Selenium in Ecosystems: Time Distribution of Phosphate and Oil

Originally, source-rock age was hypothesized as controlling Se sources, with Cretaceous-Period sedimentary rocks such as the Pierre and Niobrara Formations identified as important sources of Se. However, formations selected for our case studies range in age from the Permian Phosphoria Formation to the Miocene Monterey Formation, with significant Tertiary-Period sources in the California Coast Ranges. A compilation by age of major phosphate resources and effective petroleum source rocks (nmormalized as a percentage of total world resources of of the world's original petroleum reserves, respectively), show that some geologic ages are more important than others in determining productivity, but no apparent predictable periodicity can be discerned from them. Some 50% of phosphate resources were deposited in the Eocene and Miocene. Ninety percent of the world's discovered oil resources come from six stratigraphic intervals, with the middle Cretaceous and upper Jurassic accounting for 54%.