OR/16/004 Model assumptions, geological rules used etc
| Barron, A J M. 2016. GSI3D model metadata report for HS2 Area 3 (Newton Purcell to Thorpe Mandeville). British Geological Survey Internal Report, OR/16/004. |
The Charmouth Mudstone is regarded as corresponding approximately to the Lower Lias Clay of previous accounts (Horton et al., 1987)[1] at least in the southern half of Area 3 (Sumbler, 2002)[2], and although northwards into the Banbury and Towcester districts the interbedded limestone beds typical of the Blue Lias Formation are thought become more numerous towards the base (Edmonds et al., 1965)[3], the formation is not correlated in Area 3. It was also found that over much of the model area the Charmouth Mudstone’s base lay almost exactly at -30 m OD. However, where it lay below, it was drawn nominally at just below -30 m (as per project instructions, to allow truncation), and where it lay above, the underlying strata were drawn, also just below -30 m.
Exceptions/rules and additional polys in order of creation:
- TILMP-DMTN: usually closed beneath fingers of ALV or HEAD. Underlies main GFDMP, locally appears to be absent (e.g. part of Helper_5, Centreline between W10000 and 12000).
- WHL: all BWL merged into WHL, see Model limitations. See for example Figure 7 where the BWL (yellow) to the north was to be merged with the WHL (pale green).
- BWL: subsumed in WHL, see above.
- FMB: all BWC merged into FMB, see Model limitations.
- RLD above TY or WBRO changed into RLD1.
- RLD under WBRO retained as RLD.
- SHHB always underlies TY by definition
- GFDMP0: relatively limited coverage at base of glacigenic succession.
- TILMP1: till unit that lies above the most extensive GFDMP but not separated/distinguished on survey maps from the subjacent Till. Surface closures constructed where GFDMP is missing (Figure 9) are fitted to topographic contours where possible, but locally where the glacigenic stratigraphy is very complex (Helper_4 W7000–8000), constructed boundary cross-cuts mapped leaves of GFDMP in order to close the GU (Figure 10).
- GFDMP1: highest unit in glacigenic succession, generally overlies TILMP1, but locally coverage surface closure constructed cross-cutting very complex glacigenic stratigraphy (Figure 10).
References
- ↑ HORTON, A, POOLE, E G, WILLIAMS, B J, ILLING, V C, and HOBSON, G D. 1987. Geology of the country around Chipping Norton. Memoir of the British Geological Survey, Sheet 218 (England and Wales).
- ↑ SUMBLER, M G. 2002. Geology of the Buckingham district — a brief explanation of the geological map. British Geological Survey Sheet Explanation, Sheet 219 (England and Wales).
- ↑ EDMONDS, E A, POOLE, E G, and WILSON, V. 1965. Geology of the country around Banbury and Edge Hill. Memoir of the British Geological Survey, Sheet 201 (England and Wales).