Shelf and cold-seep limestones compared
Surrounded by an array of samples of limestone, Dr Steve Hood compares a typical New Zealand shelf limestone with an ancient cold-seep carbonate from the North Island east coast region. Cold-seep carbonates have unique fossil faunas never seen in shelf limestones as well as the aragonite mineral form of calcium carbonate as opposed to calcite
Points of interest Look out for the following micrographs in this clip:
Micrograph a: Thin section image as seen under plane polarised light of a temperate limestone dominated by bryozoans and foraminifera. Skeletons have pressure-dissolved into one another.
Micrograph b: Thin section image as seen under plane polarised light of a temperate limestone dominated by bryozoans. A tight, well cemented structure.
Micrograph c: Thin section image as seen under plane polarised light of cold-seep limestone from the east coast of the North Island. The white veins are mineralised fractures that run through the limestone.
Micrograph d: Thin section image from c above, as seen under cathodoluminescent light. The red regions are due to the presence of calcite crystals.
Micrograph e: Thin section image as seen under plane polarised light of cold-seep carbonate. The brown outer ring is fossilised microbial mat, and the worm tube cross-section is infilled with needle-shaped aragonite crystals.
Micrograph f: Thin section from e above, as seen under cathodoluminescent light.
Transcript
DR STEVE HOOD This is a typical Te Kuiti-type shelf limestone, cool-water formed around Te Kuiti/Waitomo county. Occasionally, you see large macro-fossils preserved, but the majority of the skeletal material – the shell material that you see dotted through the rock – is smashed up as a result of migration movement during storms particularly of the shells around the seafloor being redeposited and moved around. Burial diagenesispressure dissolution of the skeletons, we end up forming calcite cements around all the skeletal material, and we see there’s virtually no original holes or porosity or space around the skeletons.
The cold-seep carbonate on the other hand can be much more complex. These cold-seep carbonates can differ over a very short distance. They can often include fracturing, the strange array of fossil faunas that we don’t ever see in a shelf limestone because of this relationship with hydrocarbons.
We see interesting little bugs and skeletons that are filled with an array of minerals, typically aragonite, which we never see in a shelf limestone.
This is an interesting little slab of cold-seep limestone in that these white bands are what we call microbial mats, and this is calcified remains of bacteria that established themselves perhaps in cracks or fissures, fractures in the cold seep through which the hydrocarbons were migrating, and these are the bacteria that thrive and feed on the hydrocarbons. So we’ve got fossilised bacterial life preserved within these cold-seep limestones from on land east coast North Island.
Acknowledgement: Associate Professor Kathleen Campbell, University of Auckland
