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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 706

Last Page: 721

Title: Offshore Bar Deposits of Semilla Sandstone Member of Mancos Shale (Upper Cretaceous), San Juan Basin, New Mexico

Author(s): Neal A. La Fon (2)


The Semilla Sandstone, which crops out on the eastern side of the San Juan basin, formed as several discrete offshore shallow-marine sandbars separated laterally by muddy interbar areas. Bar sequences consist of a lower fine-grained, silty sandstone that grades upward into a coarser grained, better sorted, trough-cross-bedded sandstone. The maximum thickness of the Semilla is 21 m. Bars are elongated parallel with the shore and in the direction of dip of cross-bedding. The bars were deposited approximately 50 to 80 km from the shoreline by storms, possibly augmented but not overshadowed by tidal currents. Sand deposition was probably localized on the ancient shelf at a slight steepening of slope of the sea floor. A tidal-current origin is unlikely for the Semilla Sandston and for most other described offshore bars.

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