Friday Field Foto #75: Nonconformity in the Franklin Mountains

This week’s Friday Field Foto is from the geological wonderland of west Texas/southeastern New Mexico.

A nonconformity is a type of unconformity where sedimentary rocks directly overlie igneous or metamorphic rocks. In this case, the Cambrian-Ordovician (540-440 Ma) Bliss Sandstone sits atop the intrusive igneous rocks of the Red Bluff Complex, which are dated as 1.17 Ga. The reddish igneous rocks are in the lower right and the Cambrian sandstones are the lighter-colored rocks in the middle of the hill (the bedded rocks near the top is yet another formation but I forgot the name).

Franklin Mountains, El Paso, Texas (© 2009 clasticdetritus.com)

This photo may not look like much, but what’s really cool about this (and other outcrops along the Franklins) is that in places the lowermost Bliss sandstone beds onlap the older igneous rocks. That is, the older rocks had a undulating topography such that the lowest areas (valleys) accumulated sediment first. It is very cool to think about this ~500 million year old landscape with rolling hills.

You’ll commonly see unconformities referred to as “missing time” — that is, there is a break in the stratigraphic record (in this case, at least 630 m.y., which is more time than the entire Phanerozoic!). But, the time isn’t really “missing” … it’s simply recorded in a different way. Some time is recorded by rock, some time is recorded by surface. Think about the surface of the Earth right now — the current snapshot in geological time. The distribution (not to mention rates and potential preservation) of deposition is extremely variable. Thus, the “completeness” of the stratigraphic record is also variable. Referring to unconformities as “missing” time is misleading — there is a story there, it’s just recorded in the strata in a different way.

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check out more photographs of the Bliss Sandstone in the Franklin Mts here and here

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