Inorganic geochemical characterisation of the Cretaceous source rocks of the Eastern Cordillera, Colombia

Abstract

The depositional environment of organic‐rich mudstones in the Cretaceous greenhouse climate is a key area of Earth Systems and Energy research, as understanding the causes of carbon drawdown into these rocks has impacts for understanding the role of the carbon cycle in climate change and the formation of petroleum source rocks. The Eastern Cordillera basin in Colombia is a key region for investigating Cretaceous carbon burial in flooded continental areas and tropical regions. Both characteristics are essential to understanding the Cretaceous carbon cycle, a key question being: are the mechanisms which cause carbon burial in the open ocean similar those in more continental, tropical basins? To date, the EC is understudied, partly due to the basin’s complex history, which includes deep burial (compromising organic geochemical analysis) and uplift (complicating geological reconstruction and exposing rocks to contemporary weathering, hence compromising inorganic geochemistry). In this thesis, a combination of inorganic geochemical methods (bulk geochemistry, trace metal enrichment patterns, iron‐sulphur speciation) with high‐resolution electron microscopy and chemical micro‐analysis is applied to a unique set of Cretaceous mudrock samples from the EC with the ultimate aim of a detailed paleoenvironmental reconstruction. In a first step, an artificial weathering experiment is presented and, following interrogation of the geochemical dataset, novel filters (carbonate content and passivated pyrite particles) are presented which identify samples from Eastern Cordillera mudstones less likely to have been geochemically altered by weathering. This allows, in a second step, the robust interpretation of redox conditions, productivity and climate in the Cretaceous Eastern Cordillera basin based on bulk geochemistry, trace elements and Fe‐speciation for 208 new samples from 12 different outcrops across the basin. Also, it provides a methodology for future investigations using outcrop samples globally. New evidence is presented that the Eastern Cordillera was anoxic in the Valanginian (different from open ocean sites), likely due to the sites more restricted nature. In the Mid‐ Late Cretaceous, extremely high and prolonged siliceous and calcareous productivity drove anoxia and carbon burial. Evidence is presented that sources of nutrients to drive this productivity resulted from upwelling (Albian and Santonian); intensive chemical weathering of land (especially in the Turonian); and possibly volcanogenic sources iv (Cenomanian/Turonian boundary). In most cases, these mechanisms occur in other time equivalent facies, therefore this study suggests that global changes in climate and geochemistry of the oceans heavily influenced organic matter deposition in the Eastern Cordillera.