Several million-year-old, alluvial fan deposits near Hillsboro

Becca Goughnour, Kyle Gallant, and myself (all from the New Mexico Bureau of Geology) are currently conducting geologic mapping in the Hillsboro area. This postcard shares photographs from this mapping effort that relate to a topic near and dear to my heart: the Santa Fe Group. The Santa Fe Group refers to siliciclastic-dominated sediment that fills basins of the Rio Grande rift. In the area we are working, the mapped Santa Fe Group lies below the Plio-Pleistocene Palomas Formation and is Miocene in age (spanning ~20 to 5 million years ago). It is relatively coarse because the study area lies at the foot of the Black Range (coinciding with the western margin of the Palomas Basin). Strata are moderately cemented, conglomeratic and tend to be in 10–50 cm-thick, tabular to lenticular beds, as shown in the exposure of Figure 1. Cross-stratification and deep channel forms are rare. These sedimentologic features, the location on the basin margin, and paleoflow data are consistent with deposition on an east-sloping alluvial fan. Alluvial fans arelarge, semi-circular landforms formed when stream water spreads out at the base of a mountain range after being confined within mountain canyons, dropping its sediment
load in the process. Figure 2 shows the pinkish Santa Fe Group underlying a brown,even coarser deposit about 6-8 ft thick; this brown deposit corresponds to a Pleistocene terrace deposit, formed when the landscape was eroding rather than aggrading like it did earlier in Santa Fe Group time.