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The Permian to Jurassic Stratigraphy and Structural Evolution of the Central Cheshire Basin

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The Permian to Jurassic Stratigraphy and Structural Evolution of the Central Cheshire Basin Journal of the Geological Society, London, Vol. 150, 1993, pp. 857-870, 11 figs. Printed in Northern Ireland The Permian to Jurassic stratigraphy and structural evolution of the central Cheshire Basin D.J. EVANS, J.G. REES & S. HOLLOWAY British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Abstract: New seismic reflection data reveal details of the Permo-Triassic basin fill in the central Cheshire Basin. The seismic stratigraphy indicates that the Permian and Triassic succession is far thicker than previously estimated. Seismic data and backstripping of the preserved Permian to early Jurassic sedimentary succession suggest that at least two phases of fault-controlled subsidence occurred, in early Permian and early Triassic times. An intra-Triassic unconformity is seen on the seismic data near the top of the Sherwood Sandstone Group. This may equate with the Hardegsen Unconformity. It is possible that this unconformity was generated by syn-extensional regional uplift accompanying lithospheric thinning. The gross morphology and location of the faults on the eastern margin of the Cheshire Basin are interpreted to be, at least locally, controlled by extensional re-activation of a westwards-dipping reverse fault, possibly an extension of the Pontesford-Linley lineament. The Cheshire Basin is located in northwest England (Fig. 1). the zone of 'wet-rock head', causing collapse of overlying The basin was initiated in early Permian times and the strata. preserved fill comprises sedimentary rocks of Permian to Depth conversion and thickness estimates in the Early Jurassic age. These are flanked by Carboniferous and following account were calculated using the average interval Lower Palaeozoic rocks (Fig. 2) and for the most part velocity values quoted by Gale et al. (1984) from the overlie Carboniferous Coal Measures and Barren Measures Knutsford and Prees boreholes for the Jurassic (Smith 1985; Thompson 1985). The solid geology is poorly (3.66kms-~), Mercia Mudstone Group (4.1kms -~) and exposed, due to extensive Quaternary to Recent cover, but Sherwood Sandstone Group (4.38 km-l). All depths are deep boreholes at Knutsford, Prees and Wilkesley (Figs 1 given in metres below OD (mean sea level). and 2), provide limited insight into the subsurface relationships of Permo-Triassic and Lower Jurassic strata in Borehole data the basin. Present views broadly agree that the Cheshire Basin is a Deep hydrocarbon exploration boreholes drilled at Knuts- faulted half-graben with a NE-SW (Caledonoid) trend. ford (BP) and Prees (Trend) (Colter & Barr 1975; Jackson Basin formation was controlled by subsidence along the et al. 1987; Penn 1987) penetrate the Permo-Triassic Wem-Red Rock fault system in the east, with depositional succession in the central part of the basin (Figs 1, 2 & 5). onlap characterizing the western margin (Colter & Barr Cross-basin stratigraphical correlations have been made 1975; Colter 1978; Thompson 1985; Abdoh et al. 1990) and using gamma-ray logs from the Knutsford, Prees and perhaps the eastern margin (Hull & Green 1866; Wilson Wilkesley boreholes and sonic logs from Knutsford and 1982). Prees (Fig. 5). The sonic logs also provide a stratigraphical The geological succession in the Cheshire Basin (Figs 3 calibration of the seismic reflection data in Figs 6 & 7. The & 4) comprises the Collyhurst Sandstone Formation, the gamma-ray log for the Wilkesley borehole was converted to Manchester Marl Formation, the Sherwood Sandstone API units using the method described in Schlumberger Group, the Mercia Mudstone Group, the Penarth Group manuals (1972). and the Lower and Middle Lias (Walker 1914; Warrington et al. 1980 & references therein; Poole & Whiteman 1966 & Geological interpretation of the seismic data references therein; Cope et al. 1980 & references therein; Wilson 1982, 1993). The Penarth Group and the Lower and Middle Lias are confined to an outlier near Prees. Seismic character of the Permo-Triassic basin fill Recent seismic reflection data acquired close to the Knutsford borehole (Fig. 6a) reveal the distinctive seismic Database character of the Permo-Triassic succession in the central part of the basin. The Knutsford borehole commenced in halite collapse breccias of the associated with the Northwich Reflection seismic data Halite Formation and penetrated an apparently continuous The study area covers the central part of the Cheshire Basin Mercia Mudstone Group succession (Figs 4 & 6a). The (Fig. 2). The interpretations presented here are based upon rocks of the group are imaged as a seismically banded a grid of seismic data of variable age, quality, and line sequence, with continuous zones of high amplitude spacing, acquired using both vibrator and dynamite sources. reflections alternating with seismically quieter zones; the The overall quality is relatively poor because of halites major saliferous and mudstone units respectively. within the Mercia Mudstone Group that readily dissolve in The Bollin Mudstone Formation, Tarporley Siltstone 857 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/857/4892511/gsjgs.150.5.0857.pdf by guest on 02 October 2021 858 D.J. EVANS ET AL. VALE OF EDEN oo IRISH ":,' . , , . ''. , " . broundary of study area I I I Northwich -q I-- ~0 ,~',' g:" ~ " • Chester . • ,•, " . J IRISH SEA BASIN -. BASIN • . .... .... C re~e: ~1 Stoke-on-Trent ButkeJey Rill .~ .. , . / _ I 8 6b,7a_ APPRO×IMATE " , Stoke-on-Trent • ! . 1 ~AREAOF s uo, I: ..... I -- __-- Wilkesley,~l~ ~ .. " • NEEDWOOD ~ " "1 L. BASIN ;.~ .~..~. I • J........ Mercia MudstoneGroup -~ e { } I ' ~ .~ F~.~l.~Bufkeley HiPI Formation l \ I Sherwood Sandstone Group BAS,N-~/",2 . " . and Permian rocks i i ' J I I Carboniferous and older rocks ....... L ~ Seismic section (with Figure No.) O Borehole MDH Market Drayton Horst I Fig. 2. Geological location map of the Cheshire Basin with approximate locations of seismic reflection lines. Fig. 1. The distribution of Permian to Jurassic basins pertinent to the study area. IA ~~SH SEA CHESHIRE BASIN STAFFS. / WORCS Previous her Jacksonet a11987 AfterWarrington et al 1980. ' BASIN "English Midlands t )this study) Afte Wa mgtonet al. 1980 succession" {Hull, 1869) Formation and Helsby Sandstone Formation can usually be [ [ NW SE identified as groups of moderate to high amplitude Ormskirk Helsby I Bromsgrove I continuous reflections (i.e. reflectors a, b and c respectively I Sandstone Sandstone I Sandstone I Lower Keuper Formation Formation Sandstone on Fig. 6). The Helsby Sandstone Formation apparently ? unconformity thickens to the south of Knutsford. Wildmoor Upper The Permo-Triassic succession beneath the Helsby I Wilmslow Sandstone Mottled J Sandstone Formation Sandstone Formation gives rise to a seismically layered I Fnrmntinn Sandstone St Bees :9 oo Chester °° 5,$ U;o interval, some 1864 m (1100 ms TWTI') thick, within which o Kidderminstero o Bunter Sandstone :~, oo Pebble Beds o~°%o~$ Pebble three seismically poorly reflective zones are developed, <~oo Formation o~" Formation d~,~ Beds Formation ~.o ~.. ~ ~.o v.. 0%~o 0%~o 0%~o O°.oc~o o°,,~o o corresponding to the Wilmslow Sandstone, Chester Pebble Kinnerton Beds and Collyhurst Sandstone formations. These three Sandstone transparent zones are separated by two zones of high Formation Bridgnorth Lower amplitude, coherent reflections, corresponding to the top of -- StBees ------~-ManchesterMarl shales and -- <........~-- and -- Sandstone Mottled the Silicified Zone within the Wilmslow Sandstone -__ evaporit_es --~_- equivalents -- Formation Formation (Colter & Barr 1975), and the Manchester Marl Sandstone Formation (and lateral equivalents) plus the Kinnerton Collyhurst Sandstone Formation Sandstone Formation. Reflections from these strata are of high amplitude with good continuity and can be traced over Conglomeratic sandstone ~] Argillaceous sandstone the entire study area. The base of the Permo-Triassic formation formation succession is marked by a high amplitude, continuous event Fig. 3. Generalized nomenclature of the Sherwood Sandstone associated with truncation of underlying reflections from Group and Permian formations for the Permo-Triassic basins Westphalian strata. The tie to the reflection marking the discussed in the text. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/857/4892511/gsjgs.150.5.0857.pdf by guest on 02 October 2021 THE CHESHIRE BASIN 859 i ..... I Sandstones, dominantly aeolian I~----EZ~ Siltstones PLIENSBACHIAI~ t:; q Sandstones, dominantly fluvial I~ I Mudst°nes Ill calcareous Do -- 3500 Do o ° o Do }o o o t Conglomerates / pebbly sandstones ioooia [] t3 Halite SINEMURIAN EJDO :3 MIDDLE LIAS (UNDIVIDED) Calcareous mudstones, limestones and thin sandstones HETTANGIAN LOWER LIAS (UNDIVIDED) RHAETIAN Calcareous mudstones and limestones NORIAN --3000 PENARTH GROUP 1(UNDIVIDED) o3m Mudstones, silty limestones and fine grained sandstones of the ::~3z c::D WESTBURY and LILSTOCK FORMATIONS CARNIAN BLUE ANCHOR FORMATION VTea Green Marl") Green calcareous mudstones LADINIAN BROOKS MILL MUDSTONE FORMATION 2("Upper Keuper Marl") --\ Red-brown mudstones containing anhydrite and halite -- 2500 WlLKESLEY HALITE FORMATIDNI("Upper saliferous beds") Halite with thin mudstones \ WYCH MUDSTONE FORMATION ~(upper "Middle Keuper Marl") ANISIAN Structureless mudstones with many anhydrite nodules ¢_) Do BYLEY MUDSTONE FORMATION2(Iower "Middle Keuper Marl") Alternating structureless and laminated mudstones -- 2000 • . • . • . • . • . NORTHWlCH HALITE FORMATION 1("Lower saliferous beds")
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