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Geology Group Diary (35)

Started by johnd, October 11, 2018, 09:24:45 pm

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The Geology Group met at 10.30am on Wednesday 10 October 2018 at Merlin's Bridge Village hall. 20 members present. The topic this month was...THE CRETACEOUS ROCKS OF BRITAIN.
By the beginning of the Cretaceous period, Pangaea was breaking up accompanied by increased plate tectonic activity. The proto Pacific, Indian and Atlantic oceans were formed at this time as spreading mid oceanic ridges produced large volumes of basaltic lavas causing a eustatic rise in sea level. The Tethys Ocean began to close in late Cretaceous times as the Arabian/Indian plates drifted northwards towards Eurasia.
The Cretaceous (145 to 65 Ma) is the longest geological period since the end of the Precambrian. The Weald of south east England provides one of the best areas for the study of Cretaceous strata.. Structurally, the Wealden uplift is due to the Alpine earth movements that produced an anticline which dips gently away from its east-west axis, although the fold also plunges to the west
The High Weald is formed of the Hastings Beds which are the oldest Cretaceous rocks in the centre of the anticline. The rocks are mainly sandstones that show cross stratification, ripple marks and plant remains. These were deposited in deltas within the shallow waters of the Wealden Lake. On the shores of the lake grew conifers, cycads and giant horsetails on which dinosaurs such as Iguanadon were feeding. The first remains of Iguanodon were discovered in 1822 by Mary Mantel in the Weald of Sussex. Ironstone nodules occur within clay bands providing a source of ore for the Wealden iron industry which developed in the 16th C to provide cannon for Tudor ships that were build of oak from the Wealden forests. The area was extensively forested until the demand for charcoal for smelting and timber for ships led to the removal of much of the woodland. Hammer ponds were used to power water wheels that operated bellows and hammers for the iron works. The High Weald was the centre of the iron industry in Britain until the beginning of the industrial revolution when Abraham Darby developed the coal fired blast furnace in 1709 in Coalbrookdale. There were also numerous small quarries that provided local building stone. Bateman's House, formerly Rudyard Kipling's home, is a good example of the use of local Ashdown sandstone. The High Weald forms a major watershed separating north and south flowing rivers including the Medway and the Wey, the Arun, the Ouse and the Cuckmere. River capture commonly occurs where some rivers cut back by headward erosion and divert the headstreams of others, thus increasing their drainage system.
The Low Weald is formed of Weald Clay and forms a horseshoe shaped outcrop around the High Weald. The clay vales are poorly drained (impermeable clay) and mainly provide pastureland. Brick making was based on the Weald Clay, particularly during the 19thC when bricks were in demand for the London market.
The Lower Greensand lies above the Weald Clay and it has been worn back by erosion to form prominent scarps overlooking the clay vales. Leith Hill on the northern escarpment forms the highest point in the Weald. The sandy acidic soils generally support heathland and coniferous woodland. A spring line marks the base of the greensand where water emerges along the junction with the clay. Note that the greensand is commonly orange or brown in colour due to oxidation, but the name came from greensand containing glauconite (hydrated potassium iron silicate) that outcrops on the Dorset coast. The two most prominent formations within the Lower Greensand are the Hythe beds (buff sandstones with chert layers) and the overlying Folkestone beds (orange/brown poorly cemented sandstones). The latter often show cross stratification; evidence of deposition in shallow seas. They also contain irregular contorted beds of ironstone (carstone) that were precipitated by ferruginous waters percolating through the sandstone after it was lithified. The Folkestone beds are quarried to provide soft building sand.
The Lower Greensand was deposited under shallow marine conditions as the Wealden Lake was invaded by the sea around 115 million years ago. Later as the sea deepened the Gault Clay was laid down. This clay is one of the most fossiliferous horizons in Britain containing a rich marine fauna of ammonites, bivalves and gastropods. It forms a narrow vale at the foot of the chalk escarpment. At Folkestone Warren rotational landslipping occurs in winter when the overlying chalk is saturated and slides over the impermeable Gault Clay. In 1915 a passenger train was derailed on the coastal railway line which was buckled by a landslide.
The Chalk encloses the Weald on three sides forming inward facing escarpments along the North and South Downs. The present river system was initiated on the chalk cover which has since been eroded over the Weald. The rivers have cut gaps through the chalk scarp; for example, the Wey gap at Guildford and the Ouse gap at Lewes. Note that where the dip of the chalk is steep as on the Hog's Back, the outcrop is narrow. Where the dip is gentle or horizontal, the outcrop forms extensive undulating downland. Dry valleys are common where the water table has been lowered. The Seven Sisters on the Sussex coast are dry valleys truncated by the sea. A spring line occurs at the base of the scarp along the junction between the chalk and the underlying Gault clay. Anglo Saxon settlements developed along the spring line. Villages with suffixes such as 'ham, ton and ing' date back to this period.
Geologically, the chalk is a fine grained white limestone formed from calcareous mud containing microscopic coccoliths derived from marine plankton. The Cenomanian transgression in late Cretaceous times covered much of southern England and since there was little sand and mud brought down by rivers, the sea remained relatively clear and the chalk sediment was free from impurities. Fossils are fairly common in the Lower Chalk including ammonites, belemnites, echinoids, bivalves and brachiopods. Micraster, the heart urchin is common as are brachiopods such as terebratulids and rhynchonellids. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges, radiolaria and diatoms during the deposition of the chalk.
Mass Extinction at the end of the Cretaceous
This event is known as the K-T extinction or the K-Pg extinction (Cretaceous-Tertiary or Cretaceous- Palaeogene). It occurred 66 million years ago when an asteroid some 10-15 kilometres in diameter impacted the Earth creating the Chicxulub crater in the Yucatan peninsula in the Gulf of Mexico. The boundary marking the extinction is formed of a thin layer of sediment that is rich in iridium which is abundant in asteroids and meteorites. The sediment represents the dust and shattered rock fragments produced by the impact. Shocked quartz which is produced by intense pressure, is also present in the sediment. Luis Alvarez, an Italian physicist, first proposed the impact hypothesis in the 1980s when he identified the iridium rich clay boundary layer near the ancient Umbrian town of Gubbio. Later the Chicxulub crater was discovered in the 1990s and it provided strong evidence in support of Alvarez's research.
However, it is likely that other events contributed to the mass extinction. In the Deccan plateau in India basalt lavas were erupted at the end of Cretaceous times. These would have produced vast amounts of CO2 and SO2 and contributed to global warming with acid rain killing off the vegetation. Whilst 75% of all species became extinct, the dinosaurs are often seen as the chief victims of catastrophic events, yet many invertebrates such as the ammonites had been in decline throughout the Cretaceous. Other creatures including bony fish and placental mammals developed during the Cretaceous, survived the K-T impact and then expanded in the Palaeogene.
John Downes


6 more images of Cretaceous geology.