Aggradational lobe fringes: The influence of subtle intrabasinal seabed topography on sediment gravity flow processes and lobe stacking patterns

Seabed topography is ubiquitous across basin‐floor environments, and influences sediment gravity flows and sediment dispersal patterns. The impact of steep (several degrees) confining slopes on sedimentary facies and depositional architecture has been widely documented. However, the influence of gentle (fraction of a degree) confining slopes is less well‐documented, largely due to outcrop limitations. Here, exceptional outcrop and research borehole data from Unit A of the Permian Laingsburg Formation, South Africa, provide the means to examine the influence of subtle lateral confinement on flow behaviour and lobe stacking patterns. The dataset describes the detailed architecture of subunits A.1 to A.6, a succession of stacked lobe complexes, over a palinspastically restored 22 km across‐strike transect. Facies distributions, stacking patterns, thickness and palaeoflow trends indicate the presence of a south‐east facing low angle (fraction of a degree) lateral intrabasinal slope. Interaction between stratified turbidity currents with a thin basal sand‐prone part and a thick mud‐prone part and the confining slope results in facies transition from thick‐bedded sandstones to thin‐bedded heterolithic lobe fringe‐type deposits. Slope angle dictates the distance over which the facies transition occurs (hundreds of metres to kilometres). These deposits are stacked vertically over tens of metres in successive lobe complexes to form an aggradational succession of lobe fringes. Extensive slides and debrites are present at the base of lobe complexes, and are associated with steeper restored slope gradients. The persistent facies transition across multiple lobe complexes, and the mass flow deposits, suggests that the intrabasinal slope was dynamic and was never healed by deposition during Unit A times. This study demonstrates the significant influence that even subtle basin‐floor topography has on flow behaviour and depositional architecture of submarine lobe complexes. In addition, we present a new aggradational lobe fringe facies associations and recognition criteria for subtle confinement in less well‐exposed and subsurface basin fills.     

Facies and palaeosol analysis in a progradational distributive fluvial system from the Campanian–Maastrichtian Bauru Group,Brazil

The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km² study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin.     

Finite element computation of shape factors for Kirkham's piezometer tube method for hydraulic conductivity of saturated soil

The Kirkham piezometer tube method of measuring hydraulic conductivity of saturated soil uses a numerical constant (shape factor) to account for the relative geometrical parameters of the cavity and the soil domain. Published shape factors have mostly been determined using an electrical analogue. In this paper the application of the finite element method to shape factor determination is considered. Convergence is studied and methods of extrapolation evaluated. A determination is made of the size of finite region required to represent adequately the radially unbounded domain assumed for the piezometer method. Computed shape factors were found to be in good agreement with analogue data.     

塔里木地块库车坳陷早白垩世古地磁结果及磁倾角偏低的成因探讨

运用主成分分离及线性区段等方法 ,使早白垩世样品明显分离出二组磁组分 .叠加剩磁为喜山期重磁化 ,特征剩磁明显偏离现代地磁场方向 ,经倾斜校正后 ,有很好的一致性并通过了倒转检验 ,给出塔里木地块库车坳陷早白垩世巴西盖组古地磁新数据 .综合已有的古地磁结果 ,获得了塔里木地块早白垩世平均剩磁方向及平均古地磁极 ,阐明了塔里木地块早白垩世磁倾角明显偏低这一现象 .分析导致磁倾角偏低的诸多因素 ,认为压实作用可能是导致磁倾角偏低的重要因素之一 .     

A critical assessment of the analysis and distributions of scytonemin and related UV screening pigments in sediments

High performance liquid chromatography-mass spectrometry has been used to identify the cyanophyta-derived ultraviolet screening pigment scytonemin and its reduced counterpart in an Antarctic lake sediment. The formation of an artefact during the analysis has been demonstrated, enabling the recovery of improved estimates of the scytonemin and reduced scytonemin signals. The overall pigment composition records a primary producer community comprising both oxygenic and anoxygenic photoautotrophs, with variations in the abundance of scytonemin relative to the chlorophylls and carotenoids that are consistent with changes in response to variations in the light regime over time.     

Mass Spectrometry and reverse phase HPLC techniques for the identification of degraded fossil pigments in lake sediments and their application in palaeolimnology

Accurate identification of fossil pigments is essential if they are to be used as biomarker compounds in palaeolimnological studies. In recent years High Performance Liquid Chromatography (HPLC) has greatly enhanced the efficiency with which fossil pigments can be characterised and quantified. Using HPLC, undegraded pigments are typically identified through retention times, absorbance spectra and co-chromatography with authentic reference standards. However, lake sediments may also contain degraded pigments for which there are often no standards, and which may be difficult to identify using HPLC alone. In this study, we submitted HPLC fractions of fossil pigments and pigment derivatives collected from a meromictic lake in south west Tasmania, to a combination of Mass Spectrometry (MS) techniques including Electron Impact (EI) and static Liquid Secondary Ion MS (LSIMS) to identify their molecular ion characteristics and organic chemical composition. Mass Spectrometry permitted the detection of specific mass ions which were used to verify the identity of pigments and their derivatives. These included five carotenoids, chlorophyll a and derivatives, three previously described bacteriochlorophyll c derivatives with molecular weights of 770, 784, and 802, and two undescribed derivatives of bacteriochlorophyll c with molecular weights of 766 and 788. With these improved identifications we speculate on the pathways and modes of pigment degradation in the lake and asses the value of the degraded pigments as biomarkers. The use of MS permitted the identification of a greater number of signature pigments of algal and bacterial communities thus increasing the palaeolimnological value of the sediments. These methods are best applied in fossil pigment studies where there are a large number of unknown pigments and pigment degradation products, and where there are no authentic standards for co-chromatography. Practical suggestions for pigment MS are included in the discussion.     

The potential of using LiDAR and color-infrared aerial imagery for palustrine wetland typology and change

Wetlands are dynamic landscapes and their spatial extent and types can change over time. Mapping wetland locations, types, and monitoring wetland typological changes have important ecological significance. The National Wetlands Inventory data suffer from two problems: the omission error that some wetlands are not mapped, and the out-of-date wetland types in many counties of the United States. To address these two problems, we proposed an automatic wetland classification model for newly mapped (or existing) wetland polygons lacking typological information. The research goals in this study were (1) to develop a nonparametric and automatic rule-based model to assign wetland types to palustrine wetlands using high-resolution remotely sensed data and (2) to quantify wetland typological changes based on the wetland types obtained from the previous step. The model is a direct application of the Cowardin et al. (1979) wetland classification system without modification. The input information for the proposed model includes Light Detection and Ranging (LiDAR)-derived vegetation height and color infrared aerial imagery-derived vegetation spectral information. We tested the model for the palustrine wetlands in Horry County, SC, and analyzed 29,090 palustrine wetland polygons (101,427 ha). The model achieved an overall agreement of 87% for wetland-type classification and showed the dynamics of wetland typological changes. This nonparametric model can be easily applied to other areas where wetland inventory needs updating.