Buried early Pleistocene landscapes beneath the Pajarito Plateau, northern New Mexico
Broxton, David, E. and Steven L. Reneau
This paper describes the early Pleistocene landscapes of the northern and central Pajarito Plateau area prior to the eruption of the Otowi (1.61 Ma) and Tshirege (1.22 Ma) Members of the Bandelier Tuff, and the effects of these eruptions on the landscapes. Borehole and surface outcrop data were used to prepare preliminary pre- Otowi and pre-Tshirege structure contour and bedrock geologic maps, and isopach maps for the Otowi and Tshirege Members. The central part of this area was occupied by a broad SSW-trending valley prior to eruption of the Otowi Member. This early Pleistocene valley was flanked on the west by deeply dissected volcanic highlands of the Sierra de los Valles and on the east by a low north-trending basaltic ridge that stood approximately 500 ft higher than the valley floor. The basaltic ridge was a western extension of the Cerros del Rio volcanic field that was largely buried by the Bandelier Tuff. Gravity data and the lithology of the upper Santa Fe Group suggest that the early Pleistocene valley overlies a Tertiary graben on the western side of the Espanola basin, in turn suggesting an inherited structural control on 'valley location. Deposition of both the Otowi and the Tshirege Members significantly changed the landscape; subsequent drainage channels were commonly strongly oblique to prior channels and locally were superimposed on prior bedrock highs. Future monitoring wells evaluating the nature and extent of perched ground water systems beneath the Pajarito Plateau, and targeting specific geohydrologic horizons associated with these paleotopographic surfaces, should consider the flow paths controlled by these buried landscapes.
- Broxton, David, E.; Reneau, Steven L., 1996, Buried early Pleistocene landscapes beneath the Pajarito Plateau, northern New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 325-334.