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Geomorphology and stratigraphy of inset fluvial deposits along the Rio Grande valley in the central Albuquerque Basin, New Mexico Sean D. Connell, New Mexico Bureau of Geology and Mineral Resources, Albuquerque Office, New Mexico Institute of Mining and Technology, 2808 Central Ave. SE, Albuquerque, NM 87106, [email protected]; David W. Love, and Nelia W. Dunbar, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 Abstract incision of the Rio Grande valley in the Albu- Connell et al. 2005) that formed when the querque area. river began cutting into underlying basin Five inset levels of Pleistocene fluvial depos- The (middle Pleistocene) Edith Formation its indicating former and present posi- fill of the Ceja and Sierra Ladrones Forma- appears to be the second oldest inset deposit. tions (upper Santa Fe Group). Incised val- tions of the Rio Grande are differentiated The (middle Pleistocene) Los Duranes For- between San Felipe Pueblo and Los Lunas, mation aggraded during the eruption of the leys tend to leave a record of previous river New Mexico. All have coarse-grained, well- Albuquerque Volcanoes (156 ka) and had positions in a suite of stepped terrace land- rounded cobble-gravel bases overlain by ceased by the time of the Cat Hills lava flow forms and deposits preserved along the varying amounts of finer-grained and more (98–110 ka). The Arenal Formation is inset borders of the valley (Gile et al. 1981). An poorly sorted sediments. This paper formal- into Los Duranes Formation and is late Pleis- understanding of the timing and location of izes (with modification) three fluvial depos- tocene in age. Los Padillas Formation des- valley entrenchment depends on an assess- its informally proposed by P. W. Lambert in ignates the 15–25 m of deposits beneath the ment of evidence of how the river aggrad- 1968: Los Duranes Formation, Menaul Mem- inner valley floor and had reached nearly its ber, and Edith Formation. This paper also ed and first began to incise. Preservation current upper level by late Holocene time. The of sedimentary facies within fluvial and introduces three new stratigraphic terms to preserved sequence of deposits suggests that complete the succession of fluvial deposits substantial shifts in stream power, sediment alluvial deposits is critical to determining mapped in the Albuquerque area: the Loma- supply, and climate are responsible for each related fluvial conditions. Later episodes tas Negras Formation, Arenal Formation, and of the fluvial deposits associated with former of valley entrenchment and partial backfill- Los Padillas Formation. A fluvially recycled and current positions of the Rio Grande. ing obscure or obliterate evidence of earlier volcanic ash, exposed in the oldest and high- episodes of terrace formation. est inset deposit, the Lomatas Negras Forma- During late Pliocene time, the ancestral tion, is geochemically indistinguishable from Introduction the middle Pleistocene Lava Creek B tephra Rio Grande formed an axial river that flow- (Yellowstone caldera; 640 ka). This ash places The Rio Grande valley is an incised river ed within a few kilometers of the western the youngest age limit on the initiation of valley (Gile et al. 1981; Hawley et al. 1995; front of the Sandia Mountains, its course FIGURE 1—Schematic east-west cross section (vertical exaggeration = 5) of the piedmont member (QTsp) from the east and the Ceja Formation (Tca, the Albuquerque area illustrating the difference between the stepped ter- Tcg) from the west. Note the faulted half-graben structure with tilted thick races and fill of the Rio Grande valley and the broader interfingering of the basin fill on hanging-wall blocks. This diagram is simplified from a cross ancestral Rio Grande facies of the Sierra Ladrones Formation (QTsa) with section with well control by Connell (2006). Length of section is 46 km. February 2007, Volume 29, Number 1 NEW MEXICO GEOLOGY 13 FIGURE 2—Schematic cross section of a terrace and underlying types of sediments illustrating geomor- phic and sedimentologic terms discussed in this article. controlled in part by north-south faults Constructional terrace treads are locally sides of stream valleys or along the insides along the eastern margin of half grabens well preserved on younger terrace deposits of meander bends have been interpreted to within the Albuquerque Basin (Fig. 1; but tend not to be well preserved on older indicate that the stream has steadily cut its Hawley et al. 1995; Connell and Wells deposits. Many older terrace deposits are valley over time, leaving terraced depos- 1999; Maldonado et al. 1999; Connell 2004). locally overlain by tributary alluvium. its behind at different elevations. Paired The ancestral Rio Grande deposits (QTsa; Our purpose here is to describe and for- terraces are found at the same elevation Fig. 1) interfinger with fluvial fan depos- mally define the inset terraces and asso- on oppposite sides of a stream valley and its from the northwest (Tca and Tcg) and ciated fluvial and alluvial deposits of the commonly have some down-valley (longi- with piedmont deposits from the east Rio Grande valley in the Albuquerque tudinal) continuity. The presence of paired (QTsp). By early Pleistocene time, between area. First we define some ambiguously terraces implies that the stream stopped about 1.2 and 0.7 Ma (Connell et al. 2005), used terms. The geomorphic forms along cutting, planed laterally or aggraded in the Rio Grande began to entrench into the valley margins and their underlying response to external forcing such as chang- the basin fill just west of the modern val- deposits raise conceptual questions about es in discharge, sediment supply, climate ley. Tributary stream deposits (Qa; Fig. 1) the applications and limitations of certain change, vegetation, base level rise, or tec- prograded across much of the piedmont geomorphic terms such as terrace, geomor- tonic factors. After a change in hydraulic slope of the Sandia Mountains and buried phic surfaces, morphostratigraphic units, conditions, the stream may cut below its the older ancestral Rio Grande deposits alloformations, and landscape sediment planed level to a lower level. Zonneveld of the Sierra Ladrones Formation by early assemblages. An outline of the history of (1975) distinguishes between paired ero- Pleistocene time. During middle and late previous investigations follows the defini- sion (strath) terraces, paired accumulation Pleistocene time, the Rio Grande episodi- tions and is followed by a brief description (fill) terraces, overslagh (pass-over) ter- cally entrenched at least four times into of methods. races, and combination terraces. Bull (1991) older fluvial and alluvial deposits and distinguishes between fill terraces, fill-cut Santa Fe Group basin fill. These episodes Definitions terraces, and strath terraces, which have of entrenchment were followed by periods The term terrace is commonly applied to differing climatic and/or tectonic implica- of partial backfilling of the valley (terrace features along the margins of stream val- tions. deposits of Qr) and progradation of valley- leys. According to the AGI Glossary of The basic definition of a terrace raises border deposits. The latest episode of cut- Geology (Jackson 1997) and Leopold et al. practical questions concerning limits to ting and partial backfilling occurred during (1964) terraces are long, relatively narrow, what is or is not considered to be a terrace or latest Pleistocene and Holocene time. We gently inclined surfaces bounded on one lat- terrace alluvial deposit. We adopt the term focus on the geomorphology and stratigra- eral edge by a steeper ascending slope and terrace deposits for the genetically related phy of these inset deposits. along the other by a steeper descending sediments between the terrace tread and its Well-rounded pebble and cobble gravel slope. They are valley-contained, are narrow- alluvial base. We differ from Leopold et al. beneath each terrace level consists of sili- er than a plain, and form a step-like bench (1964) because these alluvial deposits are ceous rocks (abundant orthoquartzite, breaking the continuity in slope of a valley clearly between the basal straths and con- minor chert), subordinate volcanic rocks, margin above the level of a body of water structional treads of the terraces. and sparse plutonic and metamorphic rocks (e.g., a river). The form of a terrace includes Terrace and terrace deposits have: 1) derived from northern New Mexico (Lam- the lower (stream-facing) slope, called the well-defined lateral limits within a valley, bert 1968). Pebbles within the gravels are riser, and the low-gradient upper surface 2) thickness limits related to cut-fill epi- commonly imbricated and dip to the north- called the tread (Fig. 2). A terrace indicates sodes, 3) discharge limits associated with west and northeast, indicating a south- a former water level, but the term applies stream hydrology, 4) climatic limits related erly paleoflow direction (Lambert 1968; strictly to a geomorphic form and should to regional or global climate cycles, and Brandes 2002). Although the composition not be applied to the deposits underlying 5) time limits. Lateral limits are implied of gravel is similar among terrace depos- the form; the deposits beneath are called as the definition states “narrower than a its (and axial-fluvial deposits of the Sierra alluvial fill or alluvial deposits (Leopold et plain.” The width of the tread depends on Ladrones Formation), each deposit repre- al. 1964). the paleohydrology of the stream or river. sents a distinct and mappable unit that may The basic concept of terraces and associ- The formation of a terrace tread (associated be differentiated on the basis of landscape- ated alluvial deposits leads to further defi- with incised river valleys) implies incision topographic position, relative crosscutting nitions and implications for stream behav- through underlying deposits to form a (inset) relationships, and soil morphology ior to create various forms of terraces.

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