The eastern part of the Lower Rio Puerco area is located in the Albuquerque-Belen basin, a subdivision of the Rio Grande depression, which in turn is an extension of the Basin and Range physiographic province. The western part is in the Colorado Plateaus.

The Albuquerque-Belen basin was formed in the late Tertiary by downwarping and downfaulting. Streams eroded the Carboniferous and Mesozoic sedimentary rocks and the early and middle Tertiary volcanic rocks and deposited the sediment in the depression. Contemporary vulcanism produced local basaltic lava flows. The basin deposit and the interbedded basalt constitute the Santa Fe formation. The basin deposit consists of a maximum of 4800 feet of alluvial-fan beds, axial river gravels, and playa beds. The proportion and detailed lithology of these types depended upon the rate of uplift and erosion of the highlands, the rate of subsidence and deposition in the basin, the lithology of the source rocks, and the climate. Study of the horizontal and vertical differences in lithology in the Santa Fe formation reveals the changing physiographic and dynamic relations of the Rio Grande depression and bordering highlands during the late Tertiary. These relations are shown on two paleogeographic maps.

The western border of the Albuquerque-Belen basin is marked by a complex fault zone which was established as a zone of weakness during the early Tertiary warping of the Colorado Plateaus. The late Tertiary Santa Fe basin deposit overlapped this border, but accelerated subsidence of the basin produced faults along the old fault zone, and the Santa Fe formation is now delimited on the west by late- and post-Santa Fe faults which have a maximum displacement of 6300 feet.

The main period of late Santa Fe and post-Santa Fe deformation was followed in the early Pleistocene by a period of crustal stability, during which time the extensive Ortiz erosion surface was developed on the deformed Santa Fe and older formations. This erosion surface was graded to an ancestral Rio Grande 400-500 feet above the present Rio Grande. A paleogeographic map was prepared with restored contours to show the drainage lines on this surface within the Albuquerque-Belen basin and the plateau contry to the west.

Renewed regional deformation, marked locally by small faults, initiated a major stream piracy and the general dissection of the extensive Ortiz surface. Subsequent erosion and regional changes in base level have produced from two to four post-Ortiz erosion surfaces along the major streams of the Lower Rio Puerco area, with gradients 250–300, 175–200; 100–150, and 50–75 feet respectively above those of the present streams.

Areas covered by maps in this paper are outlined. Stippled pattern shows Rio Grande depression.

  • 1. Physiographic provinces of north-central New Mexico (Fig. 2).

  • 2. Paleogeographic map for Ortiz time (Fig. 12).

  • 3. Fault pattern of western border of Albuquerque-Belen basin (Fig. 9).

  • 4. Paleogeographic maps for Santa Fe time (Figs. 4, 5).

  • 5. Lower Rio Puerco area (Plates 8, 10).

All the erosion surfaces in the Lower Rio Puerco area are covered and in part protected by stream gravel, caliche, wind-blown sand, travertine, or basaltic lava flows. The geomorphic map of the area shows the distribution of these different covers as well as the distribution of the remnants of the surfaces themselves.

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