A morphological investigation of marine transgression in estuaries

The landscape setting of estuaries varies widely and is an important aspect of determining how they evolve. This paper focusses on alluvial estuaries in river valleys and how they respond to sea level rise. We examine the implications of marine transgression, as a response to sea level rise, where the estuary moves upwards and landwards to maintain its position in the tidal frame (so-called stratigraphic rollover). Here we encapsulate such kinematic movement of the estuary morphology using a ‘morphokinematic’ model, to assess the potential response to sea level rise and sediment supply. The model of the estuary form includes a single convergent channel, intertidal and surrounding floodplains (the valley) and allows the relative importance of the space available for deposition of sediments, the accommodation space, to be investigated as a function of rates of sea level rise and sediment supply. The transgression of the system is determined using a sediment mass balance, taking account of any supply from the river and marine environment. Model results confirm that the transgression distance, measured as the distance the entity moves landward, varies in proportion to the change in accommodation space, which mainly depends on the floodplain area. As the size of the floodplain reduces, the transgression distance is less and the system becomes much more sensitive to changes in the rate of sea level rise or changes in sediment supply. The greater demand for sediment when a floodplain is present results in greater cannibalization of the estuary form (i.e. greater landward movement) to meet the sediment demand. When the floodplain is disconnected from the estuary, the synergistic relationship is lost and the accommodation space increases. The capacity for restoration will depend on the availability of sediment and the prevailing rate of sea level rise.     

临清坳陷变换构造研究

讨论了临清坳陷内3个规模较大的调节构造带,即封丘左行传递带、马陵横向背斜调节带和夏津—腰站斜向背斜调节带的构造特征及成因机制。临清坳陷内“东西分带、南北分块”的构造特征,是由平行于裂谷轴方向的NNE(NE)向伸展正断层和垂直(或斜交)于裂谷轴方向的NW(NWW)斜向滑移断层及横向构造带等共同作用造成的;区内3个规模较大的调节构造带是造成“南北分块”的构造原因,这些调节构造带垂直或斜交于裂谷方向．通过传递带或调节带的方式调节不同伸展域间的应变,从而使区域应变守恒。     

Low‐accommodation and backwater effects on sequence stratigraphic surfaces and depositional architecture of fluvio‐deltaic settings (Cretaceous Mesa Rica Sandstone,Dakota Group,USA)

The adequate documentation and interpretation of regional‐scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of low‐accommodation settings and control of backwater hydraulics on fluvio‐deltaic stratigraphy. Exhumed examples of full‐transect depositional profiles across river‐to‐delta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of down‐dip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvio‐deltaic deposition in the Tucumcari sub‐basin (Western Interior Basin) during two regressive phases. Multivalley deposits pass down‐dip into single‐story channel sandstones and eventually into contemporaneous distributary channels and delta‐front strata. Down‐dip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multi‐storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised‐valley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping delta‐front geometries. This opposes criteria offered to differentiate incised valleys from flood‐induced backwater scours. As the incised valleys evidence relative sea‐level fall and flood‐induced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCS’ diachronous nature demonstrates that its down‐dip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative sea‐level cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret low‐accommodation settings to dominantly promote bypass, especially during forced regressions. Source‐to‐sink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative sea‐level cycle.     

The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member,Wido Island,Korea

The Daeri Member, a Cretaceous volcanic–sedimentary succession, can be divided into lower, middle, and upper parts based on vertical changes in its lithologic characters. The lower Daeri Member is composed of siliciclastic deposits formed in a semi‐arid floodplain environment, which is overlain by the middle Daeri Member consisting mainly of andesite lava flow. After the emplacement of the andesite, activities of intrabasinal normal faults created accommodation on hanging wall blocks together with the development of intrabasinal topographic relief. The upper Daeri Member occurs only in hanging wall blocks and is composed of rhyolitic volcaniclastic sediments formed during an explosive volcanic eruption. Following the eruption, owing to semi‐arid climatic conditions and the destruction of vegetation, the eruptive materials were easily remobilized and deposited by episodic sediment gravity flows, resulting in deposition of the resedimented volcaniclastic deposits with sheet‐like geometry. Away from the intrabasinal normal faults, the resedimented volcaniclastic deposits show a decrease in grain size together with changes in inferred depositional processes from debris flows to hyperconcentrated flows and supercritical sheetfloods. This suggests that the resedimented volcaniclastic deposits were stacked on alluvial fan environments induced by intrabasinal topographic relief associated with normal fault activities. In addition, episodic movement of the faults gave rise to periodic fluctuation of the accommodation and an increase in gradient of the alluvial fan surface, resulting in the development of coarsening‐upward trends in the resedimented volcaniclastic deposits. The development of the alluvial fan and the coarsening‐upward trends indicate that dynamic tectonic subsidence and concomitant changes in the intrabasinal physiographic relief influenced the depositional processes and sizes of the transported volcaniclastic sediments of the upper Daeri Member. Thus, it is necessary to carefully observe tectonic signatures in volcaniclastic successions, particularly the syneruptive lithofacies, in order to reconstruct the tectonic and volcanic histories of receiving basins.     

陆相湖盆古地形、可容空间的体积变化率与层序结构的关系

通过对东营凹陷和东濮凹陷的层序结构的差异进行了对比,并对东营凹陷和东濮凹陷的体积和地形进行了分析,认为陆相湖盆古地形、可容空间变化与总可容空间的比值(可容空间的体积变化率)与层序结构有密切关系。沙三段沉积期,东营凹陷三级湖平面变化引起的可容空间的体积变化率与东濮凹陷四级湖平面变化引起的可容空间的体积变化率相近。东营凹陷沙三段沉积时期盆地体积大、地形陡,层序的发育主要受三级湖平面变化的控制,因此层序厚度大,层序内部砂体厚度大,分布相对集中。而东濮凹陷沙三段沉积时期,盆地体积小,地形较缓,层序的发育主要受四级湖平面变化的控制,因此层序厚度小,层序内部砂体厚度小,分布广,不稳定。     

高分辨率层序地层分析中基准面变化的讨论

基准面有三个不同的概念,即侵蚀基准面、沉积基准面和地层基准面。目前对于基准面的解释和应用存在争论,在应用中尤其比较模糊。在对高分辨率层序地层学中地层基准面的解释进行分析及基础上,对其在解释以海相沉积环境为主的部分提出了质疑并就可容空间与地层基准面的关系问题进行了讨论。可容空间是层序地层学关键概念,基准面也是层序地层学很重要的概念,两者对于解释层序形成机制是非常重要的。侵蚀基准面、沉积基准面、地层基准面都是势能面,都是动力学界面,都是变化的,只是分析的基准不同、形成的机制不同。为了深入地揭示这三种基准面的特征,还应该考虑基准面的定量研究问题。     

陆相断陷盆地可容空间转换特征

陆相断陷盆地具有构造活动影响大、沉积物供给不均衡、湖盆范围小、物源多等特点。由于自身的局限性,这些因素对层序发育的控制作用更加明显,而直接影响湖盆可容空间的因素为构造沉降、沉积物供给和湖盆水体,三者的相互关系不仅使湖盆在演化过程中可容空间、水深和相对湖平面具有一致的变化,同时其变化又存在非统一性。构造沉降和水体为主控因素时,往往形成欠补偿沉积,相对湖平面和可容空间统一增加;构造沉降和沉积物供给为主控因素时,相对湖平面增加,可容空间在盆地的不同部位其变化不一致,在盆地中心可容空间增加,边缘可容空间减小,而在其间存在一个稳定区,即可容空间转换带。东营凹陷古近系沙河街组三段中亚段层序由湖进体系域和湖退体系域组成,湖进体系域为构造主控,相对湖平面和可容空间在盆地统一增加;湖退体系域沉积物供给的影响比较明显,可容空间在盆地缓坡中部和陡坡中部不变,形成可容空间转换带,该带外侧可容空间减小,内侧可容空间增加。     

喀左盆地九佛堂组露头高分辨率层序地层划分

在实测106 km剖面和对喀左盆地九佛堂组沉积特征及演化进行详细研究的基础上,依据相序关系,并结合碳酸钙、油页岩的成因意义,将九佛堂组划分了 1个长期基准面旋回和7个中期基准面旋回。在长期基准面旋回内,上升期的中期基准面旋回发育以上升半旋回为主的不完全对称型中期基准面旋回,上升与下降过渡期的中期基准面旋回,则发育近完全对称的中期基准面旋回,下降期的中期基准面旋回,则发育以下降半旋回为主的不完全对称型中期基准面旋回。经过对中期基准面旋回和短期基准面旋回发育特点和形成原因的详细研究,对代表剖面进行了中期基准面旋回对比。     

与挠曲作用相关的一种特殊裂缝形成机制

浙江省临安市上侏罗统劳村组火山凝灰岩露头中发育了很多与挠曲相关的裂缝, 地层构造简单、露头良好, 为研究与挠曲相关裂缝提供了良好的条件.通过对两处野外露头的精细测量, 发现裂缝面平直、且未被充填, 均为构造裂缝, 在剖面上裂缝的组合呈扇形.裂缝面与岩层层面夹角为44°~80°, 裂缝密度随着地层厚度的增加而增加.根据裂缝面与层面的交切关系, 将两组裂缝分别命名为: 锐夹角指示邻层相对运动方向的D裂缝和锐夹角指示本层相对运动方向的D’裂缝.通过对地层挠曲变形的几何分析, 发现两组裂缝的形成是为了调节核部地层在变形过程中产生缩短量和剩余剪切, 地层厚度与地层中产生剩余剪切成正比.结果表明, 地层厚度越大, 形成的裂缝也越多.      

可容纳空间转换系统的定量模拟

为探讨盆地两侧可容纳空间和层序叠加模式的非一致性变化,利用SEDPAK二维层序模拟软件,通过考虑控制层序沉积过程的不同参数,对可容纳空间转换系统进行定量模拟并提出新的认识.模拟结果表明,盆地两侧同期层序叠加模式可以分为"同步"和"非同步"两种,同步叠加模式多出现在层序的低位体系域及高位体系域后期,非同步叠加模式多出现在层序的水进体系域及高位体系域初期.多种地质参数的合理组合,盆地两侧同期层序均可形成非同步叠加模式.非同步叠加模式对体系域界面的识别、层序对比具有一定的参考价值.