Magnetochronology of Mid-Miocene mammalian fauna in the Lanzhou Basin,northeastern Tibetan Plateau:Implications for Asian mammal migration

The Quantougou(QTG) Fauna in central Lanzhou Basin is an important late Mid-Miocene fauna on the northeastern Tibetan Plateau margin,but its numerical age remains a matter of debate.Here,we present a new magnetostratigraphic record for a fluvio-lacustrine section to further constrain the age of the QTG Fauna.Results suggest that the studied section spans from polarity chrons C5Cn.2n to C5 n.2n or C5 An.1n,with ages of ca.16.5 Ma to 10 Ma or 16.5 Ma to 12 Ma.The QTG Fauna is located at the top of polarity chron C5r.3 r or C5 Ar.2 r,which corresponds to an age of 11.7 Ma or 12.8 Ma for the fauna.Accordingly,the associated Myocricetodontinae(a subfamily of Gerbillidae,Rodentia) is suggested to have appeared in the Lanzhou Basin at 11.7 Ma or 12.8 Ma,which is the oldest Myocricetodontinae in East Asia but is still much younger than the ～20 Ma appearance of this subfamily in West and South Asia.Our age data support the interpretation that East Asian Myocricetodontinae originated from South Asia.The QTG fauna further suggest a dry and open grassland environment,which is consistent with global cooling after the Mid-Miocene Climatic Optimum.     

Coastal landscape evolution tracked from late Holocene wave-cut benches in the Sinan Archipelago,Southwest Korea

Glacio-isostatic adjustment(GIA)and tectonic activity are important factors in the formation of marine terraces.Late Holocene wave-cut benches in the eastern part of the West Sea of Korea,also called the Yellow Sea,can be divided into two steps:531 cm above sea level(ASL)for the upper bench(T2)and 464-481 cm ASL for the lower bench(T1).Sediments on the benches a re classified into four units,and are interpreted to be beach deposits acco rding to gravel shape,textu re,and seaward inclination.The ove rlying sediment indicates that T2 was formed at approximately 530 cm ASL before 2900 yr BP,and T1 at approximately 460-480 cm ASL before 1520 yr BP.Late Holocene(4000-2000 yr BP)relative sea level(RSL)curves based on GIA models are inconsistent with the wave-cut bench elevations.Comparing T1 and T2 benches to the RSL curves of the West Sea,the upper and the lower benches were uplifted by approximately 5-8 m and 4-7 m,respectively.Although the area is several hundred kilometers away from plate boundaries,the high frequency of earthquakes in the West Sea may have induced the uplift of wave-cut benches during the last 2000 years.These indicate that the west coast of the Korean Peninsula(KP)should no longer be considered an area of subsidence,but be assigned to a regime of uplift during the late Holocene.     

新疆塔什库尔干温泉地区花岗岩体侵入与新生代构造变形:对东北帕米尔中新世构造演化的启示

帕米尔高原是受到印度-亚洲大陆碰撞、持续汇聚影响最显著的地区之一,以强烈地壳增厚和缩短、大量断裂和片麻岩穹窿的形成以及广泛的陆内岩浆活动为特征。以往有关帕米尔陆内岩浆岩的讨论多集中于对其地球化学成分及其所指示的构造背景的研究方面,而对岩浆形成与大型新生代构造之间的联系研究较少。本文通过对东北帕米尔塔什库尔干温泉地区新生代花岗岩及其围岩中锆石U-Pb和Ar-Ar年代学研究,结合该地区新生代构造变形分析,揭示岩浆侵入与区域构造变形之间的关系。U-Pb及Ar-Ar测年结果显示温泉地区花岗岩脉形成时代为中新世（11.8±0.2Ma）,其及其围岩在10.8±0.1Ma冷却到300℃左右;中新世花岗岩脉中继承锆石及围岩片岩中碎屑锆石U-Pb的年龄分布特征迥异,它们分别具有类似松潘-甘孜地体以及中帕米尔地体的物质来源。花岗岩内部几乎无变形,围岩片岩变形主要体现为近E-W向的伸展构造,反映其形成于拉张为主的构造应力环境。结合区域构造背景,推测温泉中新世花岗岩脉为公格尔山、慕士塔格峰构造单元岩石部分熔融产物,是在东北帕米尔地壳从挤压增厚向局部伸展转换的过程中形成的,此时,公格尔伸展断裂系可能已经开始发育;此后,直到6~4Ma,公格尔伸展断裂系开始快速运动,与之相伴公格尔-慕士塔格片麻岩穹窿快速折返。     

Initiation and early evolution of a forearc basin: Georgia Basin,Canada

The lower Nanaimo Group was deposited in the (forearc) Georgia Basin, Canada and records the basin's initiation and early depositional evolution. Nanaimo Group strata are subdivided into 11 lithostratigraphic units, which are identified based on lithology, paleontology, texture and position relative to both the basal nonconformity and to each other. Significant topography on the basal nonconformity, however, has resulted in assignment of lithostratigraphic units that are not time correlative, and hence, cannot reliably be used to accurately reconstruct basin evolution. Herein, we present a sequence stratigraphic framework for lower Nanaimo Group strata in the Comox Sub-Basin (northern Georgia Basin) that integrates both facies analysis and maximum depositional ages (MDAs) derived from detrital zircon. This stratigraphic framework is used to define significant sub-basin-wide surfaces that bound depositional units and record the evolution of the basin during its early stages of development. Seven distinct depositional phases are identified in the lower 700 m of the lower Nanaimo Group. Depositional phases are separated by marine flooding surfaces, regressive surfaces, or disconformities. The overall stratigraphy reflects net transgression manifested as an upwards transition from braided fluvial conglomerates to marine mudstones. Transgression was interrupted by periods of shoreline progradation, and both facies analysis and MDAs reveal a disconformity in the lowermost part of the Nanaimo Group in the Comox Sub-Basin. Stratigraphic reconstruction of the Comox Sub-Basin reveals two dominant depocenters (along depositional strike) for coarse clastics (sandstones and conglomerates) during early development of the Georgia Basin. The development and position of these depocenters is attributed to subduction/tectonism driving both subsidence in the north-northwest and uplift in the central Comox Sub-Basin. Our work confirms that in its earliest stages of development, the Georgia Basin evolved from an underfilled, ridged forearc basin that experienced slow and stepwise drowning to a shoal-water ridged forearc basin that experienced rapid subsidence. We also propose that the Georgia Basin is a reasonable analogue for ridged forearc basins globally, as many ridged forearcs record similar depositional histories during their early evolution.     

Fractures in the low porosity and ultra-low permeability glutenite reservoirs: A case study of the late Eocene Hetaoyuan formation in the Anpeng Oilfield,Nanxiang Basin,China

The Naxiang Basin, located within the Qin-and-Dabie Mountains orogenic belt in central China, is a small intermountain faulted basin, in which the late Eocene Hetaoyuan Formation in the Anpeng Oilfield is the low porosity and ultra-low permeability glutenite reservoirs. The large-scale fractures are mainly developed in the thick-bedded conglomerates and gravel-bearing gritstones, the small-scale jointed-fractures bounded by layer are mainly developed in the medium-granular and fine sandstones between the conglomerates or gravel-bearing gritstones. There also developed three kinds of micro-fractures, namely intergranular fractures, intragranular fractures and transgranular fractures, in the tight glutenites. The fractures in the glutenite reservoirs are of chiefly high dip-angles. They assume chiefly the E-W strikes, next are the NE-SW and NW-SE strikes. On the log curve, fractures usually show such responses as a decrease of the deep and shallow lateral resistivities, an increase of the acoustic time difference, a reduction of the density, an increase of the compensation neutrons, an occurrence of middle and high gamma ray and an expansion of the calipers. Fractures make contributions to the low porosity and ultra-low permeability glutenite reservoirs by improving the permeability and then porosity. Micro-fractures are the important channels connecting intergranular and intragranular solution pores, thereby making better the connectivity within the tight glutenite reservoirs. The small- and medium-scale fractures serve as the flow channels within the layers of fine and medium-granular sandstones, while the large-scale fractures serve as the main flow channels for the whole reservoir. Under the impact of the present-day stress, the NE-SW fractures perpendicular to the minimum principal stress assume a tensile state and have good connectivity, large apertures and high permeability, and are the major direction for fluid flow, thereby providing a major basis for a further development.     

山东及皖北上侏罗统及与邻区对比的大地构造意义

通过对研究区中生代地层典型剖面或代表性剖面系统考察和综合研究,对上侏罗统及其上覆、下伏岩石地层进行进一步系统清理和修订：（1）将济阳坳陷原上侏罗统蒙阴组划归下白垩统,与之相对应的是鲁西南地区为杨家庄组;将山东地区中—上侏罗统三台组界定为中侏罗统;（2）将皖北地区合肥盆地圆筒山组划入中—上侏罗统;原上侏罗统周公山组认为是跨时代地层单元（J3K1）。（3）大别山北麓北淮阳六安、金寨地区的凤凰台组、三尖铺组定为晚侏罗世早期。在此基础上建立了中国东部地区准确可行的地层对比关系,提出了与国际接轨的地层划分与对比表。对比结果表明中国东部晚侏罗世除了在南北缘存在沉积外,整个地区缺失上侏罗统。该现象表明中国东部在晚侏罗世仍为挤压应力环境甚至比早中侏罗世更为强烈,甚至一直维持到早白垩世初。     

晚泥盆世弗拉斯期—法门期之交海水温度变化的同位素证据

位于我国湖南省冷水江地区锡矿山矿区的老江冲剖面为发育良好的浅水台地相沉积，出露著名的上泥盆统弗拉斯-法门阶（Fr/Fa)界线，多年的地层古生物学工作已详细地了解了该区的古生物面貌，本研究采用了基不可一世保存完好的腕足动物Cyrospirifer uernemili壳体进行了稳定C、O同位素地球化学分析，δ^18O值为-8.4‰-9.6‰，δ^13C值为-2.2‰-0.5‰，二值在弗拉斯-法门阶交界处均发生了异常波动，将此值与其他地区的研究结果进行比较，可以认为，弗拉斯-法门期交界时所发生的剧烈而大范围的事件所造成的同位素波动可以作为大区域地层对比的标志，经过计算，推断从弗拉斯期末至法门期初，我国华南地区的古海洋发生了降温事件。     

柴达木盆地北缘石灰沟晚石炭世的四射珊瑚

青海省柴达木盆地北缘晚石炭世地层出露完好，四射珊瑚化方丰富。本文研究了该区晚石炭世四射珊瑚２６属，４９种和亚种，１０个未定种。自下而上建立了５个四射珊瑚组合带：１Ｌｉｔｈｏｓｔｒｏｔｉｏｎｅｌｌａ－Ｔｈｙｓａｎｏｐｈｙｌｌｕｍ组合带；２Ｃｇａｔｈａｘｏｍｉａ－Ｒｏｔｉｐｈｙｌｌｕｍ组合带；３Ｐｒｏｔｏｉｖａｎｏｖｉａ－Ａｍｐｌｅｘｕｓ组合带；４ＮｅｏｋｏｎｉｃｋｏＰｈｙｌｌｕｍ－Ｋｏｎｉｎｃｋｏｐｈｇｌｌａｍ组合带；５Ｐｓｅｕｄｏｚａｐｈｒｅｔｏｉｄｅｓ－Ｌｏｐｈｏｃａｒｉｎｏｐｈｙｌｌａｍ组合带。     

西藏扎布耶盐湖30.0 ka B P以来水位与古降水量变化

扎布耶湖9级大型沙砾堤记录了约30．0ka B P以来水位退缩历史，与北部拉果错、南部塔若错的垭口沉积记录了3个湖泊最后分离的时间。本文应用数字地面高程模型(DEM)，计算了扎布耶各级沙堤对应湖面期的湖水面积、体积与含盐量；分析了扎布耶与拉果错、塔若错的分-合历程，计算了各时期汇流盆地总面积；参考湖泊、冰川、孢粉、天文学等多学科关于古温度、辐射平衡的结论，得出了较为可信的计算参数。在此基础上，应用根据西藏实际情况得出的辐射平衡和水面蒸发、陆面蒸发计算模型，代入封闭盆地水量平衡方程，得出了较Kutzbach水-能方程更可靠的降雨量-水域面积／流域面积比的非线性方程，计算出泛湖期(9级沙堤，40．0～28．0ka B P)该区降雨量567 mm／a，盛冰阶时降至350mm／a以下，冰期后增至402mm／a，随后逐步下降直至约Ⅰ-1级阶地时(海拔4421m，3．53ka B P)为280mm／a(约为现代的两倍)。通过定量恢复该区30．0ka B P以来降水量变化，为认识西藏高原湖泊演化和古环境、古季风演化提供了定量依据。     

内蒙古苏尼特左旗中新世晚期孢粉记录干旱事件

对中中新统通古尔组和上覆地层宝格达乌拉组的40个孢粉样品的分析表明通古尔期晚期形成环境较早期温湿,在通古尔期向宝格达乌拉期过渡期间,灌木和草本植物花粉由19·7%增加到46%,而乔木植物花粉总数由69·5%下降为34·7%,松蒿比(A/C)由4突降为0·64,标志着干旱事件的开始。将该地区的古地磁、哺乳类化石和孢粉资料与前人研究进行对比,结果显示干旱事件发生在晚中新世约8·5Ma,这一事件在华北具有广泛的可比性。