Wright, J.K. & Cox, B.M. 2001. British Upper Jurassic Stratigraphy. Geological Conservation Review Series, No. 21, JNCC, Peterborough, ISBN 1 86107 482 4. The original source material for these web pages has been made available by the JNCC under the Open Government Licence 3.0. Full details in the JNCC Open Data Policy
Dry Sandford
J.K. Wright
Introduction
Dry Sandford Quarry lies within a nature reserve immediately south-east of the village of Cothill
Since its earliest documentation, the site has figured strongly in accounts of the Oxfordian Stage in southern England, being particularly well known through the works of Arkell (1936b, 1947b, 1935–1948). Callomon (1960) has produced a definitive account, and the sequence has also been described by McKerrow and Baden Powell (1953), McKerrow (1958) and McKerrow and Kennedy (1973). Talbot (1971) discussed the carbonate cements, and Talbot (1973a) and Johnson (1983) discussed the erosion surfaces and palaeoenvironment.
Description
The succession in the quarry is predominantly arenaceous and totals 7.5 m in thickness. An updated section is as follows (bed numbers and data on beds no longer exposed (in brackets) taken from Arkell (1936b)).
Thickness (m) | |
Stanford Formation | |
Coral Rag Member | |
10b. Flaggy, micritic limestone containing Thecosmilia annularis (Fleming) and Thamnasteria concinna (Goldfuss) in a fine-grained, slightly shelly matrix seen | to 0.25 |
10a. Tough, flaggy, bioclastic limestone containing well-preserved large bivalves in a bioclastic matrix with many coral fragments | 0.30 |
Kingston Formation | |
Beckley Sand Member | |
9. Soft, calcareous, fine- to medium-grained sand approx. | 1.0 |
8c. Upper Trigonia Bed: shelly, calcareous sandstone passing up into extremely sandy, shelly bioclastic limestone | 0.40 |
8b. Poorly cemented sand | 0.10 |
8a. Upper Trigonia Bed: shelly, medium-grained, very sandy limestone with only scattered, abraded shell fragments | 0.50–0.60 |
7. Iron-rich, shelly sand | 0.40–0.80 |
6. Lower Trigonia Bed: medium-to coarse-grained, shelly, very sandy limestone, sporadically ooidal, with well-preserved bivalves | 0.20 |
5. Poorly sorted, fine- to medium-grained, shelly sandstone | 0.70 |
(4. Gritstone, poorly fossiliferous | 0–0.45) |
(3. Interlaminated shelly sand and clay | 0.15–0.30) |
2. Sand with large calcareous concretions containing Nanogyra nana (J. Sowerby) and Lima sp. | 1.50 |
(1. Natica Band: extremely fossiliferous, decalcified gritstone (seam of clay, underlain by white sand | 1.55) |
A log of the section as seen by Johnson (1983) is given in
Bed 8: 6 Perisphinctes spp., 1 Cardioceras sp., 1 Goliathiceras sp.
Bed 7: ?3 Perisphinctes spp., 5 Cardioceras spp., 3 Goliathiceras spp.
Bed 6: 6 Perisphinctes spp., 3 Cardioceras spp., 3 Goliathiceras spp., 2 Aspidoceras spp.
Bed 5: 3 Perisphinctes spp., 3 Cardioceras spp., 1 Goliathiceras sp., 4 Aspidoceras spp.
Bed 4: 2 Cardioceras spp.
Beds 3, 2: 2 Cardioceras spp., 2 Goliathiceras spp., 2 Aspidoceras spp.
Bed 1: 1 Aspidoceras. sp.
The fauna of Bed 8, with its great preponderance of perisphinctids, indicates the Antecedens Subzone of the Sub-Boreal Province, whereas the faunas of beds 1 to 7 are typical of the Vertebrale Subzone.
The Coral Rag, which here represents the Stanford Formation, is poorly exposed at the top of the quarry.
Interpretation
The soft sands in the lower part of the Beckley Sand Member with concretions up to 0.6 m thick and 1–2 m in diameter (Bed 2) are still well exposed. These coarse, shelly, cross-bedded sands with numerous bivalves and ammonites suggest rapid accumulation in a beach environment. The underlying Natica Band, a decalcified gritstone largely composed of the casts of the eponymous gastropod, is unfortunately no longer exposed. It is an excellent marker horizon traceable locally over several kilometres at the base of the Vertebrale Subzone.
The sequence in the upper part of the Beckley Sand Member consists of alternations of very sandy, shelly limestones with medium- or even coarse-grained, slightly shelly sands
The difference in age of these two shell beds at Dry Sandford, separated by only 1 m of quartz sand (
As is often the case, the Coral Rag forms a transgressive sequence, consisting of bioclastic, coral-fragment sand laid down in shallow water, overlain by coral-rich micritic limestone. These cemented lime-mud deposits were laid down under quiet, stable lagoonal conditions, away from the marginal reef of the Oxford area, with the growth of both phaceloid or branching corals (Thecosmilia) and massive, encrusting corals (Fungiastraea and Thamnasteria) .
Ammonite assemblages collected in the quarry have enabled substantial correlation to be achieved between the different Oxfordian faunal provinces. Ammonite faunas from this site represent both the Boreal and Sub-Boreal populations (see Chapter 1, 'Oxfordian and Kimmeridgian zones and subzones', and
Conclusions
This site is the most important of those described by Arkell (1936b, 1947b) and Callomon (1960) in their establishment of the sequence of ammonite faunas in the Middle Oxfordian of the Oxford District. The abundance of stratigraphically useful ammonites at Dry Sandford Quarry, Cothill, led to this being defined as the formal standard succession for the Plicatilis Zone by Callomon (1960). Though it is no longer possible to collect ammonites, the excellence of the exposure coupled with a detailed knowledge of the ammonite faunas makes the site invaluable in any study of Oxfordian stratigraphy, palaeogeography, or palaeoecology.