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GEOLOGY SOUTH OF SUNDERLAND: 12th May 2012 A group of about 23 members met at Ryhope (NZ417531) where the leader, Andy Lane, gave an overview of the Durham Permian geology which consists mainly of about 4 cycles of marine transgression with deposition of magnesium rich limestones followed by regression and sea water evaporation. Possible causes of these fluctuations in sea level were discussed, one of them perhaps being glacio-eustatic. Some features of the Concretionary Limestone Formation were demonstrated, one of the most noticeable being frequent collapse and brecciation of the normally well bedded Formation due to solution of the underlying Hartlepool Anhydrite. For the next locality the party assembled at the car park (NZ424508) close to Seaham Hall. Some caves, fractures and faults as well as interesting solution and replacement structures in the Roker Dolomite Formation were pointed out. It was, however, not possible to tell the difference between the CLF and the RDF. In fact, interpretation was a big problem when the BGS mapped the area in the 1960's and 70's.
The third locality was at Seaham just south of the Featherbed Rocks
where rocks of the 3rd cycle (Zechstein 3), preserved on the
downthrow side of the younger Seaham Harbour Fault, were
examined. The following succession was tentatively identified,
youngest at the top: However, as with many exposures of the Durham Permian succession, interpretation of the succession is difficult due to dissolution of the evaporate horizons followed by collapse and brecciation of the overlying deposits. The rock armour at the base of the cliff was also examined. It consists of huge boulders of what is almost certainly the very handsome Norwegian rock, widely used for ornamental purposes, termed larvikite (or laurvigite) by Brogger (1890). It is characterised by its coarse grain and distinctive feldspars which exhibit a beautiful blue schillerization, interpreted as possibly the result of a cryptoperthitic intergrowth of two types of feldspars. (cf. Hatch & Wells: Petrology of the Igneous Rocks, 13th edition). Immediately on the north side of the harbour, complex recrystallisation of magnesian limestone was examined, many of the structures showing a radial pattern. The final locality visited was the Blast Beach, just south of Seaham (NZ436478) which displayed many reminders of the area's mining and industrial past, including an artificial "raised beach" made up of colliery spoil, almost all of it derived from the underlying Coal Measures. The non-marine "mussel" Anthracosia sp. was identified in one of the boulders. |
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Collapse and brecciation of the normally well bedded Concretionary Limestone Formation due to solution of the underlying Hartlepool Anhydrite can be seen to the right of the photo. |
Seaham Formation (cycle 3), towards the top left, followed by the Fordon Evaporite Formation (cycle 2), below. The Roker Dolomite Formation (cycle 2) at the base. |
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Complex recrystallisation of magnesian limestone, occasionally exhibiting radial structures. |
The Blast Beach covered in colliery waste of broken shale and sandstone boulders. |
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A view of the Blast Beach, looking south towards Nose's Point. More consolidated waste forming a wave cut platform can be seen to the right. |
