Geologic Misconceptions: “Layer-cake” stratigraphy
This post is part of the fifth installment of the geoscience blog carnival, The Accretionary Wedge. The topic is ‘geologic misconceptions’ and it is also National Pie Day. Instead of pie, I discuss cake … a very messy, uneven cake. Head on over to Green Gabbro for info and links to all the posts for TAW #5.
I don’t care if the sedimentary layers in this image “look” flat…they’re not.
Okay … maybe they are essentially “flat” over a few or even several 10s to 100s of kilometers. But … as one who studies strata, one of my pet-peeves is the portrayal of sedimentary successions as perfectly flat layers.
It is more appropriate to think of sedimentary layers as sedimentary bodies. No sedimentary layer goes on forever … they eventually pinch out, get cut out, or gradually change into a different type of sedimentary rock. As a result, these bodies have a shape to them … they are not rectangles. Nearly all sedimentary bodies, when viewed in a 2-D cross section, are some sort of wedge-shaped body. Bailey (1998) called these bodies “lenticles”, that is, they have a lenticular shape. The geometry might not be visible unless you impose a great deal of vertical exaggeration (as stratigraphers often do with cross sections) but it is there nonetheless.
Why does this bother me? Or, to put it another way, why is this important?
The “layer-cake” view of stratigraphy bothers me because it does not appreciate (1) complex patterns of deposition/erosion or (2) complex patterns of preservation. A snapshot of a depositional system (i.e., some place where there is net accumulation of sediment) looks more like this:
That’s the Lena Delta in Siberia from GoogleEarth … click on it for a nice, hi-res version.
In most cases, however, we do not see a fully-preserved delta in the rock record. The processes of long-term preservation creates a body of sediment/rock that is typically an amalgam of numerous elements of the system over a long period of time. These processes can, in some cases, “smooth” out some complexities but they also introduce some of their own.
Most geologists who don’t specialize in stratigraphy know what I’m talking about. If you do a lot of field geology and have done some mapping, you have, at one time or another, had to deal with units that have significant thickness changes or the study area. In other cases, you may have been able to treat the sedimentary layers as having essentially constant thickness across the mapped area (e.g., some epeiric sea carbonate rocks do indeed go on for 100s of km with little thickness change).
But, the point of bringing this up is not so much for geologists, but for everyone else. Of course, the term stratigraphy itself is rooted with the word for “layer”. I’m not going to get too nit-picky…that’s fine if we call them layers. But, the next time (and every time after that) that you are looking at a sedimentary “layer” remember that what you are looking at is actually a 3-D body with a complex shape and even more complex history.
UPDATE: check out the post by Hindered Settling about misconceptions that arise from 2-D vs. 3-D representations.