Islands (volcanic material). Because of its morphologic situation, recent sediments will have major pelagic or hemipelagic characters (Diester-Haass, 1975; Moyes et al., 1979) and turbidite deposits should be rare. Nevertheless, these last deposits are common in Paleogene and Miocene sediments (Dean et al., 1978). Also, several works have reported that recent sediments were controlled by bottom currents (Hobart et al., 1975; Sarnthein et al., 1982; Lonsdale, 1982; Jacobi and Hayes, 1982; Meinert, 1985; Sarnthein and Meinert, 1986) related to the North Atlantic or to the Antarctic bottom water. Microphysiographic studies show a lot of evidence such as sediment-wave fields. But these wave sediments are fine grained without typical contouritic features (Sarnthein and Meinert, 1986); and, consequently, contour current processes are not always very well recorded in facies deposits. Never theless, we must take into account these processes and their variations insofar as strong changes of geostrophic circulation occur during the Neogene (Kennett, 1982) and three major events have taken place and are recorded in Miocene deposits at Site 659. METHODS The purpose of this paper is (1) to analyze the lithologic features of the three kinds of sediments: pelagic cyclic sediments, siliciclastic dark layers and foraminifer-rich ooze layers; (2) to determine, for each