Saturday, 13 September 2014

Day 8 - A Detectives Game

Today we started mapping. The caldera covers a vast area of 77km2. Some literature has it larger at around 88km2. I suspect this difference depends on if the flank crater is included as part of the caldera or not. This area means it is too large to map in the time we have, so we will start mapping along the roads. Most of the area is lavas as already discussed. Mapping these will be a case of trying to determine which flow is which.

Today, mapping began close to the security gate we pass through every day. Here there are sedimentary deposits, reported to be lacustrine, from a time when the north of the caldera was filled by a small lake.

The first section we began to observe had three separate sedimentary units. The bottom unit was a tuff. Not a sedimentary unit in the classic sense, but a volcanic sedimentary unit, a lithology I have mentioned briefly previously. This type of deposit comes under the igneous classification with a sub-classification of pyroclastic rocks, in which sedimentary structures are often observed. The deposit is only visible because of the construction of the road. It fines up from a layer of pumice ranging from 2-5cm to grains that are 1mm in size.

Lacustrine deposits in the north east corner of the caldera

Overlying this is a sedimentary unit associated with the lake. It is very pale in colour and contains small grains of pumice and lavas up to about 3mm in size. The matrix, which is dominant, is very fine grained. The best way to describe this unit is a lithic siltstone. Within the siltstone, bedding has been preserved and fractures can be measured. This is further overlain by another tuff that again fines up. The layer of pumice at the base is absent, but there are still pumice clasts with in the tuff.
Observations made of the siltstone showed there to be changes in dips of the bedding from gently dipping to the north, to almost horizontal, to gently dipping to the south. This shows a very gently folded, almost undulating topography only observed locally. The folds would suggest a small amount of east-west compression. Even though we are in a very tectonically active region, I believe this folding is more likely associated with some sort of diaper process. Changes in temperature of the water stored at depth may cause expansion and contraction that is expressed at the surface as gentle folds, this could also apply to hydrothermal alteration of some minerals. An example of this is well recorded in the Troodos Mountains where many years ago the area was under the sea. Sea water percolated through the ocean crust altering a mineral known as olivine. The resulting mineral, serpentine, is less dense than olivine. The huge amount of alteration at this location resulted in a buoyancy effect, with the crust slowly rising over time forming the Troodos Mountains we see today. This is just an example of how mineral alteration could explain the folding observed in the siltstone.
Within the sedimentary unit there is a volcanic bomb measuring at least two and a half meters high (some of it is below the road cutting). The bomb is very green in colour with fragments of obsidian and lavas, there are also what can only be described as obsidian veins running through the bomb and obsidian around the edges. There are no displacement bands within the siltstone and the contact between the bomb and the siltstone is very sharp, with little to know alteration in the form of margins. It appears that the first tuff might have settled out of air suspension on to the surface of the lake and further settled out of water suspension. The bomb has then dropped in to the first tuff unit, landing in the lake water has given the impression of a hyaloclastite. Deposition of the siltstone followed, with deposition of the second tuff later. Because of the road cutting, it hasn’t been possible to establish what is beneath the lower tuff at this location. Further along, there is another, much larger area of alteration that is unlikely to be a bomb, but instead an area of tuff that has been altered by hydrothermal processes.

Approximately 300m to the west there is a quarry. The deposit is being actively removed for use in the workings of the geothermal project. There are some very pretty deposits now exposed in the quarry wall. At the base there is a very pale grey deposit of clay. It is very porous, so much so that quite large pieces stick to your tongue! 

Awful pic, I know. But it really does tick to your tongue

The grains are so fine that individual mineralogy cannot be identified with the naked eye or hand lens. I believe the deposit was a crystal tuff dominated by feldspar that has been hydrothermally altered. Above the clay there is a thin, dark grey band that is likely to be a volcanic ash layer, with more clay above this. Further up the quarry wall there are about 20 individual fining up layers of different thicknesses. These are tuff deposits of varying compositions and some thin sedimentary layers also. One layer consisted of small angular fragments of obsidian, likely to be a vitric tuff. Further in to the quarry (but due to workings, unaccessible) we could observe from a distance another, much thicker unit of ash and blow that what we can only assume to be more clay. The ash has a green/grey appearance which may also suggest alteration to chlorite.

The Quarry

The final location of the day is at a similar elevation to the first. Here there are tuff deposits, some of which are almost horizontal and are likely to be pyroclastic fall out deposits. Others have sedimentary structures suggesting pyroclastic flow towards the east.

Tuff deposits with sedimentary structures

I did initially think that the tuff deposits were linked to the collapse phases of the caldera. Two tuffs have been recorded in the literature (eg. Leat et al., 1984). However, these records are for tuffs up to 45km away from the caldera. The very small light particles found in a tuff can travel great distances with the wind. What has made me question whether the tuffs in the caldera are from various phases of caldera piecemeal collapse is the size of the grains within the caldera tuffs. I would expect to find very large clasts, blocks and mega breccia this close to the source. Yet they are very small to fine dust, the type of deposits you might find at or close to the distal edges. I now wonder whether the tuffs here are actually from another volcano that erupted close by after the collapse of the Menengai caldera.  Reading through the literature, I have not yet found any mention of dating for the tuffs within the caldera, however it does not mean the research has not been done. This is also the case for cosmogenic dating of the exposure of the caldera wall, dates that could then be used to correlate the distal tuffs identified as Menengai tuffs 45km away.

It may seem that we didn’t get a lot of work done. And to be honest we didn’t cover a great deal of ground. But sedimentary deposits often preserve so much detail and yield so much information. If observed properly and every little detail measured and discussed either in notes or with others, a small section of a sedimentary deposit can often take some time.  

Tomorrow, more mapping, heading to the central and southern areas of the caldera.

Lala Salama from Kenya.

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