Geology Reference
In-Depth Information
Deposit Thickens
in valley and
feathers on sides
Deposit draping
topography
Deposit restricted
in valley
e.g. Air fall
e.g. Low concentration
high energy pyroclastic flow
e.g. High conc. PF
or Debris flow (lahar)
Figure 5.2
Deposit characteristics related to topography.
Pyroclastic sediments can be deposited by pyroclastic fall (fall out) and by
pyroclastic flows and therefore a large range of currents and sorting mechanisms
can affect the resultant deposits. Fall out deposits will blanket topography,
whereas pyroclastic flows will commonly be restricted to pre-existing valley
topography, so an understanding of the deposits lateral as well as vertical varia-
tion is critical in understanding the rocks (for example, Figure 5.2). Sedimentary
bedding structures again will vary considerably depending on the type of den-
sity current (high or low concentration) which sediments are being deposited
from and from fall out type layers. Examples of common sedimentary struc-
tures are given in Figure 5.3. For example, low concentration lateral blast type
deposits (sometimes termed base surge) can blanket topography like fall out but
are recognised by their more poorly sorted character and presence of tractional
structures like cross bedding (see Figures 5.3 and 5.4).
Bedding structures vary from massive poorly sorted to highly sorted to graded
beds. A range of the grading structures that can be found is given in Figure 5.5.
With grading in primary deposits it is important to understand the different
densities of the particles within the deposits. Lithic fragments of a smaller size
will have the same density as pumice fragments (air filled) of a much larger
size, and it can be very useful to record both lithic and pumice size varia-
tions separately throughout the deposit. This is of particular importance in fall
out deposits.
5.1.2 Classification
Pyroclastic fragments are classified by the type of material and their grain-size
limits. The grain size limits used in pyroclastics is given in Table 5.1 including
reference to the phi units used in sedimentary rock classification (see Tucker).
A simple classification of and nomenclature of tuffs and ashes based on their
fragmental components is given in Figure 5.6. Note the term
tuff
(from the Italian
'tufo') is used for consolidated (ancient) rocks and
ash
used in unconsolidated
