Zechstein Group ZE


Premise The name Zechstein Group was introduced by NAM and RGD (1980) . The group as defined in this paper incorporates the lower part of the former Bunter Group. The amendments to the upper boundary are explained below.
Derivatio nominis Name derived from the German stratigraphic nomenclature, where it is applied to the marine evaporite/carbonate sequence between the Rotliegend and the Triassic deposits.
Type section Location map See figure (pdf)
  Well Rossum-Weerselo-3 (pdf)
  Location N 52°21’03.5
E 06°54’38.2
  Depth 1212 to 1804 m
  Length 592 m along hole
  Reference amended after NAM and RGD (1980) (sub-basin setting)
Additional section Location map See figure (pdf)
  Well P09-01A (pdf)
  Location N 52°32’19.4
E 03°44’52.2
  Depth 4378 to 4620 m
  Length 242 m along hole
  Reference carbonate/sandstone development
  Well Q16-02 (pdf)
  Location N 52°07’05.3
E 04°07’15.3
  Depth 3817 to 3872.5 m
  Length 55.5 m along hole
  Reference sandstone development
  Well Buurmalsen-1 (pdf)
  Location N 51°54’18.7
E 05°18’49.2
  Depth 1973 to 2069 m
  Length 96 m along hole
  Reference amended after NAM and RGD (1980) .; (claystone/evaporite development).
  Well Nederweert-1 (pdf)
  Location N 51°18’41.7
E 05°46’14.2
  Depth 2572 to 2634 m
  Length 62 m along hole
  Reference amended after NAM and RGD (1980) .; (claystone/carbonate development).
Definition Sequence consisting of evaporites and carbonates with some thin intercalations of claystone. The evaporites consist of anhydrite, rock salt and minor amounts of bitter salts, and are locally strongly affected by halokinetic movements. Towards the southern edge of the Southern Permian Basin, siliciclastics gradually replace the evaporites and, to a lesser degree, the carbonates.
Upper Boundary The upper boundary of the Zechstein Group has been placed at the base of the Lower GermanicTrias Group, which forms a well-recognisable marker on wire line logs. It marks a minor hiatus, reflecting the transition from the laterally variable nature of the Zechstein deposits into the regionally well-correlatable Lower Triassic sediments. In Germany, the base of the Triassic is placed at a slightly higher stratigraphic level (Best, 1989; R�ng (1991) ). In the United Kingdom the base of the Triassic is placed lower Cameron (1992) . There, it coincides with the base of the Dutch Zechstein Upper Claystone Formation. Where erosion or halokinesis occurred, younger units may overlie the Zechstein Group.
Lower Boundary The lower boundary of the Zechstein Group in the basin has been taken at the base of the Coppershale, a thin, black, bituminous shale bed recognized over practically the entire Southern Permian Basin and providing an excellent marker horizon. This boundary has been chosen despite the fact that the uppermost part of the Rotliegend Group may in part be genetically related to the initial Zechstein transgression (reworked sandstones and conglomerates). In the platform area in the southern Netherlands, younger strata of the Zechstein Group onlap the sediments of the Upper Rotliegend Group (e.g. Buurmalsen-1, Everdingen-1, Nederweert-1). The lower boundary can be traced into Germany (Kulick and Paul, 1987) and the United Kingdom Cameron (1993) .
Distribution Deposits of the Zechstein Group are present over most of the Netherlands’ on- and offshore. They are missing on the London-Brabant Massif, the Ringkøbing-Fyn High, the Elbow Spit High, the Texel-IJsselmeer High (see pdf) , and some smaller, unnamed highs. Its absence on the highs is partly the result of Kimmerian uplift and erosion, because some of the highs were already elevated areas during deposition of the Zechstein Group.
Age Thuringian. This age name is used in middle Europe for marine Permian deposits and was thought to cover the entire Late Permian (Harland (1990) ; Odin (1990) . According to magnetostratigraphic studies Menning (1994) , Menning (1982) , it should be equated with the later part of the Tatarian. Of the palynomorph biomarkers encountered (see Table E.1 (see pdf) ) in the Zechstein Group Lueckisporites virkkiae, Klausipollenites schaubergerii, Jugasporites delasaucei, Limitisporites and multitaeniate species are common. Nuskoisporites dulhuntyi, Strotersporites communis/richteri and Vittatina occur occasionally, in low percentages. The associations characterised by a dominance of Lueckisporites virkkiae and the presence of Nuskoisporites and Strotersporites are dated as middle to late Permian RGD (1993b) .
Depositional Setting Deposition of the Zechstein Group took place in a peri-marine to marine setting. During deposition of the Zechstein Group the basin became increasingly shallow. In the Z1 (Werra) and Z2 (Stassfurt) Formations the depositional setting varied from a lagoon to sabkha/mudflat at the margins of the basin, to deep marine in the centre of the basin. The clastics in the western offshore were deposited in an estuarine setting. The topography of the Southern Permian Basin was filled-in for the greater part by Z2 Salt. The Z3 (Leine) Formation was probably deposited in a shallow marine-environment. The higher cycles show decreasing marine influence in the basin; no carbonates are present and the claystone/halite alternations are more typical of a playa lake depositional setting.
  The subdivision of the Zechstein Group is based on the recognition of evaporite cycles and associated clastics. Five cycles containing rock-salt in the Netherlands have been assigned formation status. The names have been derived from the German nomenclature. The units defined are:
  ZE Zechstein Group
  ZEUC* Zechstein Upper Claystone Formation
  ZEZ5* Z5 (Ohre) Formation
  ZEZ4 Z4 (Aller) Formation
  ZEZ3 Z3 (Leine) Formation
  ZEZ2 Z2 (Stassfurt) Formation
  ZEZ1 Z1 (Werra) Formation
  In deviation from the formal lithostratigraphic concept, the specific nature of the Upper Permian sediments has led to the historically well-established German lithostratigraphy, based on the evaporitic cycles. A wide variety of lithologies, ranging from basin evaporites to fringe clastics is assigned to one formation of which the lower and upper boundary are almost synchronous. Within such a formation, units with a specific lithology are regarded as members. This proved to be the most useful approach for the stratigraphical subdivision of the Zechstein Group. Because of the great variety in lithology within one formation, the setting in the basin, and where appropriate the main facies development, have been indicated in the listing of the respective type and reference wells.
  A typical evaporite cycle consists of (from bottom to top): claystone - carbonate - anhydrite - halite - K-Mg salts - halite - anhydrite. In the Zechstein Group several examples of these classical cycles are found, but generally the cycles consist of several sub-cycles, and moreover, one or more components may be missing. The latter applies for instance to the Z4 (Aller) and Z5 (Ohre) Formations. It is generally assumed that the cyclicity caused more or less isochronous deposition of the main lithological elements (members) in the basin. However, within the carbonate/anhydrite bodies, diagenetic changes (calcitisation of the anhydrites or anhydritisation of the carbonates) may have blurred a synchronous boundary between the carbonate and anhydrite members.
  Although the main lithological units within each cycle are distinguished here as members, considerable facies variations may occur within each member. The Z2 Carbonate Member, for example, embraces the Main Dolomite deposited at the margin of the basin, the Fetid Limestone deposited at the slope of the basin and the Stinking Shale deposited in the deep basin.
  Towards the edge of the basin, the evaporite members are progressively replaced by claystone and sandstone members. Wire-line logs generally provide sufficient ground for recognising the basin cycles in the fringe area. For those rare cases where no cycle (i.e. formation) boundaries can be distinguished, e.g. in the outer basin-fringe-clastics, the informal formation name "Zechstein Fringe clastics" has been proposed. Correlations with the basinal facies of the Zechstein were carried out by applying the concept of sequence stratigraphy. Most confident correlations were found by tracing the maximum flooding units, i.e. the claystones at the base of the cycles.
  Halokinesis has disturbed large parts of the Zechstein sequence to such a degree that the recognition of the depositional boundaries becomes difficult. Furthermore, it has transported Zechstein rocks upwards to shallower depths, where they could become subject to erosion and subrosion. These abnormal sequences are treated in Chapter 5 of this section.
  Halokinetic or halotectonic salt movements have often disturbed the described Zechstein succession, notably the salts. Another effect of salt movement is that diagnostic units such as the Z3 Main Anhydrite Member are torn apart and floating in the salt, which brings the Z2 Salt in direct contact with the Z3 Salt Salt. However, the different salt cycles can usually still be recognised on their log characteristics, thus enabling the interpretation of folded and overturned beds. The floating units present drilling hazards, as they are often overpressured. To distinguish between the disturbed and normal Zechstein sequences, the following informal nomenclature is proposed (mainly after NAM and RGD, 1980).
  5.1 Zechstein salt (ZESA)
  Name for halokinetically disturbed sequences above the Z2 Basal Anhydrite Member, in which the depositional cycles of the Z2 Salt to Z4 Salt Members cannot be recognised anymore.
  5.1.1 Lower Zechstein salt (ZESAL)
  To be used for those cases where it is impossible to distinguish between the Z2 Salt Member and Z3 Salt Member because the basal Zechstein 3 members are absent, but where the Red Salt Clay Member is still recognisable. This nomenclature can be applied to most sequences drilled in salt structures.
  5.1.2 Upper Zechstein salt (ZESAU)
  To be used for those cases where the basal members of the (Z3 Leine) Formation are still recognisable, but where, owing to the absence of the basal members of the Z4 Aller Formation, the recognition of separate Z3 Salt and Z4 Salt Members is impossible.
  5.2 Zechstein caprock (ZECP)
  Residual rock composed of sulphates and claystone, present on top and on the flanks of salt structures. It separates Zechstein rocks from rocks younger than Early Triassic and was formed as a result of subrosion and erosion (leaching) of Zechstein evaporites. The composition of the formation depends on the insoluables and lesser soluables present, that were left as the salt dissolved. A typical section forms interval 115 - 171 m below rt. in well Zuidwending-KNZ-9 (N 53°05’18.8”, E 06°56’04.1”; Annex D-12).
References See References Permian

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
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