New Mexico Geological Society Annual Spring Meeting — Abstracts

Between the communities of Rinconada and Pilar, NM-68 is sandwiched between the Rio Grande and a 3-mile-long, l000-foot-high bluff of highly fractured crystalline rock and mass wasting deposits, known as the Pilar cliffs. The route also follows a major Neogene fault system (Embudo fault) that separates Tertiary deposits of the Taos plateau from Early Proterozoic rocks of the Picuris Range. Throw along the fault, combined with the erosional resistance of the Proterozoic rocks, has produced the precipitous cliffs.

Persistent rockfall, rockslide, and debris flow activity presents a hazard to motorists driving between Espanola and Taos. For example, in 1988, five people were killed and 14 injured when a bus crashed into a falling basalt boulder. More recently, on the evening of July 25, 1991, after several days of hard rain, 11 debris flows and numerous rockslides and falls caused NM-68 to be closed for 19 hours. Three thousand cubic yards of fill were required to repair the road, and cleanup costs were estimated at $75,000. Although 20 cars were trapped along the highway, there were no reported injuries. The largest debris flow on the highway was approximately 100 feet wide and 8 feet deep. Another flow, which was channeled through a culvert, temporarily dammed the Rio Grande. A 300+ ton schist boulder, which slid and rolled 1000 vertical feet down the Pilar cliffs, created a 45 by 15 by 15 foot impact crater when it struck the highway. After impact, the boulder came to rest near water level on the opposite side of the Rio Grande. Using a simple dynamic analysis, we estimate that the rolling and sliding boulder was traveling at about 47 mi/hr when it struck the roadway with total kinetic energy on the order of 81 million ft-Ibs. For comparison, this is about 160 times the capacity of rockfall protection nets installed along other portions of NM-68.

The Picuris Range is a fault-bounded, wedge-shaped uplift that projects westward from the southern Sangre de Cristo Mountains. The present, deeply eroded Picuris block was rapidly uplifted at about 3-5 Ma, and has remained high since that time. Fault scarps in Quaternary alluvium along the northern mountain flank indicate late Pleistocene activity.

The oldest Precambrian unit in the Pilar cliffs is the Glenwoody Formation, consisting of 600 ft of homogeneous, feldspathic metavolcanic quartz-eye schist that accumulated at 1700 Ma. The Glenwoody Formation is overlain by the 6600 ft thick Hondo Group, a metasedimentary section of cliff-forming quartzites, pelitic schists, and phyllites. All of these units are polydeformed, metamorphosed to amphibolite facies, and moderately south-dipping. The boulder that fell in July 1991 came from an outcrop of the lowermost schist unit of the Hondo Group.

North of the Picuris block, basement rocks are bounded by Tertiary sedimentary and volcanic rocks associated with the Rio Grande rift. The Tesuque Formation (Miocene to Pliocene) of the Santa Fe Group, is a sequence of poorly-sorted, weakly consolidated sand, silt, and gravel beds that ranges in thickness from 500 to 3500 ft. These deposits are capped by tholeiitic basalts of the Servilleta Formation (pliocene). Between Pilar and Embudo, where the Rio Grande has oversteepened slopes underlain by Tertiary units, landslide deposits consisting of predominantly sandstone and basalt blocks cover most of the river Valley. The 1988 rockfall boulder was derived from such a deposit. In 1991, several thousand feet of rockfall protection netting was installed in landslide deposits along NM-68 near Embudo.