Lab 2: Stratification, Sedimentary Structures - MyGeologyPage

Geology 109L, Dawn Y. Sumner
Lab 2: Stratification, Sedimentary Structures
and Interpretation of Paleocurrents
Goal: The goal of this lab is to examine sedimentary structures in rocks and interpret the
processes that formed them.
Suggested Reading:
Chapter 4 in Sedimentology & Stratigraphy (p. 37-61)
*Useful pages for Part 1: Sections 4.7-4.9 (p. 58-60)
*Useful pages for Part 2: Sections 4.2.4- 4.6 (p. 41-58)
Chapter 3 in Sedimentary Rocks in the Field (p. 28-125)
*Useful pages for Part 1: p. 34-40, 68-100
*Useful pages for Part 2: p. 30, 46-67
Introduction
Sedimentary structures are the result of a variety of depositional processes and are found
within beds, on the tops of beds, and on the bases of beds. These structures reflect the
predominant environmental conditions at the time of deposition and are important tools for the
interpretation of those environments. Structures can show us the mechanics of fluid flow, the
direction of flow, paleobiological aspect of the environment, and early compactional forces.
Some structures will show us which way was up during deposition. Others destroy earlier
structures, i.e. burrowing organisms erase ripple stratification in some cases. Structures can be
depositional, erosional, or post-depositional. They can be biogenic or abiogenic. Many
sedimentary structures are due to a combination of processes.
One of the most common sedimentary structures is stratification, which is the layering of
the sediments on the scale of less than a millimeter to many meters. Stratification can often be
recognized by changes in mineralogy, color, or grain size that mark surfaces. The geometry of
these surfaces and their spatial relationships to each other are characteristic of different
sedimentary structures and styles of deposition.
Erosional Structures
Erosion occurs on the tops of beds when the fluid flowing over them is fast enough to lift
grains off the sediment bed. The structures are created on top of the bed, and the next layer of
sediment fills in the topography on the surface forming casts of the erosional structures. The
structures commonly are recognized as casts because it can be difficult to prove that an erosive
structure formed during deposition of the rock rather than during weathering. In contrast, casts
of structures had to have formed prior to deposition of the sediment forming the cast. These are
some common erosional structures:
1) Flute casts: Flutes form when eddies in turbulent flow touch the sedimentary surface (i.e. the
viscous boundary layer breaks down due to excessive turbulence). The deep narrow
portion points upstream. It flares and shallows downstream, and is typical in the Tb and
Tc portion of a Bouma sequence. They are usually recognized as casts.
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2) Ridge, furrow, and gutter casts: Ridges, furrows, and gutters are elongate features on the tops
of beds form from turbulence. They are very elongate and do not have the well defined
up-stream scour that flutes show. Thus, they indicate the trend of the current, but do not
usually tell you the direction of flow. Ridges and furrows are on the scale of mm wide,
whereas gutters are on the scale of cm. They can be meters long.
3) Channels: These are large scale scours which are usually filled with coarser sediment than the
material the channel cuts into. River channels are an example of how large channels can
be.
4) Obstacle scour marks: These form downstream of objects sitting on a sedimentary surface.
The object produces eddies, creating high flow areas with enhanced erosion and low flow
areas with enhanced deposition.
5) Grooves: Grooves form when objects such as sticks or rocks are dragged along the surface
by the flow. The shape and size of the groove depends on the shape and size of the
object.
6) Impact or bounce marks: These form when an object hits sediment and bounces off or skips.
Impacts can be meteor-size down to pebble-size. These structures are common in the Tb
and Tc portion of a Bouma sequence. Raindrop impact structures are a special subset of
these structures that form when a scattered drops hit layers of dry mud or silt.
Depositional Structures
Depositional structures form in response to the flow characteristics of the depositing fluid
and the conditions of deposition.
1) Bedding is the smallest lithostratigraphic unit. It is simply a package of sediment > 1cm in
thickness that has some sort of change above and below it, like a compositional or abrupt
grain size change.
2) Lamination is < 1 cm in thickness, and is defined by changes in grain size, color or
mineralogy.
3) Cross-stratification appears as sets of laminae that truncate older laminae within a bed.
Cross-stratification can form from infilling of channels and scours, or by the migration of
dunes and ripples. Cross-stratification forms in silts, sands, and conglomerates and is a
very useful indicator of flow conditions. Younger laminae always cross cut older
laminae, so you can tell which way is up. Features that vary include:
a. The size range of cross-lamination and cross-stratification
b. The shape of cross-stratification/lamination: See the text book and photos on the web
page to get a sense of the different types. Note that the 2-d shape of cross stratification
varies substantially with the orientation of the surface you are looking at relative to the
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flow direction. Thus, to fully understand the shape of cross-stratification, you need to
look at least 2 surfaces that are perpendicular to each other and imagine the 3-d shape of
the bedforms.
4) Ripples, dunes and sand wave forms can be preserved on bedding surfaces. Usually, the
migration of the bedforms destroys the surfaces, but in some cases, ripple crests are
preserved by burial with sediment in the absence of erosion. You have a number of these
samples in lab. When the crests of ripples are preserved, it is easier to tell their origin.
However, you can also infer ripple type from the cross lamination within the sediment.
5) Graded Beds show a grain size change from the bottom of a bed to the top. Normally
graded bedding is coarser material at the base of the bed that fines up. Reversely graded
bedding is fine material grading up to coarser material. These structures show the change
in energy in the transport system.
5) Massive beds: as the name implies, these beds have no apparent internal structures. You may
see evidence of bioturbation, dewatering or recrystallization. When the divisions
between beds are unclear and they lack distinctive structures, we call them amalgamated.
Post Depositional Sedimentary Structures
These features occur after the sediments are deposited, but usually before lithification. They do
not include regional types of deformation, such as faulting and folding.
1) Slumps and slides occur on an inclined surfaces and show folding and thrusts in the
sediments.
2) Deformed bedding is disrupted, convoluted or contorted beds. Cross-stratification and
lamination is deformed.
3) Load structures form by one bed sinking into the one below it. Flame structures form when
mud is injected into overlying sands. Ball and pillar structures form when sand sinks
into mud and forms a defined blob. Dish and pillar structures form when sand sinks into
a previously deposited sandstone. These structures are common at the base of submarine
flow deposits like turbidites.
4) Mudcracks and syneresis cracks: Mudcracks are polygonal patterns in sediment and can be
any size. They are caused by desiccation in a wet-dry environment. The fine silt or clay
laminae shrink and crack, which then allows sediments to later fill in the cracks.
Syneresis cracks occur subaqueously due to dewatering of sediments resulting from changes
in salinity. They are usually spindle-shaped incomplete polygons.
Biogenic Sedimentary Features
Biogenic features are caused by the activity of organisms in the sediments. Using the biological
evidence available in the rocks can make determining the environment much easier.
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1) Trace fossils are marks in the sediment that result from organisms crawling, feeding,
excreting and burrowing in or on the sediment. When well preserved, they are distinctive
indicators of specific organisms and have taxonomic classification just as the organism
itself does. They can be very helpful in determining age relations and facies analysis.
2) Bioturbation is the process of mixing sediment as organisms crawl around feeding and
burrowing in the sediment. Generally, it homogenizes beds, erasing other sedimentary
structures. Sometimes defined burrows can be preserved, but commonly bioturbated
beds have a mottled or massive appearance. Bioturbation can also occur from plant roots.
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Geology 109L
Lab 2: Sedimentary Structures
Name:
TA:
Part 1: Erosional, post-depositional, and biological structures. Photographs posted on the
class web page will help you identify the structures in your lab samples.
1. T-1326
a) Sketch the cut surface of this rock and part of the bottom of the rock. Name both of the
structures. Include a scale.
b) Name the rock types and give the grain sizes at the top of the sample and at the base of the
sample (Hint: there are 2 different rock types in this sample).
c) Briefly describe how the structures formed.
2. T-1355 (G106-48) Decide which direction is up.
a) Sketch this rock with up towards the top of the page. Include a scale.
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) Name the sedimentary structure.
c) Name the rock.
3. SS71 (G106-61)
a) Sketch the bottom of this rock and name the sedimentary structure. Include a scale.
b) Add an arrow to your sketch to indicate the paleocurrent direction suggested by this structure.
c) In what sedimentary environment would this structure likely form?
d) Name the rock and give the grain size.
4. CC-1 Note the granule-sized mudrock rip-up clasts in this sample.
a) Sketch the bottom of this rock and name the sedimentary structure. **This sedimentary
structure is common at the base of the turbidites we will see on our second field trip and was
collected at our field trip site.
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) What is the difference between the base of this sample and sample SS71? How might the
flow have been different?
c) This bed was collected in an area with abundant turbidites. Which part of the Bouma
sequence does this sample contain? (Answer this question after Part 2)
d) Name the rock and give the grain size.
5. P-3457
a) Sketch part of the base of this bed and name the structures. Include a scale.
b) Name the rock type that fills the structures and the rock type that surrounds the structures.
Give the grain size of each.
c) Describe the sequence of events that produced this sample. (Hint: there are 5 steps)
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6. SS76 and SS77 (GC 67)
a) What are these sedimentary structures?
b) What is the difference between the two examples?
c) What can you say about the environment in which these structures formed?
d) Name each rock. SS76:
SS77:
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Part 2: Stratification styles. For this part of the lab, it will be very helpful to use several
movies produced by USGS geologists that show how stratification varies with the geometry of
ripple and dune crests. These movies and an explanation of them are available both on the class
web page and on the lab iBooks. Check out an iBook and review the movies before examining
these samples. I have also posted numerous photos of cross stratification on the class web page,
which will help you identify structures and understand their origins.
7. T-1377 Top on this sample is the surface with the number.
a) Sketch this rock with up towards the top of the page. Include a scale.
b) What sedimentary feature is in this rock? What does it say about changes in flow speed with
time?
c) Name the rock. Include grain sizes.
8. SS6 and SS9 Decide which direction is up for each of these rocks.
a) Sketch each sample with up toward the top of the page. Show the shape of the upper surface
and schematically sketch the dip of a few laminae relative to the shape of the upper surface.
Include scales with each sketch.
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) Add arrows to your sketches to indicate the paleocurrent directions suggested by these
structures.
c) Name the structures. What is the main difference in the water flow that deposited these
samples?
d) Name each rock and give the grains size. SS6:
9. SS1
a) Examine the surface with the sample number on it (the top). Name the sedimentary feature.
SS9:
b) What is the main difference between this structure and those in SS6 and SS9?
c) What flow conditions produce this type of structure?
d) Name the rock with grain size.
10. SS52 Decide which direction is up.
a) Sketch the sample with up toward the top of the page. Include two perpendicular surfaces to
provide a 3-d view of the cross stratification. Include a scale.
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) Add an arrow that shows the direction of flow.
c) What shape do you think the dune crest had and why? (Hint: the USGS movies will be
helpful.)
d) Name the rock and give grain size.
11. RB-1 Decide which direction is up before sketching this rock.
a) Sketch this rock with up towards the top of the page. Label the two types of sedimentary
structures. Include a scale.
b) What changes in flow occurred based on the change in cross stratification in this sample?
c) What shape do you think the ripple crests had and why? (Hint: the USGS movies will be
helpful.)
d) Name the rock and give grain size.
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12. T-1451 Decide which direction is up before sketching this rock.
a) Sketch this rock with up towards the top of the page. Label the two types of sedimentary
structures. Include a scale.
b) What changes in flow occurred based on the change in cross stratification in this sample?
d) Name the rock.
13. 106-HCS-1
a) Sketch the surface of this sample that has the up arrow on it. Name the cross stratification
type. Include a scale.
b) Does the structure indicate a specific flow direction? Why or why not?
c) Where might a deposit such as this be forming today?
d) Name the rock and give grains size.
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