SEDIMENTARY ROCKS
Prepared by T. Weiland for GEOL 1121
Sedimentary rocks - rocks formed by the accumulation and consolidation of
sediments (surficial processes).
I. Sediments - material that is deposited by wind, water or ice; material that is
precipitated from seawater; or deposits of organic origin.
A. Major Components - (clastics, biogenic and chemical
precipitates). *This is the basis for the classification of
sedimentary rocks. The name of the rock indicates how the rock
formed. It attempts to delineate the conditions of deposition and
the source of the sediments.
1. Clastic - particles derived from the breakdown of
pre-existing rocks (sedimentary, metamorphic or
igneous). Clastic sediments make up >75% of all
sediments on the earth's surface. Breakdown can be
physical, chemical or biogenic; however, dissolution
does not occur.
2. Chemical Precipitates - minerals that are
precipitated from concentrated solutions. Examples halite from brines, calcite from seawater, etc.
3. Biogenic - included with chemical sediments
(called chemical organic) in the book; however, they
deserve a separate mention. *Sediments that are
produced through the life activities of plants and
animals. Examples - CaCO3 - calcite produced by
aquatic plants and animals forming coral reefs, quartz
(silica-rich) deposits formed by the accumulation of
the test of microscopic plankton animals.
B. Characteristics of Sediments - features that can be
used to decipher the geologic history.
1. Composition - depends on the mineralogy of the
source rock, duration and intensity of weathering
(climate and relief) and the rate of burial.
*Most abundant components of sediments are quartz,
feldspars, iron oxides, clays, calcite and rock
fragments. (Low temp. minerals and fine grain size
result in greater stability.)
2. Size and sorting - depend on the mineralogy of
the source rock, distance traveled, and the method of
transport water, wind or ice). **Used in conjunction
with composition to determine rock name.
a. Size Classification Wentworth Scale
Name
Size
Clastic Rx. Name
Boulder
>256mm.
Cobble
256-64mm.
Pebble
64-4mm.
Granule
4-2mm.
Sand
*2-1/16mm.
Sandstone
Silt
1/16-1/256mm.
Siltstone (mudstone)
Conglomerate or Breccia
Clay
<1/256mm.
Mudstone or Shale
*Finer grain size often reflects a greater
distance of transport. Example continental margin deposits.
b. Sorting - range of particle size. This is
controlled by the transporting agent (wind
only transports fines - well sorted
deposits, glaciers moves everything poorly sorted deposits, and water is
variable depending on amount of energy).
3. Roundness - the degree of abrasion of a clastic
particle as shown by the amount of sharp edges on
the grain. This is also controlled by the method of
transport. Examples -wind transport is highly abrasive
resulting in highly rounded grains; whereas, ice
transport is more passive, depositing very angular
clasts.
4. Color - highly variable - dependent on the original
mineral color, weathered mineral color, percent of
organic material, etc.
II. Sedimentary Environments - the physical, chemical and biologic
conditions that exist at the location where the sediment is deposited. The
different environments on the earth (deserts, ocean floor, beach, etc) are
characterized by different types of sedimentary deposits. By characterizing the
modern sediments, we can infer how and where the sedimentary rocks of the
past, thereby extracting the geologic history from the rocks.
Handout #3
A. Continental Environments -fresh water and subaerial
conditions.
1. Alluvial Fans - fan shaped deposits of gravel, sand
and mud which form at the base of mountain ranges.
Deposition occurs as flash floods and intermittent
streams sweep the loose debris down steep
gradients. At the base of the mountains, the water
does not have enough energy to transport the
sediment over flat areas.
Characteristics - coarse-grained, poorly sorted, very
heterogeneous mixture of mostly rock fragments,
sand and mud. (Form coarse-grained unsorted
conglomerates).
2. Eolian Deposits - dunes constructed of windblown sand. Commonly found in desert areas.
Characteristics - well-sorted, quartz-rich (more
resistant), well-rounded, sandstone is commonly
formed which often displays cross-bedding.
3. Fluvial Deposits - form in association with river
systems. The great rivers of the world are the major
transporters of erosional material to the ocean basins.
Characteristics - highly variable, dependent on the
amount of energy available for sediment transport.
Recognition of fluvial deposits is generally dependent
of stratigraphic relationships.
*Channel deposits are characterized by cross-cutting
relationships with adjacent deposits, occasional
cross-bedding and coarser-grained deposits than the
flood plain or sand bars.
*Sand bars tend to have better sorted, lens-shaped
deposits of sand which form in the inside of
meanders, where the water velocity is lowest.
*Floodplains are characterized by fine-grained
deposits of mud that often have a high percentage of
organics (fertile).
4. Lake Deposits - Lakes are generally characterized
by low energy. The larger the lake, the greater the
degree of sorting and the finer-grained the sediments
deposited in the center of the lake.
*Characteristics - fine-grained siltstone and shale,
which are well stratified (layered) commonly, form in
the central portion, whereas some well-sorted
sandstone is also formed along the margins.
Exception -playa lakes - lakes in arid areas which
undergo extensive evaporation forming halite,
gypsum and anhydrite to form. (White Sands N.M.,
Great Salt Lake, Death Valley).
5. Glacial Deposits - sediments deposited by the
glaciers and their associated meltwater. The glaciers
pluck and scrape rock material as they move along.
*Characteristics - generally less sorted and rounded
than other deposits except possibly alluvial fans.
Meltwaters carry sediments farther, resulting in a
higher degree of sorting. Primary glacial deposits
often contain a wide range of size and compositional
varieties.
B. Shoreline Deposits - transitional or mixed
environments.
1. Deltaic Deposits - include deposits of mud and
sand that accumulate at rivermouths (where the rivers
terminate at the coast). Example - Mississippi River
Delta - 400 million tons of sediment are deposited
here each year.
*Characteristics - often difficult to recognize due to
the variety of sediments that occur in these
environments. They are generally fine-grained,
contain both marine and non-marine fossils, and
sometimes have mud cracks.
2. Beach Deposits - include shoreline deposits such
as lagoons, inlets, beaches, etc.
*Characteristics - usually quartz-rich, well rounded,
well sorted mixtures of mud and sand or just sand.
Cross-bedding is common in beach deposits.
Abundant marine fossils.
3. Tidal Flat Deposits - areas covered by water at
high tide and exposed at low tide.
*Characteristics - Predominantly mud deposits with
abundant marine fossils. Mud cracks, ripple marks
and burrows are also common.
C. Marine Environments - the deeper and more restricted
the water, the finer grained and better sorted the deposits.
1. Shallow Marine Deposits - includes from the
shoreline to the edge of the continental shelf.
Deposits depend on the sediment supply, climate,
wave energy, circulation and temperature.
*Characteristics - sand, calcium carbonate, and mud
are the major constituents. Well-sorted sands found at
areas of high wave energy and mud found in the
deeper more restricted areas. Marine fossils are
common.
2. Organic Reefs - ridge- or mound-like structures
built of sedentary calcareous organisms (esp. corals).
These deposits form only in shallow, warm, clear
water where there is minimal sediment input.
*Characteristics - in situ remains of calcareous
organisms.
3. Deep-marine Deposits - includes the ocean floor
beyond the continental shelf (deep-sea floor and
abyssal plains). Because these areas are so removed
from any sedimentary sources, only biogenic and
extremely fine-grained sediments are deposited.
*Characteristics - fine-grained well-sorted deposits
of shale, mudstone, chert (accumulation of siliceous
organisms). *Turbidites - sediment deposited by
bottom-flowing density currents laden with coarsergrained sediment. These deposits are characterized
by graded-bedding.
III. Diagenesis - all changes that the sediment undergoes after its initial
deposition (exclusive of weathering and metamorphism.
Lithification - process where a sediment is converted into a rock.
A. Compaction - rearrangement of particles due to increasing
weight of overlying materials (results in a reduction of pore space
and volume).
B. Cementation - precipitation of crystalline materials between
particles. The most common cementing agents include silica
(SiO2), calcite (CaCO3) and iron oxide (Fe2O3). These cements
are usually derived from circulating pore fluids that become
saturated with these minerals.
C. Mineralogic Changes - diagenesis often changes the
mineralogy of the sediment. This includes the breakdown of less
stable grains such as feldspars to form a clay matrix, the
dissolution of grains or matrix such as shell material, or the
transformation of organics to hydrocarbons.
IV. Sedimentary Rock Types - the various rock types reflect different
methods of transport, various source rocks, and different environments of
deposition.
A. Clastic Rocks - those composed of particles derived from preexisting rocks. These rocks are classified according to grain size.
1. Conglomerate - cemented, rounded, gravel-sized
particles.
2. Breccia - cemented, angular, gravel sized particles.
3. Sandstone - cemented, sand-sized particles which often
contain a high percentage of quartz due to their high
maturity. (Exception - South Florida beaches)
4. Siltstone - cemented, silt-sized particles.
5. Claystone - cemented clay-sized particles. *If the rock is
composed of two or more grain sizes, the name is based on
the most abundant size.
6. Shale - fine-grained rock composed of silt and clay which
tends to part along the bedding planes. It is the most
abundant sedimentary rock.
B. Chemical Precipitates - formed by the precipitation of
minerals from saturated solutions.
1. Limestone - CaCO3 (only a minor fraction of all
limestone appears to have formed this way).
2. Dolomite - MgCO3 - can form by the direct precipitation
of seawater or by the diagenesis of limestone by Mg-rich
pore fluids.
3. Chert - can form by the direct precipitation of seawater,
but most commonly forms by biogenic processes.
4. Evaporites - precipitates formed by evaporation of
seawater. Includes halite, gypsum and anhydrite.
C. Biogenic Accumulates
1. Limestone - can form by the accumulation of any
calcareous remains.
coquina -loosely cemented shell material coral accumulations of the hard exoskeleton secreted by coral
polyps.
chalk - soft, porous limestone made of microscopic marine
organisms (commonly deep-marine also.)
2. Chert - siliceous deposits of diatoms (microscopic plants
with siliceous
coverings (deep-sea
deposits).
3. Coal - organic deposits
that form in swamps and are
later deeply buried until
converted into a
carbonaceous rock.
http://itc.gsw.edu/faculty/tweiland/sedrocks.htm
Igneous and Metamorphic Lake Superior Beach Rocks
Meandering along Lake Superior's beaches sorting through the stones and pebbles
washed up on the shore is one of my favorite relaxing ways to pass the time . . . doing
not much at all. The ultimate prize is finding a Lake Superior Agate.
Agate Hunting & Rock Picking Directory
Part 1 - Identifying Beach Stones & Agates around Lake Superior (you are here)
Part 2 - More Beach Rock & Agate Collecting around Lake Superior
Part 3 - Identifying Agates versus Agate Wanna-Be's
Rock Tumbling Lake Superior Rocks & Gemstones
Tumbling and polishing Agates- Agate pictures & some tips on using tumbling
pellets
Lake Superior Agate Festival
Pictures of Lake Superior Agates - Moose Lake Geology Interpretive Center
Maps, Pictures, Guide to Lake Superior Agate & Rock Picking Beaches
One problem with novice rock hounds is the rocks and stones we see on the beach
don't look like the ones we find in the shops. They've been tumbled and polished to
bring out their beauty. I needed to run into someone on the beach who knows their
stones and can help me know what an agate in the rough looks like. (At the 2013 &
2014 Lake Superior Agate Festival I found lots of these sorts of folks!)
But aside from agates, I find all sorts of neat looking, interestingly shapped rocks along
the beach. I rarely come home from a trip to or around the Lake without a box full of
rocks. Once I began the habit of collecting rocks, I graduated to trying to learn more
about them. I've done some reading in an attempt to better identify what I am finding
on the beach. Two sources have been particularly helpful: Sparky Stensaas' "Rock
Picker's Guide to Lake Superior's North Shore" and Susan Robinson's "Is this an Agate."
(See our recommended Rock Hound books near the bottom of the page)
While these two books have been expertly documented and illustrated, I wish they had
shown me actual photographs of the rocks rather than drawings. Which leads to what I
am endeavor to accomplish here - giving other rock pickers actual photographs of a
number of the rocks, stones, and minerals you are likely to find along the shore.
Armed with Sparky's and/or Susan's book plus some of the pictures and descriptions
I've provided here, perhaps will help you ID what you are collecting.
By the way, if you want to print out any of these larger images, just *right click* on the
image and choose "copy image" to copy it to your hard drive. Once there you can click
on it and print it out.
~~~"Click" on smaller images for a larger picture~~~
Basalt
Basalt is one of the most common rocks you'll find, yet I love their smooth surface and solid feel in
my hand. Each one has a distinctive shape and in a variety of shades of color from blush-black to
grey. They are volcanic rock formed from lava that quickly cooled when it reached the surface. That
quick cooling is what caused it to be dense, very fine grained (tiny crystals) and smooth - although
the smooth surface is also due to glacial grinding & Gitche Gumme tossing it about for eons. That
Basalt rock you hold in your hand is over a billion years old!
Ophitic Basalt
Ophitic Basalt looks like a basalt rock that has been decorated with lighter colored little painted
petals. They come from tiny feldspar crystals that were in the lava. Because the crystals have worn
at different rate than the basalt there is often a slight mottled texture to these these stones. This
particular specimen has many feldspar crystals, others have fewer and require more close
inpsection to see them (look for the little spots that look as if they were put there by dabbing with a
paint brush).
Rhyolite
Rhyolite is a sandy-colored version of Basalt. Formed from quickly cooling lava just like basalt, it is
rich in silica and potassium, whereas basalt is poor in these minerals and richer in iron, and other
minerals. Coloration of Rhyolite will vary depending upon the mix of silica and iron and some of the
trace minerals. Some stones may be a difficult call: basalt or rhyolite?
The sample on the far right may be Rhyolite that has darker bands of flow of an iron-richer form of
lava. > > >
Vesicular Rhyolite
Frequently the molten lava flowing from deep down in the earth was filled with gases that formed
bubbles when the lava cooled - leaving a pock-marked surface on the rock. The sample to the right
shows two examples - one with many vesicules and the other with only a few.
A similar process occurs with basalt, forming vesicular basalt. Unfortunately I don't have any
samples to show here.
Amygdaloidal Rhyolite
Sometimes the holes that create vesicular rhyolite fill up with molten minerals - often calcite and
quartz - that form crystals in the cavities when the lava cools into a rock. The crystal spots are
always rounded since the holes were formed by gas bubbles.
Here is a sample ofAmygdaloidal Basalt > > >
Porphitic Basalt
At first glance, a porphitic rock looks like the amygdaloidal rocks pictured above. On closer
inspection you'll see the blobs of crystals are rectangular shaped not round. This is because the
crystals were already there when the magma was molten. Rhyolite can also occur in a porphitic
form.
Granite
Granite is formed deep underground and stays there for a time, cooling slowly. This process forms
large crystals and includes minerals like quartz, feldspar and mica embedded within the rock. It is
coarse grained and speckled. Although it appears quite different, it is similar in basic composition to
rhyolite - rich in silica and poor in iron.
Gabbro
As granite is the coarse grained, slow cooling cousin of rhyolite, gabbro is the coarse cousin of
basalt, formed deep underground as granite. My guide books say it can range from black to gray or
a mixture of black and light crystals. Sparky says to look for "weathered white flecked" dark, coarse
rocks. The white or light gray crystals you see in the sample to the right are weathered plagioclase
crystals.
Diabase
Here's a rock that is a hybrid of basalt and gabbro - diabase. Basalt results from lava that cools
quickly on the surface; gabbro results from magma cooling slowly deep within the earth; diabase
occurs in between, cooling medium slowly and closer to the surface. As a result it is medium grained
with smaller crystals than either granite or gabbro.
If you hold these guys at an angle to the sun, you can usually see some smaller sparkly crystals
reflecting the light.
Slate
This is a metamorphic rock formed by heat and presure on shale. It is dark to gray in color, smooth,
somewhat shiny, and generally flat. Look for some layering. Look closely at this sample and you can
see the lighter colored layer sandwich.
Gneiss
Gneiss is also a metamorphic rock. It can be similar in appearance to granite but can be more finegrained. What is distinctive about gneiss are the bands of minerals that form alternating colors.