The influence of a palaeochannel on seam thickness in the coal measures of Derbyshire, England

Abstract

A study of coal-bearing Westphalian B rocks in Derbyshire (East Midlands, England) has shown that a palaeochannel system influenced the thickness of both the underlying and overlying seems, and the thickness of the interseam strata containing the palaeochannel. The Top Hard/Lower Coombe Seam, which underlies the palaeochannel, contains washouts, resulting in partial or total removal of the coal and anomalous seam sections. The interval between this seam and the overlying Upper Coombe Seam shows dramatic thickness variations where the palaeochannel is present, with the interseam interval diminishing rapidly to the north away from the palaeochannel. The Upper Coombe Seam, which overlies the palaeochannel system, shows marked thickness variation where it overlies the channel, with less thickness variation outside the channel. The palaeochannel is interpreted as a distributary channel, formed on an upper delta plain to lower alluvial plain, comparable with the Atchafalaya Basin, Louisiana. There is little evidence for bedload transport within the channel, which was abandoned and infilled mainly by deposition of silt from suspension and bank collapse. These mechanisms resulted in an infill of irregular thickness, causing the thickness variation of the interseam strata and the seam overlying the channel. A thick sequence of overbank flood deposits was laid down adjacent to the channel which thins rapidly away from it. Compaction influenced the development of the channel system, and modified the post-depositional geometry. Channels similar to the one described show patterns of thickness variation, which may enable their detection in other areas. The presence of erosion surfaces, bank collapse deposits, anomalous dips, and unlaminated siltstones may also enable their delination in areas of coal mining. Isopachyte patterns of thickness of seams and interseam strata are potentially very useful in the detection of palaeochannel systems in coal-bearing sequences and a number of different patterns of thickness variation accompanying palaeochannels are briefly described.