Francis, 186-212, 2014
PETROLOGY LAB 5: Sedimentary Textures and Structures
The process of sedimentation begins with erosion to produce sediments in high standing
areas, followed by transportation of the sediments by water, wind, or ice to sites of
deposition and accumulation in low standing areas, and finishes with compaction and
cementation into sedimentary rocks. An examination of textures and structures of
sedimentary rocks enables one to constrain the transport mechanisms and the nature of
the deposition environment of the sediments.
Station 1 - Sedimentary Textures: grain size, shape, and sorting
The most important sediment textures include the size, shape, and degree of sorting of the
clastic grains making up a sediment or sedimentary rock.
Grain Size:
In a broad sense, the grain-size of siliclastic
sediments reflects the hydraulic energy of the
environment: coarser sediments are transported
and deposited by faster flowing currents than
finer sediments. Thus grain-size often reflects
distance from the source where the sediments
originated. For example, pebbles that are found
in conglomerates are often transported over
shorter distances than silt or clay. With sand
and silt-sized sediments you cannot do much
more in a hand-sample than estimate grain-size
and comment on sorting and roundness of
grains. A widely used grain-size scale is that of
Udden-Wentworth. Note that  (phi) refers to
a logarithmic transformation:  = -log2 S,
where S is grain-size in millimeters.
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Sorting and Roundness:
The degree of sorting of a sandstone reflects the transportation mechanism and the
nature of the depositional environment, and increases with increasing agitation
and reworking experienced. For example, aeolian and beach sandstones typically
are well sorted, contain well rounded grains and no matrix (they are said to be
‘texturally mature’), whereas fluviatile sandstones are moderately to poorly sorted
and may contain angular grains and matrix (they are said to be ‘texturally
immature’).
Conglomerates and breccias can be distinguished by the roundness versus
angularity of the clasts in the rock: if the clasts are rounded the rock is referred to
as a conglomerate, if they are angular it is a breccia. A conglomerate or breccia
can be either:

monomictic - the clasts are all the same type of rock.

polymictic – there are different types of rock clasts.
It is also important to note the ‘maximum clast size’, since it is often a
reflection of the hydraulic energy of the transporting current. In addition you
should examine the clast-matrix relationships in these samples:

clast-support fabric is typical of fluvial and beach gravel.

matrix-support fabric is typical of debris flows and glacial tills.
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Colour:
Color can give useful information on lithology, depositional environment, and
diagenesis. Two factors determine the color of many sedimentary rocks: the
oxidation state of iron and the content of organic matter. Iron exists in two
oxidation states: ferric (Fe3+) and ferrous (Fe2+). Where ferric iron is present it
frequently occurs as the mineral hematite and even in concentrations of less than
1% this imparts a red color to the rock. Where the hydrated forms of ferric oxide
(goethite and limonite) are present the sediment has a yellow-brown color. The
formation of hematite requires oxidizing conditions and these are frequently
present within sediments of semi-arid and continental sub-aerial environments.
Sandstones and mudrocks of these environments (e.g., desert, lakes and rivers) are
frequently reddened through hematite pigmentation and such rocks are referred to
as ‘red beds’. Many red marine sediments are, however, also known. Where
reducing conditions prevailed within a sediment the iron is present in the ferrous
state and is contained in clay minerals, imparting a green color to the rock.
Organic matter within a sediment gives rise to grey colors and with increasing
organic content the rock becomes black. Such organic-rich sediments generally
form in anoxic conditions. Thus color may provide you with additional
information about the depositional environments and should be included in your
considerations when examining a sedimentary rock.
For the samples in Station 1, estimate grain-size, sorting and roundness, look at
the color and think about what conclusions you can draw about depositional
environment and distance from the source rocks.
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Station 2 - Sedimentary Structures: Bedding
Bedding and lamination:
The characteristic feature of sedimentary rocks is the presence of stratification or
bedding, typically produced by changes in the pattern of sedimentation, such as
changes in sediment composition and/or grain size. Bedding is generally defined
as layering thicker than 1 cm, whereas finer scale layering is termed lamination.
Lamination is commonly an internal structure of a bed and arises from changes in
grain size between laminae, size-grading, or changes in composition between
laminae. Each laminae may be the result of a single depositional event. Which of
the specimens from this station show bedding and which lamination? What do
these features indicate for the depositional environment in which the specimens
were formed?
Graded Bedding:
Grading is a gradual change in grain-size upwards through a bed and typically
develops in response to change in flow velocity during a sedimentation event.
Beds with coarser grain-size at the bottom and fining towards the top are termed
normally graded, while beds that show coarser grain-size at the top and finer
grain-size at the bottom are termed reverse graded. Normal graded bedding is by
far the most common, and can be an excellent tops indicator. Reverse graded
bedding is relatively rare, and tends to occur in poorly sorted sediments that have
been deposited rapidly from sediment-charged flows. Examine the specimens at
this station from the point of view of whether they exhibit normal grading or
inverse grading? How did the flow conditions change during sedimentation?
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Station 3 – Sedimentary Structures: Bedforms, and Cross Stratification:
Cross stratification due to current ripples and dunes
The nature of the bottom surface in terms of the sedimentary structures or bedforms, like
ripples and dunes, is dependent on the current flow conditions. Bedforms are
characterized by their wavelength and/or height as ripples (=0.05-0.2 m), dunes (=0.510 m), and sand waves (=5-100 m). In aqueous flows, for a given grain size and water
depth ripples form at the lowest current velocity, followed by dunes, plane beds, and
antidunes with increasing current velocity.
Ripples and dunes are asymmetrical bedforms, which gradually move downstream as
sediment is transported through erosion of the upstream-facing side and deposition over
the bedform crest in foreset beds on the downstream-facing side. They are common in
rivers, tidal flats, delta channels, on shallow-marine shelves, and on the deep-sea floor.
The foreset beds of ripples and dunes are commonly preserved as cross stratification.
These range from planar cross strata formed through the migration of straight crested
ripples to trough cross strata formed through the migration of lunate or sinuous ripples.
In planar cross bedding, the foresets (sloping beds) dip at angles up to 30 or more, and
may have an angular basal contact with the horizontal. Trough cross bedding consists of
scoop-shaped beds with tangential bases and dips of 20-30. The asymmetry of trough
cross-bedding, and their cross cutting relationships, give excellent top and current
direction indicators.
The scale of the cross stratification reflects the
bed form that they preserve:

cross-bedding typically represents the
preserved foreset beds of sand waves
and dunes.

cross lamination typically represents
the preserved foreset beds of ripples

Climbing Ripples: Under conditions
of rapid deposition, successive
ripples may climb onto the backs of
previous ripples with little or no
erosion, producing a ‘false’ cross
bedding.
Can you identify upstream-facing side and
downstream-facing side in the specimens at this
station? What can you say about the wavelength
of the ripples? Can you determine the flow
direction?
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Station 4 – Sedimentary Structures: Sole Markings and other structures:
Sole Markings:
The bottoms of sandy beds often have markings that are indicative of current flow
and/or differential compaction:
Grove casts
Elogated ridges that represent filled depressions in
the underlying bed. Commonly elongated parallel to the
current direction, although it is typically not possible to tell
the upstream ends from the downstream ends.
Flute clasts:
Bulbous ridges that are steeper in the upstream
direction and flare downstream. They represent filled scour
depressions in the underlying bed formed by eddies in the
flow that deposited the overlying sands.
Bounce, Prod
and skip marks:
Casts of small gouge marks in the underlying bed
produced by the collision of fragments being carried
by the flow that deposited the overlying bed. They
commonly give the opposite sense of flute casts, that is
their steep sides are downstream and their shallow sides up
stream.
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Load casts:
Ball, pillow, and bulge shaped sand structures that
have sunk down into an underlying clay bed because of
liquifaction and de-watering of the clays following the
loading of the overlying sand bed. Similar to flutes, but
more irregular, and they do not have any current direction
implications. Not strictly a cast because they do not fill a
pre-existing hole.
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De-watering Structures:
Sandy sediments that are deposited very rapidly commonly
have a significant proportion of trapped water that is
expelled as the sediment undergoes compaction.
This leads to the formation of dish and pillar
structures. Dish structures are defined by dark
coloured clay-rich laminations that appear saucer
or dish shaped. They are associated with vertical
pillar structures that are also defined by darker
clay-rich material.
Flame Structures:
Sandy beds deposited by strong currents over clayrich beds commonly develop flames of the clay-rich
material that have been dragged into the overlying sand.
The asymmetry of flame structures can be used to
determine the paleo-current direction.
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Fossils, rain spots, mud cracks, and other peculiar features:
The samples at this station have interesting textures and structures that are easily
recognized:

Rain spots are small depressions with rims, formed through the impact of rain
on the soft exposed surface of fine-grained sediments.

Mud cracks are polygonal patterns on the bedding surface that formed
through shrinkage and cracking of the bed or lamina.
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