Uncovering Secrets of the Ocean Floor


A New Understanding of Sediment Fall atop Shatsky Rise

For Rachel Clark, a first-year University of Houston geology graduate student, her summers as an undergraduate were spent conducting research in the lab of Will Sager, professor of geophysics in the Department of Earth and Atmospheric Sciences, where she picked up an in-depth knowledge and appreciation for marine geology.

Rachel ClarkRachel Clark, a Ph.D. student in geology, mapped the sedimentary layers in Shatsky Rise.“Rachel, demonstrating a maturity beyond her years, dove in and learned seismic stratigraphy, mostly on her own,” said Sager, who is a faculty member in the College of Natural Sciences and Mathematics.

Shatsky Rise: Oceanic Plateau

Clark’s research, recently published in the journal Marine Geology, mapped the sedimentary layers atop Shatsky Rise, an oceanic plateau that lies 1,000 miles off the coast of Japan. With the advent of better seismic imaging and more coring data, Clark was able to put together a more nuanced sedimentation model than previously known.

Sediment Caps Overlying Underwater Volcanic Mountains

Shatsky Rise is characterized by three large groups of mountains which were formed by volcanic activity. Atop these underwater mountains are sediment caps, formed by the accumulation of millions of years of sediments settling to the bottom of the ocean floor.

“The sediment caps are very different from each other,” Clark said.

The previous assumption, based on data available at the time, was that these caps were characterized by even layers of sediments, similar to a layer cake. However, this interpretation was incomplete.

Filling in Knowledge about Sedimentary Layers

Using data from drilling reports, which includes analysis of core samples, and modern seismic data, Clark was able to piece together an overall picture of the sedimentary layers in Shatsky Rise.

“Although this research has confirmed the biggest picture from previous models, the story is more complicated than expected,” Clark said. “It’s a little bit like snowfall during a blizzard, which can be chaotic and unpredictable.”

Rather than simple uniform layers, the structures of the sedimentary caps are variable, showing lateral and internal shifts in sediment deposition.

Currents Shaped Sedimentary Caps

Clark found that currents at the bottom of the ocean played a significant role in reshaping these sediment caps, with the currents intensifying and causing erosion during four intervals during which sea levels dropped. Another factor was mass-wasting, which is downslope movement, such as an avalanche, caused by gravity, which caused extensive erosion along the sediment cap margins.

This research experience led Clark to pursue a Ph.D. in geology. She is now in her first year of a marine geology project under the guidance of assistant professor Julia Wellner. This time, though, her focus has shifted to the Antarctic, which displays different patterns of deposition and erosion.

“I’m originally from the mountains,” Clark said. “I never expected to go down a marine geology path.”

- Rachel Fairbank, College of Natural Sciences and Mathematics