Page 154 - Petrology of Sedimentary Rocks

2. Fabric
a. General homogeneity (Can the entire specimen be con-
sidered a single rock type, or are two or more types
interlayered--such as sandy streaks in a mudstone, or bio-
elastic layers in an otherwise non-fossiliferous sandstone?)
b. Packing (Is there any evidence of welding or interpenetra-
tion of elastic grains by solution on the one hand, or
spreading of grains by the growth of cements on the other
hand?)
C. Porosity both before and after cementation
d. Perfection of orientation and how it is expressed (i.e. by
micas only, or by micas and elongated quartz grains, etc.)
3. Grain size
a. Entire sediment: give median, extreme ( 100%) range, 16-
84% range, and sorting in terms of CJ +(I$ 84-4 IS) j2. Sizes
should be given first in mm (if you like they can be
converted to $ also). Is the distribution unimodal or
bimodal, and give details as to the diameter of the modes
and their relative proportions in the sediment. If there is
any correlation of size with composition, describe also.
b. Gravel fraction: give percent, median, sorting, range.
C. Sand fraction: give percent, median, sorting range.
d. Mud fraction: give percent, relative proportion of silt
versus clay, and median of silt if determinable.
e. Complete textural name (use the triangular diagram; the
final name should be e.g. pebbly claystone, or silty medium
sandstone, etc.)
4. Grain shape
a. Idiomorphism, range of idiomorphism, and variation of this
property with composition.
b. Sphericity (or elongation), sphericity range, and variation
of sphericity with size and composition.
C. Roundness, roundness sorting, and variation of roundness
with size and composition.
5. Textural Maturity. Define the stage of textural maturity; are
any inversions present (such as well-rounded but poorly-sorted
grains)? Immature stage: the rock contains over 5% clays and
very fine micas under 0.03 mm. Submature stage: Clays less
than 5%, but the rest of the rock is still poorly sorted--i.e., the
l6-84% grain size range is more than 1.0 phi units or Wentworth
grades. Mature stage: rock is well-sorted (I 6-84% range less
than 1.04) but still not well-rounded. Supermature stage: rock is
well-sorted and the quartz grains of sand size show an arithmetic
mean roundnes of 3.0~ or better.
6. Bonding agents. Relative effectiveness of each cement (or the
clayey matrix) in bonding the rock together. Cements are
described in detail later, under C3.

C. Mineral Composition
Use the following detailed outline for describing each mineral present.
It is a great aid to describe the minerals in systematic order every
time. The following order of description is suggested:

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