Nature abhors a vacuum, continuity and the status quo. Variety is the spice of life and change is the order of the day. Such things make life complex for a geologist, but also exciting and lead to puzzles, multiple working hypotheses, debate, controversy - and progress. This tale is an example of all of this, of how science is a process, not an answer, how big questions can be addressed by small-scale scrutiny, and how thin layers of sand and mud can be testaments to one of the most catastrophic events in life's history. But it's a complicated story to tell - and there's a twist at the end; I've been wrestling with it for some time; now there's nothing for it but to try to tell the tale, so here goes - it'll take a while and will come in two instalments.
We sometimes talk of rock layers being the ledgers of our planet's history, and the geologist's task as being to read those ledgers, to interpret and translate the writing. Stratigraphy, from the Greek for layers and writing, is the branch of geology that attempts to do this job, to set out in three dimensions the sequence of events recorded in different places by piles of rock layers; and really it is the study of four dimensions, since the vertical direction of those layers, assuming they have not been disturbed by the churning of tectonic plates, represents time, successively younger layers piled on older ones. The analogy is often made to a cake - "layer cake stratigraphy" refers to a simple and continuous pile of sedimentary beds; here's a beach sand, then on top of it a dune sand, and, on top of that the muds and shales deposited in a lagoon. It makes for a nice simple story: sea level dropped, the beach migrated following the receding shoreline, dunes migrated over the old beach and lagoons moved over the old dunes. The sediments in the picture at the head of this post (ignore, for the moment, my reptilian Photoshop frolics) show, from a different environment, layer cake stratigraphy, a nice, simple, pile of sand and mud layers. But the width of this outcrop is only a few meters - try to follow a particular layer for any distance to the left or right and chances are things will change, for nature abhors continuity.
A lack of continuity in the stratigraphy can occur for any number of reasons, the most brutal being that erosion has removed everything, a valley has been ripped into the ledger and we must try to find the equivalent pages on the other side. Or tectonic churning has caused a fracture and movement on a fault has displaced our precious cake deep below our feet - we must again try to find the other piece of the cake elsewhere if we are to continue reconstructing its story. Layer cakes are great and to be treasured when they occur, but, more often than not, things are not that simple.
Even without erosion or tectonic disruption, nature's dislike of continuity means that a particular layer will change laterally as we follow it along. To illustrate this, imagine that you, suitably equipped, take a stroll across the Alaskan river valley shown below, taking note of the sediments beneath your feet. In doing so, you will encounter the sands, gravels, and muds of river terrace deposits, old flood deposits, sand bars, active river channels, old, abandoned, river channels and point bars, lakes, vegetated soils, and, as you finally clamber up the side of the valley, much older rocks of the hills into which the river is carving its way.
This scene is, literally, a snapshot in time of a small part of the earth's surface - it was taken decades ago and the scene would look distinctly different today since a dynamic and powerful river system like this one sweeps back and forth across its valley continuously. There is virtually no continuity of a particular sediment whatsoever - and, if you follow the river to the sea, the sediment will change again to beaches and progressively deeper water marine environments - all representing the single snapshot in time. Now imagine the river scene captured in cross-section in a cliff face a few million years from now - the stratigraphy will show dramatic lateral (and vertical) changes, the possible continuity of individual beds difficult to follow. A thick sandstone bed in one place will become thinner and disappear laterally, or perhaps split into two, thinner, beds. Below (thanks to the Texas Bureau of Economic Geology field seminar site) is just such a cliff, showing 50 million-year-old river sediments in Wyoming. Lateral continuity? No such thing.
Now, the key to telling the story of a chunk of the earth's history from outcrops such as this is correlation - taking pieces of the puzzle and putting them together, connecting layers and piles of layers in a way that makes sense, that tells a coherent story (not, for example, one that implies glaciers reaching down onto a tropical beach). Trying to read and correlate the incomplete and discontinuous ledgers is like trying to read an article that has been chopped up. Below is part of one of the articles I shall describe in the next instalment, but I have deleted a narrow column in the middle - the valley - and a couple of lines from the column on the left - sandstone layers that disappear laterally. It's perfectly possible to correlate across the gap, reconstruct the original, and make suggestions for what the missing words might be - but it's not simple and the result is open to different interpretations. Nevertheless, the fact remains that it was a coherent article, telling a joined-up, sensible story. It's worth noting that a single, straightforward, correlation - in this case the paragraph break, is a good place to start, to anchor the puzzle.
In the first instance, the simplest thing to do when faced with the puzzle of correlating two piles of rock layers, is to match the character of individual layers - their lithology. A distinctive sandstone layer that has purple pebbles along its base on one side of a valley can be correlated with a similar sandstone with purple pebbles on the other side. In making this connection, you have perpetrated an act of lithostratigraphy - setting up a stratigraphic scheme simply by describing and using the character of the layers - for sandstones, their grain size and composition, the range of grain size, any features such as ripples, and so on. This works well in putting together a framework of the geological sequence over an area or a region, but what you have not done is to pin down the timing. Look again at the Alaskan River - your stroll took you across a wide variety of sediment types. But let's consider the partially overgrown banks of sand and gravel in the foreground: they may form a roughly continuous layer, but they were deposited at successive stages of the river's migration - they are more or less made up of the same sediments and they are right next to each other, but they don't represent a single time, a single episode of deposition. The sediments of which they are made are, in the jargon of the trade, diachronous - their age varies from place to place. Whether it's because sea level is always rising or falling, or rivers are always wandering around their valleys, or dunes are always on the move, virtually every layer in a sedimentary pile is diachronous to some extent - often dramatically. And so lithostratigraphy tells us nothing about the timing of the story, the evolution of events in the particular part of the earth that we are studying. To rectify this, we must look for clues that allow us to overcome diachroneity, to make correlations of what was going on at the same time, to reconstruct the picture of the Alaskan river landscape - we need a time framework, a chronostratigraphy. To correlate layers that document the things that were going on at the same time, fossils can often be of help. As can any means of measuring the age of a rock directly, through the different methods of radiometric dating, or an ability to calibrate layers against a timescale that we have sorted out in detail, such as the periodic reversals of the earth's magnetic field. But often, as is the case with the rocks of the Wyoming cliffs, these methods are simply not available, there are no fossils, there are no dateable minerals - we simply have to do our best to tell a story that makes geologic sense.
Correlations made on the basis of lithostratigraphy and chronostratigraphy are invariably different. Below, I've sketched a cartoon of the two sides of a valley. In this case, we've got lucky. High up in the pile is a strange rock that happens to contain newspaper fragments - it's called, lithologically, pulpite (I am, of course, making this up). Lithostratigraphy leads us to correlate the base of this distinctive layer from one side of the valley to the other. But, on examining the newspaper fragments, we find that, on the eastern side they come from 1954, and on the western side, 1963 - the pulpite is diachronous. The chronostratigraphic correlation, the important one, shows that at the time that the pulpite started being deposited in the east, sandstone, let's say with all the characteristics of a river point bar, was being deposited further west. We can now begin to put together the picture of the earth's surface at this location, the paleogeography: a map that shows a pulpite environment of deposition next to a river bank.
With every successive detail we look at, with every incremental piece of research, the story becomes clearer, the palaeogeography better-defined, the correlations more precise. But nature's games invariably mean that the puzzle is never complete, questions always linger and avenues for further work become apparent. This often is the basis for vigorous debate between different geologists working on different parts of the same puzzle, and it's what makes the science exciting. What these differing interpretations and vigorous debates don't mean is that the science is wrong, or that significant events identified by earlier work didn't occur - they simply illustrate how science works and how the picture comes more into focus as more data are gathered and fragments of the puzzle fitted into place. 250 million years ago, life on earth suffered the mother of all extinctions - 90 percent of marine species and 70 percent of land life were obliterated. Whole families and genera of creatures disappeared forever. How this took place and why, whether it was geologically sudden or occurred in a series of pulses of mortality, remain questions on which considerable ongoing research is concentrated. But the fact of the extinction is exactly that - a fact. Some of the details of how it happened are contained in the testimony of the layers shown at the top of this post, and the refinement of our understanding of this testimony continues - amid considerable debate. But the vigour of the debate and the vicissitudes of lithostratigraphy versus chronostratigraphy, in no way warrant headlines on credible science news sites such as “The catastrophe that wasn’t” - that in turn feed the frenzied celebration of the creationists. But more of that next time.
[The outcrop photo at the top of the post is from Earth magazine, June 2009. In terms of the principles of stratigraphy that I have attempted to introduce here, there are many superb sites for details, illustrations, movies, etc. A small selection that will take the interested reader further (in addition to the Bureau of Economic Geology site linked above): https://courses.washington.edu/ess456/; https://strata.geol.sc.edu/index.html; https://courses.washington.edu/ess456/2009files/FaciesIntroWalther.pdf]