Throughout the history of the Turkana Basin, the dominant hydrographic systems that have fluctuated throughout the basin have been fluvial and lacustrine systems. While this fluctuating environment has been a main theme throughout most of the Geology module, students are now turning to studying current day environments to better understand sediment accumulation in past environments. While they just recently visited Lake Turkana’s shoreline, it was now time to examine a deltaic environment. During lacustrine stages in the basin, deltas form wherever a river meets a lake. Sediment accumulation is typically high at the delta due to sediment being supplied faster at the river mouth than it can be removed by lake processes.
For this lesson, we focused on deltaic environments and different methodologies to study the sedimentary processes that are occurring in modern day environments. Deltas are very important in the Turkana Basin because there have been many important fossils preserved in paleo-deltaic deposits. In the Koobi Fora and Ileret regions in which we are predominately working, deposits from the Omo river delta can be found that date from the Pliocene-Pleistocene.
The Ileret region is, as the crow flies, rather close to the Omo river delta, but reaching this area would involve taking boats and is mostly presently located within Ethiopia. Due to limited time, the students had to look for the answers at another nearby delta that would preserve similar processes as the Omo river delta. Fortunately, we have our very own delta where the Ileret river empties into Lake Turkana! Although not as large as the Omo, it was close and just as informative. The Ileret river delta is particularly interesting because it is an ephemeral system. As such, it typically flows only during the rainy season; but when it does, it is a rather high energy braided river, and thus moves great amounts of sediments.
For our field practical, we obtained sediment cores. Coring is a method used very often in geology to obtain samples of sediments and/or rocks in an area. Though we would only be using four foot long PVC pipes to take our samples, in other areas and studies, cores can be absolutely massive, such as those used in mid-oceanic drilling and sampling.
The students were divided into three groups: each group would be taking a core in a different area of the delta, with each area being indicative of different influential processes. Two would be taken in the delta plain, which is closer to the river channel and is typified by more vegetation, and one at the delta front, which experiences more wave action from the lake.
To take an accurate core, you are trying to push the core as deep into the sample area as possible while still being able to remove it (oftentimes this is easier said than done!). Once the core has reached a sufficient depth, it must be sealed: we used a fitting cap which was also duct taped around the PVC to ensure an airtight seal. Sealing the top of the core creates suction that should hold in the majority of the sample inside the core as you remove the core from the ground. Further, as soon as the bottom of the core exits the ground, you should seal the bottom (either with a hand, cap, or other method).
Removing the core after forcing it a few feet into a claylike lake and delta bottom is much more difficult than can be imagined! The surrounding clay suctions to the PVC pipe. Teamwork was very important in successfully removing these cores!
Once the cores were removed and appropriately sealed, they were frozen immediately after returning to TBI-Ileret. This is done to better solidify the sediments and keep them accurately intact when the cores are split open. The cores were split open using a grinder, wielded by our ever helpful driver and mechanic, Mwan’gombe. He was very careful just to split the PVC pipe and not the sediments inside!
Once the core is split open, the students can then analyze what they see inside, and compare across cores what each group found. This practical is especially fun and interesting because it is almost an exact replica of a very important method used by geologists around the world!
While at the delta, visiting geologist Dr. Davis was experimenting with research of his own! Because this was his first time in the area, Dr. Davis had brought his GPR (Ground Penetrating Radar) equipment with him to conduct preliminary data inquiries. He was not quite sure what he would find but was interested in what this area may have to offer!
GPR uses radar to detect changes in underlying sediment: for example, if there is a large uniform sandstone bed, the radio waves rebound with a uniform signal. However, if there was a large fossilized femur (or other sufficiently large object) in the middle of the bed, the waves would rebound with different characteristics off of the fossil than the surrounding sediment!
Further, it was very interesting to see a geologist conduct research with high-tech equipment. This experience really brought to light the unique nature of this field school—we are not just learning about this material in a classroom, but rather learning about it as we encounter it in the field. The Earth itself is the best geology textbook!