Zechstein Research Articles

Organic petrographic investigations of the Kupferschiefer in northern Germany

Results of an organic petrographic study of the Kupferschiefer (Zechstein, Upper Permian) in northern Germany are presented. Because vitrinite occurs only sporadically in this black shale, the random reflectance ( R r) of a relatively high reflecting, vitrinite-like variety of bituminite was measured as a maturity parameter and the problems connected with this procedure were investigated. The main organic component is bituminite, generally with an abundance of more than 90%. Sporomorphs and algae are relatively scarce, inertinite is usually a trace component, and vitrinite is present only in some samples taken close to the coast of the Zechstein sea. Migrabitumen also occurs in small amounts in a few samples. The range of the bituminite reflectance values is often considerable and depends on rank, anisotropy, particle size, porosity, and shearing. Interpretation of the reflectance values is made difficult by differences of up to 0.4% R r between layers within the Kupferschiefer. Hydrothermal alteration may also be present. The difference between the reflectance of the light and normal varieties of bituminite disappears between 0.9 and 1.2% R r. Bituminite behaves in a way very similarly to vitrinite during coalification. Their reflectance is identical between 2 and 4% R r. Below 2% R r, the reflectance of bituminite is lower than that of vitrinite. Linear regression curves valid up to 4% R r were calculated for R max versus R r and for R r versus R max. Taking certain limitations into consideration, bituminite reflectance may be used as a coalification parameter.

Changes in palaeoenvironmental conditions during deposition of the Permian Kupferschiefer (Lower Rhine Basin, northwest Germany) inferred from molecular and isotopic compositions of biomarker components

Biomarker distributions based on GC and GC-MS data supplemented by stable carbon isotopic compositions based on irm (isotope ratio monitoring) GC-MS data have been used to investigate changes in depositional conditions in a number of samples from a 2 m core (Rheinberger Heide) covering the entire deposition of the Permian Kupferschiefer (Lower Rhine Basin, northwest Germany). Compound classes investigated are aliphatic and aromatic hydrocarbons and maleimides (1 H-pyrrole-2,5-diones). The core has been previously divided into four sections: TI, TIIA, TIIB and TIII based on lithology. The biomarker distributions and the δ 13C values of components indicate contributions from cyanobacteria, algae, green sulphur bacteria and higher plants (minor) throughout Kupferschiefer deposition. All of the phytoplanktonic components show 13C enrichment in the lowest section, suggesting a greater productivity. The occurrence of components derived from green sulphur bacteria (Chlorobiaceae) indicates that the entire deposition was characterised by periods of photic zone anoxia. Such conditions were rapidly established after the initial transgression of the Zechstein Sea and may have been productivity-driven, but were less prevalent during deposition of the upper core sections.

Regional distribution and geometry of salt diapirs and supra-Zechstein Group faults in the western and central North Sea

Regional mapping of major faults above the Zechstein Group salt in the western and central parts of the North Sea basin has shown that there exist at least three discrete salt tectonic zones. On the platform west of the Central Graben there is Zone 1, an area of extensional faulting above the salt, roughly parallel to the margin of the Central Graben, c. 60 km wide and continuous along strike for more than 180 km. Within Zone 1, extension (c. 6%) has created arcuate and en echelon listric growth faults, which sole within the salt layer beneath and bound blocks typically 3–7 km wide, 2–3 km thick and 7–10 km long. Zone 2 occurs in the Central Graben area, where there is considerably less faulting above the Zechstein Group salt, and a much greater thickness of salt. Tall and narrow salt diapirs occur exlusively in the three deepest parts of the Central Graben and some penetrate high into the Cenozoic section. Between Zone 1 and Zone 2 there is a third area, Zone 3, characterised by having relatively few supra-Zechstein faults, and no major salt diapirism. Zone 1 formed mainly due to gravity-driven break-up of the supra-Zechstein sedimentary wedge. Diapirism in Zone 2 resulted from increased relative bouyancy of salt at greater depths and a relatively greater supply of salt. Zone 3 represents a relatively stable region, with possibly some movement of salt and supra-salt sediments down the regional slope.

Novel porphyrins as molecular fossils for anoxygenic photosynthesis

The iron porphyrin fraction of Kupferschiefer, an organic-rich sedimentary rock deposited in the Zechstein Sea during the Permian (ca. 235Myr before present) contains three minor components characterised by 1H NMR spectroscopy. The carbon skeletons correlate specifically with members of one series of the bacteriochlorophylls d of the Chlorobiaceae, which are obligate anaerobes. Their structures therefore reveal the occurrence of anoxic conditions extending up into the zone of light penetration in the water column of the original depositional setting in the Lower Rhine Basin.

Differences in the depositional environment of basal Zechstein in southwest Poland: implication for base metal mineralization

Base metal enrichment in the Permian basal Zechstein sediments occurs in two major areas in southwest Poland, the North-Sudetic Syncline and the Fore-Sudetic Monocline. Two profiles of Kupferschiefer and adjacent limestones from the Konrad mine and from the Rudna mine were studied by petrographic and geochemical methods in order to recognize similarities, and differences, in the comparative depositional environment of both areas. In the North-Sudetic Syncline basal Zechstein was deposited in a shallow marine environment under slightly oxidizing sedimentary conditions. It is composed of 1.2m thick mergelkalk, which might be the equivalent of Werrakalk (Ca1). In the Fore-Sudetic Monocline the basal Zechstein is a black shale, the so-called Kupferschiefer (T1) with a thickness of 0.2–0.5 m. Microscopical investigations have shown that a substantial contribution of higher continental plant material is observed only in the North-Sudetic Syncline because of its location near the Zechstein sea-shore. Pyrite is present in small amounts only. In the Fore-Sudetic Monocline, apart from the former shoreline of the Zechstein sea, the organic matter is preferentially composed of degraded algae due to its deposition under deeper anoxic bottom waters. Abundant pyrite is also present. Differences in the chemical composition of the respective kerogens are reflected in the results obtained from Rock-Eval analyses. Hydrogen indices (HI) are significantly higher in the T1-section of the profile from the Rudna mine, in comparison to the basal Zechstein of the Konrad mine profile. A depletion of hydrogen is only observed in the highly mineralized section and in Rote Fäule of the Konrad mine. This indicates that hydrogen equivalents from organic matter participated in metal sulfide precipitation by thermochemical sulfate reduction (TSR). This is further confirmed by the occurrence of pyrobitumen, saddle dolomite and calcite spars. In contrast, in the mineralized section of the Rudna mine profile a depletion of hydrogen is not observed, pyrobitumen is missing and saddle dolomite and calcite spars are very rare. Instead, abundant pyrite originating from bacterial sulfate reduction (BSR) could have been the source of reduced sulfur species, available for base metal precipitation after deposition of the sediment (pyrite replacement). The results allow one to conclude that in both areas base metals were precipitated from ascending solutions due to the interaction with the reducing Zechstein barrier. Several observations argue for a post-depositional mineralization process at a temperature of less than 130°C in both areas.

Permian and Triassic rifting in northwest Europe

The Permo-Triassic roughly coincided with the life span of Pangaea. So far as Europe is concerned, Pangaea's creation began in the early Carboniferous (Visean) when the northward-drifting megacontinent Gondwana began to collide with the Iberian portion of the slower moving Laurussia, while Proto-Tethys was subducted beneath the southern margin of central Europe. Creation was complete by the end of the Carboniferous or early in the Permian with the final development of the Variscan orogenic belt, which trends from Brittany eastward through central Europe, and with the addition of western Siberia along the line of the Ural orogen (Ziegler 1989). Despite, or perhaps because of, its bulk, Pangaea was not a stable megacontinent. No sooner had it formed than it tried to break apart again. The disintegration of Pangaea had already started before the end of the Triassic with the westerly extension of Tethys between Iberia and Africa, though not yet underlain by oceanic crust, and, by early in the Jurassic, rifting was taking place between Africa and the Americas in the newly forming Central Atlantic Ocean (Ziegler 1988). Indeed, E-W extensional movements within a Proto-Atlantic Ocean possibly began as early as the Late Carboniferous (Haszeldine & Russell 1987), whilst mid-Permian extension is well documented in East Greenland (Surlyk et al. 1984). These extensional movements may have propagated southward to initiate fracturing in the Viking and Central Grabens of the North Sea, along which the Late Permian Zechstein Sea was to break into the subsiding Rotliegend basins, and towards the Central Atlantic, where a shallow seaway eventually developed early in the Jurassic. Thus in the northern half of Pangaea, the continued existence of the former Laurussia was already at risk in the Permian, although crustal separation in the North Atlantic, which possibly started in the Rockall Trough in the Early Cretaceous, was finally achieved along the line of the Reykjanes Ridge only in the Paleocene. These major events on the periphery of what is now Europe, separately and jointly, were factors that probably controlled a whole sequence of tectonic events within the continent, which, in turn, controlled its patterns of mostly terrestrial sedimentation. Climatically, the northward drift of Laurussia had carried NW Europe from a region of equatorial rain forest during the later Carboniferous to the latitudes of a trade wind desert, like the modern Sahara, in the Permian. The Late Permian basins of Upper Rotliegend and Zechstein deposition were arid. Rotliegend sediments are characterized by dune sand and the saline mudstones of a semipermanent desert lake, and the Zechstein by shallow-water carbonates and anhydrite and deeper-water halite. During the Triassic, however, brackish-water fluvial and lacustrine sediments occupied the basinal areas, although even here, halite horizons (e.g. Rrt, Muschelkalk and Keuper in the North Sea area) associated with local transgressions from Tethys, testify that arid conditions were never far away.

Gravity-driven thin-skinned extension above Zechstein Group evaporites in the western central North Sea: an application of computer-aided section restoration techniques

Normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of Mesozoic and Tertiary strata in the North Sea. In addition to the previously documented mechanisms of salt withdrawal, dissolution and differential sedimentary loading, it is recognized that gravity-driven thin-skinned extension above inclined salt layers has played an important part in North Sea basin development. Commercial section restoration software has been used to facilitate depth conversion, restoration and decompaction of seismic sections selected from an interpreted regional database in the western central North Sea, allowing validation of the interpretations, and a graphical and highly quantitative description of salt-assisted extension. Results of this work show that Zechstein Group evaporites were deposited in shallow sag basins during the Permian. Triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites. Bedded salt became folded, while mobile salt flowed to fill anticline cores. Since the early Jurassic, regional tectonic tilting related to post-rift subsidence and increasing sedimentary overburden have caused allochthonous Mesozoic and Tertiary strata to extend by gravity spreading above the mobile salt layer, which detaches the allochthon from the underlying autochthonous Late Palaeozoic rocks. Concave-up listric normal faults sole out in the salt layer, propagate into the overlying cover sewuence, and have been active at different geological times causing stratal thickening and folding within the allochthon. Antithetic and synthetic normal faults have developed, producing complex upward branching fault systems. In map view, the listric faults form curvilinear en echelon arrays, the faults linked by relay ramps. Fault blocks are typically 3–7 km wide, 2–3 km thick and 7–10 km long. Salt movement during the Jurassic-Tertiary has been driven by active extension of the cover, causing salt to fill potential voids created by fault block rotation. Thus salt highs occur beneath sites of extension. The listric faults generally dip in the same direction as the sub-salt surface, although there are also some major counter-regional faults. During extension, regional dips have increased up to about 5, which is sufficient for gravity-driven extension above a salt layer. A total extension of about 6% has occurred. The gravity-driven thin-skinned extension documented in the western central North Sea is a phenomenon which can be recognized elsewhere in the North Sea basin, and can be readily compared with similar phenomena already documented in offshore Angola, Brazil, Nova Scotia and the Gulf of Mexico.

Emplacement and diagenesis of gypsum and anhydrite in the late Permian Raisby Formation, north-east England

SUMMARY Cavities, pores and crystal pseudomorphs which are lined or filled by calcite are ubiquitous in Raisby Formation dolostones at outcrop in north-east England. Their distribution in most exposures is random on a metre scale. On a smaller scale, cavities, pores and crystal pseudomorphs are most numerous in bioclastic and resedimented beds. Deep borehole cores of the Raisby Formation contain nodules and euhedral crystals of gypsum and anhydrite of very similar size and shape to the cavities and pseudomorphs. This indicates that calcium sulphates were once abundant in the Raisby Formation at outcrop, but almost all have been removed subsequently. Calcium sulphate minerals were formed early in the diagenetic history of the Raisby Formation, predominantly by selective replacement of carbonate rocks. Gypsum was precipitated first, probably as a by-product of dolomitization, by brines refluxing basinwards through the Raisby Formation from the hypersaline Zechstein Sea during deposition of the Edlington Formation and the Hartlepool Anhydrite Formation. Later, replacive anhydrite formed following both pressure-solution and dehydration of the gypsum. During further burial, some anhydrite was replaced by barite, dolomite and sphalerite. Upon uplift of the Raisby Formation during the Tertiary, anhydrite rehydrated to porphyroblastic gypsum and this was then dissolved by meteoric groundwaters. The resultant pores were lined or filled by two generations of calcite and also, locally, by kaolinite. The earlier calcite generation is luminescent and was precipitated partly as a passive cement and partly by replacement of surviving gypsum and anhydrite, a reaction thought to have been mediated by sulphate-reducing bacteria. The later calcite generation is predominantly non-luminescent and was precipitated from near-surface meteoric groundwaters.

The Humphrey Head Borehole: Evidence for Carboniferous vulcanicity and Permian dolomitization in the southern Lake District

AbstractIn south Cumbria, Permo‐Triassic breccias and conglomerates (‘brockram’) are exposed only at Rougholme Point on the Cartmel peninsula. In 1973 the Institute of Geological Sciences Humphrey Head borehole penetrated 257 m of brockram before entering probable Upper Carboniferous sediments. The brockram consists of pebbles of carbonate, chert and basalt in a matrix of haematite‐stained quartz sand. Carbonate and chert fragments were derived from the upper part of the Carboniferous Limestone sequence exposed today nearby. Basalt clasts were derived from lavas, which appear to have cooled in a subaqueous environment, at least in part. They were locally derived and are the only certain evidence for Carboniferous volcanic activity in south Cumbria. As volcanic fragments increase in abundance towards the base of the borehole they must have been derived from the top of the succession being eroded and are probably of Brigantian age.Carbonate fragments were dolomitized soon after incorporation in the brockram, probably by saline fluids derived from the evaporative Zechstein Sea. The dolomitization was incomplete, leaving remnant limestone cores to clasts which were subsequently dissolved. The resultant vugs were infilled by dolomite, calcite and gypsum cements, which have been partially weathered from outcropping brockram, leaving hollow pebbles.

Sedimentology and Diagenesis of the Zechstein Ca2 Carbonate, Late Permian, Northwest Germany

Carbonates of the second Zechstein cycle, the Stassfurt Carbonate (Ca2), constitute north Germany's most prolific carbonate gas play. The underlying Werra Anhydrite (A1) of the first Zechstein cycle and the overlying Basal Anhydrite (A2) of the second Zechstein cycle enclosed the reservoir. An erosional sequence boundary present at the top of the A1 sulfate platform separates the A1 from the overlying Ca2 carbonate platform. Ca2 thickness ranges from 30 to 80 m on the platform, 40 to 250 m on the slope, and 10 to 40 m in the basin. Following flooding of the underlying A1 sulfate platform (Ca2 maximum flooding), Ca2 shallow-water carbonates prograded basinward. Platform, upper slope, middle slope, and lower slope facies are recognized and are subdivided into 27 subfacies. Systematic trends in facies distribution are apparent on depositional dip-oriented well log cross sections. Trends in vertical succession of subfacies types, along with the good correlation between Ca2 facies and A1 thickness, enable mapping of the updip and downdip limits of the facies. Diagenesis is important in determining Ca2 reservoir quality. Reservoir rock is typically fair to good where it is dolomite. Early calcitization (dedolomitization) of the Ca2 yields poor to non-reservoir rocks. Although calcitization in generalmore » increases basinward, lateral prediction of mineralogy within individual Ca2 facies is difficult. A multivariate statistical investigation has been initiated to determine parameters, which are important in predicting trends in calcite distribution.« less

Merrillina divergens (Bender & Stoppel 1965) -- der erste Conodontenfund aus dem Zechstein am Südharzrand

Merrillina divergens (Bender & Stoppel 1965) -- der erste Conodontenfund aus dem Zechstein am Südharzrand

The Clipper Field, Blocks 48/19a, 48/19c, UK North Sea

Abstract The Clipper Gas Field is a moderate-sized faulted anticlinal trap located in Blocks 48/19a and 48/19c within the Sole Pit area of the southern North Sea gas basin. The reservoir is formed by the Lower Permian Leman Sandstone Formation, lying between truncated Westphalian Coal Measures and the Upper Permian evaporitic Zechstein Group which form source and seal respectively. Reservoir permeability is very low, mainly as a result of compaction and diagenesis which accompanied deep burial of the Sole Pit Trough, a sub-basin within the main gas basin. The Leman Sandstone Fm. is on average about 715 ft thick, laterally heterogeneous and zoned vertically with the best reservoir properties about the middle of the formation. Porosity is fair with a field average of 11.1%. Matrix permeability, however, is less than 1 millidarcy on average and is so low that some intervals in the field will not flow gas unless stimulated. Steep dipping zones of natural fractures occur in certain areas of the field; these commonly allow high flow rates to be achieved from large blocks of low-permeability matrix. Expected recoverable reserves from the most favourable part of the field are 558 BCF and Clipper Field is now being developed in conjunction with part of the adjacent Barque Gas Field. Later development of the remainder of Clipper Field will depend upon reservoir performance in the initial development area.

The Ravenspurn North Field, Blocks 42/30, 43/26a, UK North Sea

Abstract The Ravenspurn North Field is a gas accumulation located in the Southern North Sea, Permian Gas Basin which was discovered in October 1984. It has undergone four years of appraisal well drilling culminating in the approval of the development plan in 1988. Development wells are currently being drilled and three offshore installations are planned; first gas production began in July 1990. The Ravenspurn North Field is a combined structural and stratigraphic trap. The reservoir is fault closed along a series of anastomosing oblique strike-slip and normal faults. Seals along the faults are provided by the Silverpit Formation mudstones and Zechstein Group evaporites. The reservoir deteriorates to the northwest because of thinning, facies change and increasing authigenic clay content. The Lower Leman Sandstone Formation of the Rotliegendes Group forms the reservoir. It consists of a sequence of aeolian dune, fluvial sheetflood, fluvial channels and lake margin sabkha deposits. Non-reservoir intervals are formed by playa lake mudstone sequences. Fluvial and sabkha facies dominate in the northwest while aeolian facies dominate in the southeast parts of the Field. Reservoir quality was initially controlled by lithofacies distribution. Subsequent diagenesis further modified the reservoir rock resulting in variations in the porosity and permeability. Deliverability is a function of variable permeability with two areas identified; the high deliverability area where gas wells have tested sufficient quantities for commercial production without artificial stimulation and a low deliverability area where gaswells require hydraulic fracture stimulation before significant commercial production rates are achieved.

The late Permian palaeogeography of north-east England

SUMMARY All available information has been used in the preparation of a series of palaeogeographical maps showing the evolution of north-east England at the end of the early Permian and at intervals throughout the late Permian. Most of the area initially lay near the western fringe of the Southern North Sea tropical desert basin, much of which was flooded at the beginning of the late Permian so as to create the epicontinental Zechstein Sea. With continually oscillating relative sea levels and episodic differential subsidence, contemporary coastlines fluctuated widely whilst successive progradational carbonate shelves were constructed in the first two of four main cyclic sequences; the deposition of thick salt at the end of the second main cycle almost filled the basin, eliminating most of the earlier depositional topography and ensuring that carbonates and evaporites of the third and fourth cycles were formed in a less diverse range of coastal plain to shallow marine environments.

The Kupferschiefer; an overview with an appraisal of the different types of mineralization

The Kupferschiefer; an overview with an appraisal of the different types of mineralization

Use of biological marker distributions to study thermal history of the Permian Kupferschiefer of the Lower Rhine Basin

In the Central European Zechstein Basin, the Permian Kupferschiefer has been deposited under marine anoxic conditions. From a lagoon at the southwest border of the Zechstein Sea, today part of the Lower Rhine Basin, 30 core samples have been studied by means of organic geochemical methods. Within the area investigated, the depth of the Kupferschiefer horizon increases from about 350 m in the south to about 1000 m in the north. Furthermore, the Kupferschiefer has been thermally affected in the western part by the intrusive body of the so-called Krefeld High. This geological situation opens the possibility to investigate differences of short term and long term temperature effects on the biological marker distribution within a sediment layer of a largely uniform facies type. Depth related changes in the hopane, sterane and diasterene/diasterane distributions are compared to those changes induced by the Krefeld High. The composition of aromatic steroid hydrocarbons is used to discuss time-temperature effects. It is shown that depth related values of maturation parameters can only be interpreted by taking into account post-depositional tectonic events. Samples from tectonic horst structures show higher maturation values than one would expect from the present day depth.

Element- und isotopengeochemische Untersuchungen an Kupferschieferprofilen der Sangerhäuser Mulde (DDR)

We analyzed six profiles of the copper shale from the Basin of Sangerhausen (GDR) (determination of 52 chemical elements) to understand the vertical and lateral distribution of the elements (52 determinations) and the role of “Bote Faule” in the process of mineralization was studied. Using instrumental neutron activation analysis the concentration of 27 chemical elements were determined. On basis of the calculated correlation coefficients for element pairs their binding types could be estimated and some peculiarities of the distribution of elements were established. Results of isotope geochemical investigations of the carbonate fraction support the supply of oxidic waters across the “Bote Faule” from the ingression of the Zechstein sea to late diagenesis of copper shale.

The Marl Slate: a model for the precipitation of calcite, dolomite and sulphides in a newly formed anoxic sea

ABSTRACTDetailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ13C, δ18O, δ34S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite‐rich and dolomite‐rich carbonates.C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ18O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for 18O and 1·5% for 13C in calcite. The δ34S values of pyrite are isotopically light (mean value = ‐ 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins.The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in the Zechstein Sea.

Chemical and isotopic studies of a core of Marl Slate from NE England: influence of freshwater influx into the Zechstein Sea

Summary Detailed mineralogical, chemical and stable isotope analyses have been made of single core through the Marl Slate in order to investigate the Zechstein transgression and its associated mineralization. The core (VT8), drilled by the National Coal Board off the coast of NE England, contains three subfacies: (1) dark grey, organic-rich, sapropelic siltstones; (2) organic-dolomite laminites; (3) massive finely crystalline dolostones. Two cycles of deposition which include the sequence sapropel→laminite→dolostone are present although the lower one (112 cm) is much thicker than the upper (32 cm). There are some close correlations between the lithological and geochemical variables, including the stable isotope values. The sapropelic units are characterized by high Fe content (as iron sulphide) and strongly negative δ 18 O values in associated carbonate minerals. Quartz silt and organic carbon are also higher in these units but diminish upwards in each depositional cycle. Each cycle of deposition appears to have been initiated by an influx of freshwater into the Zechstein basin which promoted surface productivity, water stratification and regional anoxic conditions. The relative change in δ 13 C to more positive values higher in the Marl Slate is independent of lithological variation and indicates progressive enrichment of 13 C in the Zechstein Sea in response to the precipitation of large amounts of organic matter. The distribution of base metals in the Marl Slate in this core strongly suggests that they were introduced by freshwater influx. Although Pb shows no clear relationship to the depositional cycles, Cu and Zn contents are relatively low in the lower sapropelic parts of the cycles and become higher in the upper parts. We take this to indicate that Cu and Zn sulphides were precipitated interstitially when more ‘normal’ marine conditions were re-established in the later part of each cycle.

Zechstein salt correlation: England-Denmark-Germany

Summary A correlation of the Zechstein strata of England and Germany is presented. It is based on strong similarities in the distribution, thickness and lithology (including lithological succession) of the various evaporites in the two sequences and, particularly, on an upwards change in colour from mainly grey to mainly red between the second and third cycles; this change probably indicates a basin-wide change in the oxidation state of the Zechstein Sea. By these criteria, it is suggested that strata of the Don and Aislaby groups of England together are equivalent to the deposits of the Werra Cycle in Germany and that those of the Teesside Group are correlatives of the German Stassfurt sequence.