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Link: Mineralogy and Crystallography
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Link: Sedimentology > Sedimentary Structures > Way Up Structures
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Cross-bedding is a series of
laminations included within a larger sedimentary bed. A directional current
allows the laminations to build upon the leeward (downstream) side of a
migrating bedform called a ripple. (Bigger versions of ripples are called
dunes, and they create cross-beds as they migrate, too.) Ripples and dunes form
in a directional current of either water or air. They accumulate at the angle
of repose for that size of the sedimentary particle, and within that medium
(saltwater, freshwater, or air). In general, it is a gentle angle of around 30°
of dip.
In terms of shape, cross-bedding is
most useful when the cross-beds show a pronounced concavity, with the scoop shape curving upward, like a smiley face. This concave-up shape is a
reflection of the cross-beds curving into a gentler orientation, approaching
parallel to the base of the main bed. We
call this “tangential,” from the geometric
term describing a line
intersecting a circle at one point. At the top of the bed, in contrast, they
are often truncated (“stopped short” or “cut off”), because the original upper tangential portion of the leeward side of the ripple has evidently been “planed
off” by post-depositional erosion.
A sketch showing how cross-beds
approach parallel with the main bed's bottom but at the top of the bed,
erosion has removed the tangential portion, resulting in a truncated contact (Figure 8). Another way of putting this is that
the angle between the crossbred and the main bed is typically small at the bottom
(close to parallel) and larger (around 32 degrees or so in dry sand) at the
top.
Figure 8: A cartoon showing the different cross-bed/bed
relationships at the upper and lower portion of the bed.
This leads to important insights, as
shown in the case study of the Mixed-Up Quartzites of Cape Agulhas.
Example 0X:
Examine this panorama of a specimen of cross-bedded red sandstone (Figure 9). There are two halves to the sample shown. One is
right-side-up. The other is up-side-down. Explore the two samples to see if you
can figure out which one is which, applying the criteria explained above.
Figure
9:
Specimen of cross-bedded red sandstone.
Really large cross-beds form via
aeolian (wind) deposition in dune fields. The shape and geopetal implications
are the same, but the cross-beds are much larger. Here is an example
(right-way-up) from coastal exposures in the western Orkney Islands:
Figure 10: Cross bedding exposures in the
western Orkney Islands
MAGIC BOX è Exceptions to the Rule
Cross-bedding can be tricky. Sometimes
cross-bed laminae accumulate without the tangential, concave-up portion, and
sometimes they preserve the upper convex-up portion at the ripple crest. Even
crazier, sometimes they accumulate not on the leeward side of a dune, but the
upstream (stoss) side! Let us take a moment to see what these complications
look like…
Image (Figure 11) showing annotation of a photograph of climbing ripples
in sandstone. A Swiss Army knife serves as a sense of scale. The climbing
ripples' foresets (leeward side laminae) build up and to the left, implying
current flow from the right toward the left.
Figure
11:
Climbing ripples in cross-sectional view, Morrison Formation, Greybull,
Wyoming.
Climbing ripples form when downstream
migration of a ripple or dune is accompanied by rapid vertical aggradation of
sediment. This tends to occur when the sedimentary load is higher than the
capacity of the current that’s carrying it. Climbing ripples are distinct
structures indicating critical to supercritical flow, but they often look about the same right-side-up as
they do up-side-down. The concave-up portion of the laminae in the lee of the
ripple is matched by the convex-up crest of the ripple itself, buried as soon
as it forms. As a result, they can mislead the historical geologist.
Consider the example in Figure 12; if you flipped the photo up-side-down, it would be difficult to tell:
The image is shown in Figure
13 shows a cross-section through three sets of climbing
ripples from modern (unlithified) sand deposits in the Mississippi Delta region
of Louisiana. Note how the largest, oldest, deepest set of cross-beds shows an
undulating set of internal laminations, and these “climb” up and to the right,
with tangential bases. This is likely indicative of supercritical flow. The the second set has a trough-like bottom that cuts into the older set of cross-beds,
and is truncated by the most recent, uppermost, thinnest set of cross-beds.
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“Which way’s up?”
With rocks, the answer is not always clear.
If layered rocks have experienced
mountain building, they may be rotated from their original horizontal positions
into vertical orientations, potentially confusing geologists, since the
principle of superposition no longer applies.
Consider the hypothetical example
illustrated in the Figure 1: you discover a vertical sequence of strata. In one
layer, you find evidence of a mass extinction event. In another layer, you find
evidence of glaciation. Did an ice age cause the extinction? Or did the
extinction somehow trigger the ice age? In order to pose intelligent questions about causality,
you need to know which one is older. The older event can influence the more
recent event, but not the other way around.
Even more extreme is when compressive
stresses associated with convergent plate tectonic settings manage to fold beds
into up-side-down positions. If the beds are up-side-down in such a tectonic inversion,
it could really throw off the interpretations of the Historical Geologist: they
would be reading Earth history backward!
In such situations, we need reliable
tools in order to accurately interpret which direction was “up” when the rock
originally formed. We
call these patterns that look different right-side-up compared to up-side-down
by the general term “way-up structures.” Some geologists also call them
“geo-petal structures.”
Way-up indicators are critical for
figuring out the correct sequence of geologic units. The help us determine
“younging direction,” the direction in which strata get younger. (This is the
same as paleo-“up” or “facing direction.”)
The welter of terminology shouldn’t
turn us off: it’s an indication that geologists put a strong emphasis on
finding and relying on way-up structures. Consider this example (Figure
2):
Three layers are shown in outcrop at
Earth’s surface, color-coded green, blue, and yellow. They are folded into what
appears to be a series of anticlines and synclines. Given superposition alone,
we would assume the lowest one is oldest, and the uppermost one is youngest.
However, way-up structures in these three units point downward as the “up” or
younging direction. Therefore, the exposed layers are part of a larger-scale
fold, and the upper (upright) portion of that fold has been removed by the
forces of erosion. Without the way-up indicators, we would have been fooled. So,
they are very important.
So, what are these
geo-petal tools, exactly?
The key thing is that a way-up structure must be display
some difference between its top and its bottom. They always look different
up-side-down compared to right-side-up.
In sedimentary
rocks, the following way-up structures can aid the historical
geologist in figuring out the paleo- “up” direction:
·
cavity
fills
·
crossbeds
·
ripple
marks
·
mudcracks
·
graded
beds
·
loading
structures
·
sole
structures
·
burrows
·
stromatolites
In igneous
rocks, there are fewer options, but a few that are handy
include:
·
vesicle
concentration
·
apophyses
·
pillow
structures
·
in
a few rare intrusions, primary structures including grading and cross-bedding(!)
Metamorphic
rocks develop no
way-up structures as a consequence of metamorphism, but sometimes primary
structures in a sedimentary or volcanic protolith can potentially survive as
discernible patterns in lower-grade metamorphic rocks. Higher-grade metamorphic
rocks are so thoroughly recrystallized that any original geopetal structures
would be destroyed.
Be cautious about leaping to big, important conclusions
from a single way-up structure. The way-up indicators shown on this page are
hopefully straightforward, but nature is vast and varied. Examples seen in the
field can frequently be ambiguous. The careful historical geologist will search
for as many examples as possible; to see if they agree with one another.
Careful geologists seek a preponderance of evidence before settling on an
interpretation.
We’ll begin with an examination of way-up structures in
sedimentary deposits, whether lithified into sedimentary rock or in an
unconsolidated state (but after deposition).
When an empty space exists in a sedimentary deposit, such
as the protected little hollow under a shell, it can be partially filled by
sediment. We call this little pocket of space a “void” or a “cavity.” If the cavity is entirely
filled with sediment, it’s useless as a geo-petal structure. It only has geo-petal
value if it is partially filled in with mud. In fact, this is geo-petal in the
strictest sense of the term: it refers to these cavity fillings. Here is how it works: the mud
settles under the influence of gravity, lining the bottom of the space (but not
the top of the space). Later, as groundwater moves through the sediment,
it carries with it dissolved ions, which may bond together, filling the
available space with crystals. In the example in Figure 3, the crystals formed are of the mineral calcite, which
occurs in a coarse form geologists call “spar.”
Figure 3: In cavity fills, mud
fills a portion of the empty space at the bottom and later diagenetic
deposition of calcite fills in the remaining empty space (at the top) with
spar.
Use this principle to examine the following Figure 4, showing a cross-section through a bed of limestone
collected in West Virginia. It has intentionally placed in a vertical position
to examine. As you will see, the limestone includes many snail shells
(gastropod fossils). A lot of them show this key pattern of infilling with two
substrates: gray limy mud
and white calcite spar.
Note that because the shells are made of calcite also, both the original shell
and the subsequent spar appear the same coarse crystalline white:
Which way is depositional “up” in this sample (Figure
4)? You can use the rotation the image to test various
orientations, until you get the mud at the (original) bottom and the spar above
it in the (original) top of the available space.
Hopefully you determined that the side of the sample next
to the pencil was originally “down” (Figure
5).
Quiz X1:
|
Question: Is this sample right-side-up or up-side-down? Explain?  |
Option A: Right-side-up, since
the bottom is filled with mud, and the empty cavity at the top is partially
filled with sparry calcite crystals. Option B: Up-side-down, since the the bottom is partially filled with sparry calcite crystals, and the top is
filled with mud. |
|
Answer: Right-side-up, since
the bottom is filled with mud, and the empty cavity at the top is partially
filled with sparry calcite crystals. |
|
|
Question: Examine this sample for cavity fills. Which way is depositional "up?"  |
Option A: The bottom of the
screen (opposite the side where the scale is) is the original depositional
"up" direction. Option B: The top of the screen (where
the scale is) is the original depositional "up" direction. Option C: On the left of the screen
(where the biggest shells are) is the original depositional "up"
direction. |
|
Answer: The top of the screen
(where the scale is) is the original depositional "up" direction. |
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Mineralogy and Crystallography: Chapter #03 and Chapter #04 A complete pdf is kept in the link mentioned below: Link: Mineralogy and Cr...
