Geology Reference
In-Depth Information
(a)
20m
Expanded view of preserved flow
front in Etendeka Basalts, Namibia
Volcanic stratigraphy
(b)
SE
NW
Tabular basalt flows
with rhyolite units
compound flows with
sediment interbeds
Sediment interlayers
Isolated dunes
Interpretation
Photo
(c)
Huab River
D2
Tabular basalt flows
with rhyolite units
D1
Tabular basalt flows
with rhyolite units
compound flows with
sediment interbeds
Basement
Karoo Seds.
~65 km
Figure 4.11
Scales of lava flood basalt heterogeneity in Etendeka flood basalts
Namibia. (a) Individual flows with their internal core and crust. (b) Lava flow
stacking at large outcrop scale. (c) Mapped out lava stacking and lateral varia-
tions on a basin scale.
parallel to the underlying floor or curving down against the direction of move-
ment, that is flat lying in the original lava. Traced upwards, such joint planes
tend to become more widely spaced and slabbly, and become more nearly
vertical or even overturned near to the surface. Steep-sided platy joints on the
surface of dacite or rhyolite extrusions may be termed
ramp-structure
.When
recording the structures in silicic flows, it is useful to produce cross-sectional
sketches to highlight the style of jointing top and bottom structures, distribu-
tions of glassy and breccia facies, and so on. Good examples of this are given
in Figure 4.12.
