Patrick Nduru Gathogo

Physical stratigraphic dating methods in the Turkana Basin —Leveraging on tectonic and volcanic events

The geochronology of Pliocene and Pleistocene deposits in the Turkana Basin, where many hominin fossils have been discovered, is largely based on isotopic dating and geochemical correlation of volcanic materials such as tephra and basalts. Age control therefore remains challenging for fossil sites that are isolated by faults and which lack suitable materials for isotopic dating or geochemical correlation. This talk will present a petrology approach, based on which key rock features can be used for the physical stratigraphic dating method at local and regional scales.

FULL TRANSCRIPT 

I am very glad for the opportunity and the invitation to come here and present some of the work that I've been doing. And this is a very important occasion for me because it's an opportunity for me to pay tribute to several people who have played a very significant role in my life, in my career particularly. And before I go into that, I'm just going to introduce what I'm going to be discussing about. So, I'll start with the normal type of chronology or framework that we have in the Turkana Basin. So, I'll mostly be discussing about Pliocene and Pleistocene in sequence and then I'll talk about petrology and what we can be able to do with petrology because that is the basis that I'm using for the method that I'll be discussing. So, it's a type of relative dating method, but if you use it very carefully together coupled with isotopic dates or paleomag and similar things, you can be able to come up with a very, very precise age estimates of a site. 

So that's what I'll be discussing and many of these ideas, they come from some of the work that Frank Brown started a while ago. And part of the work that I've been doing is basically to continue with the legacy of the work that he did. So you'll be hearing me mention him very many times during my discussion. And these are very have been very important people in my life. I met Richard Leakey at the museum when I was working there as a volunteer, and he's actually him and Dr. Meave Leakey who gave me an opportunity to go to Turkana for the first time. And Richard Leaky introduced me to Frank Brown and he recommended that I become his geology student. And that's how my university education started. And then I came to University of Utah and I was Frank Brown student for both my bachelor's, master's and PhD. Everything was under Frank Brown. And I remember when I was going to get an interview at the US Embassy, it was extremely very difficult to get a visa at that time. And I got a recommendation letter by Richard when I took it to the embassy for interview. I think that my interview was the shortest one of all the people that went there. So they just read the letter and they told me, come in the afternoon and get your visa. 

So this is a very important talk for me because it's really a reflection of the fruits and what Richard really wanted to happen with me being trained as a geologist and continuing with the work that Frank was doing. And there's several organizations and people who have contributed towards my studies through either scholarship or funding to go to the field and they include some of these institutions that are listed here and many other people. So I always start with acknowledgement because sometimes I talk too much and I don't have time to acknowledge. And my talk will be about the physical stratigraphic dating method. So, it's basically using field observation and then to refine and then relate various event geologic events that happen in the field for you to be able to come up with a date of site. And then these geological events includes both the deposition of volcanic and tectonic, and I'll be focusing a lot on tectonics and volcanic. 

So, the geologic framework of the Turkana Basin is one of the best in that part of the world, particularly for the Pliocene and Pleistocene. And that is because of the presence of a lot of volcanic ashes that are associated with pumices where you can be able to get feldspar crystals and date them and then link those ashes with glasses that you can use for geochemical correlation and stratigraphy. And for that reason, people ask why you would have any geochronology problem in the Turkana when you have such a very, very well refined stratigraphy that is mostly using tephra material. And then you can also refine that using paleomag and some other methods. 

And because of the work that has been done there, identifying various volcanic ashes, this graph here shows the distribution of all the eruption that have been happening in the Turkana Basin for the last 4 million years. And this is just based on the ashes that have been able to be correlated. So, we have almost 400 eruptions of volcanic ashes within the Turkana Basin. So that gives us a very, very well refined stratigraphy within the Turkana basin. But there are some occasions where you are not going to be able to find these volcanic ashes or anything that you can obtain absolute dates. And often you'll find your section having faults all over. And that's where you have to devise a method that you can be able to use in order to tie your isolated outcrop to another area where you can be able to obtain good dates. And that's what I'll be discussing. There are so many sites that have this kind of scenario and it happens that Frank Brown was a petrologist and one of the methods that he developed for identifying volcanic ashes was first of all to be able to sort because they get mixed as they're traveling from the sites of eruption to where they're deposited. 

And it happened that these volcanic ashes also get mixed together with sediments. So the method that I've been using, which was initially proposed by Frank is looking at the sediments that come together with the volcanic ashes and also to find some birth marks that you can be able to identify within those grains, whether it's quartz grains or feldspar. And it's a method that has been used very effectively in the oil and gas. It happens that I've worked in the oil and gas for quite a while, since 2006, so I know they use this method very effectively. And Frank said, why can't we be able to develop such a method so that we can use it together with the absolute dating method within the Turkana Basin? 

So he collected at least 3000 samples of volcanic ashes that have also the trait of grain with them. So that's what we were working with and most of them were from the Turkana Basin but the challenge with this  is that you require a lot of money to be able to carry out such a study. And for geology it can become very tricky to be able to obtain funding to do this. I'll give an example of the application of this that I was able to do in the oil and gas using the petrology method and we have proved that it can be able to work there. So, there's no reason that you should not be able to work in the Turkana Basin. So in the next few slides I'm going to show that. 

So, for you to be able to use the petrology method, you have to be able to identify where the sediments are coming from. So, for example, this is the drainage basin of the Turkana Basin. So, you have to know the characteristics of the quartz or sediments that are coming from that part of the basin and the other ones that are coming from these other parts of the basin. And that way when you find them in a fluvial deposit, you can be able to link them to their source. So that's one of the important thing to be able to know. And the sediments are going to be coming from different sources, either metamorphic rocks or igneous rocks, and from very, very different age groups. And depending on the environment in which they're formed, either quartz or feldspar or other minerals, they'll have some distinct characteristics that  will be a reflection of the source of their sources. 

And in addition to that, they might even be coming from basalts that are coming from different age intervals. But even with that, you can still be able to use the petrology method to identify class that are coming from different type of basalts. And one of the examples that I'll show you here show how that could also be very effective. So, whether you have sandstones or conglomerates or pebbles, you can still be able to use the method for that. So, one of the studies when I was finishing my PhD, I was sent for internship to work in the oil and gas industry, and I had an opportunity to do a study that was using petrology. So, within five years I had a sample collection of about a 100,000 samples, petrology samples were coming from all over the world ranging in age, from Ordovician up to present and from very different environments. 

And the objective was to see whether what you can be able to tell about the deposition environment and so many things concerning reservoir. So this was in the oil and gas reservoir. So this is what I was able to come up with. So these are the section images. So, the colors here, are a reflection of the electrofacies that they're getting from the Basin. So, you can actually see that each distinctive color that they're getting from the basin has a very unique petrologic characteristic. So, the petrology is giving you the face of the facies that you're having there, and you can actually even be able to know the deposition environment because some of these sediments, they have fossils, and you can even be able to know that their genetic transformation. So there's a lot of information that you would be able to know just from petrology. 

So that includes the original deposition of facies and then the deposition and the diagenesis that has happened over time. And from that electrofacies, which is based on logs that are drilled into the wells, you can be able to also link it with seismic data. And once you link it with seismic data, you can be able to extrapolate and take it to other parts of the basin. And seismic data will help you to be able to identify where faults are occurring and where these brick in the beds. And then you can also link that to sequence stratigraphic model and that would really cover the whole basin that you have. So, this method is very effective in the oil and gas and petrology for that reason became a very, very important tool, especially for unconventional reservoirs here in the US because you can be able to tell a lot, even just from the microscopic features. 

And this is the example that proved to me and to Frank, that you can be able to use this method to do such studies in the Turkana Basin. So, the first application of physical stratigraphic dating method, we use it to refine the age of this basalt here that used to have different name depending on where you are. So, for example, here it was called the Kataboi at Lothagam it was the Lothagam basalt and then you'll have another area, it's called the Kokoi and it had different isotopic ages that they had, they had for each ranging all the way from 3.0, maybe 3.8 all the way to almost 9 million years. 

Around 2004 when I was doing my master's, my bachelor's degree, Frank took me to one of the sites. And in that site we saw there was a shell bed. And the reason he took me to that site is because he thought it was unique because the shell beds were cupping on basalts. But what we discovered is that there is actually a chilled contact between the basalts and the shell bed. And we went to very many location within the basin and we saw the same relationship. So this showed demonstrated to us that the basalt was actually intruding into a lake and we were able to confine the age of that basalt to the age of the occurrence of that lake. And when you look at the petrology of that basalt, the mineralogy is basically identical. The texture, the fabric of the minerals identical. So for that reason, we were able to assign that basalt to about 4 million years. 

And even though, for example in this area it looks as if it's overlaying lake sediments, we see a lot of areas where the dykes were actually intruding the lake sediments. So the basalt spread throughout the basin and it was associated actually with some tectonic events that were happening in the basin. And after seeing that, we wanted to go to some other areas there, we also have basalts deposited there, and that area is Loiyangalani, we saw the same thing. There were several basalts that were erupting, and in between them are sediments. And every time you see those basalts, you would see a lake sequence developing. So all these basalts were intruding lake sequences. And what that told us that there is specific deposition environment that is associated with the eruption of basalt and most of these basalt were associated also with faulting. So, there are three things that are happening this faulting that is happening, eruption of basalt almost at the same time as the time the lake are forming. So, if you can be able to identify those markers because there'll be evident throughout the basin, then it'll be easy for you to know what time that you're dealing with because it'll be associated with a specific deposition environment, either lacustrine or you'll see a major erosion taking place around there. 

So, we saw that the association is occurring throughout during the Pliocene and because it's a major thing that is happening within the basin, if you can be able to identify that stratigraphic marker as an erosion surface or conformity, then you would be able to know what the age range that you're dealing with. And this is the occurrence of that Loiyangalani section. And you see where the intensity of basalt volcanism is occurring there. That's where you have the biggest gap in the sediments sediment occurrence within the Koobi Fora formation. So, it means that the deposition within the Turkana Basin, at least during Pliocene, was very much affected by volcanic activities that were happening. So, if you can be able to identify that horizon, even if you don't have volcanic ashes or other things, then you would be able to use it as a stratigraphic marker. And that is one of the phenomenon that we're using for dating, finding precise ages for some of these sites, including South Turkwel and other places where you have isolated outcrop that don't have volcanic ashes. 

And with that, we were also able to reconstruct the distribution of the basin sediments within the basin and know that there are a lot of sediments within the basin. And if you can be able to know where you have faulting taking place and bringing up those sediments, then it would be easy for you to be able to find outcrops or sediments. And I am running out of time here. So, once you understand this association between faulting, basalt volcanism and deposition environment, it'll be easy for you to even be able to find a new outcrop because one of the common thing between this Miocene deposits and the South Turkwel Pliocene deposits and some of these Oligocene deposits is a big fault that is going through. And it started being active all the way from Pliocene, from Miocene, mid Miocene and when you go to South Turkwel, you can actually see it being active almost about, 3.5 million years. 

So that means that if you walk around there, you're very likely to find some sections of deposition that are going to be cropping out. I'll go quite fast here and we can be able to use the same technique to understand what was happening in the basin in terms of deposition by taking advantage of cuttings and using petrology to reconstruct the deposition environment within the Turkana Basin. And this has been very effective methods in the oil and gas, so we can basically be able to use that. And one of the studies that I did here for the oil and gas is to take a section here, the vertical section and cuttings from the horizontal and basically reconstruct how the facies are developing along the stratigraphic units and even know where the faults are occurring just by using cuttings. So, it's a similar thing that we will be able to do within the Turkana Basin because we have a lot of wealth from the oil and gas and data that we can be able to use for reconstruction of that. 

And this is just an example of how you can be able to use petrology to know how some of those facies are occurring. For conclusion, so petrology will be able to help you relate and then know the timing of various events, whether they're basalt volcanism or tectonic. And if you can tie those events with a date that you know elsewhere, then you don't even need to know to have direct isotopic measurements there. You'll use that relative date as the approximate date for that interval. So, there are many other application that you can be able to use for this, which includes, for example, even for economic use, geothermal and finding many other resources just based on petrology. And the good thing about that is that you will be able to get resources even from other people working in the basin, including the oil and gas companies that are working there, and we could use some of their data to really be able to reconstruct the geology of the Turkana Basin. And so I think that's the last slide because of gone over time.

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