OXFORDSHIRE ROCKS | Zig zag Chalk Formation
Grey Chalk Subgroup
[100.5 - 93.9 Ma*]
chalk: a porous, fine-grained rock. predominantly composed of the calcareous remains of micro-organisms eg. foraminifera and coccolithophores.
Image: Zig Zag Chalk (Totternhoe Stone Member) (Ⓒ Crown Copyright)
* Ma is an abbreviation for million years
Usually a firm, pale grey to off-white blocky chalk with a lower part characterised by rhythmic alternations of marls and marly chalks with firm white chalk. Thin gritty, silty chalk beds act as markers in the sequence. The Zig Zag Chalk Formation has, on average, a higher calcium carbonate content than the West Melbury Marly Chalk Formation.
The Zig Zag Chalk has on average <10% clay content, despite its 'clayey' appearance. This is because it contains a relatively high content of clay-sized (<2 μm) silica flakes in the form of the Opal-CT mineral lussatite. Lussatite consists of extended, disordered interlayering sheets of crystalline cristobalitic and tridymitic silica tetrahedra. This structure gives the mineral an 'expanding lattice' potential which may largely explain the 'clayey' behaviour of this chalk.
The lower boundary is conformable in full sequences. The lower boundary is placed at the possible erosional surface marked by a concentration of phosphatic pebbles in the base of the Totternhoe Stone Member.
The upper surface is conformable and is redefined as the bedding plane beneath the lowest of the marls in the Plenus Marls Member of the overlying Holywell Nodular Chalk Formation.
Generally in the range 35 to 50 metres thick.
The formation is within the Acanthoceras rhotomagense to Calycoceras guerangeri zones.
The Zig Zag Chalk occurs low in the main Chalk escarpment, typically forming steeper ground than the West Melbury Chalk. Where the ground is steep, the Zig Zag Chalk is expressed by alternating buttresses and down-slope gullies creating a corrugated surface with an amplitude of as much as 3 m. This is thought to be related to the development of large-scale orthogonal joint sets in thickly-bedded chalk. The base of the Zig Zag Formation is invariably found at a negative break in slope.
Throughout the Southern Province and the Transitional Province of England.
Corresponding unit in Northern Province:
Ferriby Chalk Formation
Totternhoe Stone Member
A distinctly harder unit in the Grey Chalk Subgroup (Lower Chalk). Typically brownish-grey, fine-grained calcarenite. Has been described as ‘sandy’ because of coarse fossil fragments, not because of quartz sand grains. Thin to thickly bedded. Phosphatic in part with dark brown pellets a few millimetres across, up to nodules several centimetres across. Fossiliferous. Locally used as building stone.
Experimental Tunnel at Chinnor (1974)
experimental tunnel excavations in discontinuous rock (lower White Chalk SubGroup and Grey Chalk Subgroup) at Chinnor in Oxfordshire.
Horizontal inclinometer tubes and extensometers, installed in the shaft walls adjacent to the 210 metre test tunnel, were used to monitor ground movement and shaft-tunnel interaction during tunnel excavation. (Hudson & McCaul, 1975).
Ground behaviour during tunnel excavation was monitored in boreholes, and in a trench excavated across the tunnel line.
Zig Zag chalk exposure in Chinnor Pit 2
An informal Transitional Chalk Province that includes the Chilterns and East Anglia has been used since the Meeting on Chalk Lithostratigraphy Conference was convened at the BGS in Keyworth by the Stratigraphy Commission of the Geological Society in 2005. The Northern Chalk Province and Southern Chalk Province are primarily lithologically delineated by faunal content (Boreal in Northern and Tethyan in Southern).
The Transitional Province takes into account the affect of the Anglo-Brabant Massif on sedimentation, the interdigitation of lithological characteristics such Boreal and Tethyan faunal assemblages.
Chinnor Cement Works, South Oxfordshire
CHINNOR CHALK PITs
Chinnor Quarry is a geological Site of Special Scientific Interest which exposes fine sections of the Grey Chalk Subgroup and the basal part of the White Chalk Subgroup.
Chalk has been quarried to provide the raw products for the industrial production of Ordinary Portland Cement (OPC) at Chinnor since 1921 (just hydraulic lime production from 1908). At the height of production 5 600 tonnes per week was manufactured at the complex. Due for closure, decommissioning was delayed as construction of the Channel Tunnel called for vast quantities of cement. Rugby Cement plc finally closed the site in 1999.
Chinnor Quarry consists of three pits. Pit Three, is of particular interest. It is a narrow, sinuous deep excavation, 950M long, orientated north-east to south-west. The excavation is ~100 m wide at the southern end and ~200 m wide at the northern end, where there are seven exposed chalk faces, each one is 5-6 m in height, separated by berms. In total 35 m of the Grey Chalk Subgroup is exposed at the base of the quarry, with a further ~14 m of the overlying White Chalk Subgroup (Holywell Nodular Chalk Formation) above it.
The regional dip, and the dip of the succession exposed here is ~0.5º-1º to the south-east. The higher units appear to exhibit dips ~8º-11º north-west. There is some minor faulting in the strike-sections, However, in the dip-section at the north-east-end extensive, closely spaced normal faults (both oblique & sub-parallel to the exposure) are evident.
The Plenus Marls Member is affected by normal Faults orientated approximately north–south, with displacements of 0.5 m to over 2m to the east. The only fault that can be traced throughout the north-east exposure has a displacement of 2 m in the Plenus Marls Member, at the top of the succession, reducing to 0.4 in the Totternhoe Stone Member at the foot.
1908 William Benton founded Chinnor Lime Works and began lime burning in five beehive kilns to manufacture lime for agriculture and construction.
1919. Business was expanded to include cement making and four additional static kilns were built allowing cement production to begin two years later.
1928 The static kilns were replaced by a rotary kiln allowing production to increase to 225 000 tonnes pa.
1938 The manufacture of hydraulic lime was transferred to six modern shaft kilns, making the original beehive kilns redundant.
1960 The site is extended to ~81 hectares as British industry flourished. This site was one of just six major operational cement plants in
the United Kingdom.
1990 Changes in the construction and minerals industry made the plant unviable and a decision was taken to decommission the site by the end of the 20th Century.
Chinnor Chalk Pit (disused)
The former GWR branch line survived closure in 1966 because of the Chinnor Cement Works
Bristow, C R, 1989. Geology of the East Stour - Shaftesbury district (Dorset) British Geological Survey Technical Report WA/89/58.
Bristow, C R. 1990. Geology of Sheet ST92SW (Berwick St John, Wiltshire). British Geological Survey Technical Report WA/90/49.
Bristow, C R, 1991. Geology of Sheet ST 81 (Shillingstone-Compton Abbas, Dorset). British Geological Survey Technical Report WA/91/03.
Bristow et al. 1997. Discussion. Proceedings of the Geologists' Association, Vol.110, 68-72.
Bristow, C R, Mortimore, R N and Wood C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol.108(4), 293-315.
Bristow, C R, Mortimore, R N and Wood, C J. 1999. "Lithostratigraphy for mapping the chalk in southern England" by
Hopson, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01 102pp. ISBN 0 852725175
Kennedy, W J. 1970. A correlation of the uppermost Albian and the Cenomanian of South-West England. Proceedings of the Geologists' Association, Vol.81, 613-677.
Rawson, P F, Allen, P M and Gale, A. 2001. A revised lithostratigraphy for the Chalk Group. Geoscientist, Vol.11, p.21.
Sumbler, M G and Woods, M A. 1992. The stratigraphy of the Lower and Middle Chalk at Chinnor, Oxfordshire. Proceedings of the Geologists' Association, Vol.103, 111-118.