Low-ash Peat Research Articles

Consolidation test on peat soil of Humbang Hasundutan regency, North Sumatera Province

The purposes of this study were to determine the index properties of peat soil, to determine the classification of peat soils and to determine the consolidation coefficient (Cv ) and compression index value (Cc ) of peat soil in Nagasaribu Village, Lintong Nihuta sub-District, Humbang Hasundutan Regency, North Sumatera province. This research was conducted through a consolidation test with direct loading of 9.810 Newton and 19.613 Newton and gradually loading of 9.810; 19.613; 39.226; 78.453; and 156.96 Newton. The peat soil in Nagasaribu Village, Lintong Nihuta sub-District, Humbang Hasundutan Regency has a high water content (w), of 628.139%, specific gravity (Gs) of 1.301, moist unit weight (γ) of 10.125 KN/m3, dry unit weight (γd) of 1.390 KN/m3 and void ratio (e) of 8.157. Based on ASTM D4427-92 (2002), the sample of peat soil was classified as low ash peat because it had 0.780% ash content and 26.500% organic content. And according to the acidity level with a pH of 4,000, the sample was classified as highly acidic peat soil. The compression index (Cc ) of sample 1 was 3.044 and sample 2 was 3.574 and the consolidation coefficient (Cv ) of sample 1 was 2.155 cm2/sec. and sample 2 was 1.818 cm2/sec.

Prospects for the use of peat in metallurgy

A large number of metallurgical industry enterprises in the regions of Russia mainly use imported and rather expensive coking coal or natural gas for steelmaking. However, almost all regions of Russia have large reserves of peat, which can replace expensive hydrocarbon raw materials in metallurgical processes. For this reason, the use of peat raw materials (briquette, piece) in metallurgy is of scientific and practical interest. The properties of peat briquettes and pellets in the process of pyrometallurgical processing are set by the methods and technological parameters of their preparation. The article presents the results of a study of the influence of the type and quality properties of peat on the technological process of recovering peat compositions prepared on the basis of iron ore from the Bakchinsky deposit. Peat of the Vasyugan peat massif was used for the production of peat melting compositions. Experimental data showed that, for peat compositions, low-ash peat on the watershed is suitable. The use of peat melting compositions allows us to solve the problem of the return of ore dust and substandard ore in the smelting processes and thereby develop a rational low-waste technology.

The study of highly mineralized peat sedimentation products in terms of their use as an energy source

Peat belongs to the slowly renewable biomass resources, therefore its use as an energy source helps to reduce the negative impact on the environment. However, its use in the energy sector is gradually decreasing due to high operating costs because of the characteristics of peat. The aim of this work is a comprehensive study of the peat characteristics and the development of recommendations for its effective use. In this paper, we consider the widespread methods for studying fuel (thermo-technical and fusibility characteristics determination, X-ray fluorescence and phase methods), as well as the original approach of separation by sedimentation in liquids of various densities. The peat separation by sedimentation in liquids of various densities made it possible to obtain a low-ash peat fraction comparable in characteristics to brown coals and a highly mineralized fraction (heavy fraction). On the example of grain processing waste, it is shown that the addition of a 5% of heavy peat fraction to bran prevents the formation of strong slag deposits and can be recommended for co-burning. This allows us to consider the separation by sedimentation in liquids of various densities as an effective tool for using highly mineralized types of biomass in energetic sector.

Amelioratory plowing of peat bogs and substantiation of the basic parameters of a double-cut plow

Drained peat soils in Belarus occupy 11.3 % of agricultural land. These soils lost their genetic traits on the area of almost 260 thousand hectares, and moved into the category of anthropogenically transformed, over 190 thousand hectares of those – into the category of degraded. The most important condition for improving the properties of low-ash peat soils is an increase of ash elements level – up to 50–70 % in the plowing layer. The anthropogenically formed layer-mixed culture of soil profile with deep reclamation plowing method corresponds to this to the fullest extent. Its use is important on shallow peatlands that quickly lose their organic layer. It has been determined that the depth of such plowing is determined both by the thickness of peat bog and by the thickness of plowed mineral soil (sand or sandy loam). Based on the obtained analytical expressions, analysis of process layouts for formation of new soil profile by the plow, taking into account availability of peat soil areas according to deposit capacity, feasibility of using two types of special plows is substantiated: double-cut mounted type with plowing depth of 50–100 cm and single-cut trailed type with plowing depth of 90–150 cm. Technologically rational is plow operation according to layout with undercut equal to the width of sandy layer in sub-plowing horizon. At the same time, the interval of rational values of structural width of double-cut plow grip (on the II cut body) b k = 50–67 cm, and the width of plow grip shall not exceed 100 cm. Ratio of its actual value to the accepted design width of the bodies is in the range of (5 : 4)–(2 : 1) depending on agrotechnical requirements. The designed PTN-0.9 double-cut plow for deep amelioratory plowing has passed acceptance tests and is recommended for serial production. The result of the study is theory of dump plowing. The results allow for development of special double-cut plows for deep amelioratory treatment of shallow peat bogs, which ensures their preservation during agricultural use.

Portable X-ray fluorescence analysis of mineral and organic soils and the influence of organic matter

Portable X-ray fluorescence (PXRF) spectrometers with tube X-ray source are being used to determine the elemental composition of soils in the laboratory and the field. Most studies with PXRF have been with mineral soils and there is a need to assess PXRF applications to organic soils and peats. Using a commercial PXRF instrument and the manufacturer's soils programme we assessed performance by the analysis of various soils with a range of organic matter concentrations up to those of low ash peats. We first analysed seven certified reference mineral soils and a certified, low ash ombrotrophic peat. The data obtained for the certified reference mineral soils was definitive for Ti, Cr, Mn, Fe, Ni, Cu, and As, quantitative for K, Ca, Zn and Sr, and qualitative for Pb. Portable X-ray fluorescence analysis of the ombrotrophic peat gave satisfactory results for Cu (4.00±1.00mgkg−1, certified 5.28±1.04mgkg−1) and Pb (184±3mgkg−1, certified 174±8mgkg−1) but overestimated the concentrations of Ca, Ti, Cr, Ni and Zn by 2–3 times, and Fe by 5 times. To extend comparison beyond the only available low ash certified reference soil and further exemplify the differences between mineral soils and peat soils we analysed 183 Scottish topsoils that had a wide range of organic carbon (OC) concentrations (1.23 to 48.8% by weight) and compared the concentrations of K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Sr and Pb determined by PXRF with pseudo total concentrations determined by aqua regia extraction and inductively coupled plasma spectrometry. The results demonstrate that to maximise the potential of the PXRF instrument for use beyond mineral soils, to organo-mineral and peats, modifications to the manufacturer's calibrations should be made. We recommend validation using soils on a continuum of OC concentrations from those of mineral soils to peats covering the range required.

An experimental and modeling study of ash deposition behaviour for co-firing peat with lignite

Co-firing peat with lignite for power generation was studied at pilot-scale, focusing on the issue of ash deposition, the major concern for this application. A specially designed probe was used to measure the rate of ash deposition under similar conditions of power plant boiler operation. Fraction of peat in the feed was varied up to 100%. It was observed that whereas the ash deposition decreased in general with increasing fraction of low-ash peat, the decrease was significantly less than expected from ash content of the feed, suggesting higher deposition tendency of ash from the peat. Chlorine content in the deposited ash showed a maximum at a certain blend ratio of the feed, which could not be explained by fuel chlorine content alone. The ash and chlorine deposition behaviour has been analyzed and simulated by mathematical models where interactions between fuel chlorine, alkali and ash particles are parameterized. The models ought to be useful for control and optimization purposes, and also be useful for co-firing other fuel blends.

Neo-mineral formation during artificial coalification of low-ash — mineral free-peat material from tropical Malaysia-potential explanation for low ash coals

Abundant coal deposits have very low inorganic contents (ash yield; < 2%) and hence are almost ash-free. Inexplicable to date is that the precursor of coal, modern peat deposits, almost nowhere have such low ash yields as a result of both the inorganic mineral and biogenic phytolith contents. However, despite the common occurrence of phytoliths in modern peats, they are invariably absent in coals. It has thus been hypothesized by some investigators that modern environments are not ideal analogues for the formation of low ash coal deposits. Here we present new evidence to suggest that mineral diagenesis during coalification can transform biogenic inorganics to new minerals and that a substantial component of these new minerals could be removed in solution during expulsion of moisture with coalification resulting in low ash precursors of coal. This study presents results from artificial coalification experiments of modern tropical forest peat material with ash yields between 2–4.3 wt.%. The inorganic material of the peat consists almost exclusively of Al–Si-and Si-rich phytoliths and other bioliths; no other inorganic mineral matter is present. Compressed pellets of dried peat were deformed in a Griggs constant strain rate apparatus at a constant confining pressure of 5 kb and constant strain rate of 10 − 5 s − 1 at variable temperatures from 350 °C to 550 °C. The samples, exposed to artificial coalification processes, were then analysed by SEM, EDS, and XRD for semi-quantitative chemical analyses. The deformed material showed a lack of any biogenic silica and Al–Si-phytoliths, but contained neoformed idiomorphic quartz crystals and clays. We conclude that modern peat forming environments that have low-ash peats containing biogenic silica and other biogenic Al–Si-material can represent precursors of very low-ash coal deposits. Our experiments illustrate that during coalification, it is likely all or almost all biogenic material is transformed into new minerals, mainly quartz and clay minerals, such as kaolinite. Because natural systems are not confined to the same degree as our experiments, it is likely that part or all of the inorganic fraction migrates in solutions out of the peat or coal seam with available fluids, resulting in an almost ash-free coal deposit.

Implications of soil substrate and land use for properties of fen soils in North-East Germany Part I: Basic soil conditions, chemical and biological properties of topsoils

The objective of the paper was to analyse the implications of the origin of peat (muck) soil substrate, the current type of land use and the state of anthropogenic soil development for the topsoil properties of fens. Chemical and biological properties of peat soils of the Rhin-Havelluch lowland and the Uckermark rural landscape were analyzed. The unit water content according to Ohde and the ash content were utilized to characterize the anthropogenic development status of peat topsoils. Several chemical properties were significantly influenced by soil substrate, in particular by the proportion and kind of the mineral component. The substrate was associated with the hydrological type of mire and the soil development state. TOC/N ratio and microbial activity were increased in cases of high lime spring mires and moorshified low ash peat. The proportion of easily soluble organic carbon increased, whereas the sulphur content decreased with the soil development state. The nitrogen content and the proportions of oxalate soluble iron and aluminium reached maxima in the moorshified state. The type of land use (grassland, forest) significantly influenced the topsoil pH and the proportion of oxalate soluble phosphorus. Soils under forest were clearly determined by topsoil acidification.

Low-ash peat deposits from a dendritic, intermontane basin in the tropics: a new model for good quality coals

Many good quality, thick, low-ash Carboniferous, Cretaceous and Tertiary coal deposits have dulling-upward sequences that have been interpreted to be of ombrotrophic, lowland, coastal origin. Previous studies of potential modern analogues for thick, dulling-upward coal sequences have focused on tropical, ombrotrophic coastal peats. However, most modern coastal ombrotrophic deposits would yield “all bright” coal seams. New evidence from peat deposits of the southern intermontane Tasek Bera Basin, Western Malaysia, reveals that thick, low-ash, low-sulfur peat may originate in narrow tributary valleys with moderately steep flank gradients. These deposits may be favorable precursors to dulling-upward coals, in that they contain high wood and low-ash content at depth and medium wood and slightly increasing ash content in the upper parts. Peat accumulation in the Tasek Bera Basin began in Mid-Holocene time. During the past 4500 years, 500 cm of low-ash (<5 wt.%), low-sulfur (<0.4 wt.%) woody peat was deposited in narrow (400–700 m), elongate (3 km) tributary valleys of the southern Tasek Bera swamp system. Before onset of peat accumulation, a thick (>40 cm) white, fluvial silty clay with wood fragments was deposited. The clay-rich sediments are overlain by limnic black mud, sandy clay and dark-brown mud-rich peat up to 70 cm thick. The ash content of the mud-rich peat is as high as 31 wt.%. The high-ash peat is overlain by low-ash, sapric to hemic peat up to 5 m thick. The basal 3 m of the peat deposits contain abundant logs and large woody fragments, whereas the top 2 m have less hard wood, but more arborescent fragments of palmae, such as roots from Pandanus atrocarpus. The acrotelm (top 50 cm) has a low-to-medium ash content (<10 wt.%). With burial, the deposits of southern Tasek Bera Basin would yield a dendritic sediment pattern of sandstone, shale, carbonaceous shale and a dulling-upward, vitrain-rich coal, overlain by carbonaceous shale. Because of the dendritic nature of the Tasek Bera Basin, coal seams would most likely have a similar pattern as the Carboniferous humic coal deposits of the Black Warrior Basin in Alabama (USA). However, the good quality coals with dulling-upward characteristics would be concentrated in the southern tributaries of the basin.

97/03460 The neogene coal deposits of the Mukah-Balingian region, Sarawak, NW Borneo

Preliminary sedimentologic investigations were conducted in the Rajang River delta/coastal plain system in Sarawak, East Malaysia, to determine the processes and timing of siliciclastic transport and sedimentation in tropical, peat-dominated systems. Methods involved depth profiling and collection and analysis of bottom and suspended sediment load samples from channels and offshore areas, and coring and trenching of subaerial channel, shoreline, and backswamp features. The Rajang delta occurs in an embayment formed by the folded Mesozoic and Cenozoic sediments of the Central Borneo Massif. It covers an area of 11,000 km2 and is physiographically separated into an alluvial valley floodplain, an abandoned tidally flushed delta plain, and an actively accreting rectilinear delta/coastal plain. From 50 to 80% of the surface area is covered by peat 1 to 20 m thick. During the dry season of July/August, 1992, river channels varying from 2 to 45 m deep with discharge rates from < 100 to approximately 3550 m3/s were carrying suspended sediment loads ranging from 0 to 2281 mg/l. At present, approximately 8.2 × 1010 kg/yr of fine-grained sediment is supplied to the system and areal accretion rates are over 1.0 km2/yr. The structural configuration of the uplands/receiving basin and subsequent drainage patterns and tidal influence results in a contradictory distribution of mature, fine-grained sediments in the channels of the active delta/coastal plain, and immature, coarse-grained angular sands and gravels in the channels of the abandoned delta lobe. The contemporaneity of the peat and clastic accumulation is evidenced by the margins of the thick, domed, low ash peat deposits interfingering with and being overlain by root-penetrated siliciclastic sediments. Much of the fine-grained sediment, however, bypasses the delta plain due to in-channel density gradients. Delta/coastal plain accretion occurs by distributary mouth bar buildup, sand bar welding, and ridge and runnel accretion. All subaerially exposed surfaces are quickly vegetated. The aberrant depth and configuration of some channels suggest that drainage is, in part, controlled by contemporaneous faulting.

Distribution of radiocaesium in organic soils

Radiocaesium added to organic soils remains plant-available far longer than might be predicted from the behaviour of the element in mineral soils. To investigate this, the distributions of radiocaesium added to a low-ash peat and a peaty podzol were studied using sequential extraction with ammonium acetate, sodium pyrophosphate, sodium hydroxide, and ammonium acetate after peroxidation. Comparisons were made with the distribution reported for mineral soils. Changes in distribution between soil phases with time were measured on a sequence of samples treated with radiocaesium over a period of 4 years. Extractions were performed on soils pretreated with HF to remove mineral matter, on soil fractions enriched or depleted in mineral matter isolated physically by means of wet sieving, and on soil fractions obtained by density separation with chloroform and ‘Nemagon’. These experiments showed that, even in a highly organic peat, the mineral components were important for the eventual fixation of radiocaesium. Oxidizable organic matter corresponding to humin also strongly retained caesium.

Provenance and sediment dispersal in the Rajang River delta/coastal plain system, Sarawak, East Malaysia

Preliminary sedimentologic investigations were conducted in the Rajang River delta/coastal plain system in Sarawak, East Malaysia, to determine the processes and timing of siliciclastic transport and sedimentation in tropical, peat-dominated systems. Methods involved depth profiling and collection and analysis of bottom and suspended sediment load samples from channels and offshore areas, and coring and trenching of subaerial channel, shoreline, and backswamp features. The Rajang delta occurs in an embayment formed by the folded Mesozoic and Cenozoic sediments of the Central Borneo Massif. It covers an area of 11,000 km2 and is physiographically separated into an alluvial valley floodplain, an abandoned tidally flushed delta plain, and an actively accreting rectilinear delta/coastal plain. From 50 to 80% of the surface area is covered by peat 1 to 20 m thick. During the dry season of July/August, 1992, river channels varying from 2 to 45 m deep with discharge rates from < 100 to approximately 3550 m3/s were carrying suspended sediment loads ranging from 0 to 2281 mg/l. At present, approximately 8.2 × 1010 kg/yr of fine-grained sediment is supplied to the system and areal accretion rates are over 1.0 km2/yr. The structural configuration of the uplands/receiving basin and subsequent drainage patterns and tidal influence results in a contradictory distribution of mature, fine-grained sediments in the channels of the active delta/coastal plain, and immature, coarse-grained angular sands and gravels in the channels of the abandoned delta lobe. The contemporaneity of the peat and clastic accumulation is evidenced by the margins of the thick, domed, low ash peat deposits interfingering with and being overlain by root-penetrated siliciclastic sediments. Much of the fine-grained sediment, however, bypasses the delta plain due to in-channel density gradients. Delta/coastal plain accretion occurs by distributary mouth bar buildup, sand bar welding, and ridge and runnel accretion. All subaerially exposed surfaces are quickly vegetated. The aberrant depth and configuration of some channels suggest that drainage is, in part, controlled by contemporaneous faulting.

Development of Low-ash, Planar Peat Swamps in an Alluvial-plain Setting: the No. 5 Block Beds (Westphalian D) of Southern West Virginia

ABSTRACT Coals from the No. 5 Block coal beds (Westphalian D) of the central Appalachian basin are noted for their blocky, dull character and their low ash and low sulfur content. The beds are multiple benched, with rock partings separating benches. Individual benches have limited lateral extent and, where thick, are dominated by bright, high-ash coal at the base and dull, low-ash coal in the upper parts. The duller coals contain more exinite-group and inertinite-group macerals than the brighter coals. These coal beds are encased in sandstone units dominated by fining-upward sequences. The overall depositional setting is an alluvial-plain environment with northwest-flowing channels spaced approximately 20 km apart. The channels were flanked by clastic swamps about 7 km wide. Low-ash peat accum lated in areas of the flood plain most distant from the channels. These peat-accumulating swamps were about 8 km across. In a few instances low-frequency flood events introduced fine siliciclastic sediment into the peat swamps, depositing a thin layer of sediment on top of the peat. This sediment layer is thicker where the underlying coal is the thickest. These thick coal areas are topographically lower than surrounding coal areas. This relationship between coal thickness, parting thickness, and topography indicates that these peat swamps were planar at the time of deposition. Individual coal benches contain abundant preserved cellular tissue (telocollinite, semifusinite, and fusinite) at most locations, suggesting that robust vegetation was widespread in the swamps and that the morpholo y was planar. The high concentrations of exinite-group an inertinite-group macerals in the upper parts of benches resulted from selective decomposition and oxidation of the peat in subaerial and aquatic planar-swamp environments.

Translocation and plant availability of radio caesium in an organic soil

134Caesium chloride solution was injected into the surface of a peaty podzol at 3 cm depth with 5 cm spacing over a M2 at two upland sites, one of which had been fertilized and reseeded. The movement of radio caesium in the soil was subsequently followed by coring and/or taking out 10-cm square blocks at monthly intervals over a period of 2 years. There was very little movement of the caesium down the profile, with more than 95% remaining at the point of application. Lateral movement was also minimal. The caesium did not concentrate in the roots. Samples of herbage collected from the surface during the growing season showed a progressively decreasing concentration of caesium over the period. Total caesium removed in herbage amounted to 3.5 and 0.6% of the original present for the improved and umimproved soils, respectively. The extractability of the caesium from the 134Cs-amended peaty soil was compared with that from a low-ash peat which had been treated with the radionuclide for 2 months. Initially 1 M ammonium acetate (pH 7.2) extracted 30 and 56% of the Cs from 0–3 and 3–6 horizous of the improved peaty podzol, respectively. After 5 months only about 12 and 7% of the Cs was extracted by 1 M ammonium acetate and 4 and 1% by 0.1 M sodium pyrophosphate from the 0–3 and 3–6 cm horizons, respectively. After 24 months 1 M ammonium acetate extracted 1.6 and 3.3% of the Cs from the 0–3 and 3–6 cm horizons, respectively. In contrast, the values for the peat after 2 months were 100 and 80% for 1 M ammonium acetate and 0.1 M pyrophosphate, respectively.

Facies development in the Lower Freeport coal bed, west-central Pennsylvania, U.S.A.

The Lower Freeport coal bed in west-central Pennsylvania is interpreted to have formed within a lacustrine-mire environment. Conditions of peat formation, caused by the changing chemical and physical environments, produced five coal facies and two mineral-rich parting facies within the coal bed. The coal bed facies are compositionally unique, having developed under varying conditions, and are manifested by megascopic, petrographic, palynologic and quality characteristics. The initial environment of the Lower Freeport peat resulted in a coal facies that is relatively high in ash yield and contains large amounts of lycopod miospores and moderate abundances of cryptotelinite, crypto-gelocollinite, inertinite and tree fern miospores. This initial Lower Freeport peat is interpreted to have been a topogenous body that was low lying, relatively nutrient rich (mesotrophic to eutrophic), and susceptible to ground water and to sediment influx from surface water. The next facies to form was a ubiquitous, clay-rich durain parting which is attributed to a general rise in the water table accompanied by widespread flooding. Following formation of the parting, peat accumulation resumed within an environment that inhibited clastic input. Development of doming in this facies restricted deposition of the upper shale parting to the margins of the mire and allowed low-ash peat to form in the interior of the mire. Because this environment was conducive to preservation of cellular tissue, this coal facies also contains large amounts of crypto-telinite. This facies development is interpreted to have been a transitional phase from topogenous, planar peat formation to slightly domed, oligotrophic (nutrient-poor) peat formation. As domed peat formation continued, fluctuations in the water table enabled oxidation of the peat surface and produced high inertinite concentrations toward the top of the coal bed. Tree ferns became an increasingly important peat contributor in the e upper facies, based on the palynoflora. This floral change is interpreted to have resulted from the peat surface becoming less wet or better drained, a condition that inhibited proliferation of lycopod trees. Accumulation of the peat continued until rising water levels formed a freshwater lake within which clays and silts were deposited. The development of the Lower Freeport peat from a planar mire through transitional phases toward domed peat formation may be an example of the type of peat formation of other upper Middle and Upper Pennsylvanian coal beds.

Behavior of Inorganic Constituents During Early Diagenesis of Peats

ABSTRACT Extensive diagenetic changes occurring in peat during burial to approximately 1.5 km may result in the formation of lignite (or medium brown coal). Examination of available literature indicates very little research dealing specifically with the behavior of inorganic (non-carbon-based) compounds during this early postburial diagenesis. Because these inorganic constituents determine both the quantity and quality of ash in coals and may act as potential diagenetic agents in associated rocks, their behavior throughout the peat to lignite transformation is potentially important in several areas of study. Laboratory simulations of the early postburial diagenesis of peat offer a new approach for the study of such processes. In the present research, laboratory simulations have been used to study diagenesis of several well-characterized peats. During the experimental runs, approximately 80 g of peat were compressed and heated in a PTFE-lined reaction cell from atmospheric conditions to 2100 psi and 60°C, thus approximately simulating conditions necessary for the formation of lignite. Pressure and temperature were increased on a stepwise pattern, and prior to each increase, expelled solution from the previous interval was collected. Following completion of each experimental run, the residual solid plug was extracted from the cell intact. Extensive reorganization of organic compounds in the produced residual solids during laboratory simulations was documented by Rollins et al. (1991). In addition, extensive petrographic changes can be observed, including increases in gelatinous material, darkening of colors, and increases in reflectance. Concentrations of major inorganic constituents (Na, K, Ca, and Mg) released in solution are typically constant, or decrease, with increasing pressure and temperature for high-ash peats; whereas, minor inorganic concentrations (Fe, Cu, Zn, Mn, and Al) increase, occasionally reaching a maximum and then decreasing. With low-ash peats, concentrations generally are much lower than high-ash peats, and both major and minor inorganic concentrations typically increase throughout the experimental runs. For all peats, the ratio of major to minor inorganics decreases. The general behavior of inorganics during the diagenesis of these peats is strongly related to the composition of the original peat and to the stage of alteration. In addition, organic acids, including acetic and others, are present within the solutions. These organic acids may act as organic complexors and thus contribute to the mobilization of inorganic constituents.

The inorganic chemistry of peat from the Okavango delta (fan), Botswana: implications for low-ash coal formation

The Okavango Delta is a large land-locked alluvial fan located in the northwestern part of the semi-arid Kalahari basin of northern Botswana. The geographic position of the Okavango system is controlled by a southwest extension of the East African rift system; sediment is presently accumulating in a graben defined by northeast-bounding faults. Depositional slope and gradient are low, 1:3600, and the fan sediments cover 18,000 km 2. Seasonal floodwaters from the catchment in tropical Angola disperse on the fan creating both perennial (12,000 km 2) and seasonal swamps (6,000 km 2). Distributary channels on the fan surface are confined by vegetation, consisting mainly of Cyperus papyrus L., but these channels are unable to accomodate water flow. Therefore, water movement occurs mainly outside of the confined channels and it is this sheet-like dispersement of water which sustains plant growth and peat accumulation and preservation. The dispersal of water also results in severe water loss and over 95% of the water is lost by evapotranspiration. A complex relationship exists between the inorganic chemistry of macrophytes and that of the peat in the perennial swamps of the Okavango fan. Living and decaying plants and peat from a portion of the swamp were analyzed chemically and mineralogically to determine the process involved in peat production. Chemical changes in the macrophytes commence at the onset of senescence as a result of nutrient recycling and further changes occur due to leaching by rainwater. During subaqueous decay, major changes in inorganic chemistry occur, some components being lost, others gained, while major mass loss occurs. While major changes in inorganic components occur, during decay, peat formation does not fractionate the O/C or H/C ratios. However, the C/N ratios declines markedly during decay, probably due to bacterial activity, and reaches low values (approximately 12) in peat. Ash content of peat declines downstream from a maximum of 70 wt.% to approximately 40%. Average ash analyses yield 20% quartz, 40% kaolinite and 30% plant-derived silica, mainly from phytoliths. Therefore, 90% of the peat ash is accounted for by allochthonous detrital mineral matter and plant silica, while the remaining 10%, which includes Fe, Mg, Mn, Ca, Na, K and P is difficult to account for. Ion exchange is unimportant in concentrating these elements, and bacterial activity may be an important process for their uptake in peat. The uptake of these elements by the peat is an important mechanism for offsetting salt build-up which should occur due to the severe evapotranspiration losses. Sodium, however, is only weakly taken up by the peat and plants and accordingly sodium salts (trona and thermonatrite) dominate evaporite deposits on the delta's islands. Peat is a powerful scavenger of those inorganic components which are essential to the living plants. In the normal evolution and dynamics of the swamp system, channels and adjacent peat swamps ultimately become moribund and are abandoned. In the prevailing semi-arid climate, this abandoned peat rapidly desiccates and becomes susceptible to fire. Peat fires burn on several fronts and at varying depths, thereby reducing several meters of peat to a thin residual ash layer. Inorganic nutrients form part of the resultant ash and, through bioturbation by burrowing animals and insects, these enrich the otherwise sterile, sandy soils that characterize the substrate. Subsequent reflooding of a burnt-out area is promoted by topographic inversion of the landsurface caused by the peat fires. This study has important implications for the origin of low-ash peats and coals in general. In order that a peat swamp might form at all requires an appropriate geomorphological setting in which perennial, laterally extensive, shallow flooding can occur. This setting must survive for a substantial period of time to allow sufficient time for significant peat accumulation. An essential requirement for prolonged peat accumulation is a very low input of bedload and suspended inorganic detritus in the fluvial systems that drain into the swamp. In the proximal reaches of the swamp, ash content may be volumetrically small but large on a mass basis. Ash content declines slowly and irregularly through the system, and is controlled by the distribution of channel systems. The wide dispersal of inorganic detritus appears to be due to a low degree of flucculation, which inhibits filtration. Nevertheless, such filtration of allochthonous detritus in the proximal reaches is an important stage in the production of low-ash peats. In the distal reaches, the proportion of allochthonous detritus is very low and ash contents are accordingly also low. The composition of the ash changes significantly and its appears that metal uptake in such regions may be a function of bacterial activity. When bacterial activity declines to low levels, possibly in the most distal reaches of such a swamp or in the deeper peat, very low peats will result, which must be essential precursors to low-ash coals.

Organic-clastic Facies Relationships and Chronostratigraphy of the Barataria Interlobe Basin, Mississippi Delta Plain

ABSTRACT The lithofacies of Barataria Basin, an interlobe basin of the Mississippi Delta Plain, were established from analyses of 115 boreholes. Five organic lithofacies are distinguished by organic matter content, botanical origin and degree of decomposition. Organic-rich facies accumulated in three stages (A, B, and C) during two depositional events. Stage A developed locally, but stages B and C developed as blankets over abandoned delta lobes, protected from marine influx by a shoreline. Stage B lasted from 2,300-1,000 yr BP and ended when clastic sediments from renewed deltaic sedimentation blanketed the peats, expanding the basin about 50 km seaward. Peat accumulation of stage C lasted from 900 yr BP to the present and is currently being terminated by subsidence-induced transgression. High-quality stage B peats accumulated beneath herbaceous, freshwater-floating and emergent marshes, mainly above the footwall of a strike-oriented, reactivated growth fault. Above the footwall, the basin was shallower than on the hanging wall, where the deeper basin invited more overbank sedimentation, which increased ash content and diluted peats. Thus, both tectonic and sedimentary factors influenced organic facies. The quality and quantity of Barataria Basin peats differ from those of many deltaic coals. However, these peats accumulated in only 2,300 yr immediately after a rapid sea-level rise. Frequent delta-lobe switching increased organic matter preservation potential but did not provide sufficient time for the accumulation of thick, low ash peats. The geometries of organic and clastic facies are similar to those of some coal-bearing strata.

Fan-delta-lacustrine sedimentation and coal development in the Tertiary Ombilin Basin, W Sumatra, Indonesia

Summary The Ombilin Basin is a Tertiary intermontane basin situated in the Barisan mountain range of W Sumatra. It covers an area of some 1500 km 2 and is known for the coal resources which have been mined in the Sawahlunto area since 1891. Seismic and borehole data indicate that the basin has been filled with approximately 4600m of middle Eocene to early Miocene sediments. The coal developed early in the basin’s history (middle Eocene). During this study the fan-delta sediments below the oldest seam, the C seam, the seam itself and the sediments immediately overlying the C seam were investigated along the western margin of the Ombilin Basin in the Sawahlunto area. The oblique convergence of the Indian Ocean plate weith the SE Asia plate created a stress field of the type that can produce a primary dextral wrench fault which in this case is named the Great Sumatran Fault Zone. The Ombilin Basin is a graben-like ‘pull-apart’ basin resulting from localized extension failure and related deformation. The failure and deformation are a function of irregular strike-slip movement along the Great Sumatran Fault Zone during the early Tertiary. Tertiary sedimentation began in the Eocene following graben formation. High basement relief adjacent to the newly formed basin resulted in debris flows and fan-delta formation around the margin of the central lake. Concurrent with the deposition of coarse-grained fan-delta sediments around the basin margins, fine-grained lake sediments accumulated in the basin centre. Reduction in relief difference between the source area and the basin was associated with an expansion of the lacustrine environment. Shallowing of the lake and a humid tropical climate allowed plant colonization to take place. Peat accumulation was influenced by the underlying fan-delta lobe geometry. In interlobe areas thick sequences of low ash peat accumulated, while central lobe areas through the same interval were sites of continued sediment influx and relatively slow peat accumulation. In these areas high ash coal or carbonaceous shale are laterally equivalent to the low ash coal. A detailed investigation of the character of the coal in the C seam indicates that, while the lateral dispersion of clastic material may play some part in the variation in the ash content of the coal from one place to another, the most significant variable is the rate of accumulation of peat.

Deltaic coals: an ecological and palaeobotanical perspective

Summary Coals occur widely in deltaic sequences but are rarely of economic quality, being thin, split and with high ash content. Coals with brackish-water influence or overlain by marine rocks tend to have high sulphur values. Not all coals in deltaic sediments formed in swamps or marshes. Peats may form under a wide variety of conditions, and ecological and hydrological interpretations of mire types are essential and should be integrated with facies studies of accompanying clastic sediments. Botanical, physical and chemical data are needed for the interpretation of the coals. Considerable time may elapse between delta abandonment and the onset of peat formation. Rheotrophic or flow-fed peats may form initially on abandoned delta lobes, but if peat accumulation continues under high rainfall conditions ombrotrophic low ash peats may form as raised bogs. In this case vertical and lateral zoning of the peat (coal) may occur. In any interpretation of the coals based on analogies of modern peats care must be exercised in using ecological data from Recent plant communities, as peat-forming vegetation shows significant changes from the Devonian to Recent.