Abstract

A model is proposed which involves biogenic Mg enrichment, a stratified lake environment, and authigenic growth of minerals that led to the development of oil shale in the lacustrine Green River Formation. The chemistry and mineralogy of Ca-Mg-Fe carbonates and other minerals in oil shale are consistent with an authigenic origin. The higher content of magnesium with respect to calcium in kerogen-rich rocks is probably due to the preferential concentration of magnesium with respect to calcium by blue-green algae whose remains released these cations after accumulation on the lake bottom. These elements were available for incorporation in Ca-Mg-Fe carbonates which crystallized in lake-bottom muds, while degradation of admixed algal material led to the development of kerogen. In modern lacustrine environments, primary and secondary Ca-Mg-Fe carbonate development and stability in terms of geologic time are consistent with authigenic development of the Ca-Mg-Fe carbonates present in Green River Formation oil shale. Iron is a significant component of these rhombohedral carbonate assemblages in oil shale, and thus it limits interpretations of origin of oil shale in the context of CaCO3-MgCO3 equilibria.

The variable composition and variety of Ca-Mg-Fe carbonates in oil shale also prohibits interpretations in terms of CaCO3-MgCO3 equilibrium diagrams. Similarly, inferences regarding the chemical composition and structure of these carbonates lead to misinterpretations due either to the lack of adequate chemical data or to misunderstanding of crystallographic parameters.

Greater amounts of mineral matter in time-stratigraphic intervals in the depositional center of the Piceance Creek basin, compared to the basin margins, suggest strongly that authigenic mineral development is more important than detrital accumulation of minerals in the richer oil-shale sequences.

Systematic consideration of all of the arguments developed to promote the playa-lake model to explain the chemical, mineralogical, and geologic aspects of Green River Formation oil shale leads to the conclusion that the biogenic-chemical model proposed here is more appropriate than a strictly chemical model, because the playa-lake model so far does not consider the influence of any biogenic factors.

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