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Terraces in Milligan Gulch

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Dan Koning

Socorro County, NM
— February 22, 2021

"I have been working along Milligan Gulch with Kevin Hobbs and Kristin Pearthree (both of the NM Bureau of Geology) to make a geologic map of the Fort Craig quadrangle. Milligan Gulch is the canyon immediately south of the dirt road to Fort Craig, located about 30 miles south of Socorro. This canyon has a fantastic set of terraces, which can be correlated to terraces flanking the west side of the Rio Grande. This photograph shows a gravelly middle to late Pleistocene terrace deposit (100,000-500,000 years old) at the top of the exposure. Below is tabular-bedded, orangish tan sediment composed mainly of very fine to fine sand with minor coarser sand and 3-10% scattered pebbles (1.4 million years old). These different-aged deposits were both deposited by ephemeral stream-flow of Milligan Gulch, but the stream in the middle-late Pleistocene obviously could carry much coarser sediment. This change in apparent stream power could be attributed to differing paleoclimate regimes producing different stream discharges. Or maybe it is due to the fact that Milligan Gulch was entrenched in a canyon in the middle-late Pleistocene compared to 1.4 million years ago, when it was depositing a distal-alluvial fan at this location. On alluvial fans, channels tend to bifurcate and become more shallow downstream, and shallower flowing water can't carry sediment as coarse as that in deeper water (assuming flow velocities are the same). The white stick (actually a wooden ruler) in the center of the photo is 2 m long. The deep-orange layers above and at the top of the ruler are clay-enriched horizons of paleosols, which are common in the mapped distal alluvial fan deposits. Note how the paleosols are more erodible compared to the sandier sediment, causing the former to be in recessed layers."

— Dan Koning, Field Geologist, NMBGMR