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Stratigraphic study of the Turonian Semilla sandstone member of the Mancos shale, San Juan Basin, New Mexico, A
Kennedy, Scott H.
Kennedy, Scott H.
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2022
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2023-10-22
Abstract
Shallow marine Turonian-aged (Upper Cretaceous) sandstones were deposited within the tectonically active western margin of the Western Interior Sea: the Semilla Sandstone Member of the Mancos Shale in the San Juan Basin, the Codell Sandstone Member of the Carlile Shale in the Denver and Raton Basins, and the Turner Sandstone Member of the Carlile Shale in the Powder River Basin. The Semilla Sandstone Member is the least studied of the three, not only in terms of depositional environment, lithology, and reservoir character, but also economic potential. The Semilla is present in the southeast portion of the San Juan Basin, as well as two other locations to the east of the Basin. The member coarsens upward, starting from a siltstone at its transitional base to a medium grained sandstone with a variety of sedimentary structures preserved. Bioturbation is present throughout. Stratigraphically, the Semilla is similar to the Codell Sandstone. Based on petrographic analysis, textural and compositional maturity are low. High levels of matrix clay, as well as feldspars, chert and accessory heavy minerals are characteristic of the Semilla. The accessory mineral assemblage suggests that that the sediment was sourced primarily from the felsic basement rocks in the Mogollon Highlands to the southwest. This differentiates the Semilla from the texturally and compositionally more mature Codell Sandstone, which suggests either a difference in proximity to sediment source or difference in provenance. Both in outcrop and in subsurface, the Semilla is lenticular and forms northwest-southeast trending sandstone packages. For these reasons, the Semilla is interpreted to have been deposited in the shallow marine waters of the Western Interior Seaway as linear shelf sands, parallel to the paleoshoreline. Storm currents and waves were the dominant processes affecting the Semilla, though some tidal influence is present.
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