Happy Valentine Formation Day!

With Valentine's Day having come on Friday, I thought it would be fun to show some love to the Valentine Formation. This geologic unit, from the Middle Miocene of Nebraska, is a great fossil producer but not especially famous, so let's give it a moment in the spotlight.

On fossils in metamorphic rocks

One of the introductory-level bits of information people learn about fossils is that fossils are found in sedimentary rocks, not igneous rocks or metamorphic rocks. This works as a first-order approximation, but... it's not strictly true. For example, among igneous rocks, there are some that overlap sedimentary rocks, e.g., pyroclastic flows, ash beds, and so on. You can certainly find fossils in those. There are also occasional impressions of things in basalt flows; tree trunk molds are most common, and at least one rhino (the famous "Blue Lake rhino"; e.g., Beck 1937) has been captured this way. Tree molds are also interesting for testing our conventional definitions of paleontology in other ways: Because of the rapid cooling and hardening of the basalt, tree molds are essentially instant fossils. The rock can't get much more lithified, after all (especially compared with Quaternary sedimentary slop), yet modern examples haven't put in the time that frequently quoted definitions insist on for fossils, such as 10,000 years. In my day job we talk about paleontological resources as evidence of life in a geological context, so there is no cutoff. Where was I? Oh, yes. Basically, you can get fossils in igneous rock if the rock was playing by sedimentary rules when it formed. Don't expect them in intrusive igneous rocks, though, except in xenoliths of sedimentary host rock (e.g., McCracken et al. 2000, also featuring cores!).

For metamorphic rocks, most of us with paleontological experience will make exceptions for low-grade metamorphics: quartzites, slates, low-grade marbles, things like those. In fact, a lot of rocks commonly called "marbles" are just limestones that can hold a polish. Get much beyond that, and surely the temperature, pressure, chemically active fluids, and mineralogical changes will destroy any unsuspecting crinoid or brachiopod, right?

Well...

Obviously, the answer is "not always", otherwise there wouldn't be a post.

Reports of fossils in metamorphic rocks even as far gone as eclogite (Cavrgna-Sani et al. 2010) or migmatite (Hill 1985; the rock you get when it stops screwing around with metamorphism and gets down to serious melting, but doesn't finish) pop up every so often in the literature as a thin thread of geological quasi-Forteana. The "thin thread" is because they are not common. If they *were* common, they wouldn't be worthy of comment. You don't get a paper out of announcing that there are crinoid columnals in the Decorah Shale. Bucher (1953) is perhaps the most significant reference on the subject, and includes a good bibliography, but through no fault of its own is now more than 70 years old and likely to only get older. Labora-López et al. (2015) adds a few more recent references in its discussion, although it has its own purposes that do not include being a thorough review and update of Bucher (1953).

Aside from being a curiosity, fossils in metamorphic rocks are useful for providing age controls that are notoriously absent from many metamorphic units, and sometimes can help with correlations. Other interpretations are limited by distortion or general poor preservation of the fossils, and by the topic of Labora-López et al. (2015), the loss of information from the sedimentary matrix itself.

One interesting aspect evident from Bucher's lists is that the fossils are not necessarily confined to animals with hard parts that are particularly resistant to replacement (e.g., crinoids). There are recrystallized bivalves, for example. Of course, bivalve replacement is hardly unusual in itself, as shells of bivalves and other mollusks are prone to replacement anyway thanks to the poor stability of aragonite over geologic time, which is why their fossils are so often molds and casts. Some fossils have been replaced by more exotic materials, including echinoderm ossicles replaced by diopside and epidote in a contact metamorphism zone.

Why have fossils in metamorphic rocks not attracted much attention? A not-insignificant part of the problem is people haven't generally been looking for them. As noted by Bucher (1953), paleontologists weren't looking because they assumed that metamorphism destroyed fossils, metamorphic rocks are a pain in the neck to work with compared to sedimentary rocks, and the fossils that have been found in metamorphic rocks are not of great quality, so why bother? Meanwhile, the hard-rock specialists weren't looking because they were hard-rock specialists and weren't interested in fossils.

Still, though, even if this means that fossils are actually some order of magnitude more abundant in metamorphic rocks than generally thought, they aren't exactly falling out of schists and gneisses. Bucher had an elegant proposal: he suggested that most metamorphic rocks of appropriate age to have fossils of things like brachiopods, crinoids, and mollusks are derived from rocks that were deposited in settings with few fossils to begin with. The exact language used is more complicated and bound up in pre-plate-tectonic thought and geosynclines (well, it was 1953, after all), but you get the point: It's hard to have metamorphosed fossils if you don't have fossils to start with. Another rule of thumb Bucher proposed is that fossils can persist if metamorphism does not involve mechanical effects and the fossils are much larger than the size of newly formed minerals. The chances aren't great, but if you're face to face with a metamorphic rock that began as a sedimentary rock of Phanerozoic age, there might just be a stubborn brachiopod or crinoid in there.

References

Beck, G. F. 1937. Remarkable west American fossil, the Blue Lake Rhino. The Mineralogist 5(8): 7–8,
20–21.

Bucher, W. H. 1953. Fossils in metamorphic rocks: a review. Bulletin of the Geological Society of America 64: 275–300.

Cavargna-Sani, M., J. L. Epard, and W. L. Taylor. 1997. Discovery of fossils in the Adula nappe, new stratigraphic data and tectonic consequences (Central Alps). Bulletin de la Société vaudoise des Sciences Naturelles 92: 77–84.

Hill, M. L. 1985. Remarkable fossil locality: crinoid stems from migmatite of the Coast plutonic complex, British Columbia. Geology 13: 825–826.

Laborda-López, C., J. Aguirre, and S. K. Donovan. 2015. Surviving metamorphism: taphonomy of fossil assemblages in marble and calc-silicate schist. PALAIOS 30: 668–679.

McCracken, A. D., D. K. Armstrong, and T. E. Bolton. 2000. Conodonts and corals in kimberlite xenoliths confirm a Devonian seaway in central Ontario and Quebec. Canadian Journal of Earth Sciences 37(12): 1651–1663.

Journey Somewhat Nearer to the Center of the Earth: fossils in cores

Most of the time, when people are looking for fossils, they find them at the surface or just below. However, this is hardly the limit of where they can be found. After all, a fossiliferous formation found at the surface in one location may be buried hundreds to thousands of feet beneath other rocks and sediments somewhere else, and it doesn't stop being fossiliferous just because it's buried that far down. It just becomes much less accessible. We can get glimpses of the buried fossils through core samples.