--> --> The Importance of Detrital Dolomite in Upper Devonian Carbonates: Examples from the Dyer Formation (Northwest Colorado) and Bakken/Three Forks Petroleum System (Williston Basin)

AAPG Rocky Mountain Section Meeting

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The Importance of Detrital Dolomite in Upper Devonian Carbonates: Examples from the Dyer Formation (Northwest Colorado) and Bakken/Three Forks Petroleum System (Williston Basin)

Abstract

Detrital dolomite in the rock record generally consists of tiny fragments of dolomite less than 100 microns in size that have been transported by wind and/or water. Grains of detrital dolomite have been widely recognized as small crystal fragments in marine sandstones across the Rocky Mountain region, but similar grains can form relatively pure carbonate beds such as some of those in the Upper Devonian Dyer, Three Forks, and Bakken formations. Recognition of these detrital dolomites is based on sedimentary structures such as scours, ripple and small-scale hummocky crossbeds, injectites, and soft-sediment deformation features (microfaults, fluidized beds, flame structures, etc.) formed in the dolomite fragments. Other clues include grain size relationships with non-dolomite detrital grains (e.g., quartz silt), and petrographic textures that commonly include inclusion-rich (cloudy) abraded dolomite crystal fragments encased in clearer authigenic rhombic overgrowths. Recognizing detrital dolomite is important not only in creating an accurate depositional and diagenetic history, but also in understanding carbon and oxygen isotope data, which may be misleading if the reworked nature of the detrital dolomite grains is unrecognized. The Late Devonian in the Rocky Mountains was a particularly good time for forming widespread detrital dolomites because many of the subtle paleohighs surrounding the depositional basins were capped by older Paleozoic dolomites. Around the Williston Basin, these older dolomites occur in the Red River, Interlake, Winnipegosis, and Duperow formations. An arid climate led to common dust storms rich in silt-sized dolomite crystal fragments with the silt subsequently reworked in water. Similar detrital dolomite crystal fragments are also found in the black shales of the Bakken Formation. Once deposited and buried, these detrital dolomite crystal fragments were nuclei for syntaxial overgrowths, many of which are ferroan, and formed under reducing conditions during burial.