Measured Sections

Before I get back on the research philosophy thread, I want to make a little digression for the stratigraphy-oriented. If measuring stratigraphic sections is something you never plan on doing, your time might be better spent visiting another blog. If you haven’t seen it, I would encourage you to check out the picture Darren posted a few weeks ago on Tet Zoo of a giraffe getting on a donkey.

Still here? Your loss. It makes a great desktop.

Testability and repeatability are cornerstones of science. Although the vast majority of the people who read your publications will probably never examine the original data first hand, a few will. These readers are arguably the most important, as they will be the ones who will actually evaluate your work. It is important that your presentation of evidence be explicit enough that they can retrace your steps and understand exactly what you were looking at when you made a particular claim.

Measured sections are the basis of stratigraphic research, but curiously few stratigraphers seem to care whether or not anyone will ever actually try to identify their units in outcrop. When Bill and I began to work on the stratigraphy of Petrified Forest National Park, we wanted to use previously measured sections as well as our own. A lot of workers have measured a lot of sections in the park over the past 50 years, and it seemed a waste to re-measure them all when there were perfectly good ones available. However, matching units on paper to units in outcrop is extremely difficult.

We wish therefore to make two recommendations:

1. Provide labeled photographs of the outcrop.

2. Don’t micro-measure

The advantage of providing a labeled photograph with your units actually marked should be obvious. Here is a crappy, low-resolution photo for one of our measured sections. We identify exactly where we measured the units (off to the right for the lower part of the section, to the left for the upper part at the small mesa). Anyone will be able to use this photo to walk right up to the outcrop and examine our units, right where we described them:






Micro-measuring needs a little more explanation. Identifying units in an outcrop is inherently subjective, and you can of course break them up in various ways. However, it is easiest to identify someone else’s units when you break them up into relatively big, visually distinctive chunks. The more units you have, the harder it gets to match up described units to outcrop, especially if you are talking about a complex fluvial system like the Chinle Formation in which units can vary radically over a few meters.

For example, let’s say you have a reddish mudstone with a lot of interbedded fine grained sandstone lenses which are pretty similar to each other and vary from .5-2.5 cm in thickness. This whole package is capped by a distinct unconformity with a drab-colored conglomertic unit on top. How many units do you break it up into? I would suggest two: the mudstone with interbedded sand lenses, and the conglomerate.

However, do NOT do this for the lower unit:
unit 1: thin sandy layer about 3.5 cm thick;
unit 2: 10 cm of red mudstone;
unit 3: sandy layer 2.2 cm thick;
unit 4: 15 cm of red mudstone;
unit 5: thin sandy layer .75 cm thick
unit 6: 12 cm of red mudstone (etc…)”.

Murder. This is going to be awfully goddamned hard to replicate exactly in the outcrop, especially if these sandstone lenses are discontinuous. How will anyone else ever know exactly which sheet sands you were describing, especially if they change if you move over a few meters on the outcrop?

Granted, the labeled photograph might help another researcher find the exact beds you identified, but also think in terms of what useful information you are supposed to be getting here. To people interested in, say, the depositional system, or the overall thickness of the member these units are a part of, the more general description is a lot easier to digest, and letting them know the exact thickness of each individually laterally discontinuous sand lens is probably not going to be provide any particularly useful information. If a particular sandstone lens IS important (for example, a bone horizon) you can just identify it on the labeled photograph anyway, and mention it in the general description: “the bone producing horizon is a sheet sand about 2.1 cm thick which is about 54 cm above the base of the unit.”

Think about future researchers who want to test your work. Make it easy for them, not hard. Mapping is another underappreciated form of documentation for lithostratigraphic models I’ll be talking about when I get to our work in the park.

LNJ