Marine Permian of England - JNCC

1 Chapter 1 The Permian Marine rocksof EnglandFrom:Smith, , (1995), Marine Permian of England , Geological Conservation Review Series, No. 8, Chapman and Hall, London, 205 pages, illustrations, A4 hardback, ISBN 0 41261 080 9 For more information see: setting3 GEOLOGICAL SETTINGThis volume is concerned with rocks laid down in and near epicontinental tropical seas that covered low-lying tracts of northernEurope perhaps for 5 7 million years during the late part of the Permian period; Britain then lay deep within the Laurasian super-continent, with a climate that was both hot the early Permian and early latePermian, northern Europe was part of one of theworld s great deserts, with a topographic and cli-matic range matching much of that of the pres-ent Sahara; widespread barren uplands createdduring the Armorican earth movements weregradually ground down as early Permian deserterosion led to extensive peneplanation of vastareas of Carboniferous rocks (especiallyWestphalian Coal Measures), and post-orogenicand extensional subsidence resulted in the for-mation of extensive, sub-sea-level, inlanddrainage basins in areas now occupied by theIrish and North seas and some adjoining landareas (Smith, 1970a,b, 1979; Glennie and Buller,1983; Glennie, 1984; Smith and Taylor, 1992).

2 It was these basins, surrounded by persistentand inhospitable deserts, that were flooded toform the Bakevellia and Zechstein seas ( ).The inferred mode of creation of the two seasgave rise immediately to classic barred basins,with sills that remained sensitively close toworld sea levels. Both seas were particularlyprone to relative sea-level changes and the thicksedimentary sequences formed in them displayan extraordinary variation in their facies andextent. The sea-level changes stemmed from thecomplex interplay of several main mechanisms,including glacio-eustatic oscillations, isostaticeffects, deepening and shallowing caused byperiodic reversals in the relative rates of sedi-mention and subsidence, and, when the sillsbecame emergent during oceanic low-stands,rapid evaporative downdraw. Cyclicity, caused at least partly by these relative sea-level changes,is on a range of scales and is expressed both by incomplete marginal sequences resultingfrom repeated transgressions and regressionsand in a wide variety of carbonates and evapor-ites.

3 Some variation of lithology and biota alsostemmed from repeated and protracted phasesof basinal anoxicity, a natural outcome ofrestricted circulation in a barred basin; benthiccommunities at these times were confined tomarginal and shoal areas above an Marine and associated rocks formed inthe late Permian seas of northern Britain havebeen studied intermittently for more than 150years, but the recent search for hydrocarbons inthe Irish and North seas accelerated the rate oflearning and has revealed much new informa-tion on the stratigraphy, distribution, deposi-tional environments and diagenesis of the vari-ous lithostratigraphical units. The Marine andassociated Permian strata of the Bakevellia Sea(part of the East Irish Sea) Basin, for example,have been shown to include major deposits ofrock-salt, and their thicker counterparts in theEnglish Zechstein Basin have been shown tocomprise a complex series of progradational car-bonate aand sulphate prisms around the mar-gins, and basin-filling halite in the interior ( and Colter, 1975; Colter and Reed, 1980;Taylor, 1984).

4 The age of Zechstein and Bakevellia Sea stratain world chronostratigraphic terms is uncertainbecause these seas lay in the Boreal Realm andtheir faunas evolved almost independently ofthose in the Tethyan Realm upon which latePermian stages are now mainly based; a latePermian age is generally agreed, but it is becom-ing clear from mainly palynological evidencethat the initial Marine transgressions were prob-ably appreciably later than the early Permian late Permian boundary accepted, and perhaps aslate as mid-Tatarian (see Smith et al., 1974 fordiscussion). Marine fossils found at the variousGCR sites featured in this review are stronglyfacies-linked and throw little light on the preciseage of the few late Permian shorelines in theBritish Isles can be recognized with certainty,projection of sedimentological and thicknesstrends suggests that the Bakevellia andZechstein seas generally extended no more thana few kilometres beyond the present outcrops oflate Permian strata and remained separated by aProto-Pennine barrier (Figure ) that wanedwith time but was surmounted by the sea onlybriefly and locally during the third of four maincycles of the English Zechstein sequence.

5 TheZechstein Sea itself was divided into several sub-basins (Figure ), and early Zechstein Basinmarginal depositional environments in north-The Permian Marine rocks of England4 Figure Zechstein Sea and its environs. After Smith (1980a, fig. 1)Geological setting5east England fell into distinct Durham andYorkshire provinces, separated by the ClevelandHigh (Figure ). These two provinces form thebasis for Chapters 3 and low relief and aridity of the Proto-Pennines and most other hinterlands, togetherwith the prevalence of onshore trade winds,ensured that the Zechstein Sea in Englandreceived relatively little terrigenous sedimentand most of the late Permian Marine rocks therecontain less than 2% of siliciclastic grains(mainly wind-blown); in contrast, the hinterlandof much of the Bakevellia Sea was generallymore elevated and rugged than that of most ofthe English Zechstein Sea, and terrigeneousinput was correspondingly greater.

6 In bothbasins, occasional storms resulted in flash floods which swept coarse continental debris far across the coastal plains and led to the for-mation of marginal breccia lenses and distribution of Marine Permian strata in mainland Britain is shown in Figure , which emphasizes a great disparity between the widely scattered but mainly small occurrences in north-west England and the wide extent and almost continuousoutcrop in north-east England . This disparity is reflected in the spread of GCR sites, only one of which is in the north-west and theremainder in the north-east. Altogether, 27 sites have been identified and are described in this volume; almost all meet the exacting requirements laid down by EnglishNature, the main exception being the site at the former New Edlington Brick-clay Pits in Yorkshire, which have recently been filled but are included here pending denotificationand the choice of a suitable alternative site demonstrating comparable geological of the late Permian seas of northern England and adjoining areas, showing the persistentPennine Ridge.

7 After Smith (1992, fig. ).The Permian Marine rocks of England6 Rationale of Marine Permian siteselectionThe choice of sites described and discussed in this review was governed by a wide range of criteria, including their representativeness,their content of exceptional or unique geolo-gical features, their historical and their nationaland international importance. Together thesesites form a dynamic network that covers most ofthe geology of the Late Permian marinesequence in northern England , but some aspectsare under-represented pending the choice ofappropriately representative additional sites andthe network will undoubtedly evolve as sitesdeteriorate or are superseded. The wealth ofsuitable high-quality exposures of these rocks inthe Durham Province of north-east England , isdramatically indicated by the fact that almost allthe chosen sites meet at least two of the mainselection criteria, and some sites satisfy all thecriteria.

8 Several sites, moreover, gain additionalimportance by forming links in chains of sitesthat together form a coherent whole but which,because of the large scale, no single site couldgive full coverage. In addition to their geologicalinterest, and arising from the specialized florathat characterizes many thin soils developed onthe Magnesium Limestone, several of the GCRsites coincide with (or overlap) sites of specialbiological sites considered here include naturalexposures such as river and sea cliffs, but mostare quarries (or part of quarries) and sites include a number of separateexposures and these, and some others, exhibitFigure of Marine Permian strata in mainland setting7more than one geological feature worthy ofpreservation. Outstanding examples of this are the complex of interlinked faces in reef and associated rocks at Ford (Sunderland), that provided the key to the solution of other-wise almost intractable sedimentological andstratigraphical problems, and the spectacularand equally varied coastal cliffs between Trow Point (South Sheilds) and Sunderland;coastal cliffs at Seaham and Blackhall are other examples.

9 These coastal sites, indeed, are without parallel in Britain and most ofWestern Europe, and are unrivalled for research and teaching purposes. They aremainly in limestones and dolomites in the upper part of the Permian Marine sequence and,in addition to exposing a continuum of rock-types extending basinwards from a marginalbarrier system to those deposited near themiddle of the basin-margin slope, also furnishmagnificent examples of the disruptive effects of evaporite dissolution. Inland, most of the Marine Permian GCR sites are in rocks in the lower and middle parts of the sequence,and clearly exhibit the great lateral variation that characterizes carbonate sediments beingformed in tropical nearshore environments;these inland sites include the type sections ofthree of the main late Permian Permian Marine and associatedstrata in north-west England andadjoining areasLate Permian Marine and associated strata arewidespread beneath the eastern Irish Sea, butextend far inland only in the Solway Firth andsouth Lancashire/north Cheshire areas wherearms of the Bakevellia Sea extended sequence is up to about 350 m thick in thebasin centre, where hydrocarbon explorationhas disclosed much new information (Ebburn,1981; Jackson et al.)

10 , 1987), but thins sharplytowards the basin margins where sequences arecondensed and incomplete. Exposures on landare concentrated in these basin-marginal areas,especially in west and south Cumbria andaround Greater Manchester, and most includeintercalations of continental strata that indicateperiodic proximity to cyclically migrating on Permian strata in Cumbria has been summarized by Arthurton et al. (1978) and for the basin as a whole by Smith(1992). Additionally, many new stratigraphicaland structural data have been given by Jackson and Mulholland (1993). The nomen-clature and classification of these strata in theseveral outcrop areas have evolved independ-ently and are presented in Table Three maincycles and one possible other cycle wererecognized in West Cumbria by Arthurton andHemingway (1972) and four supposedlyequivalent cycles (BS1 to 4) were defined byJackson et al.