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Silicified volcanic ash deposits in the Nacimiento Formation at Ceja Pelon Mesa

Sandoval County, New Mexico
— August 3, 2021

These silicified beds are common in the upper Nacimiento Formation, where they often form resistant bluffs or the caps of hoodoos. Previously interpreted as pedogenic silcretes or the result of cementation by groundwater, we now think that these are volcanic ash deposits for reasons highlighted in the accompanying photos. The field photograph shows two such beds: a lower one marked by a white arrow is truncated by an upper bed marked by a blue arrow. Just below the red arrow, the lower bed is truncated and the upper bed cuts across it. The "draping" of the upper bed over pre-exisiting topography is a hallmark of volcanic ash deposits. Some erosion of the lower bed must have occurred prior to deposition of the upper bed.

We also use microscopes to understand geologic problems, and these two photomicrographs (photographs taken through a microscope) show ash shards (called tephra) in rocks collected at the site of the field photograph. These arcuate and angular clasts (about half as thick as a strand of hair from your head) share the typical morphology of volcanic ash. Their sharp edges indicate minimal transport after deposition, suggesting that they likely were preserved at the site where they initially landed after a volcanic eruption.

There are at least 30 unique beds like these in the Nacimiento Formation; each one likely was sourced from a different volcanic eruption in the Paleocene Epoch (66 to 56 million years ago). We still do not know where these eruptions occurred or what their effects might have been, but exploring such unknowns is among the joys of geology!

— Kevin Hobbs, Field Geologist, NMBGMR

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Field photograph with two volcanic ash bed (see text)
(click for a larger version)
figure
Photomicrograph of volcanic ash shard (see text)
(click for a larger version)
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Photomicrograph of two arcuate volcanic ash shards (see text)
(click for a larger version)