Abstract
In the southeastern part of the San Juan basin, the transgressive La Ventana member of the Cliff House Sandstone represents a near-vertical buildup of barrier-beach sands during a period when sediment supply was in approximate balance with subsidence and shore-marginal environments migrated landward/seaward over a limited horizontal distance. Recognition of these conditions leads to speculation regarding a corresponding back-barrier accumulation of coal-bearing strata. Using logs from both the project-sponsored drill holes and commercial oil and gas tests, the workers have compiled an 8.5-mi stratigraphic cross section oriented normal to the depositional strike. This cross section illustrates the laterally changing relationships between the La Ventana and the upper Menefee. At the north (seaward) end, the interval is principally sandstone with thin tongues of coal-bearing shale and siltstone. Southward the barrier beach sand units split, thin, and give way to increasing numbers of coal-bearing intervals. Five major La Ventana sandstone tongues and a corresponding number of upper Menefee tongues alternate through an interval of about 650 ft. The number of individual coal beds ranges from 3 to 18, and thicknesses range from about 1 to 8 ft.
Published Version
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