Over the weekend I learned of the passing of geologist Arnold Bouma last Friday. Dr. Bouma served on my doctoral committee at LSU for several years, although he was on sabbatical the semester I graduated. During my first year in graduate school I took a course from him called “Deep-water Depositional Environments”. This was a bit intimidating, as we were using a textbook that he wrote, and largely studying sedimentary sequences that were discovered by and named for him!
In 1962, Bouma described and explained a sequence of sedimentary rocks which later became known as the “Bouma sequence”. The sediments he was studying were from deep-water deposits and were mostly fine-grained shales, but they included certain characteristics that would more typically be found in fluvial (river) systems, such as graded sands and cross-bedding. Bouma suggested that these deposits were formed by turbidity currents, which are basically submarine sediment flows. Sediment-water slurries are more dense that sea water, and on a shallow slope can flow across the seafloor for great distances. Check out the lab example below (direct link):
These flows leave behind a very specific sequence of sediments. Bouma labeled the unit A-E, with A at the base:
E: Fine-grained, massive bed
D: Parallel laminae
C: Cross-bedded or wavy lamina
B: Parallel, laminated beds
A: Massive, graded, relatively coarse-grained, with an erosional base
The image at the top, from the Devonian Chattanooga Shale, I believe represents a complete Bouma sequence. Here it is with the units labeled:
It’s possible that what I’ve labeled “E” actually includes unit “A” of the next sequence. Bouma sequences are often incomplete, with younger flows eroding parts of the older sequences, or with the lower units never reaching the distal part of the flow.
It turns out that turbidity currents are probably the most significant source of sedimentary deposits in offshore deeper-water environments, such as the outer continental shelf and slope, as well as in back-arc and foreland basins. As such, understanding the origins of Bouma sequences was a critical step in the interpretation of depositional settings and tectonic histories.
Below is a link to a 2010 interview with Dr. Bouma (direct link); embedding is not working for this site: