【作者】 陈曦；

【导师】 王成善；

【作者基本信息】 中国地质大学（北京） ， 古生物学与地层学， 2009， 博士

【摘要】 白垩纪大洋红层(CORB)及其所代表的深水大洋氧化作用自提出以来一直广受关注,现已上升为白垩纪研究新的热点问题之一。对于全球性海洋事件而言,CORB的时空分布与演化规律是认识白垩纪古海洋演化的基础。目前,CORB时空演化研究主要集中于特提斯域,对其它地区的CORB分布状况还存在较大范围的盲区。同时CORB所代表的氧化程度究竟如何,仍然需要作进一步深入研究。因此,本文通过对CORB大洋钻探岩芯以及陆地露头资料进行综合整理,概括了CORB的时空分布及其在不同区域之间的共性与差异。并以西藏南部江孜盆地为例,分析了CORB在东特提斯地区的时代、岩性与沉积环境。结合地球化学手段,研究CORB所代表的氧化程度,重建白垩纪东特提斯深水大洋长周期演化特征。CORB在大洋钻探岩芯中的颜色以棕色为主,包括棕色,红棕色,黄棕色及浅棕色等,偶见红色、浅红色、黄色或橙色等,且常混有灰色或绿色夹层。岩性主要为(半)远洋粘土、软泥和白垩,亦见有泥岩、灰岩、页岩和白云岩等,偶见火山碎屑沉积物夹层。而在陆地露头中,CORB主要为浅红色、红色或紫红色灰岩和页岩。CORB的沉积古水深一般为半深海至深海,即200 m以下直至数千米,主要沉积环境为斜坡和深水洋盆,也可沉积于水深较浅的陆棚地区,CCD面之下或之上均可。沉积速率通常在几至10 mm/kyr之间,而在有浊积岩夹层存在时,沉积速率可高达30 mm/kyr。CORB的形成可能部分由于低沉积速率所致。CORB最广泛出露的时代是晚白垩世,从陆地露头来看,集中分布于Turonian早期—Santonian期,在大洋钻探岩芯中,以Campanian早期—Maastrichtian期为最广泛分布。CORB演化可划分为三个阶段,逐步由零星分布演化为区域性分布,最终达到全球分布,其启动时间分别是早Aptian晚期、Turonian早期和Campanian早期,分别紧随白垩纪大洋缺氧事件OAE1a,OAE2和OAE3。因此推断大洋缺氧事件可能为CORB沉积的重要诱因。结合古地理演化格局,晚白垩世时期,原环非洲海道打开形成南大西洋和印度洋,各大洋之间的联通性增强,本文认为,这一古地理条件促进了CORB的全球性分布。通过对江孜盆地CORB的区域对比,认为CORB在江孜地区的时代主要为Campanian期,其中南部可能略早于北部,整个盆地CORB的初始沉积具穿时性。岩性主要由红色页岩和浊积泥灰岩薄层以及杂色滑塌/滑动成因灰岩组成。稀少的底栖有孔虫种类指示其沉积古水深为半深海至深海,钙质含量较低表明其沉积于CCD面与碳酸盐溶跃面之间或紧邻CCD面之下。江孜盆地CORB的岩相组合主要为页岩与泥灰岩夹层、滑塌或滑动成因灰岩以及颗粒支撑砾岩组成。碳酸盐岩有八种微相类型:微晶灰岩、含有孔虫微晶灰岩、含有孔虫及内碎屑微晶灰岩、有孔虫微晶灰岩、微晶有孔虫灰岩、微晶内碎屑灰岩、有孔虫颗粒灰岩以及角砾岩。其主要可与标准微相3对应,即浮游生物微晶灰岩,据此推断本区CORB的沉积环境为斜坡坡脚裙的外裙至盆地环境。江孜地区海相白垩系从黑层至红层的地球化学特征,揭示了黑-红转变过程中东特提斯地区深水大洋的演化特征:黑层代表底层水和表层松散沉积物孔隙水均为缺氧状态,对应于暖湿的古气候条件。白层沉积时期大洋底层水为次氧化-氧化状态,而松散沉积物中的孔隙水主要为缺氧状态,对应暖湿与干冷交替的古气候状况,风成作用加强。红层沉积时期,底层水和沉积物孔隙水主要为氧化状态,然而在松散沉积物底部可能保有还原水膜,古气候趋于干冷,风成作用进一步加强。根据江孜地区白垩纪古海洋学演化特征,认为红层代表的氧化程度明显高于传统意义上的氧化沉积——白层,因此有必要红层代表的大洋底层水状态从氧化的大洋中区别出来,建议将其将命名为过氧化状态(hyperoxic)。本文最后部分,研究了沉积物色度对同位素事件的响应,认为红绿色度a*值与δ13C事件有着良好的一一对应关系,δ13C正偏事件常对应于a*值的负偏,即沉积物颜色偏绿,反之δ13C负偏事件常对应于a*值的正偏,颜色偏红。因此本文认为沉积物色度不仅对于CaCO3和TOC含量,铁矿物类型等古海洋指标有很好的指示作用,对于地球系统的碳循环也有很好的响应。由于CORB常沉积于CCD面之下,难以获得高分辨率δ13C记录,因此利用其色度值进行区域对比,不失为一种可行的途径。这一方法仍处于尝试阶段,还需要更多的事例的验证。更多还原

【Abstract】 Cretaceous Oceanic Red Bed (CORB) deposition has been one of the most attractive topics in Cretaceous research in the last several years, because it has a significant implication on paleoceanography evolution. As a globally distributed sedimentary succession, CORBs need to be correlated globally for their age ranges, lithologies and depositional environments. However, the temporal and spatial evolution of CORBs is not clearly documented except those in Tethyal realm. In this dissertation, to correlate CORBs globally, the author compiled the data of CORBs recovered from the DSDP and ODP sites and the cropped out in the Tethyan realm, Boreal realm and New Zealand. Nonetheless, the characteristics of CORBs cropped out in a typical area, Gyangze Basin were correlated in detail. The ages, lithofacies and depositional environments of CORBs in Gyangze Basin were described as well. The geochemical approaches were used to study the condition of bottom water during the CORB deposition as well as the paleoceanography evolution of eastern Tethyan Ocean.CORBs in DSDP and ODP sites mainly consist of clay(stones), oozes and chalks with a few exceptions of mudstones, limestones, shales or dolomites. Volcanogenetic debris could also be intercalated in the CORB sediments. Colors are mainly brown hues including brown, reddish brown, yellowish brown and pale brown, although the red, reddish, orange hues are found in some of the sites. In some cases, CORB color is characterized by alternation of different colors. However, CORB outcrops are generally composed of red, reddish or purple shales and limestones. CORBs were deposited at bathyal to abyssal depth, i.e. 200 m to thousands of meters, below or above CCD. The depositional environments are mainly slpoes to abyssal plains. Sedimentation rates vary from a few to 10 mm/kyr in most of sites, while the average rate can reach 30 mm/kyr with the tubidite intercalations. We suggest that the low sedimentation rate is a potential controlling factor of the CORB deposition.The common age of CORB is late Cretaceous, i.e. between Turonian and Maastrichtian. In oceanic drilling sites, CORBs are spread widest during early Campanian-Maastrichtian. In the outcrops, CORBs are mainly aged early Turonian-Santonian. The tempo evolution of CORB can be divided into 3 stages: sporadic occurrence, regional distribution, and global widespreading. The onsets of each stages are early late Aptian, early Turonian, and Campanian, which follow respectively the OAE1a, OAE2, and OAE3. It is suggested that the OAEs are the trigger of CORB deposition, because the significant increasing of oxygen content as well as the decreasing of carbon dioxic followed the burial of organism during the OAEs. Paleogeographically, in late Cretaceous South Atlantic Ocean and Indian Ocean were formed and resuled from the opening of circum-Africa Seaway, which extended from the western Tethys region through the North and South Atlantic into the juvenile Indian Ocean in early Cretaceous. This pattern increased the connection between oceans; therefore, we suggest that the formation of South Atlantic Ocean and Indian Ocean is one of the important contributing factors of CORB distribution.In Gyangze Basin, CORB successions share the general features described above, but they are differentiated from the others with the re-deposition of carbonate rocks. They are mainly Campanian in age. The occurrence of CORB in south is earlier than in the north basin. CORBs are composed of red shales interbedded with thin tubiditic marls bands, and variegated slide or slump limestones. Very few benthic foraminifera found in the limestones indicate that the paleodepth of CORB is bathyal to abyssal, i.e. below 200 m. The calcic carbonate is quite low in the red beds, which indicates that CORBs were deposited between CCD and lysocline, or immediately below the CCD. Eight microfacies types were identified in the limestones, which mainly belong to the standard microfacies 3 of Wilson indicating a basin or deeper shelf margin environment. According to the correlation of the facies between Gyangze CORB and Bahama slope, we suggest that the depositional environment is out slope-base-apron to basin.The Cretaceous marine sediments in Gyangze Basin were spectacularly expressed by black, white to red succession. The geochemistry of each unit was discussed in the dissertation to study the redox condition and paleoceanography evolution of deep eastern Tethyan Ocean. The black unit was deposited under an anoxic to dysoxic environment with a warm and humid climate. The white unit stands for the suboxic to oxic bottom water, but the pore water of sediments was dysoxic. Therefore, the Mn was enriched in the sediments immediately below the redox boundary that is close to the sediment-water interface. An increased eolation is suggested. The red unit was deposited under a more oxic condition, which is significantly differentiated from the white unit by the relative position of redox boundary. In the period of red unit deposition, the redox boundary is verge on the bottom of the soft sediments so that the Mn enriched as Mn-Ca under the boundary while a small portion of Fe is reduced to Fe(Ⅱ). More eolian sediments input to the basin. The Mg/Al and K/Al ratios indicate that the paleoclimate changed from the alternation of cold and warm of white unit into relative cold and dry in the red beds. The thesis suggests hyperoxic to newly name the bottom water condition of red beds, because it is distinctly more oxic than normal oxic state of the whit unit’s.In addition, the dissertation studied the relationship of color and isotope event. The red-green chromaticness a* is controlled by the content of Fe(Ⅱ) and MnCO3 in the dark gray shales and pale brownish white limestones of studied section. The yellow-blue chromaticness b* is controlled by the content of goethite. The positive excursions ofδ13C correlate to the negative excursions of a* curve. It is interpreted that the positive excursions ofδ13C is caused by the anoxic event, that represent the reduce state of the basin. Therefore, the iron presents mainly as Fe(Ⅱ) in the sediments, which in turn leads to the decrease of a* value. Otherwise, the Fe(Ⅲ) minerals, goethite or hematite lead the increase of b* or a* value. I suggest that the curve of chromaticness can be a useful tool of stratigraphic correlation, especially for the carbonate free sediments like CORBs in southern Tibet.更多还原