Things About Me Series – 04 – I Like the Old Stuff
One of the most marvelous things about geology, for me, is the opportunity to work with ancient rocks. There is something special about them. They have witnessed the assembly and destruction of supercontinents, drifting with plate tectonics, and surviving subduction zones. And today, they reach your hands.
I will never forget the moment I held a core sample from the Barney Creek Formation (McArthur Basin, NT)—rocks that are 1.64 billion years old. Let that sink in. Imagine traveling that far back in time. 1.64 billion years ago (Ga). I know there are older rocks—I have studied slightly older ones from the Tawallah Group—but Barney Creek (part of the McArthur Group) was special to me.
To truly appreciate these rocks, you need to imagine a completely different Earth. Oxygen levels were much lower, and CO2 levels were higher than today. Life on Earth was just beginning. Eukaryotic bacteria thrived in the waters, and biological evolution was slow. The Barney Creek Formation belongs to a time known as the Boring Billion (1.8 to 0.8 Ga), often referred to as Earth's Middle Ages. Personally, I don’t think it was boring at all. The literature describes this period as one of relative tectonic stability, yet evidence suggests that the Columbia (Nuna) supercontinent gradually broke apart, and the Rodinia supercontinent began assembling during this time, though exact timing remains debated.
Despite oxygen- and nutrient-poor sulfidic (euxinic) conditions in shallow marine waters, some intracontinental basins may have remained sulfur-limited, potentially contributing to the evolution and diversification of microorganisms. Redox conditions likely varied across different basins, influencing microbial development in ways that are still being studied. In this sense, the harsh conditions of the Boring Billion may have set the stage for the explosion of life that followed.
When I examine organic matter in Barney Creek sediments under the microscope, I am looking at some of Earth's earliest life forms. This organic matter gave rise to one of the oldest active petroleum systems in the world, generating hydrocarbons that may have remained trapped for over 1.5 billion years, although secondary migration and remobilization over geological time cannot be ruled out.
Despite their age, in some locations, the organic matter plots in the immature or beginning of the oil window, as determined by Tmax (°C) in pyrolysis analyses. In other locations, the organic matter is overmature, having generated gas. Geological factors such as differential depth and fluid circulation may have driven those differences.
Petrographically, the organic matter in Barney Creek sediments is relatively simple to characterize. We primarily observe lamalginite (Lam), which consists of laminated or string-like organic matter derived from microorganisms, lacking distinct morphology (classified as Type I-II kerogen). If hydrocarbons were generated, we also find solid bitumen (SB), either as an in-situ transformation of lamalginite or as material filling voids and fractures, indicating migration over varying distances.
Where would these samples plot on a pseudo-van Krevelen diagram? The image below shows two samples from the same drill core, 300 meters apart, both with total organic carbon (TOC) values above 1%. As expected from petrographic analysis, one sample contains highly fluorescent lamalginite (indicating a higher hydrogen index, HI), while the other is dominated by low-fluorescence solid bitumen (resulting in a lower HI). An interesting point to consider is that these samples predate the occurrence of vitrinite (Type III kerogen), which originates from higher plants. Therefore, any observed Type III kerogen must have formed from the thermal maturation of lamalginite and the cracking of hydrocarbons. Oil films can also be observed, indicating the presence of free oil in this sample.
Pseudo van Krevelen diagram of two samples from the Barney Creek Formation, McArthur Basin, Australia.
However, the similar Tmax values between these samples are puzzling, as no anomalies were detected in the pyrolysis data and pyrograms. One would typically expect higher Tmax values for samples with low HI and abundant solid bitumen. The kinetics of pre-plant organic matter may also differ from those of younger sediments. Since the biochemical precursors of early organic matter were different from those in later, plant-dominated environments, their decomposition pathways and thermal stability may not align perfectly with conventional kerogen kinetics models. This remains an area of active research.
Great work has been carried out by the Northern Territory Geological Survey on this topic. Check out their website if you’re interested in these ancient petroleum systems and their association with mineral systems, such as the McArthur River Mine.
Studying formations like Barney Creek helps us unravel the history of organic matter, from its deposition to its transformation over geological time. At CarbonMat, we apply the same rigorous approach to make sense of complex data, combining petrography, geochemistry, and pyrolysis to extract meaningful insights for both research and industry.
