Extensive outcrops and 14 measured sections of the Upper Jurassic Entrada Sandstone and overlying Todilto Formation in the southern Chama basin Echo Amphitheater-Ghost Ranch area show long, parallel, eolian transverse dune buildups in the upper Entrada perpendicular to the dominant wind direction as shown by cross-bed foresets. Some bevelling of the buildups occurred prior to the evaporitic deposition of limestone and gypsum of the Todilto Formation and this deposition preserved the buildups, filling in the low interdune areas.

The subsurface Entrada-Todilto of the southeastern San Juan basin was mapped over an area including 20 townships. An isopach of the Todilto Formation shows that essentially the same trend of Entrada dune buildups is present here as in the outcrop area.

Support for the mechanism of trapping in Entrada oil fields as proposed earlier by Bryant is present both in the surface and subsurface where relief created by eolian buildup of sand is preserved by the infilling of the Todilto. However, structural dip counter to regional dip along the axis of dune buildups is needed to trap oil. This dip can be provided by syndepositional faults cutting across the buildup axis as in the outcrop area where there is sudden thickening of the Todilto and of the Entrada downthrown to the fault.

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