New Mexico Geological Society Annual Spring Meeting — Abstracts

The Guaje Mountain 7.5-min quadrangle contains some of the most spectacular volcanic geology in the state, including voluminous lava and ignimbrite deposits, thick sequences of volcaniclastic sediments, and numerous faults related to the Pajarito and Embudo fault zones. The majority of the Cerro Grande bum area lies within the quadrangle, which in its current state of stripped vegetation provides exceptional exposures of bedrock and surface features. From west to east, rocks in the quadrangle record the geographic transition from pre-caldera, Tschicoma rim rocks, to valley-fill Bandelier Tuff depOSits, to rift-fill fanglomerates of the Puye Formation. Major rock units in the map area include late Miocene to Pliocene silicic lavas of the Tschicoma Formation and their associated Puye fanglomerates, Quaternary Cerro Toledo tephras and sediments, upper Bandelier Tuff (Tshirege Member), and alluvial deposits.

Three major silicic lava units of the Tschicoma Formation have been differentiated in the quadrangle, providing greater insight into the pre-caldera eruptive history of the region. The oldest (late Miocene/early Pliocene) and most voluminous unit, termed the Rendija Lavas, forms the bulk of the highlands immediately west and north of Los Alamos, including Sierra de los Valles, Guaje Ridge, and Guaje Mountain. Most likely, the formation of these volcanic highlands had an immediate sedimentary impact, resulting in a rapid eastward progradation of Puye fanglomerates into the Rio Grande Rift. At approximately 3 Ma, another episode of silicic volcanism occurred, as viscous lavas erupted from vents at/near Pajarito Mountain and Caballo Mountain along the western boundary of the quadrangle.

The Otowi Member of the Bandelier Tuff, erupted from the Toledo Caldero at ~1.6 Ma, is exposed in canyon bottoms in the southern portion of the quadrangle. In the northern half of the quadrangle (north of Rendija Canyon), however, this unit is missing, suggesting that the northern Tschicoma highlands created a depositional shadow zone during the eruption of this ignimbrite. Between caldera events (1.6 -1.2 Ma), Cerro Toledo interval deposition was widespread in the quadrangle, including tephras, conglomerates and other epiclastic sediments. Eruption of the Tshirege Member of the Bandelier Tuff at ~1.2 Ma filled in large paleovalleys along the Tschicoma highlands and created the Pajarito Plateau. Prior to the incision of modem canyons into the Pajarito Plateau, a brief interval of Quaternary sedimentation occurred upon the Bandelier Tuff, forming alluvial caps to many of the present mesa tops, including several locales in the
town of Los Alamos. Other, minor sedimentary units in the map area include landslide,
terrace, and recent alluvial deposits.

Faults in the map area transect all of the major stratigraphic units, with the older Tschicoma Rendija lavas showing the greatest measurable degree of offset. The majority of the faults trend N-S and show down-to-the-west displacement, contrary to offset along the Pajarito fault zone south of the map area. In the northern portion of the quadrangle structural trends veer to the NW, parallel to the Embudo Fault system and the Jemez
lineament.