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The depositional environment of units within the Dakota formation of the San Juan basin was determined by means of fossils, sedimentary structures, and facies relationships. Grain-size distribution, heavy-mineral content, and clay-mineral content were found to correlate well with the depositional environment in a manner similar to observations reported from modern sediments. Many depositional environments are represented in the Dakota formation, which grades from lenticular, non-marine sandstone and carbonaceous shale in the northwestern San Juan basin to regularly interstratified shale and very fine-grained sandstone in the southeastern San Juan basin.
Dakota fluvial sandstones are fine-skewed and are generally poorly sorted. Dakota beach sandstones are coarse-skewed and well sorted, while Dakota offshore marine sandstones are fine-skewed and are generally moderately well sorted. The finer particles in Dakota offshore sandstones were winnowed from beaches and were deposited offshore. Dakota fluvial sandstones characteristically are fine-skewed. The finer particles in these fluvial sandstones were not winnowed and the competence of stream flow sets an upper limit to the size of particles transported.
In a vertical series of samples from Dakota sandstones grading from nonmarine deposits below to marine deposits above, the degree of sorting increases upward into the lower part of the lowest marine sandstone (beach deposit) and gradually decreases in offshore sandstones above the beach sandstone. The change in skewness from coarse in the beach sandstone to fine in the fluvial and offshore marine sandstones is abrupt.
Dakota non-marine shales contain much kaolinite with some illite. The amount of carbonaceous matter in the shales is inversely related to the amount of illite. Dakota marine shales contain much montmorillonite in addition to kaolinite and illite.
Dakota sandstones derived from source areas north and west of the San Juan basin have a small, stable suite of heavy minerals in which zircon and tourmaline predominate. Most of these mineralogically simple sandstones are non-marine. Dakota marine sandstones, such as the Tres Hermanos and Twowells members, which were derived from source areas south of the San Juan basin, contain a suite of metamorphic minerals in addition to zircon and tourmaline.
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