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Offshore Equivalents of Upper Cretaceous Gallup Beach Sandstones, Northwestern New Mexico
In northwestern New Mexico’s San Juan Basin, shelf siltstone and mudstone enclose offshore-bar sandstones. Successively younger offshore bars appear landward at progressively higher stratigraphic levels and eventually transgress older beach sandstones. These relationships are based principally on data from 21 core holes with supplementary conventional well data and are illustrated on a cross section extending 15 miles northeast from outcropping Gallup sandstone.
The offshore bars are elongated approximately parallel with the distant beach shoreline, have no contiguous landward equivalents but grade seaward into shelf siltstone and mudstone, and are composed of parallel beds that are imbricated both seaward and downcurrent. Except where churned by burrowing organisms, the beds internally consist of high-angle cross laminae that dip parallel with the length of the bar.
The offshore bars presumably formed on a shelf at a break in slope more than 12 miles seaward from the laterally equivalent beaches. The water depth was apparently 200 feet or more, and the depositing currents flowed approximately parallel with the distant shoreline. No modern counterparts of this type of sandstone body are known to the writer.
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