Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: Example from the McMurray Formation,Canada

The fluvial–tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones: (1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial–tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley (“A” Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial–tidal channels.Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north.Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the “A” Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north.Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems.     

深部岩柱在动态扰动下力学响应的数值模拟

在复杂应力状态下对深部岩柱动态扰动的力学响应进行了数值模拟。通过改变侧压系数,来改变岩柱所受的地应力状态,进而分析不同应力状态下岩柱对外界动力扰动的力学响应;通过改变动态扰动的作用时间,来分析动态扰动时间的变化对处于不同应力状态下的岩柱稳定性的影响。数值模拟结果表明,动态扰动对巷道岩柱破坏的触发与岩柱所处的地应力状态密切相关,侧压系数越小,外界的动态扰动对其影响就越明显。动态扰动的作用时间也是影响巷道岩柱稳定的重要因素,随着应力波作用时间的延长,动态扰动给巷道稳定性带来的影响越大。     

九寨沟林下28种乡土观赏植物种子基本特征

为了筛选九寨沟旅游退化地段林下植被恢复的适宜物种,并揭示乡土植物种子基本性状特征,研究了九寨沟28种观赏性乡土植物种子基本特征 (长宽、种皮厚度、千粒重、活力、萌发率)以及低温层积提高种子发芽能力的作用.调查研究结果是:1) 28种植物种子中4种为小种子(<1 mg),53.6%是中等大小的种子(1～9.9 mg),32.1%为大种子(>9.9 mg).小种子均出现萌发,与小种子相比,种子休眠普遍存在于中等大小和大的种子中.2) 77%的物种种子活力较高(>72%),但仅有7种常规条件下能萌发;窄叶鲜卑花的萌发率最高,达到80%左右,6种植物的萌发率3%～50%.3)低温层积显著提高了蔷薇、毛果铁线莲、宽叶旌节花、假升麻的萌发率(P<0.05),也相对较早地提高了陇东海棠、鲜黄小檗、独活的萌发,但对大多数种子萌发没有促进作用.综合分析表明,九寨沟28种林下或林缘乡土植物中只有窄叶鲜卑花、毛果铁线莲、宽叶旌节花、假升麻、陇东海棠、鲜黄小檗、蔷薇等12种可直播或通过低温层积处理后直播用于旅游干扰退化地段的林下植被恢复中,而其他种类尚需进一步寻求打破休眠的措施和方法.     

卫星观测到的青藏高原雷电活动特征

利用TRMM卫星上携带的闪电探测系统获取的闪电定位资料首次对青藏高原上的闪电活动进行了研究。研究发现,青藏高原上的闪电活动呈现出大陆性气候特征,但是受其特殊地形的热力和动力特征所调制,高于93％的闪电活动发生于5—9月之间,并在夏季出现单一闪电活动峰值。随着地表加热和湿度的增加,闪电活动在5月份开始明显增加。较高的闪电密度发生在高原的中部,揭示了闪电活动与地形特征的相关性。63％的闪电活动发生于14：00—18：00(地方时,下同)之间,而只有＜3％的闪电发生于00：00—l0：00之间。闪电光辐射能的时平均值也呈现出明显的日变化,随着太阳的升起,闪电光辐射能在08：00开始增加,并在l0：00达到稳定的极大值,直到02：00开始明显减小。闪电的光辐射能在高原北部呈现最大值,高原西部呈现最小值。青藏高原上的闪电放电强度比其它地区弱得多,其差别归因于高原上较低的对流不稳定能量。     

Biogeochemistry of Major Redox Elements and Mercury in a Tropical Reservoir Lake (Petit Saut,French Guiana)

The hydroelectric reservoir of Petit Saut, French Guiana, was created in 1994–1995 by flooding 350 km² of tropical forest. When sampled in 1999, the lake exhibited a permanent stratification separating the 3–5 m thick, oxygenated epilimnion from the anoxic hypolimnion. The rate of anaerobic organic carbon mineralization below the oxycline was on the order of 1 μmol C m⁻² s⁻¹ and did not show a pronounced difference between wet and dry seasons. Methanogenesis accounted for 76–83% of anaerobic carbon mineralization, with lesser contributions of sulfate reduction and dissimilatory iron reduction. Upward mixing of reduced inorganic solutes explained 90% of the water column O ₂ demand during the dry season, while most O ₂ consumption during the wet season was coupled to aerobic respiration of organic matter synthesized in the surface waters. Inorganic mercury species represented 10–40% of total dissolved mercury in the epilimnion, but were of relatively minor importance (≤10%) in the anoxic portion of the water column. Net production of soluble organic mercury compounds in the flooded soils and anoxic water column did not vary significantly between wet and dry seasons. Methylmercury accounted for about 15% of total dissolved mercury below the oxycline. Its estimated net production rate, 0.04 mg m⁻² yr⁻¹, is of the same order of magnitude as values reported for contaminated lakes and flooded terrestrial ecosystems.     

Qualitative and Quantitative Characteristics of Modeled and Natural Oscillatory Zoning Patterns in Calcite

Four types of oscillatory zoning patterns (OZP) produced by a dynamic model are described qualitatively and quantitatively and displayed as simulated cathodoluminescence images. The behavior of the dynamic model was investigated in terms of the parameter θ, which is the ratio of diffusivities of Ca²⁺ and H₂CO₃, and in terms of the parameter γ, which is the product of θ and the ratio of concentrations of Ca²⁺ and H₂CO₃ far away from the crystal surface. Qualitatively, the dynamics of the model has been characterized by a stable focus, an unstable focus changing to a stable node or to a stable limit cycle, and by periodical behavior with constant amplitude. Quantitative characteristics, including amplitude of oscillations and duration of oscillations, change between the patterns. It is shown that the process of forming oscillatory zoning in calcite with conditions corresponding to periodical behavior with constant amplitude is very slow in comparison to other OZPs. The oscillatory zoning pattern in a natural calcite crystal is described qualitatively in terms of four general OZPs produced by the dynamic model.     

The role of shelf morphology on storm‐bed variability and stratigraphic architecture,Lower Cretaceous,Svalbard

The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.     

内陆湖盆"深水成盐"形成条件和识别标志:以东濮凹陷与现代盐湖为例

位于我国东部的许多新生代盆地,在地质历史时期发育了巨厚的岩盐沉积。尽管这些盆地的成盐模式研究已经历了40多年,但由于缺乏“深水成盐”的现代沉积模式,到底是“深盆-深水”还是“深盆-浅水”成盐,一直难以定论。为了理解“深水成盐”的控制因素,很多现代盐湖开展了水文学和水化学调查,然而这些湖盆的矿物组合各不相同,“深水成盐”的形成条件和控制因素尚不清楚。本文对死海、佛瑞湖等“深水成盐”的现代盐湖以及我国东濮凹陷沙三段的成盐特征进行了解剖,从盐度、水深、湖平面波动以及卤水分层等角度探讨了“深水成盐”的形成条件,对比分析了“深水成盐”的相对位置以及沉积物特征,总结了“深水成盐”的识别标志。研究表明,盐岩层系在岩心和测井资料上显示出多尺度的旋回性。“深水成盐”为水深较大的洼陷中心成盐,与盆缘的“浅水成盐”同盆共存。沉积相对岩盐层系的结构和组成有明显的控制作用,其中洼陷带的“深水成盐”,主要在洼陷带的卤水-湖底沉积物界面附近,析出和增生粗晶的盐类矿物,常与富含有机质和黄铁矿的暗色泥岩共存;而湖滨附近的“浅水成盐”,单层厚度薄,结晶粒度细小,通常含有较多的陆源碎屑。死海为代表的现代盐湖以及东濮凹陷等古代成盐盆地的沉积特征表明,“深水成盐”发生于湖平面的下降期,而且卤水剖面的厚度对蒸发岩的形成分布有明显的控制作用,同时具有“深水”和“深盆”特征的内陆盐湖更容易形成单层厚度大、横向稳定的岩盐沉积。本研究有助于改变人们以往对内陆湖盆成盐机理的认知,尤其是现代盐湖的卤水分层析盐特征,对解读地质历史中的其他成盐事件具有重要启示。基于现代沉积实例的“深水成盐”识别标志,可以为古代岩盐沉积模式的建立提供限定条件。     

地基InSAR技术及其典型边坡监测应用

为了开展边坡活动实时监测,研究以IBIS-L地基InSAR系统为代表的地基InSAR形变时序分析关键技术,进行针对不同变形特征的观测方案和数据处理方法的优化,并开展了奉节大树场镇灾后滑坡、神农架林区人工不稳定斜坡和备战铁矿露天采矿边坡等边坡活动的监测应用,以及备战铁矿隧道南口冰川运动的监测试验。4种类型边坡地基InSAR监测结果表明,在复杂工作情况下,地基InSAR设备具有较高的观测稳定性,在满足观测条件的情况下,地基InSAR测量精度能达到亚mm级,已成为监测边坡微小形变的重要技术手段。这扩展了形变监测模式从星载InSAR技术的“大尺度、宽覆盖”走向地基InSAR技术的“小尺度、局部精细测量”,进一步丰富了科研人员对边坡变形现象的动力学过程认识,为“星-地”联合InSAR技术监测地表形变过程奠定了技术基础。