Calcareous nannofossil stratigraphy, mass accumulation rates and bulk density of sediments from ODP Leg 173 sites

Drilling on the Iberia Abyssal Plain during Ocean Drilling Program Leg 173 allowed us to recover Upper Cretaceous through Paleocene sediments at Sites 1068 and 1069 and only upper Paleocene sediments at Site 1067, which expands considerably the Upper Cretaceous to Paleocene record for this region. Of these three sites, Site 1068 recovered uppermost Cretaceous sediments as well as the most complete Paleocene record, whereas Site 1067 yielded only uppermost Paleocene sediments (Zone CP8). Site 1069 provided a rather complete upper Campanian through Maastrichtian section but a discontinuous Paleocene record.
After a detailed calcareous nannofossil biostratigraphy was documented in distribution charts, we calculated mass accumulation rates for Holes 1068A and 1069A. Sediments in Hole 1068A apparently record the final stages of burial of a high basement block by turbidity flows. Accumulation rates through the Upper Cretaceous indicate relatively high rates, 0.95 g/cm**2/k.y., but may be unreliable because of the lack of datum points and/or possible hiatuses. Accumulation rates in the Paleocene section of Hole 1068A fluctuated every few million years from lower (~0.35 g/cm**2/k.y.) to higher rates (~0.85 g/cm**2/k.y.) until the latest Paleocene, when rates increased to an average of ~2.0 g/cm**2/k.y. Mass accumulation rates for the Upper Cretaceous in Hole 1069A indicate a steady rate of ~0.60 g/cm**2/k.y. from 75 to 72 Ma. There may have been one or more hiatuses between 72 and 68 Ma (combined Zone CC24 through Subzone CC25b), as indicated by the very low accumulation rate of 0.15 g/cm**2/k.y. The Paleocene section of Hole 1069A does not show the same continuous record, which may result from fluctuations in the carbonate compensation depth and poor recovery (average = 40%). Zones CP4 and CP5 are missing within a barren interval; this and numerous other barren intervals affect the precision of the nannofossil zonation and calculation of mass accumulation rates. However, in spite of these missing zones, mass accumulation rates do not seem to indicate the presence of hiatuses as the rates for this barren interval average ~1.0 g/cm**2/k.y.
This study set out to test the hypothesis that a reliable biostratigraphic record could be constructed from sediments derived from turbidity flows deposited below the carbonate compensation depth. As illustrated here, not only could a reliable biostratigraphic record be determined from these sediments, but sedimentation and mass accumulation rates could also be determined, allowing inferences to be drawn concerning the sedimentary history of this passive margin. The reliability of this record is confirmed by independent verification by the establishment of a magnetostratigraphy for the same cores.

在大洋钻探计划（Ocean Drilling Program）第173航次期间，于伊比利亚深海平原开展的钻探工作，使我们在1068、1069站位成功获取了白垩纪晚期至古新世的沉积物岩芯，而1067站位仅采获了古新世上部沉积物，这极大拓展了该区域白垩纪晚期至古新世的地质记录。在三个站位中，1068站位不仅采获了最顶部的白垩纪晚期沉积物，还拥有最为完整的古新世记录；1067站位仅产出古新世最顶部沉积物（CP8带）；1069站位则获得了相对完整的坎潘期晚期至马斯特里赫特期剖面，但古新世记录并不连续。
在通过分布图表完成详细的钙质超微化石（calcareous nannofossil）生物地层学研究后，我们计算了1068A与1069A钻井的质量累积速率。1068A钻井的沉积物显然记录了浊流作用掩埋高位基底地块的最终阶段。白垩纪晚期的沉积速率相对较高，达0.95 g/cm²/k.y.，但由于缺乏基准点和/或存在潜在沉积间断，该数据可能并不准确。1068A钻井古新世剖面的沉积速率每隔数百万年便出现波动，从较低的~0.35 g/cm²/k.y.升至较高的~0.85 g/cm²/k.y.，直至古新世最晚期，此时沉积速率升至平均约2.0 g/cm²/k.y.。1069A钻井的白垩纪晚期质量累积速率显示，在75 Ma至72 Ma期间维持在约0.60 g/cm²/k.y.的稳定水平。72 Ma至68 Ma期间（对应CC24带至CC25b亚带）可能存在一处或多处沉积间断，这一点可从0.15 g/cm²/k.y.的极低沉积速率得到佐证。1069A钻井的古新世剖面并未呈现连续记录，这可能是受碳酸盐补偿深度波动以及岩芯收获率较低（平均为40%）的影响。CP4与CP5带在一段无化石层段中缺失，此类无化石层段与其他多处类似层段，均会对钙质超微化石分带以及质量累积速率计算的精度造成干扰。不过，尽管存在上述带段缺失，该无化石层段的平均质量累积速率约为1.0 g/cm²/k.y.，并未显示存在沉积间断。
本研究旨在验证一项假说：即可从沉积于碳酸盐补偿深度之下的浊流沉积物中构建可靠的生物地层记录。正如本文所示，我们不仅能从这类沉积物中获取可靠的生物地层记录，还可确定沉积作用与质量累积速率，进而推断该被动大陆边缘的沉积演化历史。通过对同一岩芯开展磁地层学（magnetostratigraphy）研究进行独立验证，进一步证实了该记录的可靠性。