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| Black shales |
The Kimmeridge Clay
Is rich in lamalginite,
Whatever that is.
The Kimmeridge Clay Formation is a late Jurassic succession of organic-rich mudstones and limestones. It is the source of much of the North Sea oil.
I have spent much of the last year studying it, looking at its sedimentary structures, trying to understand the environmental conditions under which it was deposited. I'm particularly interested in biogenic sedimentary structures, or trace fossils, such as these:
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| Rhizocorallium in the Rope Lake Head Stone Band, Kimmeridge. |
I've spotted some interesting features, which I might even write up one day, but my understanding of climatology and geochemistry and sequence stratigraphy and oceanography and microbiology is so poor that I doubt it'll make much sense.
Last week, trying to address this, I was looking into the types of kerogen present in the succession, and read that the most organic-rich interval contains anomalously high quantities of lamalginite. Alginites are types of kerogen derived from algae, and lamalginite is a 'finely banded, lamellar alginite interbedded with mineral matter'.
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| The green alga Micrasterias. I have no idea if it ever produces lamalginites. |
The thing I'm puzzled about, though, is that alginites are Type I kerogens, which originate in lakes or lagoons, and the Kimmeridge Clay is a marine deposit. Some authors have suggested that it was deposited in water hundreds of metres deep, a long way from land. How can sea-sourced sediments contain such large quantities of lagoonal or lacustrine algae?
I have no doubt I'm making an elementary error somewhere here, but it's all very confusing.
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