Download origin of stylolites in upper permian zechstein anhydrite

ORIGIN OF STYLOLITES IN UPPER PERMIAN ZECHSTEIN ANHYDRITE
(GORLEBEN SAL T DOME, GERMANY)
GÜNTHER
HÄUERLEI,
'!nstitut
O1TO
BORNEMANN!
für Geologie und Paläontologie,
FRlEDRICH
Universität
Hannover,
MAtrrHE,'
Callinstr.
AND DIETER
30, 30!67
Hannover,
e-mail: [email protected]/austhal.de
2 Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2. 30655 Hannover,
3!nstitut für GeoloRie. Ruhr-Universität
Bochum. UniversitätssIr.
ABSTRACT:Stylolites in evaporites are very uncommon. In the Gorleben Zechstein salt dome, stylolite horizons have been found in the
so-called "Bänderanhydrit"
(z3HAll) at the top of the Zechstein
Hauptanhydrit (Main Anhydrite). These pressure-solution features
seemsto be related to the presence of thin magnesite-clay interlayers.
Five principal stylolite morphologies can be distinguished in the core
sections studied. The orientation of the sutured seams is generally bedding-parallel with the stylolite axes vertical to the bedding surrace. This
demonstrates that pressure-solution features clearly predate the formation of the salt dome and that there is no relation between their
formation and halokinesis. Solution rates of up to about 26% have
been calculated by measuring maximum stylolite amplitudes in certain
core intervals. Considering the bedding-parallel occurrence of the stylolites and the fact that the stylolite seams are crosscut by anhydrite
crystals that we interpret as pseudomorphs after gypsum, it must be
concluded that the stylolites formed before the dehydration of gypsum
to anhydrite was completed. The lower parts of the Hauptanhydrit
were excluded from these processes because they had already been
converted to anhydrite by the overburden of overlying Zechstein units
and by ascending compaction fluids of the underlying Zechstein 2 Salt.
We assurnethat the dehydration of gypsum to anhydrite was completed
before the end of Zechstein sedimentation and that the formation of
stylolites thus must be attributed to the same time interval.
INTRODUCTION
Chemical compaction (Lloyd 1977) by pressure solution is a weIl known
feature of diagenesis. It is most important in carbonate rocks and sandstones, where it causes reduction of porosity by generation of autochthonous cement (e.g., Wong and Oldershaw 1981; Choquette and I"' 'es 1990).
In addition to physical compaction a considerable reduction 01 ued thicknesses can also be attributed to chemical pressure solution depending on
sediment or rock composition. The susceptibility to pressure solution depends on mineralogy, so that more soluble minerals may dissolve under
minor stress conditions while less soluble ones remain (Trurnit 1968a,
!50. 4480!
MICHALZIK3
Gel7nany
Germany
Bochum, Gel7nany
1969; Niktin 1985). Carbonate minerals are among the most susceptible
components, so much information about the effects of pressure solution
comes from carbonate rocks (e.g., Bathurst 1975, 1995; de Boer 1977;
Wanless 1979; Buxton and Sibley 1981; see also references in Choquette
and James 1990).
Pressure solution is related to gravitational loading or to unilateral tectonic stress (e.g., Wagner 1913; Wagner 1964; Beiersdorf 1969; Janssen
and Friede11985; Füchtbauer 1988), resulting respectively in bedding-parallel or transversely oriented features. The products of pressure solution
are quite diverse and depend on pressure directions and intensity as weIl
as chemical composition, texture, and permeability of the sediment. Generally two fundamental styles of pressure-solution features are recognized
in carbonate rocks: nonsutured seams and sutured seams (e.g., Wanless
1979; Choquette and James 1990). Nonsutured types are smooth and gently
undulating seams of residual organic matter, clay, pyrite, or other less soluble minerals. It is supposed that smooth solution seams more frequent I y
develop in limestones with more than about 10% noncarbonate impurities
(Choquette and James 1990). However, in many cases it is difficult to
recognize whether or not this feature is related to pressure solution or to
simple physical compaction. The sutured types comprise different more or
less serrated seams known as stylolites. A special type of sutured seams
are the so-calied fitted fabrics (Buxton and Sibley 1981). Whereas the former two types are planar features, the fitted fabric is characterized by an
anastomosing network of microstylolites affecting all grains and pervading
the whole rock unit (Choquette and James 1990).
The present study focuses on the morphology, orientation, and origin of
stylolites in anhydrite rocks. As mentioned above, stylolites are cornrnon
features in limestones but are known from other sedimentary rocks as weIl
(e.g., Heald 1955; Trurnit 1969). Stylolites have even been reported from
nonsedimentary, e.g. metamorphic, units (Niktin 1985). However, to our
knowledge, stylolites in evaporites have rarely been described (Borns 1985;
Sadooni 1995) and interpreted in detail until now. In the special case of
salt domes and their suitability for waste storage, the dating of stylolite
formation may be significant in understanding the timing of fluid flow and
reduction of porosity and permeability.
FIG. 1.-Location of the Gorleben salt dome
in northern Germanv.
JOURNALOF SEDIMENTARYRESEARCH,VOL. 70, No.3,
MAY, 2000, P. 726-737
Copyrighl @ 2(XX). SEPM (Society for Sedimentary Geology)
IO7J-IJOX/OO/O70-726I$0J.OO