Miocene Española

Our major landmarks appeared in the Miocene that lasted from 23 to 5.3 million years ago. It’s hard to visualize a time when the land was relatively level with the surrounding Santa Fé, Pajarito, Colorado, and Taos plateaus. The surface would no longer have been the smooth water bed left by the retreating Western Inland Sea.

In the preceding Eocene that began 56 million years ago, the smooth edge of the Farallon plate had bumped into the buried remains the Yavapai-Mazatzal boundary. When the Picurís-Pecos fault intersected the Embudo fault, faults multiplied. The central Sangre de Cristo from Santa Fé north to Picurís began sinking while the two ends, Taos and Albuquerque, rose.

Shari Kelley and Ian Duncan have found evidence the Sandías and Truchas range were lifting again 35 million years ago, in the early Oligocene. The Picurís range recovered, and began rising a million years later.

A chunk of rock near Peñasco collapsed between the Pilar-Vadito fault on the northwest and the Santa Barbara on the southeast. The Jicarilla fault ran around the east end. The oldest surface rocks we have in this area are siltstone, sandstone, mudstone and claystone washed down from Peñasco.

These gravels cover Los Barrancos, a furrowed ridge skirted on its east by the road to Santa Fé. It’s laced with north-south fault lines starting above the modern Pojoaque river, crossing highway 84, and continuing across the Santa Cruz river. They’ve been roughly dated to the middle Miocene, 14.5 to 5 million years ago.

Coarser rocks from the Peñasco Embayment formed the bad lands on the east side of the valley north of the Santa Cruz. Perhaps they were the wash from that unnamed river, perhaps its course changed. Cobbles also spread over an old dune field in the north that had begun retreating between 11.5 and 9 million years ago.

The rock underlying the Española Basin dropped along the Pajarito fault. The half graben stayed connected on the east, tilting the block to the west and north. When it fell, the effects reverberated along the Embudo fault that runs northeast from the northern end of the valley.

Then the volcanos began. In the Arroyo Seco valley east of Los Barrancos, Ted Galusha and John Blick counted 37 layers of ash in the surrounding rocks. Tchicoma was formed between 7 and 3 million years ago.

Volcanism continued into the Pliocene era that began 5.333 million years ago. The southern Black Mesa formed about 4.4 million years ago, but the magma cooled in its neck, stopping the flow and sending it elsewhere in the Jémez. During the Ice Age, the surrounding cover disappeared, leaving the stem and a slope of debris to one side.

Rifting moved north and east. The Taos Plateau volcanic field became active about 4.5 million years ago. It threw lava that landed on that old dune field, capping the northern Black Mesa between 3.8 and 3.3 million years ago.

While the volcanos were spewing ash in the west, life continued on the other side of the basin. Grasses had multiplied in the Miocene and so did the animals that fed on them. The now extinct forms of camels, horses and rhinoceroses replaced the dinosaurs that had been lost at the end of Cretaceous mentioned in the last post.

In the Arroyo Seco basin, Galusha and Blick said the ash was never too thick to kill grazing mammals. They simply left when rains stimulated by eruptions flooded their feeding grounds. No bones are found in the ash layers or in the ones immediately above. They came back when the grasses recovered.

Notes:
Galusha, Ted and John C. Blick. Stratigraphy of the Santa Fe Group, New Mexico, 1971.

Kelley, Shari A. and Ian J. Duncan. "Late Cretaceous to Middle Tertiary Tectonic History of the Northern Rio Grande Rift, New Mexico." Journal of Geophysical Research Vol. 91:6246-6262:1986

_____ and Rachel L. Landman. "Low-temperature Thermochronologic Constraints on the Tertiary Cooling and Unroofing History of the Southern Sangre De Cristo Range, New Mexico," Geological Society of American, annual meeting, 2013.

Koning, Daniel J. Preliminary Geologic Map of the Española Quadrangle, Rio Arriba and Santa Fe Counties, New Mexico, 2002, map and report.

_____ and Kim Manley. Geologic Map of the San Juan Pueblo Quadrangle, Rio Arriba and Santa Fe Counties, New Mexico, 2003, map and report.

_____, William C. McIntosh, and Nelia Dunbar. "Geology of Southern Black Mesa, Espanola Basin, New Mexico; New Stratigraphic Age Control and Interpretations of the Southern Embudo Fault System of the Rio Grande Rift," New Mexico Geological Society, Guidebook, 2001.

McDonald, David W. and Kent C. Nielsen. "Structural and Stratigraphic Development of the Miranda Graben Constrains the Uplift of the Picuris Mountains," New Mexico Geological Society, Guidebook, 2004.

Riecker, Robert E., Peter W. Lipman and Harald H. Mehnert. "The Taos Plateau Volcanic Field, Northern Rio Grande Rift, New Mexico," in Riecker, Rio Grande Rift: Tectonics and Magmatism, 2013.

Photographs:
1. Exposed tilted rocks, road from Dixon east toward Peñasco, 8 October 2011.

2. Los Barrancos near Black Mesa Golf Course, 15 January 2012.

3. Two of the Three Sisters knobs in the Arroyo Seco valley, 2 November 2011.

4. Northern Black Mesa west of Chamita, 17 February 2013; note the different sizes of the rocks by layer.

5. Arroyo Seco valley looking east from highway, near where they built the overpass, 2 November 2011.