Sedimentary structures associated with extensional fault movement from the Westphalian of NE England

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Abstract
Summary: Although differential subsidence and extension has been well established as a control on sedimentation for the Dinantian of N England, the continuation of this tectonic regime into Silesian times is less well demonstrated. The Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England, which were deposited on an essentially flat, deltaic plain, display abundant evidence of a subtle structural control on sedimentation. Medium-scale (hundreds of square kilometres) patterns of sedimentation, notably the disposition of major fluvial channel belts, were at times strongly controlled by active, ENE-WSW- and E-W-trending fault lines. The faults represent a response to a broadly N-S directed tension and many resulted from the reactivation of earlier, Caledonide crustal weaknesses. The effects of such tectonic activity may be seen spectacularly at field-outcrop scale. At an opencast coal mine in southern Co. Durham close to the major Butterknowle fault, an exposed Upper Westphalian A sequence displays a plethora of tectonically associated structures. These include gravity slides, various dewatering structures, claystone dykes, convoluted and internally chaotic sandstone beds, desiccation cracks and signs of vertical drainage in an originally waterlogged paleosol profile, and abrupt bed thickness changes. The existence and orientation of two minor channels on the ESE side of the fault line, and other palaeoflow data, support the hypothesis that the Butterknowle fault displaced downward the area to its ESE at this time. Fractures across the area of the opencast site were found to contain high concentrations of galena, sphalerite, pyrite/marcasite and other minerals which were apparently introduced at the time of formation of the N Pennine orefield (Carboniferous-Permian).

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