Transcript Sedimentary Rocks - Uplift Community High School

Sedimentary Rocks
Quartz sandstone
Limestone
Shale
The Simple Ideal Model For
Understanding Sedimentary Rocks
• The key to understanding sedimentary
rocks is to realize that all sedimentary
processes of weathering, transportation,
and deposition are aimed at one goal reaching the three final end products of all
sedimentary processes, quartz sand,
shale (clay), and limestone (CaCO3).
How Sedimentary Rocks Are
Formed
How Sedimentary Rocks Are Made
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Imagine an average continental igneous rock, a granodiorite, as at right
(click picture to enlarge). It contains quartz, and feldspar, and mafic
minerals, a representative sampling of the eight rock forming minerals in an
igneous rock.
Now imagine we are going to do every sedimentary process to that rock
that it is possible to do, including complete weathering, and complete
transportation, sorting and deposition. The results are always the same quartz sandstone, shale, and limestone separated from each other in
different depositional environments: the beach, near shelf, and far self.
Sea diagram previous slide
Process of Creating Sedimentary
Rocks
• Sedimentary systems work this way because of two processes.
WEATHERING: Weathering is the breakdown of one mineral/rock
into another. Of the eight igneous rock forming minerals all are
subject to degradation (weathering into something else), except
quartz. Quartz, for all intents and purposes, does not weather and
will survive in the system relatively unscathed.
The remaining seven rock forming minerals all dissolve to make
the sea salty, or decompose to form new minerals stable at the
earth's surface. Orthoclase, for example, breaks down to form clay,
and the calcium in Ca plagioclase goes into solution to form CaCO3.
There are lots of other weathering products, of course, but they are
just details. The simple, ideal model predicts three end products,
quartz sand, shale, and limestone, which all together compose the
vast majority of sedimentary rocks.
Process Continued
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TRANSPORTATION AND SORTING: The second process is sorting during
transportation. The sand and clay, beginning as a poorly sorted mixture, are
separated more and more as they travel down stream away from the source. Quartz
sand, which rolls and bounces along the bottom, does not transport as easily as clay
which travels in suspension. And the CaCO3 is dissolved and therefore just travels
with the water.
The result is, during transportation these three weathering products do not
transport equally well, and become separated. The final separation takes place at the
ocean shoreline (image to right). Here we see river transported sediment entering the
ocean. Waves crashing on the beach keep the sediment continuously stirred up. The
quartz, being relatively heavy, settles quickly to the bottom, the clay remains in
suspension until it drifts to the quieter near shelf, where if finally settles to the bottom
to form shale.
(see picture next slide)
Finally, the dissolved CaCO3 precipitates out of suspension in the far shelf, beyond
the range of sand and clay to form limestone. The calcite is deposited because plants
and animals extract it from sea water and use it to build their skeletons. After death
their calcite skeletons form the limestone sediment.
Final Separation
Sorting
Where different
Materials are separated
Finally, the dissolved CaCO3 precipitates out of suspension in the far shelf, beyond
the range of sand and clay to form limestone.
The calcite is deposited because plants and animals extract it
from sea water and use it to build their skeletons. After death their calcite skeletons
form the limestone sediment.
How to Classify Sedimentary
Rocks
• A Basic Sedimentary Rock Classification
•
A good classification is based on some theory that
explains how the rocks form, and are related to each
other. We want to group together rocks that form by
similar processes. The most general theoretical model
we have for sedimentary rocks is the simple ideal model.
The basic classification is based on that model.
The complication with sedimentary rocks is they form
from such a diversity of processes that straight forward
classifications are difficult. They are unlike igneous rocks
where a relatively straight forward Texture and
Composition classification leading directly to
interpretation is possible.
Reasoning Behind Classification
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The simple ideal model for the evolution of sedimentary rocks says there
are three end products, three attractors, that all sedimentary processes are
working to reach - quartz sandstone, shale, and limestone.
The three attractors in the simple ideal model are not isolated, however;
each one stands for a class of weathering products. For example:
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Quartz sandstone = all visible grains, including such ones as
incompletely weathered feldspar from the granodiorite in the simple ideal
model.
Shale = all clay sized grains (clay is a generic name; there are many kinds
of clay minerals as well as other minerals that are clay sized)
Limestone = all dissolved minerals, including not only calcite CaCO3, but
also halite (table salt; NaCl), and gypsum (CaSO4 . H2O) among others.
On the chart below we can see these additional weathering products
Classification of Sedimentary
Rocks
So far, so good. But now we have to do some mixing and splitting.
Sedimentary rocks are generally divided into three great categories,
siliciclastic (or simply, clastic) rocks, chemical rocks and
biochemical rocks. Their relationships to the three divisions
from the simple ideal model are shown in the figure below.
Observe how visible grains and clay sized grains mix together to
form clastic rocks, while minerals in solution split to form chemical
and biochemical rocks.
Classification of Sedimentary
Rocks
Silicicastic Sedimentary Rocks
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(Silici)Clastic rocks are composed of weathering products that do not
dissolved into water, have silica (SiO2) as one of their major components,
and are transported either by rolling along the bottom, or in suspension.
Because of this the VISIBLE GRAIN and CLAY SIZED GRAIN weathering
products in the chart above tend to be mixed, and deposited together. So
we group them together as SILICICLASTIC rocks
Silicicastic Sedimentary Rocks
Continued
• Clastic rocks (sandstones, shales, etc.) are classified on
two criteria - texture (grain size), and composition (that
is, QFL).
Shale, of course, is both fine grained, and composed
of clay, so the name "shale" incorporates both properties
of texture and composition.
For the coarser sizes, where the mineral grains can
be seen by eye or with a handlens, a complete name
consists of two parts. For example, an " arkose
sandstone " is a rock composed of sand sized particles,
and a large percentage of those particles are composed
of feldspar.
.
WENTWORTH SCALE.
• Clastic particles are divided into size categories based on the
WENTWORTH SCALE. This scale has been in use for over a
hundred years and is universally recognized. The middle size is 2
mm, the boundary between sand and gravel. Note that the size
categories get geometrically larger, and smaller, from 2 mm. Sand,
for example, ranges from 1/16 mm to 2 mm, while granules range
from 2 mm to 4 mm, and pebbles from 4 mm to 64 mm. The reason
for this is we want to group together particle sizes that can moved
more or less together by running water.
The Wentworth scale is straight forward, and with a ruler for scale
it is relatively easy to classify the rock. One simplification, and one
complication, though. The simplification is, we generally group all
the greater than sand sized particles into one category - gravel,
unless there is a specific need to distinguish these grain sizes. The
complication is, gravel is divided into whether the grains are angular
(breccia), or rounded (conglomerate).
Chemical and Biochemical
Sedimentary Rocks
• To deposit minerals that are in solution, they must some how come
out of solution and this happens two ways. Either they precipitate
directly from sea water (usually because it is evaporating and
concentrating the salts) - CHEMICAL ROCKS.
Or " plants" and " animals" extract the dissolved minerals from
the sea water to make skeletons, their skeletons eventually
becoming part of the sediment as BIOCHEMICAL ROCKS. So,
minerals in solution split into two categories
CARBONATES:
• These are composed of the mineral calcite (CaCO3 calcium carbonate), and are thus all known as
carbonates. Note on the chart that there are many of
these, that they form by both chemical and biochemical
processes, and that they tend to be mixed together in
various combinations in the rocks. They are extremely
abundant and important.
OTHER CHEMICAL ROCKS
• : These rocks fall into two categories. Chert is a siliceous rock
(composed of SiO2) that forms from the recrystalized skeletons of "
animals " (single celled radiolarians, and glass sponges) or single
celled " plants " (diatoms, silicoflagellates). And although the silica
comes from skeletons to become chert it must be chemically
recrystallized, thus putting it in the chemical category (but it is
confusing).
Rock salt (halite; NaCl) and gypsum (CaSO4 . H2O) originally
are dissolved in the sea water, thus making the sea salty. When sea
water evaporates in a closed area, such as a lagoon, the salt
concentration becomes very high, supersaturated, and precipitates
out. The process is common in desert areas, with examples today in
the Red Sea and Dead Sea in the Middle East, both highly saline.
BIOCHEMICAL ROCKS
Peat and coal because they come from plant remains
are biochemical rocks, but unlike all the other
chemical/biochemical rocks peat and coal always form in
the presence of clastic rocks - sandstones and shales.
This discussionillustrates part of the difficulty of
developing a completely consistent classification.
Sedimentary rocks form in so many different ways, from
so many different processes that coming up with one
scheme that is inclusive and yet straight forward is not
easy. There always seem to be exceptions to the rule
that have to have to be explained individually
Sedimentary Rocks Features
Stratification
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Internal bedding structures These are sedimentary structures which are
best seen looking at a side view of a sedimentary rock or sequence of
sedimentary rocks.
Stratification (or layering) is the most obvious feature of sedimentary
rocks. The layers (or strata) are visible because of differences in the color or
texture of adjacent beds. Strata thicker than 1 cm are commonly referred to
as beds. Thinner layers are called laminations or laminae . The upper and
lower surfaces of these layers are called bedding planes
Graded bedding
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Graded bedding results when a sediment-laden current (such as a turbidity
current) begins to slow down. The grain size within a graded bed ranges
from coarser at the bottom to finer at the top. Hence, graded beds may be
used as "up indicators".
Cross-beds
• Cross-stratification is a general term for the internal bedding
structure produced in sand by moving wind or water. If the individual
inclined layers are thicker than 1 cm, the cross-stratification may be
referred to as cross-bedding. Thinner inclined layering is called
cross-lamination. Cross-stratification forms beneath ripples and
dunes. The layering is inclined at an angle to the horizontal, dipping
downward in the downcurrent direction. Hence, cross-beds may be
used as paleocurrent indicators, or indicators of ancient current flow
directions. Cross-beds usually curve at the bottom edge, becoming
tangent to the lower bed surface. The upper edge of individual
inclined cross-beds is usually at a steep angle to the overlying
bedding plane. Hence, cross-beds may also be used as "up
indicators".
Cross-Bedding
Trace fossils
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Trace fossils or ichnofossils include tracks, trails, burrows, borings, and
other marks made in the sediment by organisms. They are bioturbation
structures formed as the activities of organisms disrupt the sediment. As
organisms tunnel through sediment, they destroy primary sedimentary
structures (such as laminations) and produce burrow marks. Bioturbation
continuing over a long period of time will thoroughly mix and homogenize
the sediment. Through this process, a laminated sediment can be altered to
a massive, homogeneous sediment with no readily discernable layering or
other sedimentary structures.
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