Understanding Of Depositional Processes Research Articles

Eutrophication and contamination dynamics of Schweriner See, NE-Germany, during the past 670 years – A multi-proxy approach on lacustrine surface sediments and sediment cores

A multi-proxy study on sedimentary records from Schweriner See (See = lake), NE-Germany, covering the past 670 years (from 1350 CE to today) combined with sediment surface samples to better understand lake internal dynamics enables to reconstruct local and supra-regional eutrophication and contamination trends. Our approach shows that a thorough understanding of depositional processes is crucial for core site selection since at Schweriner See wave- and wind-induced processes in shallow water areas (e.g. reworking) or carbonate precipitation resulting from groundwater inflow may have altered the desired (in this case anthropogenic) signal. In Schweriner See both eutrophication and contamination have been directly influenced by sewage and population dynamics of the city of Schwerin and its surroundings. A higher population density resulted in increased sewage volume, which was discharged directly into Schweriner See since 1893 CE. Maximum eutrophication was reached in the 1970s but a solid improvement in water quality only occurred after the German reunification (1990) as a combined result of a decrease in population density and the connection of all households to a new sewage treatment plant, which stopped the discharge of sewage waters into Schweriner See. These counter measurements were traced within the sediment records. Eutrophication and contamination trends were detected within the lake basin as shown by remarkable similarities in signals between several sediment cores. To get an understanding of regional contamination tendencies east of the former inner German border in the recent past we compared our results with sediment records from the southern Baltic Sea area, which show similar contamination trends.

Fringe or background: Characterizing deep-water mudstones beyond the basin-floor fan sandstone pinchout

ABSTRACT Mud dominates volumetrically the fraction of sediment delivered and deposited in deep-water environments, and mudstone is a major component of basin-floor successions. However, studies of basin-floor deposits have mainly focused on their proximal sandstone-prone part. A consequent bias therefore remains in the understanding of depositional processes and stratigraphic architecture in mudstone-prone distal settings beyond the sandstone pinchouts of basin-floor fans. This study uses macroscopic and microscopic descriptions of over 500 m of continuous cores from research boreholes from the Permian Skoorsteenberg Formation of the Karoo Basin, South Africa, to document the sedimentology, stratigraphy, and ichnology of a distal mudstone-prone basin-floor succession. Very thin- to thin-bedded mudstones, deposited by low-density turbidity currents, stack to form bedsets bounded by thin packages (< 0.7 m thick) of background mudstones. Genetically related bedsets stack to form bedset packages, which are bounded by thicker (> 0.7 m thick) background mudstones. Stratigraphic correlation between cores suggests that bedsets represent the distal fringes of submarine fan lobe elements and/or lobes, and bedset packages represent the distal fringes of lobe complexes and/or lobe complex sets. The internal stacking pattern of bedsets and bedset packages is highly variable vertically and laterally, which records dominantly autogenic processes (e.g., compensational stacking, avulsion of feeder channels). The background mudstones are characterized by remnant tractional structures and outsize particles, and are interpreted as deposited from low-density turbidity currents and debris flows before intense biogenic reworking. These observations challenge the idea that mud accumulates only from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets (< 0.7 m thick) are interpreted to mainly represent autogenically driven lobe abandonment due to up-dip channel avulsion. The thicker background mudstones separating bedset packages (> 0.7 m thick) are interpreted to dominantly mark allogenically driven regional decrease of sand supply to the basin floor. The recognition of sandstone-prone basin-floor fans passing into genetically linked distal fringe mudstones suggests that submarine lobes are at least ∼ 20 km longer than previously estimated. This study provides sedimentological, stratigraphic, and ichnological criteria to differentiate mudstones deposited in different sub-environments in distal deep-water basin-floor settings, with implications for the accurate characterization of basin-floor fan architecture, and their use as archives of paleoenvironmental change.

Internal architecture of a till sheet, Tiskilwa Formation, north‐central Illinois, USA: Application of architectural element analysis

AbstractThick till sheets deposited during the Quaternary form significant aquitards in many areas of North America. However, the detailed sedimentary heterogeneity and architecture and depositional history of till units are not well understood. This study utilizes architectural element analysis to delineate the internal sedimentary architecture of the Tiskilwa Formation exposed at two outcrop sections in north‐central Illinois, USA. Architectural element analysis facilitates systematic delineation of sedimentary architecture based on the nature of facies contacts and change in facies associations, delineation of unit geometries and understanding of depositional processes at different scales of resolution; making architectural element analysis suitable for the sedimentological analysis and palaeoenvironmental reconstruction of subglacial deposits. Eleven facies types are identified in this study, including sand, gravel and diamict facies that record a suite of subglacial depositional processes. Detailed analysis of facies contacts (bounding surface hierarchy) and change in facies associations allows the delineation of five architectural elements, including coarse‐grained lens, coarse‐grained sheet, mixed zone, diamict lens and diamict sheet elements. The spatial arrangement and genetic interpretation of elements, and their spatial relationship with fifth‐order bounding surfaces, allows the delineation of five larger scale architectural units (‘element associations’), which can be mapped in the local study area and record at least three stacked successions of meltwater accumulation and till deposition. The results of this study can be utilized for architectural analysis of till sheets and provide insight to groundwater flow pathways through till in the study area and elsewhere.

3-D seismic geomorphology: Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea

Three-dimensional seismic geomorphology provides an indication of a reservoir's internal and external architecture. This furthers an understanding of depositional processes and allows a prediction of fluid flow during hydrocarbon production. Seismic images of four fluvial reservoirs from Widuri oil field show a range of features related to point-bar accretion and stacking pattern. Lateral variations in reservoir quality and thickness follow scroll-bar patterns associated with lateral migration of a moderate- to high-sinuosity meandering river. These variations control fluid flow during primary and secondary hydrocarbon recovery. Interpreted bar and abandoned channel dimensions, compared with meander map form and wavelength, are consistent with observations of modern-day examples, which provides confidence in the depositional model. The high seismic resolution of the Widuri reservoirs provides useful analogs for other subsurface reservoirs from similar depositional environments.

About this title

Manganese mineralization is diverse in occurrence, origin, mineralogy and geochemistry. These variations reflect differences in the processes of formation and depositional environments, which in turn are a response to changes in the land — ocean — atmosphere system over geological time. As such, manganese deposits can act as markers of major events in the dynamic evolution of the Earths surface. Modern manganese accumulations provide insights into key factors controlling manganese deposition that cannot readily be determined from examination of ancient ores. A knowledge of oceanic currents, ocean chemistry or small-scale variation in physicochemical patterns of recent basins, for example, may extend our understanding of depositional processes in the past. Equally, the study of Precambrian deposits not only elucidates ancient mechanisms of manganese metallogenesis, but also helps to unravel the impact of comprehensive environmental changes on metal deposition on a scale not realised in younger geological times. The volume includes a review of the range of terrestrial Mn deposits and their relative abundance through geological time. This is followed by six papers on Precambrian deposits that cover the manner in which Mn mineralization reflects changes in planetary and environmental chemistry. Ten papers on Cenozoic deposits show the wide diversity of genetic types of the younger deposits, from nodules, crusts and stratabound layers, to deposits precipitated from hydrothermal fluids. Five papers in the last section consider the geochemistry and mineralogy of Mn deposits. Experimental and modelling approaches to Mn geochemistry and mineralogy can further aid our understanding of the formational and depositional processes involved and thereby our interpretation of deposit metallogenesis.

Precambrian to modern manganese mineralization: Changes in ore type and depositional environment

Manganese mineralization is diverse in occurrence, origin, mineralogy and geochemistry. These variations reflect differences in the processes of formation and depositional environments, which in turn are a response to changes in the land-ocean-atmosphere system over geological time. As such, manganese deposits can act as markers of major events in the dynamic evolution of the Earth's surface. Modern manganese accumulations provide insights into key factors controlling manganese deposition that cannot readily be determined from examination of ancient ores. A knowledge of oceanic currents, ocean chemistry or small-scale variations in physicochemical patterns of recent basins, for example, may extend our understanding of depositional processes in the past. Equally, the study of Precambrian deposits not only elucidates ancient mechanisms of manganese metallogenesis, but also helps to unravel the impact of comprehensive environmental changes on metal deposition on a scale not realized in younger geological times.

Generation of a Synthetic Vertical Profile of a Fluvial Sandstone Body

Abstract Any stratigraphic section or bedding sequence can be synthesized if there is a transition procedure from one lithology or bedding type to another, and if thickness distributions of the different lithologies are known. Stratigraphic sections of a fluvial sandstone body were synthesized with five bedding types: cross-bedding, massive beds, parting lineation, ripple mark and mudstone. The transition procedure from one bedding type to another used dependent, Markovian random processes which have a memory that extends one step backward in the depositional process. As observed in nature, median grain size and sand wave thickness (cross-bedding and ripple mark) decline upward in the synthesized sections as proportions of the different bedding types change. Grain size and permeability were also incorporated into the sections. By changing the transition procedures, bed thickness distributions, rate of upward decline or sand wave height and length, different types of sections can be synthesized, thus making it possible to model many different sedimentation problems. Introduction This paper describes a general method for synthesizing stratigraphic sections and bedding sequences of sedimentary, metamorphic or igneous origin. Synthetic generation is of interest for several reasons. Close correspondence between real and synthetic sections suggest that the factors used in the synthesizing model may indeed be the correct ones, thus giving the investigator a check on his assumptions. Rapid, inexpensive simulation of many stratigraphic sections permits one to synthesize a rock body (sandstone or carbonate reservoir) or, on a larger scale, the fill of a sedimentary basin. Harbaugh gives an example of mathematical simulation of a carbonate basin. He simulated the basin in the hope that improved prediction would follow better understanding of the depositional processes. From the petroleum engineer's viewpoint it seems reasonable to believe that the synthetic generation of rock properties and their distribution in a reservoir should be relevant in the study of reservoirs. Any stratigraphic section or bedding sequence can be generated provided there is a transition procedure from one lithology or bedding type to another and provided the thickness distributions of the different units are known. The transition procedure involves random processes that are either independent or dependent. If the depositional process is independent, previous deposition will have no influence on present deposition. However, if it is dependent, past deposition will influence either present or future deposition. Such a dependent depositional process can be thought of as having a memory that extends backward in time through one or more pulses of deposition. A process with a memory can be described by a Markov process. Because the concept of memory or dependence appears to be in accord with our understanding of many depositional processes, Markov processes were used to synthesize the bedding sequences of this study (see Appendix). The above methods are perfectly general and are appropriate for any stratigraphic section or bedding sequence: bedding types in a beach deposit, an evolving carbonate bank or the changing lithologic fill of a thick geosyncline sequence. We chose to synthesize a vertical profile of a fluvial sandstone body because its characteristics were well documented, much was known about fluvial processes and fluvial-deltaic sandstone bodies constitute an important class of petroleum reservoirs. CHARACTERISTICS AND ORIGIN OF FLUVIAL CYCLE The fluvial cycle has been well documented in recent years by Bersier, Allen and Visher. Deposits from fluvial cycles range from 10 to 150 ft or more in thickness and are characterized by a "fining upwards": coarse sandstones with occasional conglomerates grade upward into medium- to fine-grained sandstone, and hence into siltstone and mudstone. SPEJ P. 243ˆ

ORIGIN OF ALLUVIAL FANS, WHITE MOUNTAINS, CALIFORNIA AND NEVADA1

(1963). ORIGIN OF ALLUVIAL FANS, WHITE MOUNTAINS, CALIFORNIA AND NEVADA. Annals of the Association of American Geographers: Vol. 53, No. 4, pp. 516-535.

Depositional Topography--Examples and Theory

Depositional topography which in profile resembles the classic delta is demonstrated with electric well-log cross sections and isopach maps in the late Pennsylvanian and early Permian of western Texas. Distinctive limestone beds can be traced readily over the undaform (top-set beds), and down the clinoform (fore-set slope) where they thin and commonly disappear, or in some places grade into resistive black shale which extends far out onto the fondoform (bottom-set beds). The limestones are thickened in many places along the outer edge of the undaform into organic reefs (and banks) which are termed undaform-edge reefs. These grew when relative sea-level rose, which caused most terrigenous detritus to become trapped in the stream valleys and estuaries. The clay and sand were deposited at sea mostly when its level fell, or after it had remained stable for some time. These two predominant lithologic types--limestone, and shale with sandstone--recur again and again, producing an hierarchy of sedimentary cycles that reflect the sea-level fluctuations. The cycles vary across strike from mere changes in the rate of deposition on the fondoform through more typical cyclothems to presumed changes in the rate of erosion in the source areas. Waves and currents on the undaform sorted the detritus supplied it into dissimilar fractions, a kind of sedimentary differentiation. The coarser fraction, potential reservoir rock, ordinarily remained on the undaform, but at times large amounts were carried onto the fondoform. The finer fraction ordinarily settled in deeper water beyond the edge of the undaform, especially on the clinoform. The younger prominent clino limestones occur successively farther west, thus recording the progressive filling-in of a relatively deep sea by undaform advance. This process, plus sedimentary differentiation, produced a distinctive vertical stratigraphic succession identical with what has been termed the geosynclinal sequence. The essential conditions for this sequence are merely a sea floor well below the depth of wave agitation and a good supply of terrigenous detritus. Similar depositional topography is evident in many other places, including the Mesozoic and Tertiary of Texas and Louisiana. The major undaforms in this region constituted the continental shelves of their time, and the clinoforms the continental slopes. Recognition of depositional topography improves stratigraphic correlations, and the understanding of depositional processes and history. Reservoir beds related to the topography can be mapped and traps sought along them, and the relations of depositional topography to structure may disclose structural features.