The Haughton Impact Structure (HIS) formed 39 Ma ago. The target rocks include Lower Palaeozoic aged limestone that contain small quantities of solvent extractable organic matter. The target bedrock, hydrothermal deposits and limestone clasts within impact melt breccias all yield fossil organic matter that has been thermally matured to varying degrees. But unusually for a terrestrial impact crater, molecular biomarkers survive. While many other impact structures on Earth (e.g. Gardnos) possess lithologies with associated organic carbon, thermal maturation prior or subsequent to the impact has destroyed any biomarkers that were present (note that this is not necessarily due to the impact itself). In other instances the biomarkers present are found in hydrocarbon accumulations and have migrated through the structure (e.g. Siljan). The HIS is unique in not experiencing subsequent regional metamorphism, furthermore the sedimentary organic matter is mostly present within the target rocks as inclusions of organic matter and not a free continuous petroleum phase (it has not migrated).
The extraction and analysis of biomarkers from impactites from the HIS radically alters how an organic geochemist, looking to detect ancient molecular evidence of life, views the surface of a planetary body. For craters of a certain diameter (23 km), impact cratering, far from being an agent that obliterates the biomarker fossil record, can be shown to be a geological process more akin to tectonic activity: it has the potential to exhume geological formations that would otherwise be buried – albeit at the cost of a relatively minor increase in thermal maturity.
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