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

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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......

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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.

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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") .o...' o'..'o. Halite with thin mudstones ? Hardegsen unconformity BOLLIN MUDSTONE FORMATION 2("Lower Keuper Marl") Red-brown mudstones and siltstones

TARPORLEY SILTSTONE FORMATION 1("Keuper Waterstones") Siltstones interbedded with mudstones and sandstones -- 1500 including the MALPAS SANDSTONE SCYTHIAN • .,,:...... ,.... • . • HELSBY SANDSTONE FORMATION1 ("Lower Keuper Sandstone") Sandstones with subordinate mudstones. Comprise a lower, often pebbly fluvial Delamere Member, and an upper, aeolian Frodsham Member

?BULKELE¥ HILL SANDSTONE FORMATIONI("Keuper sandstone passage • ...... -...... Top silicified beds") Sandstones, aeolian and fluvial, interbedded with mudstones ...... • ....- ...... -. • • . WI/MSL0W SANDSTONE FORMATION (rUpper Mottled Sandstone") 1000 ?/: _ _ Sandstones, non-pebbly. Upper part dominantly aeolian, lower part dominantly fluvial ., . ., ...... " - '." ' b".: ' , '.: '6 o ... o. .-o... .o- CHESTER PEBBLE BEDS FORMATION l"Bunter Pebble Beds") Conglomerates and pebbly sandstones

__ -----

• . .~--~.~ . KINNERTON SANDSTONE FORMATION1 (upper "lower Mottled Sandstone") .... Sandstones, generally pebble-free, dominantly aeolian Z RUSHTON SPENCERbreccia at base

undivided MANCHESTER MARL FORMATION (and equivalents) 500 ...... -,~, • ¢-r • , ..,. Mudstone with thin limestones in lower part. In south passes laterally into Idd ¢'1 •..":"..i.i.".".:i •... siltstones and sandstones o. .-...... • . .. ,.... • o QO ° 'o' . ' o COLLYHURST SANDSTONE FORMATION (lower "Lower Mottled Sandstone") Sandstones, both aeolian and fluvial Late Variscan . unconformity COAL MEASURES AND BARREN MEASURES (UNDIVIDED) WESTPHALIAI~ Mudstones sandstones and occasional coals

0m 1. Warringtonand others 1980 Scale shows approximatethicknesses in the Preesarea 2. Wilson 1993 3. Localunconformity on footwallblocks of the Wem-Red Reck FaultSystem 4. Localunconformity, North CheshireBasin Fig. 4. Generalized vertical section and litho-stratigraphy of the study area (details of the ages for the Helsby Siltstone, Tarporley Siitstone, Bollin Mudstone and Northwich Halite formations and the Wych and Byley Mudstone formations, from Wilson (in press) and Benton et al. (in press)).

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LITHOSTRATIGRAPHY PREES

WILKESLEY l O GR 160 160 BHCS 40

Lower Lias and Penarth Group

5OO Brooks Mill Mudstone Formation jJ

KNUTSFORD Wilkesley Halite Formation j/f/-j 0 B"cS Byley/Wych Mudstone Formations [ lOOO ? faulted

E Northwich Halite Formation

Bollin Mudstone Formation

.... Tarporley Siltstone Formation Ii- ..... :--,t -- ? Helsby Sandstone Formation

1111111 faulted interval , Wilmslow / _.

Sandstone (top Siltcified Zone)

Formation

Chester Pebble Bed Formation Kinnerton Sandstone Formation Manchester Marl Fm ~.~ lateral equivalents t

Collyhurst Sandstone Formation

/- ? a\~eOZmCs ,~ LO~e~ Westphalian Barren Measures / Coal Measures

KB: 46m AOD

Fig. 5. Correlation between the Knutsford, Wilkesley and Prees boreholes, based upon sonic and gamma-ray logs.

base Permo-Triassic is poor at Knutsford. Seismic data possibly the Wem fault itself, within the Wilmslow suggests that this borehole intersects a fault near the base of Sandstone Formation. If this is the case, the borehole would the Collyhurst Sandstone Formation. intersect the base of the Permo-Triassic succession in the Seismic data to the west of Stoke-on-Trent and in the footwall block of this fault, with the throw on the fault at Prees area are of good quality. The seismic stratigraphy is the base Permo-Triassic being in the order of 1000-1200 m. virtually identical to that near Knutsford (Figs 6b & 7), The eastern end of the seismic profile is very noisy at the indicating that the geological succession varies little between location of the Prees borehole, and the postulated fault is these areas. From these data, the Mercia Mudstone Group not visible. However, some of this noise is due to reflected in the southern areas of the Cheshire Basin is estimated to refractions or fault plane reflections cutting through the be 1400-1500m thick, and the combined Sherwood section at between 900 and 1600 ms TWTT. These indirectly Sandstone Group and Permian formations are estimated to support the presence of a fault. Further support for the be 2600-2800 m thick. presence of such a fault in the Prees borehole comes from These interpretations, which indicate that the base borehole correlation. Permo-Triassic is some 4000-4500 m deep to the west of Prees, are at variance with published interpretations of the Prees area based on the Prees borehole, in which the basin Borehole Correlation floor is at 3506 m (Gale et al. 1984; Colter & Barr 1975; The gamma-ray and sonic logs of the Knutsford, Wilkesley Scott & Colter 1987). This difference could be explained if and Prees boreholes are shown in Fig. 5. Taking the base of the borehole intersects a previously unidentified fault, the Northwich Halite Formation as a datum, they can be

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confidently correlated to just beneath the level of the Helsby the level of the Wych and Byley Mudstone formations. Sandstone Formation. In the Prees borehole, the Tarporley These formations in the Wilkesley borehole are approxim- Siltstone Formation shows a relatively low gamma response ately 327 m thick (Fig. 5). However, in Prees, only 10 km in the upper part, probably reflecting a more silty or sandy distant, the same formations appear to be only 174 m thick, succession than the lateral equivalent in the Knutsford whilst those above and below maintain a relatively constant borehole. Below the Helsby Sandstone Formation in the thickness. Knutsford borehole, a low gamma signature and rather There have previously been differing interpretations of serrated sonic log response characterize the part of the the stratigraphical or structural relationships of strata of Wilmslow Sandstone Formation above the Silicified Zone. Permian, Westphalian and Lower Palaeozoic age in the This upper part of the Wilmslow Sandstone Formation is Prees borehole. Gale et al. (1984) suggest that a basinal apparently very thin or absent in the Prees borehole, the borehole did not intersect a fault, remaining wholly in the gamma-ray log showing a higher, more serrated response, succession. Wilson (1982) inferred that the contact between more akin to that in the Knutsford borehole below the Top the Permian and Westphalian is stratigraphical but that the Silicified Zone. Indeed, below this level the logs from the boundary between the Westphalian and Lower Palaeozoic is Knutsford and Prees boreholes are remarkably similar in faulted. Interpretations by Colter & Barr (1975) and Scott & both the character and thickness of units, although the Colter (1987) inferred approximately 160 m of Westphalian gamma-ray responses in the Prees borehole are generally strata to lie unconformably upon Lower Palaeozoic rocks. higher. The Wilmslow Sandstone Formation in the Prees West of Prees and elsewhere in the basin there is ample borehole is relatively thin compared to the succession in evidence from the seismic reflection data that Westphalian Knutsford. It is interpreted to be tectonically thinned by the aged sequences greater than 160m thick exist beneath the fault or faults inferred from the seismic section (Fig. 7b). Permo-Triassic. This study supports the interpretation of This may cut out up to 500 m of the succession. Wilson (1982), that the contact between the Westphalian The Chester Pebble Beds, Kinnerton Sandstone and and the Lower Palaeozoic rocks in the Prees borehole is a Collyhurst Sandstone formations can be readily correlated faulted one. between the Prees and Knutsford boreholes which appear almost identical in thickness and log character in both The Helsby Sandstone Formation and its boreholes. Although gamma values are lower for the relationship to an intra-Triassic unconformity postulated Manchester Marl Formation or equivalent in the Prees borehole, the log and seismic character indicate that a Seismic data (Fig. 8) suggest that the Helsby Sandstone lateral, possibly more sandy/silty equivalent (?the Bold Formation locally rests with angular unconformity on a Formation; see Thompson 1989), persists farther south in sequence of rocks which generate seismically distinct, the basin than suggested by previous workers (Colter & moderate to high amplitude reflections. These reflections Barr 1975; Gale et al. 1984; Thompson 1985; Penn 1987; are commonly laterally persistent and are interpreted as Scott & Colter 1987). representing well-bedded strata, which conformably overlie Log correlation within the Mercia Mudstone Group the seismically transparent Wilmslow Sandstone Formation. suggests that the Prees borehole intersects a further fault at They have an estimated maximum preserved thickness of

Mudstones and salts o( the Mudstones and salts of the Mercia Mudstone Group Mercia Mudstone Group

0.5 TarporleySiltstone Fro. Tarporley Siltstone Fm Helsby Sandstone Fro. ? Hardegsen unconformity Helsby Sandstone Fm -- ? Bulkeley Hill Sandstone Fm. ? Hardegsen unconformity Wilmslow ? Top Silicified Zone 1,0 (Lr) Wilmslow Sandstone Fro. Sandstone Chester Pebble Beds ? Top Silicified Zone Kinnerton Sandstone Fm. Formation Manchester Marl Fm Equivalents Chester Pebble Beds Fm. Collyhurst Sandstone Fm. Kinnerton Sandstone Fm. Manchester Marl Fro. Equivalents

Collyhurst Sandstone Fro.

base Permo-Triassic Z.0 Barren Measures/Coal Measures

L5 Fig. 8. Seismic reflection line illustrating the development of a sequence boundary showing truncation of underlying reflections. The sequence boundary is interpreted as the Hardegsen Unconformity, present at the base of the Helsby Sandstone Formation. Folding of strata, which occurs within the hangingwall of the fault, is attributed to the regional uplift of the basin during extension.

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between 200 m and 220 m, and have not been penetrated by Hill Sandstone Formation. The rocks represented by these any boreholes. reflections are best developed in the hangingwall block to At most, the erosion surface associated with the angular the fault zone that defines the eastern margin of the basin unconformity may cut out some 850-900m of strata. (Figs 8 & 10). Laterally it passes into areas where such strata are South of the study area, the Helsby Sandstone apparently absent but evidence for angular unconformity is Formation may be absent in north Shropshire, east of the poor; the base of the Helsby Sandstone Formation is imaged Were Fault Zone. The Tarporley Siltstone Formation simply as a laterally persistent high amplitude reflection overlies aeolian sandstones formerly known as the Grinshill overlying the seismically transparent, dominantly aeolian, Sandstones (Thompson 1985, p.125), which Thompson upper Wilmslow Sandstone Formation (Fig. 9). (pers. comm.) assigns to the Wilmslow Sandstone The exact stratigraphical position of this unconformity in Formation. This suggests that the Helsby Sandstone the most complete successions cannot be identified directly. Formation was not deposited in the southeastern margins of However its position can be inferred from a detailed the Cheshire Basin because of uplift associated with the comparison of the outcrop and seismic stratigraphy. intra-Triassic unconformity (e.g. Fig. 10). Thompson (1970) recognized three members within the Helsby Sandstone Formation, exposed mainly to the north and west of the study area. The basal Thurstaston Member Basin development and structural evolution. consists of aeolian sandstones, interbedded with fluvial sandstones. The overlying Delamere Member consists of fluvial sandstones; at or near its base are the pebbly beds Burial history: methodology described by Poole & Whiteman (1966) as the Keuper A composite stratigraphical section representing the greatest Sandstone Conglomerate. Overlying the Delamere Member preserved Permian to mid-Lower Jurassic succession within is the Frodsham Member (Thompson 1969), which consists the study area was compiled from borehole and field data. of aeolian sandstones. Both Thompson (1970, p.161) and The 200m of the ?Bulkeley Hill Sandstone Formation Poole & Whiteman (1966) recognized a pronounced erosion believed to be present beneath the intra-Triassic unconfor- surface at the base of the Delamere Member. However, mity and not penetrated by the boreholes was added to this. Thompson (1970, p.161) considered that this might This summation indicates that a total of some 4500 m of represent only local erosion at the base of a major channel, compacted Permo-Triassic strata may be present in the whereas Poole & Whiteman (1966) regarded it as marking a central Cheshire Basin, which agrees well with the estimates regional unconformity. from seismic data. Dividing the dominantly aeolian sandstones of the The densities, porosities and thicknesses of the youngest Wilmslow Sandstone Formation from the mixed aeolian and strata preserved in the basin give a guide to their maximum fluvial sandstones of the Thurstaston Member is difficult. depth of burial, as they do not decompact significantly Consequently, Earp & Taylor (1986, p.15) assigned the during uplift and erosion of younger rocks. Densities of Thurstaston Member to the Wilmslow Sandstone Formation 2.52Mgm -3 obtained from borehole information for and redefined the base of the Helsby Sandstone Formation mudstones and siltstones within the Lias Group and the at the base of the Delamere Member. Brooks Mill Mudstone Formation were compared with It appears that the regional unconformity seen on the depth-density curves derived from similar Triassic and seismic data represents the only major break in the Jurassic lithologies in the Wessex Basin (Chadwick 1985a). succession seen at outcrop, i.e. the erosion surface at the This gives an estimate of the thickness of eroded base of the Delamere Member. If this is so, Poole & post-Middle Lias strata of approximately 2.2km, which Whiteman's (1966) interpretation of the erosion surface is compares well with estimates made using sonic log data correct, and the base of the Delamere Member would be the from boreholes. It also compares well with depth of burial natural place to define the base of the Helsby Sandstone estimates offshore, which suggest that some 2 km of strata Formation. may have been removed (Bushell 1986; Woodward & Curtis At Bulkeley Hill, west of the study area (Fig. 2), the 1987). The estimate of 2.2 km of eroded strata was added to erosion surface at the base of the Keuper Sandstone the composite Permian to middle-Lower Jurassic stratal Conglomerate (the Delamere Member of Thompson 1970) thicknesses to more realistically model the decompaction cuts out not only 30m of the Wilmslow Sandstone and burial histories. Formation, but also 20 m of hard, flaggy, brown sandstones The observed basin fill of Permian to Early Jurassic age and red green mudstones of fluvial origin, which are was then decompacted by a back-stripping method similar to interbedded with aeolian sandstones (Poole & Whiteman that of Sclater & Christie (1980). This permits the 1966). The latter rocks conformably overlie the aeolian sedimentary section to be restored to its original thickness at dominated upper part of the Wilmslow Sandstone the end of each stratigraphical interval. Burial history curves Formation and were regarded by Poole & Whiteman (1966) were generated illustrating the basin evolution for both as a largely fluvial sequence, lying between the Wilmslow sediment loaded and sediment starved cases (Fig. 11). No Sandstone Formation and the Delamere Member, which attempt was made to estimate water depth. they named the Keuper Sandstone Passage Beds. These Because the detail of the removed Jurassic, Cretaceous strata were renamed the Bulkeley Hill Sandstone Formation and Tertiary succession is unknown, the curves are only by Warrington et al. (1980). The moderate to high displayed to the end Mid-Lias. However, studies of the amplitude, in places laterally persistent reflections imaged Morecambe Bay gas field suggest an uplift of some 600 m at below the Helsby Sandstone Formation on the seismic data the end Jurassic (Stuart & Cowan 1991), with some 1500 m (Fig. 8) may represent a thick development of the post-Cretaceous uplift (Cowan pers. comm. 1992). interbedded fluvial and likely aeolian facies of the Bulkeley Details of the Permian cycles, stratigraphical nomencla-

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t ...... J Measures i , ..... J "~t ; ~ i '~ ~i~" P'~. ' - " . . .. , ~----~ -- -- ' I 0 1L ...... ~/" .... I I ...... ~ 1 i 0 ~ rep s,ici~,o." I

--:=2: : :-_ , • Perrrlo.'/-ri~'_.." .' .'." ." ." ." .' ." -2.0 2.0

Westphalian P-Tr = Permo-Triassic

-2.5 2"5t 7 L__J lkm

3.0

Fig. 9. Seismic reflection line and line drawing illustrating the nature of the eastern margin of the Cheshire Basin onto the Market Drayton horst. Syn-sedimentary normal faults of Permo-Triassic age developed as a result of, and detach onto, a putative re-actived Variscan compressional structure (thrust). Note in this area, the major thickening of the Sherwood Sandstone Group and Permian formations appears to occur across a fault (the Bridgemere fault; Rees & Wilson pers. comm.) to the west of the Wem fault.

ture and correlations are from Smith et al. (1974, 1986). The Chronostratigraphical details are not available for the stage and zonal details of the Mercia Mudstone Group are Collyhurst Sandstone Formation. It is post-Westphalian but based upon Wilson (in press) and Benton et al. (in press & otherwise its age is only constrained by the overlying late references therein). The Jurassic part of the curves (Fig. 11) Permian Manchester Marl Formation. Therefore, sub- was calculated using the stage and zonal details in the sidence data are only reliably calculated from the onset of references cited above. Chronological calibration is based the deposition of the Manchester Marl Formation (Fig. 11), upon Forster & Warrington (1985) and Hallam et al. (1985), which was at about 260.5 Ma (Forster & Warrington 1985). with ages recently supported by Odin & Odin (1990) and Even if deposition of the Collyhurst Sandstone Formation Claou6-Long et al. (1991). started at the beginning of the Permian, subsidence during

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NNW SSE Permo-Triassic basin subsidence Subsidence curves cannot be accurately determined for the Thin Wilmslow Set Fm main part of the Permian succession due to a lack of Frodsham Mbr (Helsby Set Fm) ~~ chronostratigraphical control. However, there are indirect indications that fault activity might have taken place in early Permian times (cf. Tonks et al. 1931; Thompson 1985; Earp - -- -- 4Ep~per part) (includesthe Thurstaston Mbr, / ;'. " . . ", and older Fms .- ~------__--~--~ ,HelsbySstFmJofrhompson 1970, /" "~~d de-rFms::: & Taylor 1986). Local breccias are documented towards the ~~_-_-_-_-_~_~~_o-~------"-~ _-~--_/..4 . ~../:-::,'//J'. '- ~. " :-.-,• - . : base of the Collyhurst Sandstone Formation (Hull 1869; ~o:%°~~--Wilmslow Set Fm -_--__./. : ~:" :, ~ ., .::- Taylor et al. 1963 and references therein), and in the lower •~o,,o~%~o~ o o----10wer0art----- .-..."'. .:"..:: , --.. ' : .' .'- 75m of the Collyhurst Sandstone Formation in the Chest erPebble eB dsFm ,/rQ~,~%~-~"y. ~ Dominantly'aeolian Knutsford borehole, very coarse-grained and pebbly KinngrtonSet Fm ~"7~,~'°?/ :: ~--:~ Dominantlv fluvial sandstones occur. Some of these breccias probably represent local alluvial fan deposits interdigitating with the more w Unconformzty' j~1~: . . ' . " , . .:"- ,..: " Conglomeraticrocks w~//~. i..i i ~n°e°~ Chester Pebble Beds characteristic aeolian sandstones of the Collyhurst Sand- Fm Formation i :~/ . '. . . ~ Westphalian and Mbr Member ~" ~ / " " I.'.'.1 olderrocks stone Formation (Thompson 1985). It is probable that most of these coarse deposits were derived from active fault Fig. 10. Interpretive sketch section illustrating the relationship of scarps generated during early Permian extension. strata to the intra-Triassic unconformity recognized in the seismic A conglomerate or breccia, containing clasts derived reflection data. from the Midlands, occurs at the top of the Collyhurst Sandstone Formation in the northern part of the Cheshire its deposition was probably slightly faster than in late Basin (Tonks et al. 1931; Taylor et al. 1963). This may Permian times. From regional considerations (below) it is represent an extensional pulse towards the end of early inferred the onset of extension occurred some time during Permian times which led to the marine incursion marked by mid-late Rotliegend times (Fig. 11). the base of the Manchester Marl Formation, itself marking a

miili0ns of years before present 280 270 260 250 240 230 220 210 200 190 180 1 l t ! J l 1 J_ _ 1 _1 PERMIAN I TRIASSJC ~- JURASSIC

Lower Upper J Scyt a Anisian Lad e Carnian No an Rhaetianan._g~man...... Sinemurian ~ liensbschia..... Toarcian "Rotliegend" I I I I I t ~'~'~ b I I~ ~ I I Manchester ~ .k~l WSF I-~:l~!mlzl ~- -£- Wikesey I BrooksM .~ ~ o~J • ...... I ~ I - .... Collyhurst Sandstone Formation i -~ i'n= uOHF i"-nt-~ i --n", i i --n" m I Hahte Fm. ] Mudstone Fro. I=.o =o- ==~l i o0-

~F T F T ?F T x

1000 1000

\ s "B.~.._

...... --to---- ~-_~ sediment starved 2OO0 20(]0 \ ,~-.m ...... % \ Abbreviations: \ period of uplift (max erosion 850-900m)

NHF Northwich Halite Fm. 30OO -3(100 BMF Bollin Mudstone Fm. e,\ TSF Tarporley Siltstone Fro. \ HSF Helsby Sandstone Fm. \ BHF Bulkeley Hill Sandstone Fro. ~--,,

40OO WSF Wilmslow Sandstone Fm. "- -4000 CPBF Chester Pebble Beds Fm. "~'~ KSF Kinnerton Sandstone Fm. ~- ~ PG Penarth Group ~-,o- ---~ MMG Mercia Mudstone Group 5OO0 SSG Sherwood Sandstone Group sediment loaded -5OO0

F Episode of fault controlled subsidence

T Episode of dominantly thermal subsidence

6O00 6110O Fig. 11. Burial history (subsidence) curves for the Permo-Jurassic strata in the central Cheshire Basin. Examples for sediment loaded and sediment starved (water filled) basins are shown.

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period of slower (thermal) subsidence rates. By end- which reveal that the greatest thickness variations occur Permian times, the base of the Collyhurst Sandstone below the Mercia Mudstone Group. Formation was at an estimated depth of 1200 m (Fig. 11). The period 243-205Ma marks a time of renewed Major thickness changes occur within the sequences of subsidence but at decreased rates, when the finer grained the Permian and Sherwood Sandstone Group across the sedimentary rocks of the Mercia Mudstone Group were series of faults defining the eastern edge of the basin. A deposited (Fig. 11). Whilst there is evidence of the seismic line (Fig. 9) to the southwest of Stoke-on-Trent (for Northwich Halite Formation being affected by syn- location see Fig. 2) illustrates the structure of part of the sedimentary faulting near Winsford (Tucker & Tucker eastern margin of the Cheshire Basin. A series of faults 1981), this is minor compared to the earlier fault movement. throw the base Permo-Triassic down to the west, from Any subsequent major movement on the former syn- outcrop in the east, to a depth of 2.0-2.1 s TWTF (between sedimentary faults appears to post-date the preserved basin 4.4 and 4.6 km), some 16 km to the west. The oldest rocks fill (see Fig. 9). which form the thin Permo-Triassic succession on the The widespread development of the Helsby Sandstone footwall blocks comprise the 70 m-thick Rushton Spencer Formation and Mercia Mudstone Group over the sites of the Breccia Formation, formed largely of locally derived former synsedimentary faults, onto the basin margins, with Carboniferous detritus (Thompson 1985), and the Kinnerton little thickness variation, reveals a period of less active Sandstone Formation. Commonly both these formations are faulting and represents the more gradual thermal subsidence absent and the Chester Pebble Beds Formation directly phase of extension. The geometries of the restricted overlies Carboniferous rocks. This unconformity may relate pre-Helsby Sandstone and the more widespread post- to a period of uplift and erosion on the footwall blocks Wilmslow Sandstone sequences which transgress the basin towards the end of the Permian or earliest Triassic. Whether margins resemble the classic steer's head profile developed intra-basinal erosion created an unconformity at the base of during rift and thermal subsidence phases. the Chester Pebble Beds Formation, as suggested by Colter The subsidence curve indicates that strata of the Lower & Barr (1975), is unknown, but regional uplift resulting Lias were deposited during a period of increased subsidence from a compressive event is considered unlikely, given the rates, which slowed during deposition of the Middle Lias subsidence data. (Fig. 11). It is not clear if this period of enhanced The onset of the main phase of fault controlled subsidence was accompanied by normal faulting. subsidence (Fig. 11), is coincident with the deposition of the Chester Pebble Beds (Steel & Thompson 1983). This phase Permo-Triassic basin controlling structures of extension was widespread. Chadwick (1985b) recognizes an early Triassic (Scythian) phase of extension in the A seismic line across the eastern margin of the Cheshire Worcester Basin, as do Badley et al. (1988) and Steel & Basin, southeast of Stoke-on-Trent, reveals steeply-dipping Ryseth (1990) in the northern North Sea, the latter reflections between 0.1 and 0.65 s TWT/" (Fig. 9). Seismic recognizing successive bouts of extension followed, in later character and regional ties suggest that these reflections may Triassic times, by thermal subsidence. be derived from Westphalian Coal Measures. Their Fault controlled subsidence continued during deposition relationship to the Carboniferous succession imaged at the of the Wilmslow Sandstone Formation as evidenced by the extreme eastern end of the line, where the Keele Formation dramatic changes in thickness of this formation from basinal (Barren Measures) crops out, suggest that they are thrust to marginal areas (Fig. 9). Active faulting is evidenced north over that formation. Exact displacements of the Westpha- of the area near Alderley, where conglomerates within the lian strata are difficult to assess because of poor velocity Wilmslow Sandstone (the Alderley Member of Thompson control, the complex structure and unclear seismic character (1970) or Engine Vein Conglomerates of Warrington & matches. Thompson (1971)) represent a localized alluvial fan Beneath the main structure, the Carboniferous Coal developed along the Red Rock Fault scarp. However, the Measures and Barren Measures appear tectonically thick- Chester Pebble Beds Formation, Wilmslow Sandstone ened by further smaller reverse faults. These structures are Formation and ?Bulkeley Hill Sandstone Formation considered to be analogous to the many other compressive represent a fining-upwards sequence (Fig. 4). This is Variscan structures seen in the area, which range in size interpreted as reflecting progressively decreasing relief from the Potteries syncline to the numerous small thrusts around the basins of NW Europe (Steel & Thompson 1983). seen in coal workings in the North Staffordshire Coalfield There is evidence that prior to the deposition of the (Taylor et al. 1963; Evans et al. 1968). It is suggested that Helsby Sandstone Formation, the intrabasinal erosive event, during the Variscan, compressive and probably transpressive already described, removed strata deposited during the fault movements on the main westward-dipping fault (?putative controlled subsidence phase. Uplift associated with the Variscan thrust on Fig. 9) carried Westphalian Coal unconformity is indicated by rejuvenation of source areas, Measures eastwards over the Keele Formation. The surface supplying exotic pebbles present in the pebbly sandstones expression of this fault may be the Hodnet fault, which and conglomerates of the Delamere Member of the Helsby following subsequent Permo-Triassic extension phases, has a Sandstone Formation. A minimum of 200m of strata, normal displacement to the west (see below). The Hodnet interpreted as the equivalents of the ?Bulkeley Hill fault is a continuation of the Pontesford-Linley lineament Sandstone Formation, is estimated to have been removed in (Woodcock 1984) which is a prominent lineament on gravity some areas (see above). An uplift of this amount is and magnetic data (Lee et al. 1990) and has a history of tentatively shown on the subsidence curves in Fig. 11. This movement and reactivation since at least mid-Ordovician uplift and the resulting erosive event are interpreted as times (Wills 1978; Woodcock 1984; Soper et al. 1987). marking the cessation of stretching and dominantly fault In western and northwestern Britain, re-activated controlled subsidence. This is supported by the seismic data, Caledonoid-Charnoid basement fractures are thought to

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have controlled basin formation during Permian to Jurassic switching of stress fields would appear to be unreasonable times (e.g. Wills 1978; Thompson 1985). The orientation of given the short time period (refer to Fig. 11). the Hodnet fault suggests that during the Variscan it was Ziegler (1990) suggested the Hardegsen Unconformity subject to a transpressive stress regime under which the was related to doming associated with wrench movements Pontesford-Linley lineament was re-activated, producing on faults connecting sedimentary basins. Such a mechanism the reverse faulting of the Westphalian sequences and the is unlikely to have caused the unconformity recognized in formation of a positive flower structure by the end Variscan. the Cheshire Basin as Chadwick et al. (1990) suggest that The syndepositional normal faults in the hangingwall block onshore UK, Permian to early Jurassic basins developed of the Variscan reverse fault developed during Permo- under roughly east-west continental extension, i.e. the Triassic extension (Fig. 9). It is suggested that during this extension direction was at high angles to the basin margin extension, the underlying compressional structure on the faults and basement fractures. eastern margin of the basin collapsed in a manner similar to The intra-Triassic unconformity described here appears that discussed for the thrusts and transpressional structures to truncate sequences deposited during the active rifting within the basement beneath the Wessex and Worcester stage, both within the basin, in the hangingwall block basins, controlling the location and development of the adjacent to the fault, as well as over horsts and basin basin margin syndepositional normal faults (Chadwick et al. marginal fault blocks. Lateral heat loss or flexural warping 1983; Chadwick 1985c & 1986; Chadwick & Smith 1988). (e.g. Dewey 1982) might explain localized uplift and erosion of basin margins. In addition, local unconformities on the crests of footwaU blocks may be generated by footwall uplift Discussion (e.g. Jackson & McKenzie 1983; Badley et al. 1984, 1988). However, these models do not explain the kind of basin interior unconformity described here. Origin of the intra-Triassic unconformity Syn-extensional regional uplift is provided for by the The intra-Triassic unconformity is of late Scythian age. An lithospheric extension models of Chadwick (1986), Barr unconformity at the base of those formations formerly (1987) and Kusznir et al. (1991). However, the latter two known as 'Keuper Sandstone' and their equivalents appears models focus on footwall uplift and appear not to specifically to be widespread in southwest England and the western and explain or predict an unconformity such as the intra-Triassic central English Midlands (e.g. Geiger & Hopping 1968; unconformity described above, which occurs in the Warrington 1970; Warrington et al. 1980, Table 4) and has hangingwall as well as the footwall block. It seems possible been equated with the Hardegsen Unconformity in that the conflicting geometric and isostatic properties of Germany (Audley-Charles 1970; Warrington 1970; Wills developing extensional basins may cause flexural isostatic 1970). rebound of the basin itself, resulting in erosion from both Whether or not the intra-Triassic unconformity in footwall and hangingwall blocks. This may induce further southwest and central England and the Hardegsen flexural-isostatic rebound, which could account for the Unconformity in Germany are the same is still debatable as generation of the unconformity seen between rift and neither have been traced across the southern North Sea thermal subsidence sequences in the Cheshire Basin. Basin (Brennand 1975 & discussion p.311 therein). Nevertheless, should they be equivalent, the available evidence suggests that the importance of the Hardegsen This work was carried out as part of the BGS resurvey of the Unconformity diminishes westwards from Germany towards Stoke-on-Trent district and the ongoing BGS Cheshire Basin project. We are grateful to both Enterprise Oil plc and Shell UK for the centre of the southern North Sea Basin. In the East Irish their kind permission to publish the examples of seismic data from Sea Basin, the effects of the Hardegsen Unconformity are the Cheshire Basin. Thanks are due to R.A. Chadwick and minimal or absent (Jackson et al. 1987). colleagues in the Tectonics and Database Group for helpful It is difficult to determine where the greatest relative discussions and additionally to A. Whittaker, A.A. Wilson, C.N. uplift occurred in the UK at this time, because in many Waters, G. Warrington and D.B. Thompson for their constructive areas no Scythian strata are preserved. However, generally, comments on earlier versions of the manuscript. The paper the effects of erosion at the intra-Triassic unconformity in benefited from the review of two anonymous referees. We thank the southern UK seem to be clearest in the Cheshire, C.J. Evans for assistance in the conversion of the Wilkesley Worcester, Stafford and Wessex basins, in areas surrounding gamma-ray log. Decompaction studies were possible using software these basins and around the western margin of the onshore developed with ODA funding. Figures were drawn by C.G. continuation of the North Sea Basin (east of Nottingham). Murray. The work is published with the permission of the Director, The relationship of the unconformity to the overlying British Geological Survey (NERC). and underlying formations is similar to that described in NW Germany (Trusheim 1963, figs 2-5, 7 & 8) and in the Worcestershire and Warwickshire area (Wills 1976). The References unconformity is overlain by sedimentary sequences inter- ABDOH, A., COWAN, D. & PILKINGTON, M. 1990. 3D gravity inversionof the preted as having been deposited during the thermal Cheshire Basin. Geophysical Prospecting, 38, 999-1011. subsidence phase of extension. The thickness of strata AUDLEY-CHARLES, M.G. 1970. Triassic palaeogeographyof the British Isles. removed is unknown. Quarterly Journal of the Geological Society, London, 126, 49-89. Whilst relative uplift is obvious, the cause of this process BADLEY, M.E., EGEBERG, T. & NIPEN, O. 1984. Development of rift basins and the consequent unconformity is less clear. If it resulted illustrated by the structural evolution of the Oseberg Feature, Block 30/6, Offshore Norway. Journal of the Geological Society, London, 141, from a short-lived compressive event which occurred 621-627. between the main extensional phase of subsidence and the , PRICE,J.D., RAMBECHDAtlL, C. ~/~ AGDESTEIN,T. 1988. The structural main phase of thermal relaxation subsidence, then the rapid evolution of the northern Viking Graben and its bearing upon

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Received 1 August 1992; revised typescript accepted 23 January 1993.

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