What is a Marker Bed?

I’ve had a couple requests to give a few more examples of what a geological marker bed (a.k.a. marker horizon) actually is, and more fully explain why I chose Marker Bed as the title for my book. Probably the best way to start would be to go through the definition, explain what it means, and move on from there.

A topical question of the day before we begin: why do we even care? The answer is easy. Geologists pay attention to marker beds because they are one of the most definitive tools we have to correlate geologic history in a global framework. Since figuring out what happened — and when it happened — is what nearly every study of the earth is all about, marker beds are some of the most important and useful tools we have to sort out earthtime. So yes, you’d better believe geologists care. And not just us Bozos who swing rock hammers: since just about everything we need and use comes from the earth, its spatial and temporal reality impacts all of us. Even you…

Anyway, as it says on Marker Bed’s title page, a marker bed is a “widespread and unique event or horizon that represents a relatively brief span of geologic time.”

Let’s start with event or horizon. In this case, we are talking about a layer of rock or other earth material that can be identified and mapped as a specific rock unit, and that was the result of a specific geologic event. An obvious example would include a bed of ash deposited during a volcanic eruption.

Unique is pretty obvious: If the layer of rock looks and is chemically similar to everything around it, it won’t do much to help us recognize it as distinctive, and then be able to correlate it with similar materials found elsewhere.

Widespread: Since a marker bed is one of the primary tools earth scientists use to link geologic events around the globe, in this case widespread indicates how important it is that the event or horizon can be found — and recognized — in locations at great distances from each other. It’s one thing to be able to say that the same event was happening on both sides of the creek, but it’s a much bigger deal to be able to say that it was happening on both sides of the world!

And lastly, the definition talks about it representing a “relatively brief span of geologic time.” If the unique situation occurred continuously for tens to hundreds of millions of years, it’s not really going to help us tie down a specific happening at a specific time in earth’s history. It’s so much better if the event or horizon is the result of something that happened very quickly. Again, like a bed of ash from a volcanic eruption.

So, let’s consider a few examples:

Since we’ve already rubbed up against volcanic eruptions, let’s start with them. I’ve already covered volcanoes in varying degrees of detail in several earlier posts, and a major eruption plays a significant role in Marker Bed itself.

Every volcanic eruption ejects materials which are chemically different from essentially all other eruptive events. If the eruption is big enough, and the ash fall spreads far enough, these can make incredibly distinctive and valuable marker beds.

The triggering eruption I write about in Marker Bed is related to the Yellowstone Supervolcano — the largest felsic volcano on our planet — but there are many other volcanoes that could also deposit a widespread and unique layer of pyroclastic debris in the rock record.

kaBoom, indeed!

Another example: Possibly the most famous marker bed, and the one which almost all of us have at least heard of, is the asteroid impact that killed off the dinosaurs. The sequence of events seems so obvious now.

Iridium is a rare and unusual element to be found in the rocks of the earth’s crust, but it is common in asteroids. In 1980 (and with the support of chemists Frank Asaro and Helen Michel), physicist Luis Alvarez and his son Walter (a geologist, so we’ll choose to like him) discovered a thin layer of iridium-rich clay in Italy that dated to the end of the Mesozoic era.

Through quite a bit of work and research, they proposed that this indicated the impact of a large asteroid, which filled the atmosphere with debris, blocked the sun’s energy, and led to a nuclear winter scenario… which resulted in the extinction of — not only T-rex — but about three-quarters of all life on earth. (Click here for an earlier post that discusses this event in detail.)

Initially controversial, the theory gained widespread acceptance after the discovery of the massive Chicxulub crater in Mexico, consistent with their impact hypothesis.

The fun part, and what makes this geologic event so important as a marker bed, is that the debris that filled the atmosphere settled out over the next decade or so, leading to a thin layer of iridium-rich clay just about everywhere on the planet. So, if you’re mapping in Australia, or China, or Wyoming, or wherever, and you find a thin layer of iridium-rich debris, you can be pretty certain that you are at the end of the Mesozoic about 66 million years ago (give or take a week).

A final example of a marker bed (and the one that leads to the title of my book) are unique lifeforms.

Sherman, set the WayBack Machine for 1974. As Susie and I approached graduation from the university, we were penniless and starving, mortally sick of Ramen and bean burritos, and I was in serious need of gainful employment. I was pretty certain a position with an oil company in Southern California would be available as soon as the ink was dry on my sheepskin: analyzing single celled microscopic organisms in sedimentary materials, and dating them through earthtime.

As it turns out, these types of creatures (forams, diatoms, coccolithophores, and similar tiny creatures) fill the oceans of the world and evolve very quickly. This includes the shape and appearance of their shells, which sink to the seafloor when they die. As such, they make incredibly useful marker beds in the search for oil!

(Piss and moan all you want about how the oil companies rape and pillage the environment — and they do — but we are all dependent on their product. You don’t think this is right? Neither do I, but I can at least take some comfort from the knowledge that I didn’t vote for the oily Bozos who have sold their souls (and my grandkid’s future) to Big Oil.)

Anyway, back in the early-70s, gas had just skyrocketed to 40¢ a gallon, and working in petroleum was still considered a useful and worthy life’s goal by nearly everyone. But… needless to say, forty hours a week hunched over a microscope did not float my boat!

So, being young and invincible, Susie and I moved to the backs and beyonds of Southwestern Oregon, where I’ve spent the last fifty years exploring for earth resources (still raping and pillaging, at least according to some), sharing my love of the planet with students of all ages, living entwined with nature, and having one hell of a lot more fun than I would’ve had with Union Oil.

But I digress — a failing that is all too common to the old and feeble-minded…

Back to unique lifeforms: There have been quite a few different organisms that have made brief visits through earthtime. All of them were unique and represented, in most cases, an instant in evolutionary history. Sadly, not all of them were very widespread, so their value as a marker bed is often limited.

But think about people! Are we unique? Are we widespread and leaving our stuff littering the surface of the plant nearly everywhere? And lastly, are we going to exist for only “a relatively brief span of geologic time?”

Certainly, the first two qualify humanity as one of the best marker beds that I’ve ever seen or even considered. The final requirement involves our longevity, and I leave it to your wisdom to sort this one out for yourself (although I do share my thoughts in the Afterword at the end of Marker Bed).

Before we go, I’m forced to make a shameless plug for my book (Word-of-Mouth remains my marketing strategy).

I found a new 5-Star comment on Marker Bed’s Amazon site a couple days ago that made my heart smile:

This book will dramatically change how you view our earth and our future. I’ve been reading science fiction for about 40 years now and it is extremely rare for a book to surprise me with the overarching theme. Whereas ‘historical fiction’ tells a compelling and colorful story about a historical event, Marker Bed is ‘future fiction’ and gives a compelling and colorful story of our future.

Added bonus, YOU WILL LEARN LOTS AND LOTS ABOUT EARTH PROCESSES as the story unfolds. If only there was a category for an educational, earth science, futuristic, prophetic, fiction/nonfiction book.

Enjoy the read (Xoix expects no less of you).

Many thanks to whomever wrote this (I have no idea who it was), and yes: I too struggled to find an appropriate category.

If you haven’t already obtained your copy of Marker Bed, I encourage you to do so now. Click here to go to the Amazon website and lock in your copy.