Day 0- West road log: Sevilleta west: Pre-meeting road log from Sevilleta National Wildlife Refuge visitor center to San Acacia, Cerritos de las Minas, west Mesa, and San Lorenzo Spring
— R.M. Chamberlin, D.W. Love, B.J. Harrison, V.W. Lueth, B.A. Frey, S. Williams, and S.D. Connell

This pre-meeting field trip emphasizes Neogene landscape evolution and the four-dimensional complexity (spatial and temporal) of upper Cenozoic rift structures in the southwestern Sevilleta National Wildlife Refuge (Sevilleta NWR). Rift-related geomorphic anomalies in the southern Sevilleta NWR include well-exposed axial river beds at Loma Blanca (axial river beds are commonly mantled by younger piedmont slope deposits), the dome-like Cerritos de las Minas uplift, a well-developed pediment surface west of Cerritos de las Minas, and the topographically high spur of Riley travertine at stop 3, which was in a basin floor setting 3.2 my ago.

Stop 1 is on the barren, nearly planar pediment surface west of Cerritos de las Minas, which is cut across steeply west-dipping conglomeratic sandstones of the Miocene upper Popotosa Formation. An obvious question for Stop 1: Is this recently exhumed erosional surface an expression of the Socorro magma body? The answer is complicated by older structural patterns, such as the west-trending Las Minas arch of Miocene age, which is coincident with the pediment surface. Small eruptive centers of late Oligocene and middle Miocene age occur on the flanks of the older Las Minas arch, which supports the interpretation that magma may be rising under the modern pediment surface. Stop 2 is at an excellent exposure of the large displacement (2.9 km), low-angle, dip-slip Silver Creek fault. The exposure is located about 100 m south of where Alamillo road begins to rise onto the Silver Creek footwall, a north-striking hogback of steeply west-tilted Oligocene volcanic rocks. Stop 3 will be our lunch stop on the high, travertine-capped spur of West Mesa (as named by Sevilleta NWR), which provides a spectacular vista across the Rio Grande rift from the Colorado Plateau to the east-rift flank uplifts of the Sacramento Section of the Basin and Range Province. Stop 3 has all the elements needed for a classic “arm-waving stop”. Cenozoic landscape and structural evolution of the Sevilleta NWR will be summarized at Stop 3. Stop 4 will be a short hike, about 300 m uphill into a narrow early Miocene strike-valley basin (ca. 19 Ma) that was apparently buried in middle Miocene time (ca. 15-18 Ma) and later partly exhumed in Pliocene to Pleistocene time. Field relationships indicate that the monolithic andesite-clast conglomerate filling the shallow strike-valley basin was derived from the immediately adjacent footwall. Stop 5 will be at San Lorenzo Spring, a perennial spring formed where sandy, saturated arroyo-fill deposits wedge out eastward onto a bedrock sill formed by a hogback of Oligocene dacite lava. For structural geologists, Stop 5 provides a “hands on” look at en echelon sinistral shear veins in the dacite lava. The shear veins and surrounding map relationships support the interpretation that the spring lies along a NE-striking zone of sinistral shear associated with a southeast-down relay ramp of late Oligocene to early Miocene age. At Stop 5, “mother nature” has also provided well exposed vertical striations on nearly vertical fault surfaces in the dacite lava, just to test your understanding of Andersonian fault mechanics.

This field trip traverses four geologic maps of 7.5-minute quadrangles, namely San Acacia (Machette, 1978a), Lemitar (Chamberlin et al., 2001) Silver Creek (Cather and Read, 2003) and San Lorenzo Spring (Chamberlin, 2004). Some additional map data is from Chamberlin, unpublished reconnaissance mapping (stops 2 and 4).