Page 157 - Petrology of Sedimentary Rocks

0. Structures, etc.
. Sedimentary structures (bedding, lamination, cross-bedding,
slumping, etc.).
2. Tectonic structures (joints, etc.).
3. Weathering and alteration.

E. Interpretation and Paragenesis
To a large extent these conclusions will be tentative until a rather
large suite of related samples has been built up. Nevertheless it is
possible to gain a great amount of information from one or a few
slides. To that aim the following outline is suggested:
I. Source area
a. Geology (based upon quartz types, presence and abundance
and character of all the minerals present). Give estimate
of the proportions of the rock contributed by each source.
b. Relief and tectonic state (based upon mineralogy, grain
size, maturity).
C. Climate (based chiefly on feldspars and micas and heavy
minerals and clay minerals).
d. Length of transport or distance of source area (based
partly upon size of largest grains, and shape).
2. Depositional area
a. Environment of Deposition (beach, river, delta, dune, la-
goon, etc.)
b. Depth of water, strength and persistance of currents,
salinity, rapidity of burial, effect of organisms).
3. Diagenetic and post-diagenetic changes
a. Age relations and mode of origin of authigenic constituents.
b. Effects of intrastratal migrating fluids.
C. Effects of post-emerqent weathering.
F. Economic importance
I. What are the Dotentialities of this rock as a reservoir for oil or
water? What is the present porosity and how did it originate?
What is the grain-size of the pores and how well are they
interconnected? Does the petrographic character (e.g. whether
phyllarenite or orthoquartzite, or type of cementing material)
suggest a favorable porosity trend in a given geographic direc-
tion?
2. What valuable clues does the specimen offer to correlation in the
way of odd minerals or characteristic varieties of minerals?

For rapid descriptions, skimping should be done only under
mineral description; but one should notice especially the follow-
ing i terns about these minerals: Quartz (possible bimodality in
size; average roundness and range; is there a break in the
roundness distribution; correlation of roundness with size; domi-
nant extinction and inclusion types; what is the relative amount
of plutonic versus metamorphic and vein volcanic types). Feld-
s ar (size and shape with respect to quartz; average freshness
%- range of freshness within the same species--look especially
an
for kaolinization; do the several species of feldspar present
differ as to average freshness. size. or roundness). Chert (size
and shape with respel C t to quaitz; a;e several varietiespresent,
such as oolitic, spicul i tic, etc.). Micas (abundance of the several

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