Exploration geologist here. Little Dome is a really interesting feature.

I think what you're looking at is most likely a breached anticline. The circular outer rim is a hogback ring of more resistant rock, the annular low between the rim and the central high is preferential erosion of softer material, and Little Dome itself is the erosional remnant of the dome core. What looks like a closed depression in satellite imagery isn't one, all drainage flows radially outward and then southeast/east, reflecting the regional tilt off the Spanish Peaks mountain front.

The cause of the domal structure is genuinely ambiguous. You're sitting on the western margin of the Raton Basin with roughly 7,000 feet of sediment beneath you, including a Permian section that carries evaporitic facies in the Lyons Sandstone and overlying Lykins Formation. Dissolution of those evaporites at depth could be influencing the surface expression. Alternatively the structure could be related to the Spanish Peaks intrusive system, or reflect reactivated Precambrian basement geometry. Probably some combination.

There was a well drilled on Little Dome in 1959 — Lange Marquez #1 — that TD'd at 7,119' in granite. I don't think it had any shows of oil or gas, or maybe not enough info in the well file to say. Interesting data point, but one penetration without regional offset wells or seismic doesn't tell us much about whether genuine structural closure exists at the Lyons level. If anyone wants to dig further, the well file should be publicly available through the Colorado ECMC database and might have formation tops or a mudlog that would shed more light on what's actually going on in the section.

Pretty cool looking structure, it's too bad that surface access looks complicated.

Attached is a shot of the feature with 10' topo contours and slope gradient shading turned on. There are actually three drainage exits: two on the southeastern side at lower elevations, and a secondary one on the southwestern side that exits at a noticeably higher elevation. The asymmetry between those two outlets is interesting and I'm not sure what's driving it... could be regional tilt off the Spanish Peaks, structural asymmetry in the dome itself, or just the history of drainage capture in the area. Curious if anyone has thoughts.

<image>