Sedimentology and stratigraphy of the Durham coal measures, and comparisons with other British coalfields

Abstract

A Westphalian (Coal Measures) sequence 900 m thick is preserved in the Durham coalfield of N.E. England, and includes the Westphalian A, B and C stages. This succession is condensed with respect to those of the central Pennine coalfields further south. The Durham Coal Measures were deposited on a coastal, deltaic plain, characterized by having little or no relief. This plain was crossed by bifurcating distributary channels, which were separated by shallow water lakes and bays. Trunk distributaries, probably of low-moderate sinuosity and perhaps 5 kms wide, fed sinuous major distributaries of up to 3 kms width. These in turn supplied minor distributaries, often sinuous in form, which were up to 100 m wide and fed shallow water deltas. Interdistributary lakes and bays were generally less than 10 m deep, up to 20+ kms wide, unstratified and often anoxic. These shallow basins were in filled by crevasse splay and minor delta sedimentation. Infilled lake and bay surfaces and abandoned channels were rapidly colonized by vegetation, and thick (perhaps up to 40 m) seams of peat were able to accumulate, often diachronously, over wide areas (100s of sq. kms).During Namurian and Lower Westphalian times, lower delta plain conditions prevailed, in which interdistributary bays were open to marine influence so that deltaic sands could be reworked to form quartz arenitic shoreline sandstone bodies. As a result of south ward deltaic progradation, this environment evolved into an upper delta plain which persisted through much of the Westphalian A and B, although during Upper Westphalian A times, the plain developed characteristics transitional to those of a fluvial plain. Occasional marine incursions caused only temporary drowning of the upper delta plain surface to 10-15 m depth. Sedimentation was controlled on the large scale (100s of sq. kms) by patterns of deltaic sediment distribution, on the medium scale (10s of sq. kms) by a combination of structurally- and compaction- induced subsidence, and on the small scale by local sedimentary processes. A comparison with other areas of Westphalian outcrop in Britain shows strong similarities to the depositional environment found in Durham.