The Geology of the Speeton Clay
The highest beds of the Speeton Clay - a description by Charlie Underwood.
The uppermost part of the Speeton Clay, the top B Beds and A Beds, are the least well known part of the succession, but are amongst the most interesting intervals. Spanning the Aptian to Middle Albian, this is the only part of the Speeton Clay that can be compared with marine sediments in southern England, and can be seen to be very different.
This interval is generally only exposed in small fault and slip bounded slices along the base of the cliff to the East of the mouth of Speeton Beck. All of the exposures are very temporary, and may be exposed and then re-covered or eroded by the sea within days. The map therefore shows localities where the interval has been seen during the 1990s, and may not be where exposures are seen in the future.
For this reason it has taken over 150 years of observations to build up a succession of the upermost Speeton Clay, and there are still several gaps. Therefore do not EXPECT to go to look at the top Speeton Clay, but count it as a bonus if you see it. For more detail see:
Mitchell, S. and Underwood, C. J. 1999. Lithological and faunal stratigraphy of the Aptian and Albian (Lower Cretaceous) of the Type Speeton Clay, Speeton, N.E. England. Proceedings of the Yorkshire Geological Society, 52, 277-296
and
Underwood, C. J. and Mitchell, S. 1999. Albian to basal Cenomanian (Cretaceous) onlap of structural highs, N.E. England. Geological Magazine, 136, 682-696,
and references therein.
The base of the Aptian is rarely seen and follows on from the Upper Barremian part of the Upper B Beds. These are finely laminated black shales with at least two paler bands with carbonate concretions. The shales are rich in pyrite and contain small uncoiled ammonites and frequent belemnites.
The basal Aptian is marked by a brown sandy bed (one of the very few cases of quartz sand within the Speeton section) with some large concretions. It can be recognised by common part-phosphatised ammonites Prodeshayesites). More commonly seen is the higher part of the Aptian. Most easily recognised is the pale tough marl commonly referred to as the Ewaldi Marl after the common belemnite Neohibolites ewaldi. Other fossils are less conspicuous, but crushed inoceramid bivalves, terebratulids and occassional ammonites are present. At the base is a strongly burrowed surface overlain by a lenticular pebble bed of reworked phosphates and broken belemnites.
This may on occassion be seen overlying hard black marls. These are somewhat pyritic and have some phosphate nodules. Belemnites are very rare, but crushed Prodeshayesites are present. When sieved, the top of this interval is seen to be a mass of planktic forams. The contact between the Aptian and Albian has never been recorded.
The lowermost part of the Albian is not commonly seen, and comprises dark brown-black silty and micaceous shales with small phosphates. Both macro and microfossils are very rare. This is overlain by a conspicuous bed of black-green clay full of glauconite and small black phosphates. This is commonly referred to as the 'Greensand Streak'. As the base of this forms a line of weakness, slip surfaces commonly run along or immediately below it. It therefore common to see this directly overlying Aptian or even repeated several times in the same section.
Immediately above this is a reddish band rich in glauconite, sometimes associated with a discontinuous limestone band. The remaining Lower Albian is composed of grey-brown clays with two intervals with pinkish bands. Rather difficult to sort out in small exposures, the sudden incoming of very common specimens of small species of Neohibolites makes a useful marker band. Other fossils are uncommon, although there is a rich microfauna and shark's teeth are not uncommon in sieved samples. This interval is the most commonly seen part of the A Beds. Of additional interest is the presence of small phosphates coated in baryte in several beds.
The uppermost A beds are of Middle Albian age and are only seen as small slices. This interval consists of grey-brown clay with common burrowing. In many ways it resembles the Ewaldi Marl, with the commonest fossils being belemnites, inoceramids and terebratulids. It may be distinguished by the shape of the common belemnite - the Middle Albian species being allied to Neohibolites minimus and rarely being over 6 cm long with a very blunt apex (rare specimens have a very slender epirostrum covering the apex, however). N. ewaldi is up to twice the length and is more gently tapering. The microfauna is also very different. If you are lucky, you may encounter bedding surfaces rich in crushed hoplitid or hamitid ammonites.
The contact with the Red Chalk is sharp, grading to a dark red hard marl over several cm. It has, to our knowledge, only been seen once.
As there is so little known, ANY additional observations are of great interest, especially additional ammonite records. It is therefore asked that these be donated to a museum and other Speeton Clay workers informed.
Web-page maintained by Mike Horne, Hull, UK.
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