The Geology of Royston Cave
A Report by: Haydon W. Bailey, Thomas Fogerty, Liam Gallagher & Nick Pierpoint of Hertfordshire Geological Society
The Chalk of Royston Cave
Results of a survey conducted by members of Hertfordshire Geological Society, January 2022.
Regional Geological Setting
The geology of the Royston area is quite unique, which makes the identification and dating of the Royston Cave chalk regionally important. The geology falls into two time periods; first the deposition of the chalk itself which took place during the Late Cretaceous period between 100 million and 66 million years ago and subsequently the major Anglian glaciation which occurred during the Pleistocene Period between approximately 470,000 and 420,00 years ago. The following description of chalk deposition is taken from Bailey & Wood (2010).
Late Cretaceous Chalk
The familiar white chalk seen in quarries and natural exposures across Hertfordshire is a very fine-grained, extremely pure (more than 98 per cent CaCO3), soft white limestone. The calcium carbonate is in the form of the mineral low-magnesian calcite. The acid-insoluble (i.e. non-carbonate) residue comprises clay minerals such as montmorillonite and illite and subordinate, slightly coarser detritus, predominantly consisting of quartz.
The white colour of the greater part of the chalk results from the insignificant content of clay and iron, while the commercially important brightness or reflectivity is a function of the fine grain-size (Hancock, 1975). Although the chalk was previously thought to be either a deep-sea deposit like the modern Globigerina Ooze, or a shallow-water inorganic aragonite precipitate of Bahama Banks type, the electron microscope has shown that up to 90 per cent of the sediment is composed of tiny calcite crystals, a few microns across, derived from the disintegration of complex, ring-like structures known as coccoliths. Coccoliths are secreted by the Coccolithophoridae, a family of highly specialised unicellular algae. Larger particles of biogenic calcite are also suspended within this coccolith ‘flour'. These comprise nannofossils such as complete coccoliths and, at some levels, the enigmatic Nannoconus; plus microfossils, including calcispheres (dinoflagellates with calcareous skeletons e.g. Pithonella), foraminifera and ostracod valves; and bryozoan, shell, and echinoderm debris.
Siliceous-walled microfossils such as radiolarians are only rarely preserved in the chalk (e.g. Hill and Jukes-Browne, 1886, Hill and Monkton, 1896), but must also have been present albeit in relatively small numbers when compared with the abundances recorded from North Sea chalks of comparable age (Hampton et al., 2010). Particularly important components are the minute calcite prisms derived from the mechanical breakdown of shells of the bivalve Inoceramus. At current-winnowed horizons, these reach rock-building proportions producing chalk that is gritty to the touch. Other macrofossils are a relatively insignificant proportion of the total sediment and tend to be concentrated at particular horizons.
The Royston area has long been known for the occurrence of chalk masses dipping at high angles contrary to the regional dip which is normally less than 5 degrees (Hopson et al., 1996). Where chalk is found dipping at high angles less than 40 degrees, such as at the Barkway and Reed Chalk pits, biostratigraphic and lithostratigraphic evidence indicates that the chalk is of late Turonian age, from levels within the Lewes Formation around the Chalk Rock (Blezard et al., 1967). It has even been suggested that the marl seams within the lower part of the Lewes Formation, e.g. the Caburn (Reed) Marl and the Southerham Marl, may have functioned as decollement surfaces along which blocks of younger chalk were effectively “bulldozed” by the Anglian ice sheet as it progressed southwards from the Wash and over the northern Chiltern escarpment.
It is difficult to obtain an accurate dip reading on the chalk within Royston Cave, so by obtaining an accurate age for the chalk at this location, it should be possible to indicate whether it is in situ or forms part of a glacially affected block.
In order to obtain an accurate age determination for the chalk within Royston Cave it was proposed by Hertfordshire Geological Society members that extremely small samples could be taken and analysed for their coccolith (calcareous nannoplankton) content. This approach was agreed by both Royston Town Council and Historic England and a sampling programme was established.
On January 8th 2022, Dr Haydon Bailey, Dr Liam Gallagher, Nick Pierpoint and Tom Fogerty, all of Hertfordshire Geological Society (HGS), visited Royston Cave together with Jane Dottridge (HGS member and Cave guide) and Nicky Paton (Royston Cave Manager).
Measurements were taken using a laser scanner, in order to construct an accurate lithological log of the chalk succession in the cave. This is illustrated in Figure 3.
Two key marl seams were identified (Marls M1 and M2) and measurements of other, nine minor, marl/clay partings were calibrated against these. The two main marls can be seen in Figure 1.
A total of four “pea” size chalk samples were collected (Samples RC#1 – 4) at selected points within the chalk succession in order to establish a number of stratigraphic data points using the coccolith fossils known to occur in the chalk. Analysis of the nannofossil content of these samples was undertaken by Dr. Liam Gallagher and Tom Fogerty.
Calcareous Nannoplankton Results
All four samples analysed yield assemblages that are diverse, abundant, and moderately well preserved (relative to southern England, Transitional Province, and North Sea Basin area chalks).
Each assemblage is overwhelmingly dominated by Watznaueria barnesiae; a taxon that dominates most calcareous nannofossil assemblages from its inception in the Bajocian to its extinction in the Maastrichtian. Also common in each sample are the Prediscosphaera species P. cretacea, P. columnata, and P. ponticula in association with Eiffellithus turriseiffelii, Tranolithus orionatus and Zeugrhabdotus howei.
Less common, but important stratigraphically, are the polycyclolithaceans (Eprolithus and Quadrum species) E. floralis and E. eptapetalus that are particularly common in the upper two samples.
The record of Quadrum gartneri in samples #RC2 - #RC4, but not in #RC1 suggests that the former samples are Zone UC7 (Burnett, Gallagher & Hampton, 1998) or younger, whereas sample #RC1 (the lowest sample analysed) is Zone UC6 or older in the Early Turonian, close to the New Pit Chalk Formation/Holywell Nodular Chalk Formation boundary. It is likely that sample #RC1 is restricted to Subzone UC6b as it contains Eprolithus eptapetalus which does not range below this subzone but lacks both Lucianorhabdus maleformis & Marthasterites furcatus. Quadrum intermedium, a marker for Zone UC5 and above is recorded in all samples, but Zone UC5 restricted taxa Axopodorhabdus albianus & Helenea chiastia are not reported.
It is likely that samples #RC2 - #RC4 are restricted to Zone UC7 in the Early Turonian, base part of the New Pit Chalk Formation as the marker taxa for Zone UC8 (Eiffellithus eximius & Kamptnerius magnificus) are not recorded.
The chalk of Royston Cave lacks the abundant Mytiloides shell debris so characteristic of the Holywell Nodular Chalk of this region, indicating that the Royston Cave section is younger than this and most probably within the New Pit Formation.
The Zone UC7 date provided by the new nannoplankton analyses for samples #RC2 - #RC4 indicates that the section sits within the latest part of the Early Turonian substage. This dating and the fact that sample #RC1 may well be within Subzone UC6a, suggests that the distinctive Marl M1 at Royston Cave may well be a lateral equivalent of the Odsey Marl described in the quarries at Steeple Morden (Hopson et al., 1996, fig. 14) and in the Hitchin railway cutting as defined by Hopson et al. (1996, Figure 15.).
This correlation, illustrated in Figure 4, is further supported by the almost flint free nature of the chalk above the Odsey Marl at both Royston Cave and in the Steeple Morden quarries. Hopson et al., (1996) describe very rare finger flints in the Plantation Quarry at Steeple Morden, and this is exactly the type of flint recovery seen in the ramp section above Marl M1 in Royston Cave.
In conclusion, the eight metre section of chalk in Royston Cave is established to be of Early Turonian age, within the basal part of the New Pit Chalk Formation. It is considered that Marl M1 is a lateral equivalent of the Odsey Marl, with the flint poor, marl seam rich chalk above this, equating to the highest chalk encountered at the top of the Steeple Morden Station Quarry.
The authors would like to thank both Royston Town Council and Historic England for permission to collect the small chalk samples which allowed us to undertake the nannoplankton analyses. The results of these analyses are included in this report. Our thanks also go to Nicky Paton (Royston Cave Manager) and Jane Dottridge (Cave guide) for their help in initiating and facilitating this project.
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