Sciency Words: Facies

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:

FACIES

So I’m currently reading a paper entitled “A Field Guide to Finding Fossils on Mars.”  Basically, if you’re hoping to dig up some fossils on Mars, you need to know where to look.  This paper is all about which “facies” are the most likely to have well preserved Martian organisms inside them.

I have to admit I’m having a tough time with the paper.  My first question, and perhaps your first question as well: what the heck is a facies?

The word facies comes straight from Latin, where it meant (believe it or not) face.  It could also mean facial expression or the generalized appearance of a thing.  According to this article from the Encyclopedia Britannica, Danish scientist Nicholas Steno was the first to use facies as a geology term in 1669, but it was Swiss geologist Amanz Gressly who reintroduced the term in 1838, leading to its modern usage.

Gressly was conducting geological research in the Jura Mountains, which lie along the border between France and Switzerland.  It was already known that there were different layers of rock stacked on top of each other.  We call these strata, and it’s now widely recognized that different strata correspond to different time periods in Earth’s past.

But Gressly noticed that, in addition to the strata stacked vertically on top of each other, there were also different “stratigraphic units” arranged horizontally beside each other—the facies, as Gressly decided to call them.  Gressly is quoted in this book as having written:

I think that the petrographic or paleontological changes of a stratigraphic unit in the horizontal are caused by the changes in environment and other circumstances, which still so powerfully influence today the different genera and species which inhabit the ocean and the seas.

In other words, if you find different facies within the same strata, then you’re looking at different environments or ecosystems that existed at the same time, side by side: a lakebed next to a forest, for example.

Or at least that’s what Gressly originally intended the word facies to mean.  But according to that same Encyclopedia Brittanica article, the term has since been generalized “[…] to encompass other types of variation that may be encountered as one moves laterally (e.g., along outcroppings of rock strata exposed in stream valleys or mountain ridges) in a given rock succession.”

So if you’re going fossil hunting on Mars, you want to look for rocks formations dating back to Mars’s Noachian Period—that’s when Mars had lakes and rivers and oceans of liquid water on its surface.  Rock formations from the very early Hesperian Period would also be good.  There was still some liquid water sloshing around at that time.

But within Noachian or Hesperian-aged strata, which facies should you look for?  Well, I’ll have to get back to you on that one.  As I said, I’m having a tough time with this paper, but I am determined to get through it!

P.S.: Bonus Sciency Word!  Those same Jura Mountains where Amanz Gressly did his geological research also gave us the name for Earth’s Jurassic Period.