Geoscience Reference
In-Depth Information
from mapping) a place where either the top or the bottom of
the sill steps up or down the stratigraphy (Figure 7.7, p. 144). A
lava fl ow cannot do this (but beware of a lava fl ow emplaced
unconformably onto a tilted and eroded sedimentary
succession; Figure 7.7b). You should be alert for such local
discordances in a sill. These may give clues to the large-scale
shape of the body; in particular, major sills intruded into
sedimentary deposits tend to be saucer shaped, by virtue of
discordant steps that allow them to be both thicker and deeper
towards their centres.
cone sheets
7
(a)
magma chamber
A lava fl ow may be emplaced over a previously weathered
surface, and in turn its top may have been weathered before it
became buried by a subsequent lava fl ow or by a pyroclastic or
sedimentary deposit. Although a sill may be intruded along a
previously weathered horizon, the top of the sill itself cannot
become weathered (except if it is subaerially exposed sometime
after intrusion). Therefore, a weathered (typically reddened,
maybe even developing into soil) top to a concordant igneous
sheet is strong evidence that it is a lava fl ow.
ring dyke
(b)
magma chamber
Figure 7.5
Block-view of (a)
cone sheets and (b) a ring dyke,
showing their typical outcrop
pattern and relationship to a
magma chamber (which would
solidify to form a pluton). The
ground surface shown in (b) has
not yet been eroded, and the
circular depression bounded by the
ring dyke is a volcanic caldera.
Figure 7.6
The base of a sill (Teesdale, Co. Durham, UK). Buff
coloured fi ne-grained sandstones occur below the level of the person's
shoulder. The base of the sill is not actually in the obvious recess at
head-height where, especially on the weathered surface, the colour
changes. This is a thin siltstone layer overlain by a further
c
. 1 m of
sandstone. Close study in the fi eld with the aid of a hand lens reveals that
the base of the sill is at the level indicated by the arrows. (David A.
Rothery, The Open University, UK.)
