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The role of iron in mudstone diagenesis; comparison of Kimmeridge Clay Formation mudstones from onshore and offshore (UKCS) localities

Joe H. S. Macquaker, Charles D. Curtis and Max L. Coleman
The role of iron in mudstone diagenesis; comparison of Kimmeridge Clay Formation mudstones from onshore and offshore (UKCS) localities
Journal of Sedimentary Research (September 1997) 67 (5): 871-878

Abstract

Siliciclastic muds and mudstones commonly contain 5% iron. At deposition, most of this iron is in the oxidized form, Fe (super III) , whereas in ancient mudstones it is predominantly in the reduced form, Fe (super II) . In most fine-grained siliciclastic sediments iron reduction is an important process during burial diagenesis. A combination of geochemical and petrologic techniques has been applied to organic-rich mudstones of the Kimmeridge Clay Formation to investigate this valence change. These sediments were collected from a variety of depositional (shallow shelf to deep graben) environments and diagenetic (0.5-4.5 km burial) settings, and our analyses have shown that significant Fe (super III) survives burial to depths of the order of 4 km. At these depths Fe (super III) is located (together with Al) in dioctahedral micaceous clays (notably smectite). In such sites, it is apparent that Fe (super III) is very difficult to reduce. Our data suggest that diagenetic redox reactions are unlikely to be significant in the burial interval 0.5-4.0 km. It is also apparent that substantial iron reduction took place early during burial diagenesis with precipitation of pyrite, siderite, and ferroan carbonates in the sulfate-reduction, methanogenic, and decarboxylation zones, respectively. All these minerals are found in the studied mudstones from the shelf localities. In contrast, within the graben mudstones, pyrite is the dominant Fe (super II) -rich mineral species present. We propose that the different style of diagenesis in the two settings was produced by a fraction of the reactive iron (i.e., that contained within soil sesquioxides) being converted to the pyrite prior to sediment remobilization. Resedimentaton then allowed a second phase of sulfate reduction with replenished pore-water sulfate from the anoxic bottom graben waters. The remaining reactive iron was converted to pyrite, thereby preventing precipitation of Fe-rich carbonates in the deeper diagenetic zones. Hence, the diagenetic iron-mineral assemblages in the different facies are quite distinct. Diagenetic assemblages present in ancient siliciclastic sediments thus offer valuable insights into both sedimentary and diagenetic processes, but the links are complex and must be interpreted with care.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 67
Serial Issue: 5
Title: The role of iron in mudstone diagenesis; comparison of Kimmeridge Clay Formation mudstones from onshore and offshore (UKCS) localities
Affiliation: University of Manchester, Department of Earth Sciences, Manchester, United Kingdom
Pages: 871-878
Published: 199709
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 37
Accession Number: 1997-072217
Categories: Geochemistry of rocks, soils, and sedimentsSedimentary petrologyOceanography
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
N50°00'00" - N55°45'00", W06°30'00" - E01°30'00"
N51°00'00" - N61°10'00", W04°00'00" - E11°00'00"
Secondary Affiliation: University of Reading, GBR, United KingdomBP Exploration, GBR, United Kingdom
Source Note: Section A: Sedimentary Petrology and Processes
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199724
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