The mother of all extinctions and the perils of correlation (1)

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
notdone 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’tmean 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. Howthis 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 Earthmagazine,
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): http://courses.washington.edu/ess456/;
http://strata.geol.sc.edu/index.html;
http://courses.washington.edu/ess456/2009files/FaciesIntroWalther.pdf] SIGNATURE

Comments

suvrat (2009-08-24):

nice illustrations of facies and correlation. awaiting part 2

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2009/08/the-mother-of-all-extinctions-and-the-perils-of-correlation-1.html