Clastic Detritus

NOTE: This post was included in Classic Science Papers 2008 Challenge, which has posts about a classic paper across many disciplines. That list was expanded and became the first installment of a new science blogging carnival called “The Giant’s Shoulders”. See more about the carnival here and see the expanded first installment on A Blog Around The Clock here.

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I finally found made some time to finish the second installment of the Theoretical Stratigraphy series. The first installment looked at a paper from 1964 and dealt with the concept of baselevel as it applies to sedimentation and the long-term preservation of sedimentation (or, stratigraphy). For this post, I will take you back to an even earlier paper:

Barrell, J., 1917, Rhythms and the Measurements of Geologic Time: Bulletin of the Geological Society of America, v. 28, p. 745-904.*

Before I get into the ideas of the paper, the first thing to note is the style of writing. Simply look at the page range in the citation above … these old-school scientists were verbose! If you’re looking for concise statements and something you can digest quickly, this is not the place to go. But, if you’re in the mood to read some great scientific prose and dive deep into an idea, these old papers can be a delight. Here’s the opening sentence:

Nature vibrates with rhythms, climatic and diastrophic, those finding stratigraphic expression ranging in period from rapid oscillation of surface waters, recorded in ripple-mark, to those long-deferred strirrings of the deep imprisoned titans which have divided earth history into periods and eras.

How great is that?

Just as a note, the term ‘diastrophic’ is a no-longer-used term referring to the deformation of the Earth’s crust … so, in essence, Barrell is citing climate and tectonics as the two major forcings on sedimentation and stratigraphic patterns. I’ll come back to this later.

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STRATIGRAPHY?

If you’re new to this blog or stratigraphy in general, it’s worth taking a moment to discuss it. What is stratigraphy? I googled it just to see what comes up first:

• Merriam-Webster online => “Geology that deals with the origin, composition, distribution, and succession of strata; the arrangement of strata.”
• Answers.com => “The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks.”
• Wikipedia => “The branch of geology that studies rock layers and layering (stratification)”.

Those are fine, I suppose … but these are rather bland definitions that miss the essence in my opinion. Fundamentally, stratigraphy asks the question: how is time recorded in sedimentary rocks? When you look at a succession of sedimentary rocks you are looking at a record of the Earth’s surface through time. If you’re interested in paleobiology or Earth history, for example, understanding how the passage of time is recorded in those rocks is critical. Ideally we would use absolute dating techniques to date every single layer of sediment. Since we can’t do that yet (that’s a challenge to the geochronologists), we need to make inferences and interpretations about the passage of time from the characteristics of the rocks^.

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BASELEVEL

Barrell’s paper is one in a long lineage of published works that contribute to the development of the field of stratigraphy we know today. If you’ve read Theoretical Stratigraphy #1, this passage should sound familiar:

The deposition of nearly all sediments occurs just below the local baselevel, represented by wave base of river flood level, and is dependent on upward oscillations of baselevel or downward oscillations of the bottom, either of which makes room for sediments below baselevel.

Note that Barrell attributes baselevel to a single geomorphic level — in this case, wave base of river flood (see first post for more on baselevel). You’ll also note the definition for what stratigraphers now term accommodation, which is simply the space available for sediment to accumulate. You’ll hear stratigraphers talk about systems with high or low accommodation, fluctuations in accommodation, and so on. One of the most intuitive, and thus oft-cited, examples of accommodation change is attributed to changes in sea level. When sea level goes up, for example, accommodation on the shelf increases. That is, there’s more room for marine sediments to accumulate. Changes in sea level have certainly had a huge impact on the stratigraphic record; however, I would caution against jumping to that conclusion in all cases. But that’s a topic for another day.

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COMPOSITE CURVES AND RHYTHMS

So, Barrell discusses baselevel and accommodation, but the aspect of the paper I wanted to highlight for this post is his discussion of, as he puts it, the rhythms of geologic time as recorded in sedimentary rocks. As far as I know (please correct me if I’m wrong) Barrell was the first to explicitly discuss composite rhythms within the context of what’s preserved in the stratigraphic record.

The concept of composite rhythms is explained by Barrell with this statement:

Nature pulsates with many rhythms, small and large, fast and slow. Their combination gives a varied curve which, if the rhythms are incommensurable in period, may never recur in quite the same combination.

Even better, this is illustrated in the key figure from Barrell’s paper below (click on it for a bigger version). The vertical axis represents baselevel and the horizontal axis is time. Barrell is combining three curves of different wavelengths and amplitudes into one composite curve (by the way, if you’ve taken a stratigraphy or sequence stratigraphy class, this figure should look familiar; it has been reproduced many times over the years).

The concept of composite curves, in general, was not new in 1917. But, Barrell applies it directly to processes of sedimentation and preservation. He shows a generic stratigraphic column on the left side of the figure. On the top of the image, he’s showing which time intervals are actually recorded by sediment accumulation (with the vertical black bars). Remember, this is a theoretical example in which sedimentation (and preservation of sediment) occurs during the times of maximum baselevel rise on the composite curve. Note how the black bars at the top are clustered (bounded by dashed lines) during the rising limb of the intermediate curve. And then each individual black bar is tied to the highest frequency curve. The idea of preservation on the rising limb of a baselevel curve was later tested quantitatively with numerous and broad-ranging modeling efforts (see Paola, 2000; Sedimentology, v. 47 for a great review of the history and types of modeling in stratigraphy research). Conceptually, this idea is still generally accepted in the stratigraphy community.

Barrell’s top figure illustrates the inherent spasmodicity of sedimentation and preservation – it occurs in fits and starts. Those fits and starts occur across a broad range of time scales. Although Barrell illustrates this idea with the composite curve of three distinct ‘frequencies’, he addresses complexity within a continuum:

The curve, to correspond to nature, should be imagined as less regular and with more orders of rhythm.

In other words, each component curve would be much more erratic and there would be many more of them. More recent research has attempted to address this from interpretations from natural systems and results from modeling. If you’ve read this far, you might be wondering what the origins of these theoretical curves are.

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WHERE DOES THE RHYTHM COME FROM?

So, if oscillations in baselevel control stratigraphic packaging, what controls oscillations in baselevel? Back in the beginning of this post, I mentioned how Barrell cited climate and tectonics as two major controls on the rhythms of sedimentation. Barrell briefly discusses origins of climate rhythms:

Another method [of investigation] is that of the detection of rhythms in parts of the sedimentary series, and the correlation of these rhythms with known climatic cycles.

He then talks about the climate cycles as a function of orbital precession and solar energy — Milankovitch cycles essentially. This is around the same time that Milanković, himself, published work related to these cycles. It might be interesting from a history of science perspective to try and track the development of the ideas within the context of who published what and when.

Many small oscillations and some of even larger magnitude are, however, dependent upon climatic change.

I’m not going to go into an exhaustive review of climate and sedimentation in this post. There are numerous papers and parts of textbooks that go into that topic in more detail#. We still don’t have a good handle on how exactly climatic rhythms translate into stratigraphy. While there may be cases where physical patterns in sedimentary successions do “match” with temporal patterns of climate variability (e.g., varved sediments interpreted as product of Milankovitch cycles), most depositional environments likely have a complex suite of intermediary processes that result in a low signal-to-noise ratio, so to speak, as a result of varying types of processes and timescales.

Investigating the relationship of process-response in sedimentary systems is still an active and vibrant area of research. One of the purposes of this post is to show that these ideas have been around for quite a while. It is fascinating to read a paper like Barrell’s because it can more clearly reveal the state of current research. If you’re interested in this field, I definitely recommend reading this paper for some important historical context.

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* Unfortunately, I do not have a digital copy of this paper, only a slighly tattered and poor-quality photocopy version. If anyone finds a link to a digital version somewhere, please put link in comments below

^ Check out a great book about stratigraphy that is written in relatively non-technical language called The Nature of the Stratigraphical Record by Derek Ager. It is a fairly short read with a playful and quite enjoyable tone.

# A text by Andrew Miall called The Geology of Stratigraphic Sequences is just one of many good references. Please comment below about others that you like.

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