Alluvial Complexes Research Articles

Sequence Stratigraphy of Subcontinental Reservoirs of the Upper Part of the Tanopcha Formation of the Minkhov Field (North of Western Siberia)

Abstract ––The purpose of this paper is to present the results of sequence stratigraphic modeling of subcontinental reservoirs of the Tanopcha Formation upper part (Lower Cretaceous) of the Minkhov field, located in the northern part of the Messov oil and gas region of the Gydan oil and gas region. The study is vital due to the need for additional exploration and commissioning of hydrocarbon reserves in the lower part of the Pokur suprahorizon in the north of Western Siberia. The modeling is based on sedimentological columns for four wells, in which the object of study was characterized by a core with sufficient completeness. The structure of seven sequences has been studied. Sequences PK 2-2, 2-3, 2-4, 2-5, 2-6, and PK 3-1, in accordance with the classical concept, are divided into three systems tracts: lowstand (LST), transgressive (TST), highstand (HST). The boundaries of the sequences are outlined along the erosive surfaces in the bottom of the alluvial complexes. LST tops – on contacts of alluvial facies with tidal, marsh, lake sediments, indicating flooding of the coastal plain at the beginning of transgressions. TST tops – on contacts of facies of the tidal plain with alluvial deposits, indicating the beginning of normal regressions. The main reservoirs of the object are large belts and covers of alluvium of amalgamating channels as part of the LST. Additional reservoirs are thin bodies of tidal genesis in the composition of TST, and alluvial genesis in the composition of HST. The bottom of the PK 3-2 sequence was outlined on the contacts of coastal-marine TST sandstones with tidal and marsh facies, indicating the event transgression of the coastline. One of the main reservoirs of the field, the TP1 formation, is connected to the TST of the PK 3-2 sequence. The boundaries of the selected sequences can be traced on seismic sections of acoustic impedance. Depending on the temporary thickness in the impedance field, the sequences are displayed as two-, three- and four-layer sequences of layers, which opens up the possibility of their further study by the method of wave field inversion.

Palaeoenvironmental reconstruction of the early Anisian from sedimentology and plant remains in the SE Iberian Range (E Spain)

The end of the Permian period is characterized by the largest mass extinction ever recorded in marine and terrestrial sedimentary rocks. The oldest plant macro-remains after the P–T boundary recorded in the Castilian Branch of the Iberian Range after this mass extinction belong to the Eslida Formation of Anisian age. These associations have been recovered at four localities and include sphenophytes (Equisetites, perhaps Neocalamites or Schizoneura), conifers (Volzia, Albertia, Pelourdea), lycophytes (Pleuromeia) and a possible seed fern (Peltaspermum).Facies associations and sedimentological analysis of the Eslida Formation have allowed for distinguishing four architectural elements: CH (channel deposits), LA (lateral accretion), CS (crevasse splay) and FF (overbank fines). The largest fossil remains are associated with CH and LA architectural elements and high-energy events, and they form part of the basal lag or reactivation surfaces. Smaller and well-preserved fragments are associated with the CS architectural elements.A sedimentologic, taphonomic and palaeoclimatic analysis reveals gradation from riparian hygrophytic vegetation to semi-arid xerophitic vegetation across the alluvial complexes and a progressive aridification through time. A comparison with other areas shows that this association is similar to coeval floras of Iberia and of the “Grès à Voltzia” Formation of northeastern France.

Les bryophytes du corridor alluvial de la vallée du Rhône : flore, végétation et fonctionnalité

Bryophytes of the Rhône alluvial complexes : flora, vegetation and functionality. Bryophytes are still too rarely used in ecological diagnostic, in particular in alluvial systems. The surveys carried out on a section of the Rhône, going from Jonage to Montélimar, allowed to draw up a first synthesis of the bryoflora and to describe the bryophyte vegetation of the Rhône alluvial complexes. The river dynamics (erosion, sedimentation, flooding) has fundamental consequences from floristic and vegetation points of view. The ecological constraints (hydric, nutrients), expressed from the main channel up to the flood plain, are responsible for a high floristic and vegetation richness. The anthropization, although a factor of enrichment, represents a major degradation of the typical bryoflora of the connected alluvial system. In highly artificialized systems, the disconnection allows the preservation of refugium habitats. The preservation of the bryophytic heritage rests on a subtle balance between re-energization of the river and management of the human pressure.

Microzonation of Potenza (Southern Italy) in terms of spectral intensity ratio using joint analysis of earthquakes and ambient noise

A temporary seismic network composed of 11 stations was installed in the city of Potenza (Southern Italy) to record local and regional seismicity within the context of a national project funded by the Italian Department of Civil Protection (DPC). Some stations were moved after a certain time in order to increase the number of measurement points, leading to a total of 14 sites within the city by the end of the experiment. Recordings from 26 local earthquakes (Ml 2.2−3.8 ) were analyzed to compute the site responses at the 14 sites by applying both reference and non-reference site techniques. Furthermore, the Spectral Intensity (SI) for each local earthquake, as well as their ratios with respect to the values obtained at a reference site, were also calculated. In addition, a field survey of 233 single station noise measurements within the city was carried out to increase the information available at localities different from the 14 monitoring sites. By using the results of the correlation analysis between the horizontal-to-vertical spectral ratios computed from noise recordings (NHV) at the 14 selected sites and those derived by the single station noise measurements within the town as a proxy, the spectral intensity correction factors for site amplification obtained from earthquake analysis were extended to the entire city area. This procedure allowed us to provide a microzonation map of the urban area that can be directly used when calculating risk scenarios for civil defence purposes. The amplification factors estimated following this approach show values increasing along the main valley toward east where the detrital and alluvial complexes reach their maximum thickness.

The use of NEXTMap Britain for geological surveying in the Vale of York

Abstract The NEXTMap Britain digital elevation model (DEM) has opened many new opportunities that considerably help and enhance the way we undertake our geological mapping of bedrock, structure, and superficial and artificial deposits. The dataset has been successfully integrated into the digital and conventional mapping workflows of the Vale of York mapping team. A variety of visualization and analysis techniques have been applied throughout the mapping process. These techniques include an initial appraisal of NEXTMap with a comparison to existing geological mapping to define the field mapping strategy and site-specific manipulation using Tablet PCs. NEXTMap interpretation has made an important contribution to the understanding of the extensive glacial and proglacial deposits found in the Vale of York; such as sand bodies resting on lake deposits, and identifying details within morainic and alluvial complexes. For bedrock mapping, NEXTMap has been used to identify landform features that relate to the underlying geology, such as breaks in slope, the extent of escarpments, hillcrests and dip slopes, to provide an overview of the landscape and to save time in mapping out features in the field. Techniques have also been developed to automatically generate these landform features. The dataset has also been used to identify areas where landsliding has occurred, for the accurate mapping of artificial ground and as a key surface for three-dimensional (3D) geological modelling.

Mapping North African landforms using continental scale unmixing of MODIS imagery

We describe the production of a landform map of North Africa utilizing moderate resolution satellite imagery and a methodology that is applicable for sub-continental to global scale landform mapping. A mosaic of Moderate Resolution Imaging Spectroradiometer (MODIS) apparent surface reflectance imagery was compiled for Africa north of 10° N. Landform image endmembers were chosen to characterize ten different types of vegetated and unvegetated desert surfaces: alluvial complexes, dunes, dry and ephemeral lakes, open water, basaltic volcanoes and flows, mountains, regs, stripped, low-angle bedrock surfaces, sand sheets, and Sahelian vegetation. Multiple Endmember Spectral Mixture Analysis (MESMA) was applied to the MODIS mosaic to estimate landform and vegetation endmember fractions. The major landform in each MODIS pixel was identified based on the majority endmember fraction in two- or three-endmember models. Accuracy assessment was conducted using two data sources: the historic Landform Map of North Africa [Raisz, E. (1952). Landform Map of North Africa. Environmental Protection Branch, Office of the Quartermaster General.] and Landsat Thematic Mapper (TM) data. Comparison with the Raisz landform map gave an overall classification accuracy of 54% with significant confusion between alluvial surfaces and regs, and between sandy and clayey surfaces and dunes. A second validation using 20 Landsat images in a stratified sampling scheme gave a classification accuracy of 70%, with confusion between dunes and sand sheets. Both accuracy assessment schemes indicated difficulty in vegetation classification at the margin of the Sahel. A comparison with minimum distance and maximum likelihood supervised classifications found that the MESMA approach produced significantly higher classification accuracies. This digital landform map is of sufficiently high quality to form the basis for geomorphic studies, including parameterization of the surface in global and regional dust models.

Palaeoenvironmental interpretation of a Pleistocene alluvial succession in central Poland: sedimentary facies analysis as a tool for palaeoclimatic inferences

A Pleistocene valley‐fill alluvial succession deposited in the Kleszczów Graben, central Poland, has been studied in the Belchatów openpit mine. The succession, palynologically documented to represent the Drenthe/Warthe interstadial, consists of three alluvial complexes whose component lithofacies associations indicate a fluvial system evolving from temperate‐climate meandering river to transitional‐type shallow braided network, to periglacial well‐developed braided river influenced by aeolian sand supply. The study suggests that the abundance of fine‐grained overbank deposits, occurrence of peats/palaeosols and fining‐upward cyclothems are diagnostic attributes of perennial meandering river alluvium, which may indicate temperate climatic conditions. Periglacial braided river alluvium is recognizable by an admixture of wind‐derived sand grains with aeolian surface textures and by the occurrence of ice‐wedge features, indicative of cold climatic conditions. The distinction between the two basic types of alluvium is aided by the analysis of architectural elements and palaeocurrent directional data. The study demonstrates that sedimentological facies analysis can be a useful tool for the recognition of palaeoclimatic changes in Pleistocene alluvial successions.

The infill of a supradetachment(?) basin: the continental to shallow-marine Upper Permian succession in the Dolomites and Carnia (Italy)

In the Dolomites and Carnia (eastern Southern Alps), the Upper Permian succession is represented by red beds of Val Gardena Sandstone, grading upwards and eastwards into the evaporitic and carbonate deposits of the Bellerophon Formation. An overall transgressive pattern is shown by the general trend of the depositional setting, which evolved from flashy alluvial fans, through multiple-channel bedload rivers, mixed-load sinuous rivers, terminal fans, coastal sabkha and evaporitic lagoon, to a shallow, low-gradient marine ramp. The inferred fluvial regime was subject to rapid and erratic fluctuations in discharge. Palaeosols are represented by calcic soils, and suggest a warm to hot, semi-arid or dry subhumid climate with strongly seasonal rainfall. Vertic features are associated with more inland alluvial complexes: they are missing in the terminal-fan deposits, suggesting greater aridity in lowland (coastal) areas. The Bellerophon Fm. consists of two units: a lower evaportte-bearing unit, deposited in a barred basin, and an upper shallow-marine carbonate unit, laid down on a very low-energy, low-gradient ramp. Five third-order sequences and the lower part of a sixth sequence, collectively showing a backstepping pattern, have been identified in the succession. Due to the presence of a very low-gradient ramp margin, and the consequent ineffectiveness of relative sea-level falls in producing large-scale erosion in coastal areas, it is suggested that, rather than eustatic changes, degradational episodes marking sequence boundaries in the red-bed succession reflect changes in the climate-modulated intrinsic variables of discharge and sediment supply, and/or tectonic uplift. Base level rises in, red-bed sequences are recorded by upward change from thick channel-belt sandstone bodies with an often high connectedness ratio, to progressively thinner channel deposits, ranging from isolated ribbons to channel-belt sandstone bodies potentially reaching high width/thickness ratios, encased in a comparably greater volume of overbank fines. This trend is thought to reflect the change from a confined geomorphic setting, with a limited area of potential avulsion, to an unrestricted setting with rivers free to move extensively; in addition, it documents the transition from an inland fluvial system with laterally migrating perennial or semi-perennial channels, to an ephemeral network of randomly migrating and frequently avulsing small terminal-fan distributaries, through a drastic downstream decrease in channel depth and discharge. Identification of key sequence-stratigraphic relationships within the red-bed succession was mostly aided by the presence of easily recognizable and regionally correlated marine and marginal marine bed packages, traceable landwards into alluvial deposits showing faint traces of tidal activity, interpreted as the equivalent of marine maximum-flooding sediments. They may grade upwards into progradational fluvial packages showing basinward increase in thickness. The Upper Permian deposits of the Southern Alps are considered part of an Upper Permian-Lower Triassic, second-order, structurally controlled sequence. The location of the basin on a thickened, previously active crust, affected by thermal perturbance after the last stages of the Variscan orogeny, the relatively reduced thickness of the basin fill, and predominantly long, transverse drainage networks, mostly derived from the denuded Insubric footwall, all suggest that sedimentation took place in a supradetachment basin, with a major detachment fault located in the palaeo-Insubric belt.