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Strata are not flat

Students in introductory geoscience courses typically first consider stratigraphy as a pile of horizontal layers or beds. Fundamental tenets of geology such as the principle of superposition and the principle of original horizontality help stamp an image of so-called layer-cake stratigraphy into the minds of incipient geoscientists. Moreover, introductory structural geology classes show diagrams with layers of uniform thickness and character to unambiguously illustrate types of folds and faults.

But this is wrong. It is more appropriate to think of sedimentary layers as sedimentary bodies. No sedimentary layer extends out uniformly across the planet, they eventually pinch out, get cut out, or maybe transition into a different type of sedimentary rock. Thus, these sedimentary bodies have a shape to them, they are not rectangles. Nearly all sedimentary bodies, when viewed in a 2D cross section, have wedge-shaped edges. Bailey (1998) called these bodies ‘lenticles’ after their lenticular shape. The geometry might not be visible unless you impose a great deal of vertical exaggeration (as stratigraphers often do with seismic reflection profiles) but it is there nonetheless. The layer-cake view of stratigraphy does not consider:

1. complex patterns of deposition and 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 the Landsat image of the Lena Delta below.

In most cases, however, we do not see a fully and perfectly preserved depositional system in the rock record. Processes related to the preservation of a body of sediment or rock results in an amalgam of numerous elements of the system over a long period of time. These processes can, in some cases, ‘smooth out’ complexities but they also introduce a few of their own.

Considering stratigraphy as a 3D configuration — or architecture — of sedimentary bodies is important for subsurface characterization. In some cases you might have sufficiently high-resolution seismic reflection data to explicitly map such geometries. In other cases you will be asked to interpret (predict!) them.

Romans__Lena_River_Delta_-_Landsat_2000.jpg

References
Bailey, R (1998). Review: Stratigraphy, Meta-Stratigraphy, and Chaos. Terra Nova 10, 222-230. DOI: 10.1046/j.13653121.1998.00192.x

Acknowledgments
This essay is modified from a version that first appeared as a blog post: ageo.co/14Tlbas

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