Structure of the Culm Basin : rapid mapping of the Tiverton Sheet and the latest Variscan inversion in Devon

The Silesian (Upper Carboniferous) sedimentary rocks of the Culm Basin, to the west of Tiverton, belong to the Crackington and Bude Formations. These comprise turbidite and debrite sandstone, interbedded with mudstone. In the field the formations are differentiated on the basis of thickness and weathering characteristics of the sandstone packages. In the study area although bedrock is commonly obscured by overlying superficial deposits, the character of these deposits can be used as an indicator of bedrock geology. Using both bedrock and superficial analytical techniques, the Crackington / Bude boundary has been mapped in the study area. Tracing of large scale structure has been helped by the use of shaded relief digital terrain models, however in areas of moorland the distinctive featuring is not present and there are no indicators of the underlying bedrock structure. Geophysical data, in particular a recent gravity survey, provide considerable information on the structure of the Culm Basin. It is clear that the east-west trend of the Tiverton Trough, depicted by a negative gravity anomaly over the Permo-Triassic infill, continues westwards in the Silesian sedimentary rocks. Further west of the Tiverton Trough an area of relatively high gravity covers the area of Witheridge and Rackenford moors. North of this is a negative gravity anomaly whose northern boundary lies close to the crop of the basal Crackington Formation and whose southern margin follows the Crackington / Bude boundary. The strong east-west linear nature of the gravity anomalies indicates that the structure controlling the northern boundary of the Tiverton Trough was probably active during Variscan inversion. The gravity high over Witheridge is thought to be related to the presence of dense, earlier Palaeozoic rocks that form a structural high underlying the Silesian; given its linear nature and its alignment with the northern boundaries of both the Tiverton and Crediton troughs.