The structural and depositional context and a new age estimate of the type Sierra Ladrones Formation in the southern Albuquerque Basin and vicinity, central New Mexico

The type Sierra Ladrones Formation is exposed between two north-trending, Quaternary normal faults and crossed by the east-flowing Rio Salado, on the southwest side of the Albuquerque Basin and the northwestern side of the Socorro Basin of the Rio Grande rift in central New Mexico. This article summarizes recent work on the two sedimentary facies of the type Sierra Ladrones Formation— fluvial facies of the ancestral Rio Grande and the piedmont alluvial facies interfingering with and overlying it. We obtained a 40Ar/39Ar age spectrum for detrital sanidines, indicating a maximum depositional age of 5.36 Ma in the lowest exposures of fluvial facies near the type area. Outcrops of the Sierra Ladrones Formation fluvial facies are bracketed by the Loma Pelada (west) and Loma Blanca (east) faults; these continue across the boundary between the southwestern Albuquerque Basin and northern Socorro Basin. The fluvial facies was transported to the south into the Socorro Basin, whereas the piedmont alluvial facies was transported north-northeast between the faults and on the Loma Blanca hanging wall. The apparent source of most of the piedmont alluvial facies south of the Rio Salado is the uplift of the Lemitar Mountains, its northern extension, and the Cerritos de las Minas. North of the Rio Salado, the piedmont alluvium has contributions from other sources, the Ladron Mountains and an ancestral Rio Salado. Uplift of Loma Blanca on the boundary between the two structural basins apparently has eroded all the overlying piedmont deposits seen to the north and south, whereas a >85 m thick section of piedmont deposits is preserved on the hanging wall of the Loma Blanca fault, with a vertical offset of at least 155 m. The piedmont alluvium on the hanging wall of the Loma Blanca fault within a kilometer of the fault contains no debris from the fluvial facies of the footwall. Two to three km east in the hanging wall, and down-section in piedmont and basin floor facies, pebble conglomerates exhibit paleo-transport ranging from northeast to southwest, suggesting a distributary system spread from a source to the northwest and north. Apparently the system was not influenced by a possibly uplifting basin divide to the south. Although basin-fill outcrops on both sides of the Cliff fault in the southern Albuquerque Basin were included in the alluvial piedmont facies of the Sierra Ladrones Formation, recent work demonstrates that these deposits were deposited by the ancestral Rio Puerco (Ceja Formation) and Rio Salado drainages. The two drainages flowed southeast to join the ancestral Rio Grande, and perhaps their distal distributary system pushed the course of the Rio Grande eastward against the Joyita Hills uplift by ca. 3 Ma. The Ceja Formation crops out on the west side of the Rio Puerco Valley along the sides of Cañada Colorado and east-flowing drainages to the south. The Rio Puerco could only have advanced into this area after the axial river had abandoned the type Sierra Ladrones area. Piedmont alluvium similar to the Sierra Ladrones alluvial facies overlies these Rio Puerco pebbly sands of the Ceja Formation on both sides of the Loma Blanca fault, and both were transported to the southeast. 239 New Mexico Geological Society Guidebook, 72nd Fall Field Conference, 2022, p. 239–251.