OR/13/050 Model assumptions and geological rules

Gow, H, Cripps, C, Thorpe, S, Horabin, C, and Lee, J R. 2013. Model metadata report for the Somerset levels 3D geological model. British Geological Survey Internal Report, OR/13/050.

• For the purposes of geological modelling tidal flat deposits have been delineated into two units (tfd2 and tfd1) which are separated by peat. In reality, the tidal flat deposits form part of a continuum and little stratigraphic sub-division is evident. These units are mapped as TFD on BGS 1:50 000 scale DiGMapGB datasets.
• It is believed that the modern alluvium is too extensive to be attributable to the action of river deposition alone. In particular, there looks to be a straight, arbitrary line drawn delimiting the alluvium with other deposits; this can be seen just south of Bridgwater. This suggests that there is some discrepancy in the meaning of the term ‘alluvium’. It is thought that alluvium was used a blanket term for all fine grained siliciclastic deposits at surface in this area, particularly in the higher reaches of the basin, even if it was thought that the ‘alluvium’ had a marine origin of deposition. However, for the purposes of this model, we have kept to the original mapping of the alluvium.
• The unit unknd (Unknown Drift) is mapped as ‘Higher Estuarine Alluvium’ on the published Wells 1:50 000 geological map, and it is mapped as TFD1 on BGS 1:50 000 scale DiGMapGB dataset. As the code TFD1 was already in use and to distinguish it from other TFD units, it was decided to assign the unit a map code that did not imply a mode of origin. Green (1965) proposes an estuarine environment of deposition, and that the units post-date the Head deposits, and pre-date the TFD deposits. The units in the model and GVS adhere to this hierarchy.

• Outside of borehole control, there is poor lateral heterogeneity due to the nature of the superficial deposits. This means that which means that most of the model is constructed using conceptual knowledge, and it is likely that there is more complexity to the superficial deposits than has been modelled.
• Due to the discontinuous nature of the peat deposits within the tidal flat sediments, tfd1 and tfd2 were reconstructed through all cross-sections that contain superficial deposits beneath a depth of c.0 m OD. This coincides broadly with the base of peat2 which is interpreted as a former land-surface. Where this peat was not encountered, the base of tfd2 was constructed around 0 m OD with some variance on a section-to-section basis (+/-3 m). All underlying tidal flat units are correlated as tfd1. In certain areas, such as the smaller valleys, the relationship between tfd2 and tfd1 was rationalised as the same rules could not be applied. In these areas the tidal flats were either removed or reduced to tfd2.
• Further to the above point, the nature of the peat has led to a number of possible models being proposed as to the number and extent of the peat deposits. In some areas it is unclear whether a peat at surface will continue laterally underneath the tidal flat deposits, or become a separate unit below the tidal flat deposits. During modelling, both ideas were put forward and the current position as modelled is the one believed to be correct. With further study (greater borehole interrogation, more cross-sections, GIS study of the bases of all peat layers) this may prove to be incorrect.
• The depths of superficial deposits within the basin were taken from the borehole data where possible. Where insufficient evidence exists, the rockhead elevation model surface (RHEM 2009) was used as a guide.