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Kansas Geological Survey Open-file Report 2003-82 |
Chapter Five : Depositional Models
Depositional models are intended as teaching tools, mental concepts, and temporary fixed points in nature (Miall, 1999). The function and utility of a model aids in the distillation of many observations for ease of comparison, and serves as a framework and guide for future investigations (Walker, 1976). In the subsurface, models serve as predictive tools for reconstructing sparsely observed systems or for interpreting preliminary results.
5.1 Previous Models and Work
Previous studies of the development of Pennsylvanian coal beds in the mid-continent have shown that coal accumulation is influenced by several general environmental factors (eg. climate, sea level change, basin subsidence, sediment accumulation, and depositional environment). Depositional environment reflected in the type of mire in which peat developed is believed the most important control on distribution, thickness, and quality of coal (Wanless et al., 1969; McCabe and Shanley, 1992).
According to Wanless et al. (1969) distribution of Pennsylvanian coals are controlled by environmental patterns such as widespread deltas, unfilled channels, estuaries, coastal marshes, barred and non-barred coast lines, cut-off stream meanders, coastal plains exposed after regression, and pre-Pennsylvanian topographic irregularities. Flores (1993) suggested that when studying coals in the ancient, depositional orientation, average thickness, areal extent, and geometry of coal beds are reflective of the environment of deposition, and can be used as a predictive model. Conversely, McCabe and Shanley (1992) stressed equal or greater importance on the concept that the type of mire in which peat accumulated is reflected in the characteristics of coal beds. With the mire concept, low-ash coals are predicted to have formed from raised mires, while high ash coal formed in low-lying mires. Marine influenced mires will have higher sulfur contents while inland mires will tend to be protected from influence of marine water during coalification, and will have reduced sulfur contents (< 2.5 %).
Pennsylvanian coals are widely distributed throughout the mid-continent, and have been correlated for hundreds of miles (Wanless et al., 1969). Transition into or out of coal from other lithologies is relatively sharp. The abruptness in which coal accumulation is initiated and terminated has been attributed to climatic shifts, such as changes in humidity, precipitation and temperature (Wanless et al., 1969). During the Pennsylvanian, the mid-continent is believed to have been a vast level plain near sea level. This plain was subject to frequent extensive marine transgressions, when the sea covered most of the continental interior (Wanless, 1969). The occurrence of frequent marine transgressions likely played an important factor in development and demise of the numerous thin Cherokee Group coals.
Previous work also suggests that most Pennsylvanian coals accumulated in situ (Wanless, 1969). Support for this interpretation is from observed rooting into underlying rock such as underclay or seat earth, shale, sandstone or limestone (Staub and Cohen, 1970). The origin of underclay beneath many of the coals is a subject of debate, in relation to depositional or post depositional weathering, and classification as a soil (Wanless et al., 1969). In general, most Pennsylvanian underclays are accepted as, originally deposited outside the basin of peat accumulation, and as a soil under swampy conditions. The underclay is subsequently altered by leaching during peat accumulation, and not directly related to upland soil development (Wanless et al., 1969).
Many studies conducted in the last decade have been in relation to the understanding of coal deposits within a sequence stratigraphic framework due to the increased interest in the hydrocarbon potential of coals (Aitken, 1994; McCabe and Shanely, 1994; Boyd and Diessel, 1995; Bohacs and Suter, 1997; Diessel, 1998). A widely excepted view is that preservation of peat is dependent on near equal rates of increasing accommodation and peat production (McCabe and Shanely, 1994; Boyd and Diessel, 1995; Bohacs and Suter, 1997). Additionally, for mires, peat will not continue to accumulate with only an increase in accommodation and therefore an increasing water table is needed for sustained peat growth (Aitken, 1994; Bohacs and Suter, 1997). An increasing water table is strongly controlled by sea level rise and the precipitation/evaporation ratio (Aitken, 1994; Bohacs and Suter, 1997).
With an understanding of the delicate balance between peat production, accommodation and sea-level rise, coal seams can be predicted within a sequence stratigraphic framework. Base-level falls, typically occurring during early lowstand and late highstand systems tracts, lead to a loss in accommodation, incision, and valley formation, causing low peat preservation (Boyd and Diessel, 1995; Bohacs and Suter, 1997). When accommodation rates are significantly above peat production rates, mires will become stressed and inundated by clastics or stagnate water, due to base level rises typical of the mid-transgressive systems tract (Bohacs and Suter, 1997). During periods of aggradation, typical of late transgressive and early highstand systems tracts, peat-producing mires may block marine transgressions and stabilize coastlines for longer periods of time, leading to higher preservation of peat (Diessel, 1998; McCabe and Shanely, 1992).
Diessel (1998) has also applied sequence stratigraphy to amalgamated coal seams in Australia, where the coal is interpreted as forming over multiple sequences. Basinward marine splits in the coal seams are interpreted to represent prograding stacking patterns, and a marine split above a ravinement surface and angular unconformity is thought to be a sequence boundary (Diessel, 1998). In the case of the Cherokee basin coals, none of the coals observed appear to be amalgamated.
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Last updated December 2003
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