Guest Series - 02 - Palynofacies - Organic Matter from another perspective by Alex Wheeler

There are many tools a geoscientist can use to examine the organic content of their rocks. Organic petrology, biomarkers, carbon isotopes are but a few. As a palynologist, my own journey into the world of organic matter began with "palynofacies", which I would describe as something of a close relative organic petrology.

Professor Coelophysis would have made a great palynologist if his arms could reach the microscope.

Palynofacies

Palynofacies refers ALL of the acid-resistant particulate organic matter in a rock. After palynological processing to remove minerals (traditionally but not always done with dangerous and toxic acids like HF), one is left with an assemblage of organic components, a melange of woody fragments, plant cuticles, pollen, spores, planktonic cysts, fungi and even parts of animals. These are comparable to the vitrinites, inertinites and liptinites we see in organic petrology. So how does palynofacies compare to organic petrology and why would one use it?

How we see in organic matter in reflected light (left) vs transmitted light (right).

Cross-section of a pollen grain in fluorescence (left) vs a polar view of a pollen grain on a palynological slide (right).

Applications

The aim of palynological processing is to concentrate the organic matter and separate it from the mineral component of the rock. This is useful in rocks with low organic carbon content and opens one up to a wide variety of target lithologies and depositional environments, from fluvial sediments to marine mudstones and carbonates. Palynofacies is widely used in oil and gas exploration for basinal and palaeoenvironmental reconstruction. But outside of industry, it also has many uses in examining climatic and environmental change. With a high enough sample resolution, it can provide insight into sea level changes or climate cyclicity. It can be applied to mass extinction research in both the terrestrial and marine realms, wildfire patterns, oceanic anoxic events, etc. The sky would appear to be the limit.

Limitations

However, there's no such thing as a silver bullet in science. Palynofacies comes with limitations and pitfalls one must be wary of. For example, the famous “APP triplot” otherwise known as the “Tyson ternary diagram” is a great way of visualising palynofacies data. But it was developed based on marine strata from the Jurassic basins of England, which means it is not necessarily applicable as is to any basin across the vast breadth of the Earth and across geological time. There are many factors that can control why your data plots in a certain field that may not directly correlate with the interpreted palaeoenvironments of the original plot. You should constantly question your data and interpretation. Is your amorphous organic matter marine or terrestrial in origin? Are the palynomorphs you see mainly pollen, spores or dinoflagellates? How does your palynofacies data compare with the geochemical data? Has your material been affected by transport and taphonomic biases, high burial temperatures or weathering? I've reviewed papers where the authors have plotted terrestrial data and directly used the fields of the Tyson triplot to draw their conclusions. I find this "fire-and-forget" approach to data presentation and interpretation is comparable to the petrographic indices often used in coal petrology on wildly variable coals across a vast span of geological time.

The APP triplot is a powerful tool for visualising the relative abundance of amorphous organic matter (AOM), palynomorphs and phytoclasts when used thoughtfully.nd out

In the end palynofacies is just another tool in our arsenal. It is up to us as responsible geoscientists to pay scrupulous attention to our data, analytical techniques and interpretations. We won't always get it right (in fact, we will often be wrong!), but we can give ourselves a few more confident steps through the murky mists of deep time.

Recommended reading:

Batten, D. J., & Stead, D. T. (2005). Palynofacies analysis and its stratigraphic application. In Applied stratigraphy (pp. 203-226). Dordrecht: Springer Netherlands.

Traverse, A. (2007). Paleopalynology (Vol. 28). Springer Science & Business Media.

Tyson, R. V. (2012). Sedimentary organic matter: organic facies and palynofacies. Springer Science & Business Media.

Find Alex here:

https://de.linkedin.com/in/alex-wheeler-9311a239

https://www.researchgate.net/profile/Alexander-Wheeler

https://scholar.google.com/citations?user=NEjTUgkAAAAJ&hl=en