| Computer Code: |
FRSA |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Guadalupian Epoch (PUG)
— Lopingian Epoch (PUL) |
| Lithological Description: |
The Freshfield Sandstone Member comprises generally soft sandstone to coarse siltstone with sporadic interbedded mudstone. The sandstone is pale red to orange-brown, rarely pale grey or with colourless quartz grains, generally very fine to fine-grained, clean, with subangular to subrounded, moderately well-sorted grains of moderate sphericity set in a variable silica or dolomite cement. Two main facies, not always mutually exclusive, appear to be present:
1. Sandstone that generally lacks mudstone partings, has fair visible porosity and possesses a lower sonic velocity (e.g. 110/20-1, 110/13-1) than the Rottington Sandstone Member and the remainder of the Manchester Marls Formation. This type (interpreted here as dominantly aeolian) appears to increase in proportional thickness towards the Llyn-Rossendale Ridge.
2. Sandstone that has a poor visible porosity, contains numerous higher gamma mudstone partings and exhibits a linear sonic velocity trace similar to that of the bounding rocks (e.g. 110/11-1, p.l 15; 110/12a-l); this sandstone is interpreted here to comprise mixed aeolian and fluvial components. |
| Definition of Lower Boundary: |
The base of the Freshfield Sandstone Member is placed at the downward change from sandstone to mudstone or siltstone comprising the remainder of the Manchester Marls Formation (e.g. 110/11-1), or, in very limited areas, the remainder of the Barrowmouth Mudstone Formation (110/3b-4).
On wireline logs, the base of the member is characterized by the abrupt downward increase in gamma values. The sonic shift at the boundary is variable. Commonly, there is a downward increase in velocity at shallow depths of burial and/or where the member is largely aeolian (e.g. 110/20-1, 110/13-1), but at greater burial depths and/or where fluvial sandstones are an important component, a small downward decrease may be evident (e.g. 110/6b-l). |
| Definition of Upper Boundary: |
The top of the Freshfield Sandstone Member is placed at the sharp downward change from micaceous sandstone containing numerous thin interbedded mudstones (Rottington Sandstone Member) to cleaner, less micaceous, sandstone of the Freshfield Sandstone Member (e.g. 110/20-1, 110/13-1), generally with fewer mudstones.
On wireline logs, the boundary is positioned at the abrupt downward change from high velocity, moderately high gamma sandstone (with numerous thin higher gamma mudstone spikes) to lower gamma sandstone, which commonly exhibits a lower velocity (e.g. 110/20-1, 110/13-1). |
| Thickness: |
The Freshfield Sandstone Member thickens progressively southwards from 3 m in 110/3b-4, through 6.5 m in 110/6b-l, to 44 m in 110/20-1. The lateral equivalent onshore is 36.5 m thick in Formby 1 (Kent, 1948). |
| Geographical Limits: |
The Freshfield Sandstone Member is preferentially developed on the northern flanks of the Llyn-Rossendale Ridge, but occurs throughout the south and south-central parts of the East Irish Sea, and also, as equivalents, in onshore areas to the east; it has not been recorded north of 110/3b-4. Lateral equivalents also occur south of the Llyn-Rossendale Ridge in the northeastern part of the Cheshire Basin (e.g. Knutsford 1, Colter, 1978), where they have been termed the "Kinnerton Sandstone Formation (upper 'Lower Mottled Sandstone')" by Evans et al. (1993), or included within the Chester Pebble Beds Formation (Triassic) (Warrington et al., 1980). |
| Parent Unit: |
Manchester Marls Formation (MM)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
Irish Sea well 110/13- 1: 1669-1685 m (5475-5529 ft) (Jackson and Johnson, 1996). |
| Reference(s): |
| Jackson, D I and Johnson, H, 1996. Lithostratigraphic nomenclature of the Triassic, Permian and Carboniferous of the UK offshore East Irish Sea Basin, British Geological Survey, Nottingham. |
| Evans, D J, Rees, J G, and Holloway, S. 1993. The Permian to Jurassic stratigraphy and structural evolution of the central Cheshire Basin. Journal of the Geological Society of London, Vol. 150, 857-870. |
| Meadows, N S, and Beach, A. 1993. Structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir: the Triassic Sherwood Sandstone Group in the Irish Sea. In: North, C P, and Prosser, D J (eds.) Characterization of Fluvial and Aeolian Reservoirs, 247-264. Geological Society, London, Special Publication No. 73. |
| Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, P E, Robinson, P L, Shotton, F W and Taylor, F M. 1980. A correlation of the Triassic rocks in the British Isles. Special Report of the Geological Society of London, No.13. |
| Colter, V S. 1978. Exploration for gas in the Irish Sea. 503-516 in Key-notes of the MEGS-II (Amsterdam, 1978). Van Loon, A J (editor). Geologie en Mijnbouw, 57. |
| Cowan, G. 1993. Identification and significance of aeolian deposits within the dominantly fluvial Sherwood Sandstone Group of the East Irish Sea Basin, UK. 231-245 in Characterization of Fluvial and Aeolian Reservoirs. North, C P, and Prosser, D J (editors). Geological Society, London, Special Publication, No. 73. |
| Kent, P E. 1948. A Deep Borehole at Formby, Lancashire. Geological Magazine, 85, 253-264. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
