Variation with respect to petrological differences in a thin band of upper carboniferous non‐marine lamellibranchs

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Abstract
The development of an impersistent band of non‐marine shells immediately underlying the Gastrioceras subcrenatum marine band and referred to Carbonicola ? lenisulcata (Trueman) and C. ? aff. bellula (Bolton) is traced over much of the area of outcrop of the Lower Coal Measures in Yorkshire and Lancashire. The fauna of an underlying local leaf in the Sheffield area is referred to Carbonicola aff. protea Wright. The shells of both faunas show allometric growth with respect to their height and length.Differences in the petrology of the main non‐marine shell band in five localities have been investigated optically by Professor K. C. Dunham and Mr. T. Deans, and by chemical analysis. What appear to be regional changes in the nature and range of variation of the shells reveal a correlation with petrological change closely comparable with that found in passage upward through the lower parts of cyclic units of sedimentation in the Soft Bed — Bassy Mine succession (Eagar 1947). Changes in the variation of the faunas, when they are traced horizontally on this single horizon, are however, of considerable smaller magnitude than those of the latter. Thus a fauna of dominantly anthraconaioid elongate shells, occurring in fine‐grained richly carbonaceous shale, shows greater variation and an increasing proportion of relatively short Carbonicolae? as it is followed into shaly mudstone with larger detrital constituents and less carbonaceous matter. The relative proportions of carbonaceous matter, varying inversely with thickness of the band and amount of shells per unit thickness, suggest that sedimentation caused reduction in size in two localities. The significance of the clay‐mineral content and of pyrite, which is unusually abundant and present with framboidal texture in the band, is brieflly discussed. It is suggested that the water was brackish during the formation of the band and that possibly unusually saline or near‐marine conditions may have obtained.

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