On balanced and unbalanced accommodation/peat accumulation ratios in the Cretaceous coals from Gates Formation, Western Canada, and their sequence-stratigraphic significance (original) (raw)
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The Crowsnest coalfield is a separated structural coalfield at the East Kootenay basin within southeast British Columbia. Selected Jurassic-Cretaceous two coals seam S-10 and S-C of the Mist-Mountain formation were investigated from the points of coal petrography to construct a Paleo-limnological setting. Twenty-two channel coal samples were assembled and measured from S-10 and S-C coal seams. Vitrinite reflectance measuring of the S-10 coal concern to low-volatile bituminous (1.61, on average). While the vitrinite reflectanc measurements of S-C coal seam show an average value of 0.98 related to high-volatile bituminous. Petrographic analysis demonstrates that the S-10 coal seam appears to be rich in inertinite than the S-C coal seam that appears rich in vitrinite. Applying coal facies indices of Tissue Preservation Index, Gelification Index, Ground Water Index, Vegetation Index and coal facies diagrams for the studied coal seams aid to suggest a condition of the depositional Paleo-environments. The results promote Paleo-depositional sites of telmatic to limno-telmatic setting of rheotropic systems swamp. The obtained results show a trend of increase in both herbaceous flora and anoxic waterlogged limo-telmatic setting toward the younger coal seam S-C than the older S-10 coal seam.
International Journal of Coal Geology, 1989
The regional coalification pattern of Lower Cretaceous strata in parts of the Canadian Foothills Belt and adjacent foreland has been determined by vitrinite reflectance measurements. The present study focusses on two coal-bearing sequences, Bluesky-Gething and Gates Formations, both of which contain coal resources of economic interest. The vitrinite reflectances (Rmax) in the B luesky-Gething Formation range from 0.76% (highvolatile A bituminous) to 2.55% (semianthracite). Rank changes from top to the base of Bluesky-Gething Formation follow lst-order regression lines. Factors influencing the rate of increase of Rmax per depth interval (coalification gradient) include paleogeothermal gradients, the rank level under investigation, conductivities of host rocks, and thicknesses of coal seams. Time-depth (burial) curves for the Lower Cretaceous Bluesky-Gething Formation suggest that the regional coalification pattern for the top of the formation results largely from variations in the depth and/or duration of burial beneath Maastrichtian-Tertiary foredeep deposits. Coalification largely predates deformation. The coalification pattern is more complicated for the base of the Bluesky-Gething Formation because thickness changes in the Lower Cretaceous Bluesky-Gething interval locally have a greater effect on the rank than does the regional change in the thickness of Maastrichtian-Tertiary sedimentary wedge. Detailed rank studies on laterally continuous coal seams of the Gates Formation showed that coalification levels were largely established before folding and thrusting started. Isoreflectance lines of the beds run parallel to the bedding of the folded strata. Seams collected from various Geological Survey of Canada Contribution No. 12088.
Bulletin of Canadian Petroleum Geology, 2002
Coal-bearing strata from the Lower Cretaceous upper Mannville Group (south-central Alberta) were investigated in order to evaluate the nature of coal-bearing non-marine to marginal marine sediments developed in an intermediate accommodation setting, located centrally within the Alberta Foreland Basin. Downdip and lateral correlations to the northwest and east link upper Mannville Group strata to the Falher sequences and the Waseca to Lloydminster sequences, respectively, and indicate that a higher order of stratigraphic subdivision, controlled by transgressive-regressive cycles, must also be present in the upper Mannville. Stratigraphic analysis of the upper Mannville Group in the study area, based on over 1200 km of borehole crosssections and 50 cores, revealed a number of features that can be considered characteristic of intermediate accommodation in non-marine sediments. These include abundant, compound coal seams and numerous incised valleys with even distribution of sediments between incised valleys and adjacent interfluves. The incised valleys may correlate laterally into horizons within the compound coals, indicating that the coal seams contain sequence boundaries within them and therefore span relative sea level cycles. However, the occurrence of coal seam splits in multiple directions suggests that seam splitting in the upper Mannville Group was at least partially controlled by differential subsidence. This is attributed to fault reactivation on the underlying irregular Paleozoic basement, and maps of inferred fault planes indicate a horst and graben style of extensional faulting. Fault planes appear to be associated with relatively steeply dipping basement topography, and areas of thickest cumulative coal preferentially occur above horst blocks. Although these particular features may be unique to the Mannville Group, they suggest that underlying structures and paleotopography are likely to exert a strong influence on sedimentation patterns in intermediate accommodation settings, because the rates of vertical accretion are insufficient to suppress the effects of differential subsidence. Other expressions of basement control are the localization of single and compound incised valleys along structural lows. Coal composition analysis is based on photometric and maceral analyses of three upper Mannville Group coals. The aim was to test various methods of identifying small-scale accommodation trends in the coal and use them to identify a characteristic accommodation signature for each coal. The Glauconite, Medicine River and informally named Hackett coals were shown to be significantly more complex than simple 'transgressive' or 'regressive' style coals. They comprise a number of wetting-and drying-upwards cycles representing repeated episodes of peat deposition under rising or falling accommodation conditions, with or without internal hiatuses between the cycles. These accommodation cycles are driven by changes in groundwater levels that are in turn hydraulically linked to relative sea level, and thus form the basis for identifying a characteristic non-marine sequence stratigraphic style.
2010
Terrestrial sediments are difficult to correlate because they have high laterally variability and lack easily identifiable chronostratigraphic surfaces. However, systematic variations in the petrographic properties of paralic coal can be used as a proxy to recognize changes in accommodation. Paralic peat cycles characterized by wetting-upward and drying-upward behaviour, linked to variations in the groundwater table, are interpreted as transgressive and regressive coals, respectively. They are associated with a range of terrestrial stratigraphic surfaces that record responses to changing accommodation. A combination of these coal parameters, together with the facies characteristics of the surrounding terrestrial and marginal marine sediments, enables recognition of distinctive high-resolution sequence stratigraphic signatures. This in turn provides a previously unavailable ability to correlate stratigraphic units from their down-slope marine position, through the shoreline zone, and into the terrestrial realm. Coal-bearing rocks from the Lower Cretaceous upper Mannville Group and Falher Member of the Western Canadian Sedimentary Basin provide an opportunity to investigate high resolution stratigraphic correlation in terrestrial to marginal marine rocks. Maceral properties of these coals reveal that they are more complex than simple 'transgressive' or 'regressive' style coals. They comprise a number of wetting-and drying-upwards cycles representing repeated episodes of peat deposition under rising or falling accommodation conditions, with or without internal hiatuses between the cycles. These accommodation cycles are driven by changes in groundwater levels that are in turn hydraulically linked to relative sea level, and thus form the basis for identifying a characteristic terrestrial sequence stratigraphic style, which has potential as a correlation tool.
Recognition of peat depositional environments in coal: A review
International Journal of Coal Geology, 2020
Peat depositional environments, the sites where and conditions under which peat accumulates, significantly influence a resultant coal's physical properties, chemical composition, and coal utilization behavior. Recognition of peat depositional environments for coal is a challenging endeavor because coal's observed compositional properties not only result from a variety of geological processes operating during peat accumulation, but also reflect the influence of adjoining or external depositional sedimentary environments and alteration during later diagenesis and/or epigenesis. The maceral or microlithotype composition of any one layer of peat can be the product of years or decades of plant growth, death, decay, and post-burial infiltration by roots in addition to the symbiotic, mutualistic, parasitic, and saprophytic relationships with non-plant biota, such as arthropods, fungi, and bacteria. The overprint of increasing thermal maturation and fluid migration through time on the resulting coal can make these relationships difficult to recognize. Therefore, published models based on maceral composition alone must be used with great caution. Lipid compositions, even from lipid-poor low-rank coals, can provide important information about depositional environments and paleoclimate, especially if combined with the results of organic petrography and paleontological studies. Just as sulfur derived from seawater provides environmental clues, the ratios of two particularly relevant trace elements rather than a single trace element can be used to interpret peat depositional environments. Epigenetic minerals, as well as their corresponding chemical compositions should not be used for such a purpose; similarly, resistant terrigenous minerals deposited during peat accumulation in many cases should be used with considerable caution. The interactions of the biota present in the peatforming ecosystem, often determined using palynological and geochemical proxies, and their interpretation in the context of geography and paleoclimate are important means for deciphering peat depositional environments. Overall, a combination of evidence from geochemistry, mineralogy, palynology, and petrology of coal and from stratigraphy, sedimentology, and sedimentary facies of related rocks is necessary for accurate and comprehensive determination of depositional environments. The need for interdisciplinary studies is underscored by peat compositional properties, which have been greatly affected by various processes during the syngenetic, diagenetic or epigenetic stages of coal formation.
Introduction to Applied Coal Petrology
Introduction to Applied Coal Petrology
Coal is a combustible sedimentary rock, composed essentially of lithified plant debris. The plant debris was originally deposited in a swampy depositional environment to form a soft, spongy sediment called peat. However, physical and chemical processes brought about by compaction and elevated temperatures with prolonged burial at depths of up to several kilometers and over periods of up to several hundred million years then changed the peat into coal through a process referred to as coalification or rank advance. The properties of a given coal can be related to three independent geological parameters, each of which is determined by some aspect of the coal's origin. As discussed more fully by authors such as Ward (1984), Diessel (1992a), Taylor et al. (1998), and Thomas (2002), these parameters are briefly defined as follows: l Rank. Coal rank reflects the degree of metamorphism (or coalification) to which the original mass of plant debris (peat) has been subjected during its burial history. This depends in turn on the maximum temperature to which it has been exposed and the time it has been held at that temperature and for most coals reflects the depth of burial and geothermal gradient prevailing at the time of coalification in the basin concerned. Heat flow from nearby igneous intrusions, however, may also play a part. l Type. Coal type reflects the nature of the plant debris from which the original peat was derived, including the mixture of plant components (wood, leaves, algae, etc.) involved and the degree of degradation to which they were exposed before burial.
Petrography of Ardley Coals, Alberta - Implications for Coalbed Methane Potential
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Palaeogeography, Palaeoclimatology, Palaeoecology, 1994
Benched channel samples and continuous block samples of the Hub and Harbour seams from the Sydney Basin, Nova Scotia were studied to define the variations in petrography, geochemistry and palynology of the seams and to interpret the depositional environments of the precursor mires. Petrographic compositions of the seams are very similar with high vitrinite content, low inertinite, moderate liptinite and negligible clastic mineral content. Pyrite content is moderate. There is little variation in petrographic composition between benches in either seam. There are marked variations in petrographic compositions at the lithotype scale. Petrographic facies parameters indicate deposition of the bulk of both seams under fairly uniform conditions in relatively wet mires. Unusually wet or dry periods were rare. The palynological assemblages of both seams indicate a middle to late Westphalian D age. The common to abundant palynomorphs are derived from "arborescent" and "herbaceous" lycopsids, tree ferns and other ferns(?), and sphenopsids. The palynological assemblages support the petrographic interpretation of relatively wet mires. Cordaites pollen (Florinites) is rare, suggesting that there was little contribution from plants growing in dry or upland areas. The absence of the Densospore miospore phase, and the petrographic data, indicate there were no periods in which domed mires occurred. The distinctive dull bands occurring in the Hub and Harbour seams are characterised by low vitrinite contents, high inertinite content, high liptinite and common detrital minerals. Sporinite, showing varying degrees of degradation, is commonly highly concentrated in thin bands. The geochemistry is marked by much higher m values than normally reported for coals but other indices are typical for coals of this rank. The palynological assemblages of the dull bands are dominated by tree fern spores and other ferns(?) with only a minor contribution by sphenopsid spores. The dull bands were deposited during periods of inundation of mires. Relatively low mineral content indicates that the distributary channels which flooded the mires were quite distant from the depositional sites of the samples.