The geological structure of Hampshire

The Structure of Hampshire is not simple. Early [understandings] based on surface structures have since been disproved, and no consensus has yet been reached as regards the true [picture] .
However; some [conclusions] can still be drawn.

• Early Interpretation
• Comparisons with the South West of England
• The Variscan orogeny
• Structures trending approximately East-West
• Structures trending approximately north-west - south-east

Early Interpretation

Initially, using surface survey, the whole of Hampshire, with the adjoining areas of Dorset and Wiltshire to the west and north-west, were considered to be a simple basin of continuous subsidence throughout the Mesozoic and early Tertiary. This area is known as the "Hampshire Basin", and is [possibly in some way related to the Paris Basin and or Weald Basin, but i do not know]
However; comparisons with the structure of [??Qatar? where?] in the Middle East led to hopes of oil, and boreholes were sunk to investigate further. The results indicated a far more complex structure.

The floor of the major 'basin of deposition', originally taken to be continuous, is in fact divided up into a number of smaller subsiding basins ("grabens"), separated by blocks of stable material ("horsts").

The outlines of these ancient features can be detected in pre-Mesozoic rocks, and they affected Mesozoic sedimentation (including the Cretaceous Chalk) and that of more recent times, and also controlled the position and alignment of folds and faults visible at the surface.

return to top

Comparisons with the South West of England

Whilst we still have little evidence of the deep seated rocks under Hampshire, comparisons with the structure of south-west England, where older rocks are nearer to the surface, may indicate what is under Hampshire.
Intense northward compressions in the Variscan orogeny (toward the end of the Palaeozoic) caused tight folding. Since then, the ancient rocks have acted as relatively rigid stable blocks, with no further folding - although they have [suffered | experienced ] some faulting during Mesozoic times.
More compression and folding associated with the Alpine orogeny affected the younger rocks in Miocene times, but this did not affect the deep-seated Palaeozoic rocks, or their structures.

Devonian and Carboniferous rocks, similar to those of s-w England, underlie much of the Hampshire Basin. However, in the northern and eastern parts, the rocks are different [uh? how?]. The boundary between the two provinces may be an ancient fault line.
[I need more explanation and a sketch, or to drop this para altogether]

South of that ancient fault, is the Variscan province, [more explanation] and there the structures can be divided into two main groups, that trend approximately

• east-west, and those that trend roughly
• northwest - southeast

return to top

The Variscan orogeny

return to top

Structures trending approximately East-West

The faults with an e-w trend were probably the result of the late Variscan orogeny. The effects are widespread, and can be seen in the Devonian, Carboniferous and Permo-Triassic rocks in south-west England. It is believed that the similar structure is reflected by the fault-bounded, east-west trending, river-valley systems in Hampshire.

A number of these faults are accompanied by folds with distinct surface expression as ridges of high ground - such as at Winchester, Fordingbridge and across the Isle of Wight (but not the Portsdown anticline).
Until recently, it was believed that these folds were due to northwards compression during the Alpine orogeny.

More recently, it has been suggested that the structure is a result of re-activated basement (deep-seated, ancient rocks) faulting, with the younger sediment "draped over".
Rather than the structure being a flat layer that crumpled under compression during the Alpine orogeny, it is possible that the structure was already faulted, and as younger sediment was deposited on the older structure, further movement caused the present surface-expression of folding.
[Really? possibly could be better explained]

return to top

Structures trending approximately north-west - south-east

Faults with a north-west - south-east trend are well known in the south-west of England, and have been linked to faults of similar trend in Dorset. Many of the east-west features just described appear to be truncated and displaced along north-west - south-east lines.
There are small nw-se folds under Southampton water, and the major east-west structure of the Portsdown anticline swings into a nw - se direction under Southampton, and continues along the line of the Avon valley.
This line probably belongs to a suite of faults that originated in the Variscan orogeny, and affected the pre-Mesozoic rocks.

return to top

For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk