Rhenium-osmium geochronology and geochemistry of ancient lacustrine sedimentary and petroleum systems

Abstract

The Re-Os geochronometer is a valuable tool able to provide precise depositional ages of marine organic-rich sedimentary rocks as well as the generation age of hydrocarbons. In addition, Os isotopes afford vital insights into seawater Os fluctuations and provide the ability to fingerprinting hydrocarbons to their source. This thesis presents new research on extending the Re-Os geochronometer to lacustrine sedimentary and petroleum systems, which provide exceptionally high-resolution records of continental geological processes as well as significant hydrocarbons sources.

Lacustrine organic-rich sedimentary rocks possess similar Re and Os abundances to those found in marine successions. New Re-Os depositional dates from the Eocene Green River Formation are in agreement with Ar-Ar and U-Pb ages of intercalated tuff horizons. As also documented in marine systems, precision is controlled by the variation in initial 187Os/188Os and the range of 187Re/188Os ratios, which are controlled by depositional setting and type of organic matter. The Os isotope composition of lake water at the time of deposition suggests the lake was dominated by inputs from continental runoff, giving insights into continental weathering.

The Green River Formation is the source rock for the Green River petroleum system in the Uinta Basin. Three types of hydrocarbons are derived from this petroleum system; oils, tar sands and gilsonite. Re-Os geochronology of these different hydrocarbon phases broadly agrees with previous generation models. Hydrous pyrolysis experiments yield similar Re and Os transfer systematics to the natural system observed and importantly reinforce that Os isotope compositions are directly transferred from source to hydrocarbon, providing a powerful correlation tool.

Lacustrine units are also important records of Earth’s climate system. New geochemical and Re-Os data from the late Mesoproterozoic Nonesuch Formation yield a Re-Os depositional age of 1078 ± 24 Ma, while Os isotope compositions strongly support existing evidence for a lacustrine setting. Fe-S-C systematics suggest that the Nonesuch Formation was deposited from an anoxic Fe-rich water column.