Virtual Field Trip
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Unit G3 at Chimiche - details
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Let's take a closer
look at the lithology in Unit G3.
Enlargement of Figure 32. The coin
is 23 mm in diameter. There are lots of holes on the weathered surface of this
rock. We need to look at them carefully - this is more difficult when we don't
have the actual outcrop or sample in front of us.
Don't forget to zoom in by clicking on the left-hand picture and scrolling around.
- Is the whole rock vesicular? It
might appear so at first, but study the shapes of the holes closely.
- The irregular shaped holes over
much of this surface have angular rather than rounded shapes: for example
D and E, which makes them less likely to be vesicles.
- In some of these angular holes,
such as A, B, C, there is a somewhat rounded clast of
soft material. The rounding is likely to be due to weathering: the outer part
of the clast has been removed, and the true shape of the clast is the angular
outline of the hole.
- Thus, we can conclude that most
of the holes in this rock represent weathered-out clasts. As in unit G1, most
of the clasts are of phonolitic pumice.
- In the large hole where the coin
rests, the upper part of the hole shows the remnant of a large pumice clast.
The holes in this, seen at F, are the more rounded shapes characteristic
of vesicles.
- Look at G. This pumice
clast was weathered out after its vesicles had been infilled with secondary
minerals. What shape are the vesicles? What type of pumice is this? We will
see another example in Figure 34.
- The fine ash-grade matrix between
these clasts is harder (probably due to cementation during diagenesis), and
stands out on the weathered surface, forming a network effect between the
weathered-out clasts, as in area E. Estimate the percentage of matrix in this
image. This will be important when we interpret process.
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This page is maintained
by Roger Suthren. Last updated
8 March, 2021 2:38 PM
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