4 Results
4.9 Diagenetic minerals and their microscopic relationships
Diagenetic mineral formations were studied by optical methods (thin sections, SEM) and mine-ral chemistry (SEM-EDX, microprobe). The focus was on the sandy layers of the clay-rich sedi-ments with a large fraction of diagenetic minerals. Six samples, three from 'Brauner Dogger', two from Opalinus Clay and one from the underlying Lias were selected for this purpose. Tab.
4-37 gives a summary petrographic description of the diagenetic minerals for the seven samples.
The detailed description with macro- and micrographs is presented in Appendix D.
Tab. 4-37: Summary of samples studies for diagenetic analysis.
Depth [m] Formation Rock description Diagenetic minerals Special features 763.58 Variansmergel
- SiO2 Pyrite concretions
827.35 Wedelsandstein
Formation Sandy limestone and silty claystone, 833.69 Opalinuston Silty claystone,
calcite-cement with 886.90 Opalinuston Micrite with
bioclasts and silty claystone layers, calcite veins
- calcite and Fe-calcite - pyrite
982.13 Arietenkalk Biosparite with abundant diagenetic
The main diagenetic mineral and cementing phase is calcite which is generally enriched in Fe (1 – 2 wt.-%), termed Fe-calcite here. The detrital and biogenic calcites show lower Fe content (0 – 0.7 wt.-%). The abundance of diagenetic Fe-calcite it related to the clay-mineral content: the larger the clay-mineral content, the more dispersed and less abundant Fe-calcite is and some-times it is restricted to the hollows in bioclasts, composed of calcite and/or aragonite (cannot be distinguished). The occurrence as continuous cement between silty components and biogenic fragments is visualised in Fig. 4-73a. The contact between the Fe-calcite-cemented silt lenses and the uncemented clay-rich parts is also shown (Fig. 4-73b). Besides the dense structure, diagenetically formed fine-grained kaolinite may have prevented calcite from forming in the clay-rich parts.
The Mg content in calcites is rather constant and rather low (0 – 0.5 wt.-%) in all samples except for SLA 833.69 (0.19 – 1.22 wt.-%). It is slightly lower than that of biogenic carbonates.
From the textural relationships between Fe-calcite and the other diagenetic minerals it can be deduced that:
Fe-calcite formed after pyrite (Fig. 4-73a), siderite, quartz and ZnS
Fe-calcite formed before dolomite/ankerite and Ba-Sr sulphate (Fig. 4-74b, Fig. 4-76).
(a) (b)
Fig. 4-73: Sample 833.69, Fe-calcite (blue); (a) continuous matrix between silty components and biogenic fragments; (b) contact between Fe-calcite cemented silt lens and uncemented silty claystone.
(a) (b)
Fig. 4-74: SLA 799.12 (a) The circular calcite/aragonite structure is a fossil wormhole, calcite is formed after pyrite; (b) Ba-Sr sulphate filling interstices between Fe-calcite clusters.
Brighter areas (red arrows) are richer in Ba compared to the darker areas.
Pyrite
Diagenetic pyrite is present in all samples (<< 1 – 5 vol.-%). It occurs as massive concretions, framboids (Fig. 4-75) and replacements of fossil calcite/aragonite structures (Fig. 4-74a). From the textural relationship between pyrite and the other diagenetic minerals it can be deduced that:
Pyrite formed before Fe-calcite (Fig. 4-75a), dolomite/ankerite (Fig. 4-75b) and Ba-Sr sulphate.
Pyrite in relation with siderite is difficult to evaluate because of their common occurrence.
Observations suggest that both pyrite formations occurred after siderite (sample 833.59) and before siderite (SLA 763.58).
(a) (b)
Fig. 4-75: (a) SLA 763.58, accumulation of pyrite framboids, red arrows mark areas where Fe-calcite has overgrown some tiny pyrite grains; (b) SLA 827.35, a dolomite/
ankerite rhomb crystallizing around the pyrite framboid.
Dolomite/ankerite
All samples except for SLA 866.90 contain rhombohedral and dispersed dolomite/ankerite crystals. These are more abundant in less densely calcite-cemented sediments, with modal abun-dances of (<< 1 – 3 vol.-%). Dolomite/ankerite grains may also form cements of silty or sandy components (Fig. 4-76a).
From the textural relationship between pyrite and the other diagenetic minerals it can be deduced that:
Dolomite/ankerite formed after pyrite, Fe-calcite, siderite (Fig. 4-76b), after quartz and after kaolinite.
Dolomite/ankerite formed after Ba-Sr sulphate (SLA 982.13) or before Ba-Sr sulphate (SLA 827.35).
(a) (b)
Fig. 4-76: (a) SLA 827.35, cementing dolomite/ankerite (bright blue), coloured thin section;
(b) SLA 763.58, zoned dolomite (core)/ankerite (rim) crystal clearly crystallised after the rhomboidal siderite.
Siderite
Siderite was observed in sample SLA 763.58 and SLA 833.69. In SLA 763.58 rhombohedral crystals are dispersed in the rock, whereas in SLA 833.69, siderite concretions are common.
They generally form hollow structures. From the textural relationship between pyrite and the other diagenetic minerals it can be deduced that:
Siderite formed before Fe-calcite (Fig. 4-77a) and before dolomite/ankerite (Fig. 4-76b).
Siderite may have formed before and after pyrite (see above).
No contacts between siderite and other diagenetic minerals were found.
(a) (b)
Fig. 4-77: (a) SLA 763.58: Siderite crystals (bright grey) embedded in a Fe-calcite matrix (Fe-cc), red arrows: hollow siderites, yellow arrows: Mg-rich carbonates precipi-tated in the hollow siderites, BSE image; (b) SLA 833.69: Siderite corona (poly-crystalline) with an empty core filled with an ankerite rhomb.
Ba-Sr sulphate
Ba-Sr sulphate occurs in five of seven samples (SLA 763.58, SLA 799.12, SLA 827.35, SLA 833.69 and SLA 982.13) with modal abundances smaller than 1 vol.-%. Generally, Ba-Sr sul-phate has an interstice-filling texture (Fig. 4-78a) and is therefore considered to be a late phase in the relative age succession of the diagenetic minerals. Ba-Sr sulphate inter-grown with detri-tal K-feldspar was observed in samples 827.35 and 982.13 (Fig. 4-78b).
The composition of Ba-Sr sulphate in SLA 837.35 and SLA 833.69 was measured with SEM-EDX. These analyses indicate highly variable content in Ba and Sr within the same sample and also between the two samples.
(a) (b)
Fig. 4-78: (a) SLA 982.13: The largest Ba-Sr sulphate occurrence observed in the 6 studied samples. (b) SLA 827.35: Ba-Sr sulphate inter-grown with detrital K-feldspar.
From the textural relationship between Ba-Sr sulphate and the other diagenetic minerals it can be deduced that:
Ba-Sr sulphate formed after all other contacting diagenetic minerals (Fe-calcite, pyrite, dolomite/ankerite, quartz).
No contacts between Ba-Sr sulphate and siderite and Ba-Sr sulphate and kaolinite were observed.
Quartz/chalcedony
Diagenetic SiO2 occurs as replacement (probably mainly as chalcedony) of fossil calcite/
aragonite fragments (Fig. 4-79a), as quartz growth rims on detrital quartz grains or as consti-tuent of a fine-grained diagenetic matrix (Fig. 4-79b). Diagenetic SiO2 was formed early in the diagenetic sequence, clearly before dolomite/ankerite and Ba-Sr sulphate.
(a) (b)
Fig. 4-79: (a) SLA 799.12: Partially replaced calcite/aragonite structures, the textures of these SiO2-replacements are typical for chalcedony, optical micrograph; (b) SLA 827.35:
Tiny diagenetic euhedral quartz grains forming a fine-grained matrix between detrital quartz grains, this fine-grained quartz generally occurs together with diagenetic kaolinite.
Kaolinite and needle-like phase (illite?)
Both phases were observed in SLA 827.35. Together with a needle-shaped SiAlKMgNaFe-phase, kaolinite is abundant and forms a fine-grained diagenetic matrix between silty and biogenic components (Fig. 4-80a), predominantly in detrital clay-poor areas and where Fe-calcite cementation is not dense. The needle-like SiAlKMgNaFe-Phase (maybe illite, according to the EDX-spectrum) is less abundant but like kaolinite seems not to be compacted.
These diagenetic silicates were also formed early in the diagenetic sequence, thus before dolo-mite/ankerite. The temporal relationships between kaolinite and Fe-calcite are not clear.
(a) (b)
Fig. 4-80: (a) SLA 827.35: Tiny kaolinite crystals (red arrows) are forming a matrix between detrital quartz grains, BSE image; (b) SLA 827.35: Sheet- or needle-like crystals (red arrow, according to SEM-EDX analysis could be illite), BSE image.
Other minerals
Diagenetic sphalerite (ZnS) was observed in SLA 799.12. The other rare occurrences observed have a detrital appearance. It appears that the diagenetic ZnS grain formed before Fe-calcite.
Ti oxide was found at several instances as rounded detrital grains. However, in SLA 827.35, Ti oxide with idiomorphic grain faces was observed suggesting a diagenetic origin. No temporal relationship with other diagenetic minerals could be deduced.
Two different Pb-rich phases (Pb-Fe and Pb-Mo phase) were noted in SLA 833.69, both of them filling interstices between pyrite grains in the pyrite-concretion.
Sample SLA 799.12 (Humphriesioolith Formation)
The high diagenetic Fe-calcite-cement content and the enhanced distances between the detrital grains are typical for hard grounds, seafloors that receive a very low detrital input and have time to consolidate by prolonged carbonate cementation.
Summary of diagenetic sequence
Fe-calcite is the main cementing phase and formed early in the diagenetic sequence. Later dolomite/ankerite also partially formed cements in the silty and sandy parts. In clay-rich parts, kaolinitic cement formed before dolomite/ankerite, as noted in one sample. The precise tem-poral relationships are difficult to unravel from the optical data. There are at least two genera-tions of diagenetic mineral formation (presented in the order of their relative abundance):
Fe-calcite, pyrite, siderite, SiO2, kaolinite
dolomite/ankerite, Ba-Sr sulphate.