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Stratigraphy of the Westhoughton opencast mine

The Westhoughton Exposure Two separate sites of exceptional preservation were identified during the study, namely Bickershaw and Westhoughton, and these were subsequently published on (Anderson et al. 1997; Anderson et al. 1999). The Westhoughton site proved to be the more important of the two as direct observation of the fossil-bearing strata was possible. The fossils from Bickershaw originated from the spoil tips of the closed Bickershaw deep mine. The accompanying image shows the site as it was early in 1997. Three coal seams were exposed at the time, the deepest being the Trencherbone, then the Wigan 2ft and finally the Wigan 4ft seam. However, virtually all of the fossil material originated from the roof shales of the Wigan 4 ft seam. The most unusual aspect of the site is that the fossil bearing shales which form a roof to the underlying coal, appear to have been deposited extremely rapidly.
This is deduced from the presence of large upright trees in life position directly above the coal seam, a general lack of sedimentary structures in the roof shales and a distinct lack of trace fossils.

An upright Sigillaria tree In-depth investigation of in situ concretions in the face of the open cast pit, indicated that two distinct faunas were preserved above the Wigan 4 ft coal at Westhoughton. At the base of the roof shales, directly adjacent to the coal seam itself, there is a predominance of terrestrial plants and animals. These include extinct orders of arachnids such as trigonotarbids and phalangiotarbids, scorpions, millipedes, arthropleurids and insect wings. At one particular horizon, the cone Lepidostrobus is extremely common, found in elongate, cigar-shaped nodules. As the nodules increase in size up succession, the fauna became more aquatic in composition evidenced by the presence of freshwater shrimps, horseshoe crabs, euthycarcinoids and fish scales. What appears to have happened is that breaching of natural river levees resulted in rapid and widescale flooding of the area now represented by the Wigan Four Foot seam. Whether this flooding was due to tropical storms is still open to question, but it is interesting to speculate whether this deposit may have formed as a result of a Carboniferous tropical storm!

Non-marine bivalves and the presence of economically exploitable coal seams associated with the fossils allow precise stratigraphic assignment to be made on these sites of exceptional preservation. At Westhoughton, the bivalve assemblage indicates an uppermost Westphalian A age (Lower Modiolaris Chronozone, regularis faunal belt).

The non-marine bivalves from the Bickershaw site suggest a level at about the junction of the pseudorobusta Subzone and crista-galli Subzone. This is important as it helps specify which coal seam in the subsurface bore the roof shales with siderite concretions, namely, the Haigh Yard coal.

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