Third-day road log from Los Alamos through the southeastern Jemez Mountains to Cochiti Pueblo and the Rio Grande
— Fraser E. Goff, Steven Reneau, Margaret Ann Rogers, Jamie N. Gardner, Gary Smith, David Broxton, Patrick Longmire, Giday WoldGabriel, Alexis Lavine, and Scott. Aby

The third day will focus on recent studies of the geology and hydrogeology of the Pajarito Plateau. Much of this work has been supported by the Environmental Restoration Project of Los Alamos National Laboratory and by parallel projects of the New Mexico Environmental Department. The Pajarito Plateau contains sites of weapons research and development created during the Manhattan Project of World War II and later projects of the "Cold War." Environmental surveillance projects have been undertaken on the Laboratory for many years, but in recent years, it has been necessary to remediate more and more Laboratory sites. In order to implement proper clean-up activities, fairly detailed studies of the Bandelier Tuff, overlying geologic units (including soils), and older geologic units have been undertaken. In addition, several studies of various groundwater systems have revealed new information on the behavior of fluids in the main aquifer beneath the plateau, and on perched aquifers and the vadose zone in the upper few hundred feet of the plateau. Papers on each of these subjects appear in this guidebook. A geologic map of Los Alamos National Laboratory was recently published by M.A. Rogers (1995).

The first stop of the day, at Guaje Pines Cemetery on the north side of Los Alamos, allows examination of the Rendija Canyon fault, the only place in Los Alamos County where a fault scarp is developed on late Quaternary alluvium. Intensive studies of movement along this fault suggests that a magnitude 7 earthquake occurred at either 8 or 23 ka. Stop 2 provides an overview of upper Los Alamos Canyon, and the Otowi and overlying Tshirege Members of the Bandelier Tuff. The internal stratigraphy and cooling history of these units will be discussed, as well as erosional patterns in the canyon, and surface- and ground-water flow and their influences on contaminant transport. A short distance to the east, Stop 3, at the Pueblo Canyon overlook, we will further examine the lithological variation and emplacement history of the units recognized in the Tshirege Member of the Bandelier Tuff. The Conference then moves south to the White Rock Canyon overlook (Stop 4 ), a spectacular gorge eroded by the Rio Grande through Pliocene and Pleistocene volcanic rocks. The geomorphic history of this canyon is complex and fascinating, involving repeated damming of the river to produce a succession of lakes, as well as a history of slumps and landslides, and springs related to the hydrogeology of the Pajarito Plateau. Moving southwest across the Plateau to Ancho Canyon (Stop 5), we have the opportunity to examine and discuss Pleistocene volcanism, soil development, a zeolitized ground-water table, flu vial sedimentation and its relationship to Quaternary paleoclimatology in this region, and Pajarito Plateau hydrogeology and environmental geology in one of the numerous canyons incised into the edge of the Plateau.

We then proceed through part of Bandelier National Monument to the "Dome Road" and into the undeveloped southeastern sector of the Jemez Mountains, where exposures of early volcanic and volcaniclastic rocks of the Jemez volcanic field may be observed. These rocks, part of the Keres Group, consist of basalt to rhyolite domes, flows, breccias, pyroclastic deposits, and derivative sediments about 6 to 13 Ma in age. The area near the junction of the Dome Road and NM-4 was devastated by a serious fire that threatened the National Lab in May 1996, after the road logging was completed; the effects of the fire will be apparent during the field conference. Stop 6, at the Cochiti Canyon overlook, provides a good panorama of the Keres Group volcanic rocks and structures. Stop 7 (an optional stop) reveals an overview of the Rio Grande valley to the south, including interbedded fluvial, volcaniclastic, and volcanic lithologies marking the transition from the volcanic geology of the Jemez Mountains to the sedimentary basin-fill facies of the Rio Grande rift. The Bandelier Tuff and geomorphic features of Cochiti Canyon are very much in evidence here, as well.

The route continues south past Cochiti Lake, Dam and Pueblo, to Stop 8, at scenic Tent Rocks, composed of late Miocene Peralta Tuff and Cochiti Formation volcaniclastics, highly eroded into cones and pedestals. Here the Field Conference ends, a few miles from Interstate-25.

We thank Heddy Dunn and Georgia Strickfaden of the Los Alamos Historical Society for some of the historical comments provided for this road log.