THE STRUCTURE OF THE BURNSALL-CRACOE DISTRICT AND ITS BEARING ON THE ORIGIN OF THE CRACOE KNOLL-REEFS

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Abstract
Summary: The structures found in the Cracoe reef-limestones are considered in relation to those found in the overlying Bowland Shales and Grassington Grit, and also to those found in the district as a whole. Although there is evidence of widespread breaks in the succession, angular unconformity occurs only in the reef area proper, where Bowland Shales and Grassington Grit overlap against eminences of reef limestone within which “pre-shale” dips can be demonstrated. While it is not possible to prove conclusively whether these dips are tectonic or are original dips of deposition, the latter explanation is considered to be the most consistent with the evidence found within this restricted district and also with existing knowledge of Lower Carboniferous reef limestones as a whole. It is concluded that an original knoll-reef topography, with original dips, was partly buried by Lower Bowland Shales which, however, lie with no disparity of dip on adjacent non-reef limestones; that intermittent uplift, with minor faulting, and with some erosion of reef limestones and of the limestones forming on the edge of the bank to the north, occurred in P 2 and early E 1 times with the formation of boulder beds in the Bowland Shales; that this erosion surface was completely buried by the deposition of the Upper Bow-land Shales and Grassington Grit; and that the whole of this succession was later folded and faulted during the Armorican earth movements. Dips in the Bowland Shales due to differential compaction over the underlying irregular surface are likely to be present and to be of the order of 10–15 degrees at a maximum. Such dips have, however, been obscured by the later earth movements.

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