New Mexico Geological Society Annual Spring Meeting — Abstracts

We present stratigraphic and geomorphic evidence to refine and update the stratigraphy of post-Santa Fe Group deposits in the Albuquerque Basin. Terrace deposits of the Los Duranes, Edith, and Menaul formations were previously defined by Lambert (1968, Univ. of New Mexico, Ph.D. dissertation, 329p.) in the Albuquerque area. New geologic mapping delineates five terrace deposits along the Rio Grande Valley between San Felipe Pueblo and Los Lunas, New Mexico. These quartzite-dominated, rounded pebbly to cobbly sand deposits can generally be differentiated on the basis of landscape-topographic position, inset relationships, soil morphology, and height of the basal contact (above the Rio Grande) as detennined from outcrop and drillhole data. We informally adopt three additional stratigraphic terms to clarify and extend Lambert's terrace stratigraphy. The highest and presumably oldest preserved Rio Grande terrace is informally called the Lomatas Negras alluvium (Qrl). Unit Qrl is typically <20 m thick. The top is generally eroded and projects 65-75 m above the Rio Grande. The Edith formation (Qre) is a 3-12 m thick and longitudinally extensive marker along the eastern margin of the Rio Grande Valley, between Albuquerque and San Felipe Pueblo. The base of Qre is about 12-24 m above the Rio Grande, contains Rancholabrean fossils, and is likely inset against QrI. Soils developed on gravelly to sandy piedmont deposits overlying Qre exhibit Stage III+ carbonate morphology, suggesting a middle Pleistocene age for Qre. The Los Duranes formation (Qrd) is up to 52 m thick, overlain by the 98-110 ka Cat Hills basalt (Maldonado et aI., 1999, NMGS Guidebook 50, p.175-188), and locally buries flows of the 156±20 ka (Peate et ai., 1996, Geophys. Jour. Int., v.23, n.17, p.2271-2274) Albuquerque volcanoes basalt. Thus deposition ofQrd ended between ca. 160-100 ka. The terrace tread on top of unit Qrd (~42-48 m above the Rio Grande) is about 12-32 m higher than the top of Qre. Geologic mapping and comparison of subsurface data indicate that the base of Qre is about 20-25 m higher than the base of Qrd, suggesting that Qrd is inset against Qre. Unit Qrd represents a major aggradational episode that may have locally buried Qre; however, Qre could possibly mark the base of an aggrading Qrd. The Menaul formation (Qrm) is a thin lens of fluvial gravel that overlies piedmont deposits and Qre. The base of unit Qrm is about 23-41 m above the Rio Grande. Soils on piedmont deposits overlying Qrm are generally similar to Qrd; however, differences in parent material texture make soil-based correlations somewhat ambiguous. Similarities in height (above the Rio Grande) and soil development on Qrd and Qrm suggest that these two units may be correlative and are probably associated with the same aggradational episode; however, definitive crosscutting relationships have not been demonstrated. The lowest preserved terrace deposit is the Arenal alluvium (Qra), which is 3-6 m thick. The top of Qra is about 15-21 m above the Rio Grande, contains weakly developed soils with Stage I to II+ carbonate morphology, and is interpreted to be inset against Qre, rather than correlative to Qre as originally interpreted by Lambert. This terrace was probably deposited during late Pleistocene time (ca. 71-28 ka). The inner valley floodplain (Qrp, Los Padillas alluvium), which contains up to 25 m of sand and gravel with minor mud and clay lenses that was probably deposited during latest Pleistocene through Holocene time (ca. 20-4 ka).