Geology Reference
In-Depth Information
Table 2.1
Summary of key features to look for in the basic descriptions of
igneous outcrops (to be used in conjunction with Table 3.6).
Plutonic rocks
Look out for any internal boundaries/contacts that denote different phases of
intrusion and determine their relative ages (cross cutting relationships - chilled
margins). Emphasise contact relationships. For example, concordant or
discordant/transgressive. Use foliations, fabrics, the presence of xenoliths or
roof pendants, plus the state of disturbance of the country rocks as some of the
criteria used to establish the mode of emplacement of the pluton. For example,
stoping, diapirism, forceful vs. passive intrusion.
If internal modal variations and layering is present look for features that
enable you to establish the mode of formation of the layering, for example
cumulative settling vs. in-situ growth, and look for way-up criteria. Use
graphic logs to document the internal variation.
Use the presence/absence of miarolitic cavities (crystal lined irregular cavities
or vugs), pegmatites, and so on to infer the depth of emplacement of the
pluton.
Record and measure any jointing, foliation, fabric, grain-size variation,
flow-banding.
Shallow intrusions/hypabyssal rocks
Measure the thickness, attitude and orientation of all intrusive sheets.
Orientation data for dykes can be plotted on rose diagrams. Cumulative dyke
thickness can be used to assess levels of crustal extension.
Emphasise contact relations. Many bodies are transgressive and or varying
orientation such that they are best referred to as 'sheets' in the field.
Record internal structures (see above). Look for evidence of processes such as
flow-differentiation. Look for chilled margins: glassy chilled margins.
Volcanic sequences
Lava sequences and volcaniclastic/pyroclastic sequences lend themselves to
graphic logging.
With lavas record the number of flows, their internal structure and
composition, their thicknesses and the presence of any inter-lava features such
as boles (paleo-sol horizons) that denote significant time gaps between flows
and ash horizons, signifying more explosive activity.
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