LABS # 7 AND # 8 - SEDIMENTARY ROCK THIN SECTIONS
1. LITHOCLASTIC BIOSPARMICRUDITE, WACKESTONE
Explanation - This is a very informative slide for a number of reasons.
We know that the energy level of the environment fluctuated. We have
allochthonous sediments (the detrital quartz and feldspar) incorporated
within fine-grained autochthonous limestone. The quartz varies from
angular to rounded grains, also showing various energy levels affecting
the quartz prior to its being dumped into the relatively low-energy
carbonate environment. Also note the presence of detrital plagioclase
within the slide (blocky grains with multiple twins under crossed
nicols). Another highly informative mineral type is represented by green
grains (under plane light and crossed nicols) scattered in the slide.
This is glauconite. As you remember, it is a strictly marine (well
almost, anyway), relatively shallow-water indicator. Some of the
glauconite is detrital, but on other portions of the slide the glauconite
is interstitial, indicating an autochthonous (authigenic) origin for much
of the glauconite. This is important for our interpretation of this as a
shallow marine environment, as glauconite can be eroded and redeposited
elsewhere. This slide exhibits a textural inversion (sand grains in a
micrite matrix). It also seems that the rate of sedimentation was high
since the sand and bivalve shell fragments (thin, elongate, curved
features) are jumbled together within a mud-supported micrite matrix.
Lithoclast Provenance - Quartz may be "plutonic", with some "volcanic"
quartz present? Most of the quartz has straight extinction with few
vacuoles. Also note the minor presence of plagioclase feldspar, possibly
suggesting a "plutonic" source. However, note that some quartz grains
appear to retain a hexagonal-bipyramidal shape and possible embayments
may be present (i.e. "volcanic"). There also appears be some recycled
quartz (note that some of the grains are very well-rounded,
monocrystalline, with straight extinction). Therefore, it looks as if
there were multiple sources of parent material. The calcite is
autochthonous.
Porosity - There is slight fracture porosity within the lower center of
the slide. There is also some moldic and vuggy porosity, although at
least some of this may be an artifact of thin section preparation rather
than "true" porosity.
Diagenetic Features - There is compaction of both the quartz and micrite.
Dissolution of micrite is indicated by the vugs. Cementation is
indicated by the presence of sparite and lime mud is recrystallized to
sparite. Inversion and/or recrystallization is indicated by the
replacement of shells with calcite.
2. QUARTZ ARENITE
Grain Size = [2(6.3)/27 + 2(6.3)/34 + 2 (6.3)/28 +2(6.3)/32]/4 = 0.42
millimeters = medium sand
Sorting = moderate; High sphericity; Subangular to Rounded; chemically
and physically mature
Explanation - This is a very clean quartz sandstone which exhibits
prominent grain suturing due to pressure solution. Note that many of the
more sutured grains also have a sheared appearance. The sheared nature
may be less a function of provenance than due to deep burial and other
diagenetic processes. Although the quartz arenite contains very little
clay it exhibits only moderate sorting with quite a range of quartz sizes
in the specimen, with small grains between larger ones. However, before
speculating about depositional energies check out the slide closely.
Some polygranular quartz seems to have recrystallized to more stable,
smaller grains. Therefore, sorting in this case may be as much (or more)
a function of diagenetic processes (i.e. pressure solution and
recrystallization) as to depositional environment.
Provenance - There are many quartz grains that appear to be "stretched
metamorphic". However, these may be due to diagenesis rather than
provenance (a typical problem; see explanation below). The quartz has
strong undulose extinction. Some internal "borders" of individual grains
are crenulated. Some vacuoles and microlites are present.
Porosity - There is a little intergranular porosity, but most porosity
was destroyed through compaction and pressure solution.
Diagenesis - There is tremendous compaction of quartz, dissolution of
quartz due to compaction and pressure solution, and suturing of quartz
grains. Quartz cement is precipitated between the grains and there are
quartz overgrowths. Note how diagenesis masks the roundness and other
textural characters of the grains.
3. BIOOOMICSPARRUDITE, GRAINSTONE (PACKSTONE O.K.)
Explanation - This slide is a limestone in which sparite is dominant with
some micrite found between a few of the grains. The sparite can be
readily distinguished from the micrite under crossed nicols by its
crystalline character. The round grains in the slide are oolites. Note
the radial nature of these grains. This is probably due to polymorphic
inversion from aragonite to calcite. The elongate grains are shell
fragments, especially prominent are fragments of ribbed bivalves. These
grains were deposited in a relatively high energy environment (probably a
carbonate beach or shoal). Also characteristic of a shallow marine
environment is the micritization apparent in this slide. The skeletal
fragments are surrounded by micrite envelopes, formed by the
disintegration of the skeletal material through chemical alteration by
boring blue-green algae (cyanobacteria). In fact, several small mollusc
fragments in the lower left hand corner of the slide exhibit very small
bore holes along the margins of the shells produced by these endolithic
algae.
Provenance - Limestone is autochthonous.
Porosity - There is minor intergranular porosity apparent (however, most
pores were filled with sparite so porosity is low).
Diagenetic Features - There is compaction of the fossils and oolites.
Sparite is precipitated (i.e. cementation) and with possible
recrystallization present. Under higher magnification, note that both
drusy (needle-like) and blocky cement types are present, indicating two
generations of cementation processes. There is polymorphic inversion of
aragonite oolites to calcite (note the "starburst" pattern of the oolites
characteristic of this process). There is replacement of shells with
calcite. Boring of shells by organisms have formed "micrite envelopes";
i.e. dark "haloes" of micrite surrounding the grains.
4. FELDSPATHIC WACKE/GRAYWACKE
Grain Size = [2(6.3)/28 + 2(6.3)/29 + 2(6.3)/31 + 2(6.3)/33]/4 = 0.42
millimeters = medium sand
Sorting = poor; Sphericity = low; Roundness = angular to subangular;
Physically and chemically immature
Explanation - Under plane light, note the dingy clay minerals between the
grains. There is also some diagenetic chlorites present, characterized
by their green color and fine-grained nature. Most of the grains consist
of quartz. These grains are clear under plane light and grey, greyishblue and straw-yellow under crossed-nicols. Many of the quartz grains
exhibit strong undulatory extinction. There is also abundant plagioclase
and potassium feldspars. The plagioclase is clear to slightly dingy
under plane light, and is primarily bluish-grey under crossed nicols and
exhibits the distinctive plagioclase twinning. There is also quite a bit
of potassium feldspar scatttered throughout the slide. An especially
large grain of potassium feldspar is in the upper right quadrant of the
slide. This grain exhibits the characters seen in smaller feldspars in
the slide, with blocky cleavage, a slightly dingy appearance under plane
light due to diagenetic alteration, and gray coloration under crossnicols. There are also a few flecks of biotite. These are brown to
green under plane light and are slightly pleochroic. Under crossed
nicols, the biotites exhibit parallel extinction with slight development
of "birdseyes", small circular areas within the biotite grains that do
not go extinct with the rest of the crystal.
Provenance - There are a couple of grains of microcline in this specimen
(notice the gridiron twinning). This would suggest, in part at least, a
probable sialic igneous (i. e. granite) provenance. There is also some
biotite in the specimen, suggesting either plutonic igneous or schistose
metamorphic provenance. However, there is also indication of more than
one provenance for this specimen. Notice that there are both grains with
straight extinction, and others that show strong undulatory extinction.
The grains with straight extinction often show tiny inclusions and
microlites, possibly indicating plutonic igneous origin. The strongly
undulose grains may be a product of provenance rather than diagenesis
(otherwise, wouldn't all grains exhibit strong undulose extinction as a
result of terminal diagenetic events?)
Porosity - The porosity is relatively low in this specimen. It has poor
sorting, with the pores filled with matrix. However, many grains on this
thin section exhibited "original" fracture porosity, although this
appears to have been largely lost through cementation processes.
Diagenesis - The most obvious diagenetic features of this rock involve
the geochemical alteration of the grains by pore fluids. Notice how
vacuolization has taken place within many of the feldspar grains, by
which water percolated along cleavage planes and fractures, producing
voids within the grains. Also, under plane light notice how dingy the
feldspars appear. Diagenetic alteration, in the presence of water, has
altered the feldspars to clays (this is often termed kaolinization,
montmorillonitization, or illitization depending upon the clay product).
Also, notice how a number of the clays have been altered to chlorite,
another obvious diagenetic feature of this rock. Although there are
numerous quartz grains exhibiting strong undulatory extinction in this
slide, this may be due to provenance rather than diagenetic process (see
discussion above).
5. SILTSTONE
Explanation - Most of the grains within this slide are quartz as
indicated by the blue-grey color and undulatory extinction. The silt is
set within a clay matrix which tends to show up as grey-green blobs on
the slide under plane light, and possess either grey or orange, yellow,
purple and blue interference colors under crossed nicols (characteristic
of the illite and smectite clays). Also, there are a number of
plagioclase grains on the slide as characterized by their multipletwinned crystals. In this sense, this siltstone is neither texturally
nor compositionally mature.
Provenance - "Common" or "Plutonic" quartz appears to be present. Under
higher magnification, note the "straight extinction" with few vacuoles
present. The blocky, unaltered feldspars may also indicate a "plutonic"
source.
Porosity - There is some development of intergranular and fracture
porosity on the specimen.
Diagenetic Features - There is compaction and minor cementation of both
the quartz and feldspars. However, some apparent "interparticle
porosity" here may actually be due to plucking grains during thin section
preparation.
6. DOLOMITE/ DOLOSTONE
Explanation - One-half of several of these slides have been stained with
a combination of Alizarin Red S (which causes calcite to stain red) and
Potassium Ferricyanide (which causes ferrous iron to appear blue). This
slide is largely unstained, although the slight bluish-purple tinge
probably indicates a very slight iron content. The dolomite forms an
interlocking mosaic of crystals. It's a good thing that this one was
stained. Anhedral interlocking crystals of dolomite are practically
indistinguishable from similar-shaped crystals of sparry calcite. Of
course, Alizarin Red S will always do the trick for differentiating
dolomite from calcite. This dolomite appears to have been pretty brittle
and competent stuff since there appears to be quite a few fractures in
it. However, you have to watch thin section structures because sometimes
"fractures" and "moldic or vuggy porosity" can actually be produced
during cutting and grinding the thin section!
Provenance - Dolomite probably formed by polymorphic inversion of calcite
(see below).
Porosity - There is some fracture porosity present, and slight
intercrystalline porosity developed between the dolomite crystals.
Diagenetic Features - This sample probably represents polymorphic
inversion of calcite to dolomite (a process termed dolomitization). Note
the "crystals" and "intercrystalline material" under plane light. There
is probable dissolution and recrystallization of dolomite "cement"
between the "grains" of dolomite. Compaction is indicated by the many
fractures present on the slide.
7. FELDSPATHIC WACKE (LITHIC WACKE O.K.)
Grain Size = [2(6.3)/21 + 2(6.3)/16 + 2(6.3)/16 + 2(6.3)/17]/4 = 0.73
millimeters = coarse sand
Sorting = poor to moderate; Sphericity = low; Roundness = subangular to
angular; Chemically and physically immature
Explanation - In many ways, this slide is similar to specimen # 4. Under
plane light, note the dingy clay minerals between the grains. There is
also some diagenetic chlorites present, characterized by their green
color and fine-grained nature. Most of the grains consist of quartz.
These grains are clear under plane light and grey, greyish-blue and
straw-yellow under crossed-nicols. Many of the quartz grains exhibit
strong undulatory extinction. There is abundant plagioclase and
potassium feldspars as in # 4, but there is also microcline present in
specimen # 7. The plagioclase is clear to slightly dingy under plane
light, and is primarily bluish-grey under crossed nicols and exhibits the
distinctive plagioclase twinning. The potassium feldspars exhibit blocky
cleavage, a slightly dingy appearance under plane light, and gray
coloration under cross-nicols. The microcline is characterized by the
distinctive "gridiron twinning", by which the crystals exhibit a
"checkerboard" pattern under crossed-nicols. There is much more biotite
in this specimen versus that seen in # 4. These are brown to green under
plane light and are slightly pleochroic. Under crossed nicols, the
biotites exhibit parallel extinction with good development of
"birdseyes", small circular areas within the biotite grains that do not
go extinct with the rest of the crystal. Although I will accept the name
"lithic wacke" for this specimen, total feldspar content is greater that
that of biotite "lithics". Therefore, the proper name of this specimen
should be feldspathic wacke.
Provenance - Although specimens # 4 and # 7 are quite similar in
appearance, there may be some differences in provenance. Although many
quartz grains in # 4 show straight extinction, virtually every quartz
grain in # 7 exhibits strong undulose extinction. Also, there is
abundant mica in this specimen, several of which still exhibit
multilayered "book-like" structures. These may represent small pieces of
schist, suggesting a schistose metamorphic provenance. However, there is
also some microcline present in this sample, which is likely of plutonic
igneous (i.e., granite) provenance. Therefore, multiple sources of parent
material may be indicated.
Porosity - There is some intergranular (interparticle) porosity between
the sand, and some fracture porosity (see upper left of slide).
Diagenesis - As in specimen # 4, the most obvious diagenetic features of
this rock involve the geochemical alteration of the grains by pore
fluids. Dingy feldspars and alteration of feldspars to chlorite and
other clays indicate that considerable water-rich diagenetic processes
influenced this specimen. Also, at least some (and maybe a lot) of the
undulose extinction of the grains may be due to diagenesis, as it is
apparent that this specimen was really "crunched" after deposition.
Therefore, it is difficult to differentiate features of this specimen due
to provenance versus those created during diagenesis.
8. POLYMICTIC EXTRAFORMATIONAL PARACONGLOMERATE
Explanation - Hold the slide up to the light before placing it on the
microscope stage. Note the presence of several pebbles indicating the
conglomeratic nature of this specimen. I called this a paraconglomerate
as the pebbles are not touching. The numerous quartz grains and lack of
matrix (at least clay matrix) caused me to ponder an orthoconglomerate
classification rather than a paraconglomerate. Also, as a thin section
is a two-dimensional surface it is possible that on the original rocks
some clasts were touching (remember that discussion?). A major purpose
in classifying rocks is to define transport energies and depositional
environments. The presence of numerous quartz grains in contact with one
another and the sparcity of clay matrix in this specimen indicates a
considerable amount of current activity (also note that the sand and
gravel is well-rounded). Despite this, I called this specimen a
paraconglomerate as the pebbles do not touch. I also called the
congomerate polymictic and extraformational since the pebbles are made of
polygranular quartz and the quartz sand is almost certainly of different
provenance.
Provenance - There are some reworked sedimentary chert clasts (i.e.,
recycled sedimentary provenance). Most of the quartz appears to be
"common" or "plutonic". The "common/plutonic" quartz has straight
extinction, with some vacuoles present and a few microlites. There also
seems to be a number of probably recycled sedimentary sand-sized quartz
grains (note that some of the grains are very well-rounded,
monocrystalline, and with straight extinction).
Porosity - There is not much development of porosity here. Some of the
quartz grains exhibit slight intergranular porosity between them.
Diagenetic Features - The quartz grains indicate some compaction and
dissolution. Note the curved contacts between the grains due to pressure
solution due to grain-to-grain contact and dissolution. Cementation is by
silica. There is also authigenesis and/or cementation by hematite as
indicated by the brown stains under plane light.
9. MUDSTONE/ SHALE/ ORTHOCLAYSTONE
Explanation - This slide is composed almost entirely of clay minerals
(pretty dingy and nondescript stuff, huh?), although quite a few very
small quartz grains may be distinguished under higher magnification.
There are some small fractures within the clays. The quartz silt-filled,
rounded structures (see center and upper left edge of slide) are most
likely cross-sections of silt-filled burrows. Other burrows may be
present along the left edge of the slide, showing up as elongate or oval
stained features. A slight suggestion of fine silt laminae are present
in the lower portion of the slide.
Provenance - Impossible to determine optically.
Porosity - There is not much porosity, as the fractures were later
infilled with silica cement.
Diagenetic Features - The clay is compacted. It is fractured with
precipitation (dissolution, cementation) of quartz within the fractures.
Bioturbation may be indicated by the possible presence of burrows.
10. LITHOCLAST-BEARING DOLOMITIC PELMICRITE, PACKSTONE (WACKESTONE O.K.)
Explanation - Some portions of this slide have been dolomitized. The
dolomite is unstained and under crossed nicols exhibits either gray or
pretty yellow, orange, pink and green interference colors. However, some
of the rounded- to oval dark micritic areas of the slide are partially
infilled with quartz silt. These probably represent burrow fillings.
Other quartz silt (clear under plane light, blue-grey with undulatory
extinction under crossed nicols) can be seen within clasts of limestone.
The small ovoid micritic grains I interpret as pellets (or peloids).
Other larger clasts I interpret mostly as intraclasts. There may be a
single, sparite-filled cross-section of a small bivalve shell in the
lower center of the slide. Although much of the material between the
grains is lime mud (micrite), there is also considerable microcrystalline
sparite and dolomite present.
Provenance - The limestone is autochthonous. The few quartz grains
present appear to be "plutonic" or "common". This is indicated by the
straight to slightly undulose extinction of the quartz, with few vacuoles
present.
Porosity - There is not much porosity here; there may be some
intercrystalline between the dolomite crystals.
Diagenetic Features - There was compaction of the pellets. Some
microstylolites are present in the lower left of the slide (notice around
the edges of a couple of the intraclasts). There is slight dissolution
of the micrite and cementation by sparite. There is considerable
replacement of calcite by dolomite (unstained parts) and "cementation" by
interstitial dolomite. Bioturbation is indicated by the minor presence
of burrows and the presence of numerous pellets left by burrowing
detritus-feeding animals.
11. ORGANIC LITHOCLASTIC PELSPARMICRITE, WACKESTONE (PACKSTONE O.K.)
Explanation - This specimen is as allochthonous as it is autochthonous.
Between the red-stained calcite portions of the slide there are numerous
quartz silt grains, as well as dark organic-rich clays. However, much of
the material seems to have been deposited within the fractured limestone,
as much of the limestone on opposite sides of the clay-filled fractures
match up. Therefore, there were at least two depositional events; the
first deposited lime mud, the second organic shale and silt. These
events were separated by a diagenetic event in which the limestone was
fractured (and possibly partially dissolved) prior to the deposition of
the clay and silt. As concerns the limestone, the micritic nature of the
limestone is readily apparent and the only allochem type abundantly
present seems to be pellets/peloids. There also seems to be some minor
recrystallization to sparite within the lower left portion of the slide.
Provenance - The limestone is autochthonous. The quartz appears to be
"common" or "plutonic". This is indicated by straight extinction and the
presence of a few vacuoles. A couple of tiny grains of microcline are
present (see gridiron twinning), also suggesting a plutonic source.
Porosity
abundant
There is
replaced
- There is not much porosity apparent. There were initially
fractures, but these have been infilled by organics and clays.
slight intercrystalline porosity developed where the dolomite
the calcite crystals.
Diagenetic Features - There was compaction of the pellets. Dissolution
and cementation by sparite is indicated, and there are microstylolites
apparent on the specimen. Calcite was recrystallized to dolomite, as
indicated by the unstained portions of the slide which are surrounded by
red-stained calcite. These unstained areas exhibit calcite-like
interference colors. There was considerable bioturbation with mixing of
organic and micrite muds and also indicated by the presence of pellets of
detritus-feeders.
12. PELBIOMICSPARRUDITE, PACKSTONE (GRAINSTONE O.K.)
Explanation - This specimen was stained with Alizarin Red S and Potassium
Ferracyanide. The presence of a red tint, coupled with purple stain,
indicates the presence of calcite and iron. Although not abundant, the
dingy tint of the intercrystalline material indicates that some micrite
is present. The abundance of grain-supported fossils and the slight mud
content (but less than 10%) led me to call this a packstone. A wide
variety of fossils may be seen in this slide including small club-like
bryozoans (see lower right side of slide) and numerous cross-sections of
bivalve shells. The other fan-shaped, porous structures may also
represent bryozoans, although some calcareous algae also has this
appearance. Pellets/peloids are represented by sand-sized grains of dark
micrite. There are also dark, quartz silt-filled round- to oval burrows
present and, along with the pellets, also indicate bioturbation by
detritus feeders.
Provenance - Limestone is autochthonous. Quartz silt appears to be
"common" or "plutonic", with straight extinction and minor vacuoles
present.
Porosity - There is not much porosity. Formerly there were tiny
fractures, but these have been infilled with sparite.
Diagenetic Features - There is abundant evidence of compaction/pressure
solution here, with fracturing and microstylolites present. There is
cementation by sparite with recrystallization of lime mud to this
coarsely crystalline sparite. Dissolution is indicated by the presence
of microstylolites, well illustrated on the left edge of the slide as
brown, hematite-stained areas of organics and silt. Bioturbation is
indicated by the presence of silt-filled burrows, especially seen along
the left lower edge of the slide.
13. ANHYDRITE
Explanation - The anhydrite crystals in this slide range from elongate
shreds to roughly tabular shapes due to the nature of anhydrite cleavage.
Under crossed nicols, it produces colorful interference colors and it
"twinkles" as the stage is rotated under crossed nicols (watch out since
calcite also "twinkles" when you rotate the stage as you have probably
noticed in other slides). However, calcite would never have the
"shredded" appearance of these anhydrite crystals.
Provenance - Probably autochthonous, although anhydrite is often formed
through dehydration of gypsum (in which case it would be diagenetic).
Porosity - There is some intercrystalline and fracture porosity, although
this may be an artifact of the thin section grinding process rather than
"true" porosity.
Diagenesis - Some compaction is probably indicated by the orientation of
the anhydrite crystals. Some pressure solution and dissolution is
indicated at the boundaries of the grains.
14. CHERT
Explanation - Note the slight brownish color of this specimen under plane
light. Although many cherts are nearly colorless under plane light, it
is not uncommon for specimens to exhibit various shades of brown or
reddish-brown due to the presence of iron-bearing minerals. In crossednicols, notice the microcrystallinity of the specimen and its black-gray
speckled appearance. This is diagnostic of chert. As chert is basically
amorphous, it should be nearly isotropic in crossed-nicols (as seen in
this specimen). This specimen was fractured at some time during its
diagenetic history, after which microcrystalline quartz was precipitated
within the fractures. Notice that the quartz crystals within the
fractures are slightly larger than those of the chert.
Provenance - Although
there is no reason to
fact, some structures
fossils, suggesting a
we cannot be certain (without more detailed study),
debate an autochthonous origin for this chert. In
may represent cross-sections of relatively large
biogenic origin for this rock.
Porosity - Fracture porosity was originally developed in this specimen.
This provided a conduit for dissolution to take place, and resulted in
the development of larger vugs along the fractures. However, there is
essentially no porosity currently seen in this specimen, as the fractures
were later filled with quartz precipitates.
Diagenesis - Fracturing and dissolution (as indicated by infilled vugs)
are apparent in the specimen. Within these fractures and voids,
secondary microcrystalline quartz has been precipitated from pore fluids.
15. GLAUCONITIC SANDSTONE
Grain Size = [2(6.3)/28 + 2(6.3)/34 + 2(6.3)/38 + 2(6.3)/30]/4 = 0.39
millimeters = medium sand
Sorting = moderate; Sphericity = moderate to high; Roundness = subangular
to rounded; Chemically mature and physically immature
Explanation - The lab worker who made this slide had some real problems.
Glauconite is a very soft clay-like mineral, so when a thin section is
ground the glauconite grains tend to be plucked out rather than ground
down. Look at the slide under plane light. Note the round "ghost
crystals" where the glauconite has been plucked out. Also notice that in
these voids there are black "crosses", which appear to rotate under
crossed-nicols when the stage is rotated. These weird isotropic areas
were created due to the holes in the specimen during slide processing.
Of course, these thin section preparation problems will influence your
textural interpretations, such as grain size, sorting, and "true"
porosity. The glauconite in this slide is green to greenish-brown under
plane light and crossed nicols. The other grains in the slide are
quartz. These are clear under plane light and gray to straw-yellow under
crossed nicols. Although many minerals may be associated with
glauconite, quartz seems to be especially common within the shallow
marine environments where glauconite forms.
Provenance - The quartz in this slide is allochthonous, whereas the
glauconite is an early diagenetic mineral. The slide preparer had such a
tough time with this specimen that I hesitate to suggest a provenance for
the quartz. The embayments and shattered appearance of the grains may
actually be a relict of the thin section-making process rather than a
reflection of provenance. However, many of the quartz grains show strong
undulose extinction, so we can probably rule out an igneous plutonic
provenance. However, glauconites are (usually) very specific in terms of
provenance / depositional environments. They indicate marine, normal
salinity, weakly-reducing conditions with slow rates of deposition and 50
to 1,000 meters water depth.
Porosity - Most of the "intergranular porosity" in this specimen is
probably due to thin section production, where the glauconites were
plucked out during the grinding process. Many of the quartz grains have
abundant fractures in them, but this may be due to the fact that the lab
technician shattered the quartz grains trying to get the glauconites thin
enough to see some structures in them.
Diagenesis - Again, the plucking and shattering of grains by the lab
technician makes interpretation of diagenesis tougher. However,
glauconite is a diagenetic mineral that usually forms in the presence of
organic matter at or near the sediment-water interface in marine
environments. Reworking of sediments by organisms, another early
diagenetic process, is suggested by the presence of at least one
grapestone-like structure incorporating several glauconite grains,
probably indicating the ingestion of the mud by detritus-feeding animals.
16. OIL SHALE/KEROGEN SHALE
Explanation - Hold this slide up to the light before placing it under the
microscope. Notice the coloration, fine-grained nature, and lamination
that characterize this specimen. Under plane light and crossed-nicols,
the medium to dark-brown material consists of clay minerals. Notice the
tiny quartz grains imbedded within the clays, especially apparent under
higher magnification. The opaque black material in this slide is
kerogen, a hydrocarbon type that consists largely of disintegrated plant
debris. The kerogen material in this slide appears to be vitrinite, so
we could possibly get some information concerning paleothermometry if we
had a scope with reflectance-measuring capabilities (maybe we will buy an
instrument for the next seds class, huh? - oh boy, would they have fun!).
There are some slightly larger crystals of material associated with the
kerogen that are clear under plane light. Under higher magnification and
under crossed-nicols they yield low-order interference colors. As these
are relatively small crystals it is difficult to determine their precise
composition. However, it is probable that they represent one or more
varieties of evaporite minerals, which are commonly associated with oil
shale.
Provenance - Kerogen shale is typical of lacustrine environments, and the
fine laminations and abundant hydrocarbons and kerogen in this specimen
would suggest such an origin for this specimen.
Porosity - There is virtually no porosity in this specimen, although
there are a few small fractures in the lower left of the slide.
Diagenesis - Compaction of the shale would be an early diagenetic feature
of this specimen. Imbedded within the dark organic layers, microcrystals
of pyrite are scattered throughout the slide. Such crystals are often
secondary precipitates within anaerobic sediments, as would probably be
the case here. The precipitation of the tiny evaporite crystals may
also, in part, be diagenetic.