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SEAN D. CONNELL
New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801
DAVID W. LOVE
New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801
FLORIAN MALDONADO
U.S. Geological Survey, Box 25046, MS 913, Denver Federal Center, Denver, CO 80225
PATRICIA B. JACKSON-PAUL
New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801
WILLIAM C. MCINTOSH
New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801
MARTHA C. EPPES
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131

The Santa Fe Group (SFG) constitutes the synrift basin fill of the Albuquerque Basin, New Mexico. Conceptually, SFG deposition is thought to have ceased when the Rio Grande and its tributaries incised into a surface of maximum basin aggradation during early Pleistocene time. We suggest that the top of the uppermost SFG in the Albuquerque Basin is diachronous in nature, and that termination of SFG deposition may be recorded on a number of constructional surfaces of different ages. If our suggestion is valid, then differentiation of the SFG from younger deposits require a better understanding of the temporal and spatial distribution of such surfaces. We interpret that the temporal and spatial distributions of such surfaces are strongly influenced by major intrabasinal faults. Deposits that post-date SFG aggradation may be distinguishable by their position in the landscape relative to known geomorphic surfaces; some young deposits are inset, others may prograde across older surfaces. We discuss new data from four such inferred terminal SFG geomorphic surfaces in the vicinity of Albuquerque and the Isleta Indian Reservation.

The Sunport geomorphic surface (SP) is about 90-115 m above the Rio Grande. Soils on the SP generally exhibit Stage III to IV carbonate morphology and are commonly developed on eolian deposits that overlie ancestral Rio Grande fluvial deposits containing early Pleistocene fossils, transported (upper and lower) Bandelier Tuff clasts, and a 1.2 Ma ash (HC-C, PL, Fig. 1). Projections of the highest (oldest) known (middle Pleistocene) ancestral Rio Grande terrace are about 20-30 m below the level of the SP. Terrace treads to the north, at similar heights above local base level in the Rio Grande, Rio Chama, and Rio Jemez valleys, locally overlie the 0.6 Ma Lava Creek B ash, as do terrace remnants to the west and south along the Rio Puerco and Rio Grande valleys. This apparently widespread inset terrace relationship suggests that the Rio Grande and its tributaries had begun to incise into the upper SFG between 1.2 and 0.6 Ma.

The Llano de Manzano (LdM) geomorphic-surface complex contains at least three locally distinguishable surfaces underlain by Manzano-Mountains-derived piedmont deposits that prograde over Bandelier-pumice-bearing ancestral Rio Grande deposits, and are thus interpreted to be younger than the SP. Soils developed on the LdM generally exhibit Stage III+ to IV carbonate morphology. Intermediate-height constructional piedmont-slope surfaces are locally preserved across the Hubbell Springs fault zone (HC-U, HC-C, Fig. 1). With the notable exception of major tributaries, such as Tijeras Arroyo, Hell Canyon Wash, and Abo Arroyo, nearly all smaller drainages on the Manzano Mountains do not incise the SFG. Instead, sediments from these drainages commonly bury SFG deposits and do not record long-term incision by the Rio Grande. We infer that aggradation and episodic soil formation continued locally on the LdM into the middle Pleistocene. We also infer that aggradation was controlled by the LdM, which acted as a regional base level for most streams draining the Manzano Mountains. We further infer that aggradation was locally influenced by episodic movement along neighboring intrabasinal faults.

The Cañada Colorada geomorphic surface (CC) is recognized only on the Hubbell bench (HB-S, Fig. 1), where it forms a high (about 10-35 m above Cañada Colorada, about 30-50 m above local LdM surface) piedmont-slope underlain by very thick (2-3 m) calcic soils exhibiting Stage III+ to degraded Stage V(?) carbonate morphologic development. Deposits underlying the CC are well cemented and unconformably overlie Permo-Triassic rocks. Although no age control is known for the CC, the deeply embayed nature (sinuosity ratio of ~3.7) of drainages crossing the Hubbell Spring fault zone and very strong soil development suggest that this local top of SFG deposition may have stabilized prior to development of the LdM, perhaps during Pliocene, or possibly late Miocene time.

The Llano de Albuquerque geomorphic-surface complex (LdA) is a gently south-sloping mesa about 110-215 m above the Rio Grande and is interpreted here to comprise one of the oldest constructional surfaces in the Albuquerque Basin. Soils on the LdA generally exhibit Stage III+ to IV carbonate morphology and are commonly developed in eolian sediments. The 3.0 Ma Cat Mesa basalt is overlain by 19 m of ancestral Rio Puerco sediments that predate the LdA (CM, Fig. 1). Pliocene fossils are reported at similar stratigraphic intervals beneath the LdA. The Tshirege ash (Bachman and Mehnert, 1978, p. 288, GSA Bulletin, v. 89, n. 2, p. 283-292) is in alluvial deposits, derived from the 1.2 Ma Los Lunas volcano, that overlie a moderately developed (Stage III) calcic soil formed in ancestral Rio Puerco deposits. This buried soil is interpreted to represent a buried correlative of the LdA soil. Thus, the LdA and underlying ancestral Rio Puerco deposits are older than 1.2 Ma in the study area, and may possibly be pre-Quaternary in age. A pre-1.2 Ma age of the LdA is supported by the exposures of Bandelier Tuff-bearing ancestral Rio Grande sediments that overlie Rio-Puerco-derived strata in Tijeras Arroyo.

Earthquake recurrence intervals for faults cutting these four geomorphic surfaces have been based on estimated ages of calcic soil development. Our data suggest that these surfaces are much older than previous age estimates. Thus, intrabasinal faults cutting these surfaces may possibly be less active than previously thought. For example, the LdA was estimated to be ~500 ka (Machette, 1985, GSA Special Paper 203, p. 1-21). We demonstrate in the Los Lunas area, it is older than 1.2 Ma. The age of the LdM and SP were estimated to be ~320 ka, but could be 2-3 times as old, at least locally. The relatively weak degree of soil development for Plio-Pleistocene surfaces such as the LdA may be due to periodic deposition and stripping of the rather extensive overlying eolian sand cover.

 

Figure 1. Schematic stratigraphic sections and elevations of geomorphic surfaces (in meters above mean sea level) at selected measured sections, illustrating topographic, stratigraphic, soil-morphologic, and age relationships across the Albuquerque Basin on the Isleta Reservation. Roman numerals indicate pedogenic carbonate morphologic stage. Approximate stratigraphic section locations are recorded in UTM (NAD 83) coordinates. Fault locations and offsets are shown schematically.

Connell, S.D., Love, D.W., Maldonado, F., Jackson, P.B., McIntosh, W.C., and Eppes, M.C., 2000, Is the top of the Santa Fe Group diachronous in the Albuquerque Basin? [abstract]: U.S. Geological Survey, Open-File Report, in Cole, J.C., ed., U.S. Geological Survey Middle Rio Grande Basin Study -- Proceedings of the Fourth Annual Workshop, Albuquerque, New Mexico, February 15-16, 2000: U.S. Geological Survey Open-File Report 00-488.

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