Sedimentology and its applications within the UK opencast coal mining industry

Abstract

The opencast coal industry seeks to identify and extract near-surface coal reserves. Within the UK Pennine Basin, most coal deposits suitable for opencast exploitation range from middle Westphalian A to lower Westphalian C in age. These deposits dominantly accumulated in an upper delta plain setting in which subsidence rates and depositional setting formed the primary controls on the initial distribution, thickness and quality of coal reserves. Hence an appreciation and analysis of the sedimentary setting and controls on sedimentation are important in the delineation of potential areas of thick, persistent coals. The calculations of in situ coal reserves and coal to overburden ratios are important objectives during exploration for prospective opencast coal sites. Coal reserves are detrimentally affected by early, depositionally related factors such as as wash-outs, seam splits, channel bank collapse and compaction faults. The thickness and type of interseam sedimentary rock (overburden) is directly related to the original despositional environment and the nature of the overburden can change substantially across the working area of an opencast site, reflecting sedimentary facies variations. The various sedimentary facies have unique characteristics and adversely influence mining in a variety of ways, including highwall stability problems, seam gradient changes, groundwater problems, interseam interval variations and facies-specific discontinuities. Many of these problems are related to the presence of palaeochannels. Palaeochannels occur at different scales and contain a variety of fills, and their identification is an important aspect of site exploration. Sedimentary facies mapping, incorporating facies analysis, the construction of isopach maps and palaeocurrent studies can play an important part in an opencast exploration programme. Facies maps have many applications which may be invaluable in site planning, including the delineation of areas of sedimentary disturbance, improved understanding of seam thickness and quality variations, and changes in the interseam strata.

A quantitative approach to fluvial facies models: Methods and example results

“…Allen, 1965;Brierley, 1996;Collinson, 1996;Miall, 1996;Bristow et al, 1999;Bridge, 2003Bridge, , 2006North & Davidson, 2012; and references therein). These bodies thin away from the channel margins, as they interfinger or grade laterally into other elements, and they tend to have flat, sharp and slightly erosive bases (Mjøs et al, 1993;Jones et al, 1995;Bristow et al, 1999); although tabular bedding is common, internal accretion surfaces usually downlap, dipping at low angle to angle of repose, as they record the progradation of the splay onto the floodplain or into standing bodies of water (Miall, 1996;Bristow et al, 1999;Bridge, 2006).…”

Crevasse splays from the rapidly aggrading, sand‐bed, braided Niobrara River, Nebraska: effect of base‐level rise

“…Crevasse splays form an important component of fluvial sequences ( Allen, 1964; Smith, 1983; Smith & Perez‐Arlucea, 1994; Willis & Behrensmeyer, 1994) and are considered to be of economic importance in hydrocarbon reservoirs ( Mjos et al ., 1993 ) and coal exploitation ( Horne et al ., 1978 ; Ethridge et al ., 1981 ; Flores, 1983; Guion, 1984; Fulton et al ., 1995 ; Jones et al ., 1995 ; Jorgensen & Fielding, 1996). However, very few examples of recent crevasse splay deposits have been described ( Coleman, 1969; O’Brien & Wells, 1986; Farrell, 1987; Smith et al ., 1989 ; Smith & Perez‐Arlucea, 1994).…”

The 20,000-years-long sedimentary record of the Lesina coastal system (southern Italy): From alluvial, to tidal, to wave process regime change

“…Crevasse splays occur when, during high energy fluvial stages, the river flow exceeds the volume of the channel, producing breaching of the levees, and generating a rapid spread-out of sediments of various grain size into the inter-channel areas (Bridge, 2003, 2006; Bristow et al, 1999; Collinson, 1996; Miall, 1996; North and Davidson, 2012). In modern analogues environments, these deposits form characteristic fan-shaped bodies (Bristow et al, 1999; Jones et al, 1995; Mjøs et al, 1993).…”