New Mexico Geological Society Annual Spring Meeting — Abstracts

Internal structure of the northern Albuquerque Basin is characterized by a deep, asymmetrical (half-graben) basin that is tilted eastward and flanked by relatively shallow structural benches (central North Graben block and North Albuquerque and Laguna benches, to the east and west, respectively) that step up to bordering uplifts (Sandia to the east and Colorado Plateau to the west). Structural interpretations based on seismic surveys and deep test drilling throughout the basin indicate that most of the major basin-bounding and intrabasin faults have curved surfaces of normal displacement that flatten markedly with depth (listric-fault geometry) and appear to "detach at the base of the brittle crust" (Russell and Snelson, 1994, GSA Special Paper 291, p. 105). Dips of these "master" faults are as low as 15 to 20° at depths of 30,000 to 35,000 ft (about 10 km) below sea level.

The primary master-fault component is designated the Rio Grande fault (May and Russell, 1994, GSA Special Paper 291, pp. 113-123), which has more than 30,000 ft (10 km) of vertical offset. Analyses of geophysical and sample logs from deep water wells (up to 1000 m) in the east Albuquerque area indicate that this fault approaches the surface along a narrow zone under the east edge of the Rio Grande Valley near 1-25. A secondary (Sandia) component of the master fault system continues eastward and approaches the modem land surface just west of the Sandia Mountain front near Tramway Blvd.; however, nelmer component appears to have been active in the late Quaternary. On a basinwide scale, the updip segments of the hanging-wall block (Laguna bench, west of the Albuquerque Volcanoes) has a hinged (inner-basin) margin that flexes down to the east and is cut by relatively steep and shallowly penetrating faults. As the basin pulled apart, clockwise (eastward) rotation of the hanging-wall block produced the very deep and complexly faulted North Graben block. The Albuquerque structural bench forms the footwall block of the Rio Grande (primary) master fault and the hanging-wall of the Sandia (secondary) master fault. This bench tilted slightly eastward during the latest Miocene to late Pliocene interval (7-2 Ma?), rotating clockwise away from the deep-basin axis. Tectonic unloading of the Sandia Mountain (footwall) block along the eastern basin margin probably started in the middle Miocene and is reflected in the high topographic relief of that "rift shoulder uplift" (May et al., 1994, GSA Special Paper 291, pp. 125-134).

The role that geologic structure (basin tectonism) has played in controlling the position of the ancestral Rio Grande is clearly illustrated in the four basin-wide hydrogeologic cross sections that are displayed in this poster session. Stratigraphic and structural interpretations are based on analyses of geophysical logs, samples, and other subsurface data from 35 deep wells drilled in the area (Hawley and Haase, 1992; and Haneberg and Hawley, 1995, NMBMMR OF 387 and OF 402). The asymmetrical half-graben morphology described in the Albuquerque Basin is characteristic of most basins of the Rio Grande rift (Mack and Seager, 1990, GSA Bulletin, v. 102, pp. 45-53; Cather et al., 1994, GSA Special Paper 291, pp. 157-170). This style of large-scale structural deformation directly influences the distribution patterns of the major environments of deposition observed in basin-fill sequences (e.g. piedmont-slope alluvial, and basin-floor playa-lake or fluvial braid-plain deposits). The thickest documented sections of ancestral Rio Grande facies of the upper Santa Fe Group are preserved as a stacked sequence of braided river-channel deposits that is as much as 300 m thick and three miles (5 km) in width beneath the eastern "Heights" of metropolitan Albuquerque. The observed eastern limit of the fluvial facies throughout the northeastern Albuquerque Basin is only about 3 mi west of the Sandia Mountain front. This sequence, which ranges in age from about 1 to 5 Ma, is characterized by extensive beds of sand and pebble gravel and relatively small amounts of silt and clay. Clasts are primarily derived from upstream (northern NM) source areas.

Active eastward rotation of the (hanging-wall) "North Graben and Albuquerque Bench" blocks along the Rio Grande-Sandia master fault zone positioned the ancient fluvial system near the eastern edge of the half-graben and above the western margin of the Sandia Mountain (footwall) block during much of Pliocene time (2-5 Ma?). The uppermost 200 to 300 ft (60 to 100 m) of the 1000 ft (300 m) stacked-channel sequence contains pumiceous fragments of volcanic rocks derived from late stage eruptions of the Jemez Caldera that culminated with the catastrophic emplacement of the Bandelier Tuff about 1.1 and 1.6 Ma ago (Goff et aL, 1989, NMBMMR Memoir 46, pp. 381-434). During that interval, the ancestral Rio Grande rapidly shifted westward toward the position of its present valley; and, concurrently, the apron of coalescent-fan deposits forming the Sandia piedmont slope prograded westward across the old fluvial plain. This sequence marks the final stages of basin flliing (Santa Fe Gp deposition), and is almost identical to the sequences described in the southern Rio Grande rift by Mack and Seager (1990). Blair and Bilodeau (1988, Geology, v. 16, pp. 517-520) observe this depositional style along the episodically active margins of asymmetrical basins in several structural settings. They proppse that initial or renewed tectonic activity maintains the fluvial (axial stream) depositional environment in the deepest half-graben zone adjacent to the footwall (e.g. Sandia) block. Rapid basinward shift in the locus fluvial deposition and progradation of the piedmont alluvial apron over the former basin-floor surface only occurs "during tectonically quiescent phases" when mountainward rotation of the hanging-wall block has essentially ceased.