CEEES/SC 10111-20111
Sedimentary
Rocks
Laboratory
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here for a PDF file of
the lab manual chapter for this Lab.
Sediment: loosely packed,
unconsolidated mineral and/or rock fragments.
Sedimentary Rock: sediment
that is lithified by compaction and cementation.
Weathering
of pre-existing rocks via mechanical and chemical means produces
sediment that is transported via water, wind, or ice to a basin of
deposition where it is deposited in horizontal layers or strata.
The composition and texture of many sedimentary rocks provide clues to
the:
Original source of the sediment;
Type and extent of the weathering processes by which the source
rock was broken down;
Type of agent (water, wind, ice) that transported the sediment
and, sometimes, the relative duration of transport;
Physical, chemical, and biological environment in which the
sediment was deposited;
Changes that may have occurred after deposition.
Clastic sedimentary rocks are
classified on the basis of Grain Size.
Grain Shape:
relative terms such as angular, sub-rounded, and well-rounded.
Sorting:
refers to the similarity in size of all grains – gives clues to the
transportation history.
Well-Sorted: all
grains essentially the same size.
Poorly-Sorted: grains are a
variety of different sizes or may show a set of larger grains imbedded
in a finer grained matrix filling the voids between the larger
particles.
Types of Sediment
Clastic (Dertrital) Sediments:
consist of rock fragments, mineral grains, and clay minerals that have
been
transported to a site of deposition as clay, silt, sand, and gravel.
Clasts – broken pieces of rocks and
minerals.
When freshwater rivers carrying clay hit saltwater, clays clump
together and settle out.
Chemical Sediments: Form from
dissolved
ions (e.g., evaporates from sea water). Forms Rock Salt and Rock
Gypsum.
Biochemical Sediments: form through
biological
chemical precipitation. For example, coal
from plant material; bioclastic
limestone.
Environments of Deposition
Important for reconstructing Earth history, climate
variations, and
exploration for ore deposits. Table 4.2 and Figure
4.2 shows the main depositional environments
for
sedimentary rocks.
Sedimentary Structures
These are features in sedimentary rocks that formed
during or
after deposition of the sediment, but before lithification.
They provide evidence of the transporting agent and the environment of
deposition.
Stratification
(bedding or layering)
Graded Bedding
Cross Bedding
Tracks/Trails/Burrows
Rain Drops (recent)
Ripple Marks
Salt Casts
Mud Cracks
Fossils
Lithification –
“rockification”
– accomplished by compaction, cementation,
and recrystallization processes that cause the grains to interlock.
Classification
Two broad categories:
1) Clastic or Detrital
2) Chemical & Biochemical.
Many sedimentary rocks have a clastic
texture with the “bits” held together with a crystalline cement.
Although many chemical and biochemical sedimentary rocks have clastic
textures, some have crystalline textures, revealed by careful
examination of the sample to see interlocking crystals.
Chert & Flint =
microcrystalline
monomineralic (quartz) rocks.
Texture in silt – shale sized clastic rocks depends on amount of
compaction.
When first deposited, clay minerals
(mica “wannabees”) and mica fragments mimic a pile of loose
leaves.
Compaction creates a Mudstone, which breaks into
characteristically
blocky particles. Mudstone may show layering or is massive.
Shale
= greater compaction and the rock more easily breaks into sheets due to
better alignment of the clay minerals – this means the rock is fissile.
Recognizing
Minerals
in Sedimentary Rocks.
How to Identify Sedimentary
Rocks.
A. If grains or crystals are visible without a hand lens and:
1. The rock scratches glass, use Table 4.4.
2. The rock won’t scratch glass, use Table 4.5.
B. If grains or crystals are not visible, or barely visible with a
hand lens:
1. The rock is a shale or mudstone (Table 4.4).
2. It is a chemical or biochemical rock (Table 4.5).
If Table 4.4
is chosen, determine the grain size and mineral composition.
These terms refer to grain size:
Claystone
Mudstone
Siltstone
Shale
Sandstone
Conglomerate
These terms are sandstones with specific mineral compositons:
Graywacke
Quartz Sandstone
Arkose
When naming a sedimentary rock, it is most informative if both mineral
composition and grain size are given. For example:
Quartz Sandstone
Granite-Pebble Conglomerate
Coarse-Grained Graywacke.
For clastic sedimentary rocks with clasts >2 mm, it is termed “Breccia” if
the clasts are angular, and "Conglomerate"
if they are rounded.
Different types of Limestone
Laboratory Specimens
1. Quartz Sandstone
2. Rock Gypsum (Gyprock)
3. Coal
4. Calcareous Mudstone/Claystone
5. Oolitic Limestone
6. Chert
7. Limestone
8. Conglomerate
9. Shale
10. Sandstone
11. Coquina
12. Breccia
13. Chalk
14. Fossiliferous/Bioclastic Limestone
15. Banded Sandstone