| Computer Code: |
HPH |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Frasnian Age (DR)
— Frasnian Age (DR) |
| Lithological Description: |
Reddish purple, green and grey calcareous, silty mudstones with calcareous nodules and a few thin fine-grained sandstones.
Poole (1977) regarded the formation as the deposits of a shallow water lagoon subject to periodic drying and oxidation. Only fish fragments are recorded, suggesting a freshwater lacustrine environment. |
| Definition of Lower Boundary: |
The lower boundary is placed at the base of thermally metamorphosed mudstones of the Hopcroft Halt Formation. The basal about 7 m of the formation are thermally metamorphosed, becoming increasingly hard downwards and eventually converted below 961.27 m to dark purplish grey marble with serpentinite, calcite and hematite in veins and patches. This marble extends to 966.70 m, where it rests on a very hard, dark purple hornfels with a contact dipping at 45° with an underlying basaltic intrusion. |
| Definition of Upper Boundary: |
The upper boundary is placed at the conformable upward passage of mudstones with desiccation cracks of the Hopcroft Halt Formation into the calcareous sandstone-shale facies of the overlying Holt Farm Formation. |
| Thickness: |
28 m drilled thickness. Dip measurements of 5° recorded in the lower beds of the overlying Holt Farm Formation suggest a true thickness slightly less. |
| Geographical Limits: |
Steeple Aston, Oxfordshire. |
| Parent Unit: |
Portishead Subgroup (POB)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
From 938.89 m to 966.90 to 967.00 m in the IGS Steeple Aston Borehole [SP 42NE/12; 4687 2586]. |
| Reference(s): |
| Poole, E G. 1977. Stratigraphy of the Steeple Aston Borehole, Oxfordshire. Bulletin of the Geological Survey of Great Britain, Vol. 57, 1-85. |
| Barclay, W J, Davies, J R, Hillier, R D, and Waters, R A. 2015. Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin. British Geological Survey Research Report, RR/14/02. 96pp. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
