Introduction to Sedimentary Structures - Part 1

Sedimentary Bedding
and Structures
Sedimentary structures are features found within or on the surface of a
sedimentary bed that formed during or following deposition and provide
information pertaining to depositional environment or burial history.

Photo by W. W. Little
Bedding
The most significant feature of sedimentary rocks is that they form
layers. These layers are like the pages of a book and provide the
history of the earth’s surface (stratigraphy).

Bedding Scale
Scan from Stow
Bed thickness is typically tied
to depositional process and is,
therefore, an indicator of
depositional environment. To
assist in describing sedimentary
rock units, a formal
classification of bed thickness
has been established.

Photo by W. W
Bed Nesting
Often, thinner beds are nested within thicker beds, which, in turn, are
nested within yet thicker beds, indicating multiple levels of
depositional patterns.

Vertical changes in bed thickness are indicators of changes in
depositional environments and can be indicative of the depositional
system or of variations in base-level.
Bedding Patterns

Massive Bedding

Bed Shapes
Bed shape is an indicator of the depositional conditions under
which the bed formed.

Tabular Beds

Wedge-shaped Beds

Bedsets
Typically, beds occur in sets in which the beds are
genetically related to one another.

Heterolithic Bedding
Heterolithic bedding refers to finely-interbedded grain sizes, such as
sand and mud and can occur at a variety of scales.

Types of Sedimentary Structures
• Bedforms
• Surface markings
• Sole marks
• Biological structures
• Soft-sediment deformation
• Diagenetic structures
• Unconformities

Bedforms
Bedforms are produced as sediment actively accumulates
during and following fluid flow and are characterized
internally by a variety of sedimentary structures, such as
cross-bedding.

Lower Flow Regime Bedforms
Flow-regime bedforms are those that
are produced by a moving, non-
viscous fluid, such as water or wind.
Bedform size tends to increase with
increasing flow rate. Flow regime is
also influenced by grain size.

Ripple Classification (size)
Terminology is not completely fixed, but ripples give way to dunes
with increasing flow velocity. Some schemes have an transitional
form, referred to as sand waves.
Ripples are 0.5 to 3.0 cm in height with
wavelengths of 5 to 40 cm. They are
typically found under low to moderate flow
velocities in sand that is less than 0.7 mm
in diameter.
Dunes are over 3.0 cm in height with
wavelengths of at least 40 cm. They
typically form under moderate to high flow
velocities in relatively deep water and
sand that is more than 0.2 mm in
diameter. Dune height and spacing is
related to water depth.
Both ripples and dunes tend to be straight-
crested under lower flow velocities and
sinuous under higher velocities.
Another classification uses the
terms microforms (e.g. ripples),
mesoforms (e.g. dunes), and
macroforms (e.g. bars).

Ripple Classification (morphology)
Morphological ripple classification is based on plan geometry and
increases in complexity with shallower depths and higher flow
velocities.

Wave-generated RipplesCurrent-generated Ripples
Ripple Classification (process)
Ripple classification is based on plan morphology as related to
interpreted process of formation.

Asymmetrical (Current) Ripples

Symmetrical (Oscillation) Ripples
Symmetrical ripples are typically produced by oscillatory motion of
waves. In addition to their symmetry, they can often be distinguished
from current-formed ripples by bi-directionally-dipping cross-laminae.

Interference Ripples
Surface ponding can lead to the development of interference ripples.

Megaripples

Hummocky (Cross) Bedding (HCS)
Hummocky cross-bedded sand is produced mostly on the shallow sea
floor during storms by a combination of current and oscillatory flow,
resulting in aggradation of mounds and swales, mostly from vertical
accretion.

Hummocky Cross-bedding Animation
http://walrus.wr.usgs.gov/seds/bedforms/

Cross-bedding
Crossbedding is layering that dips between the upper and lower
boundaries of a sedimentary bed and is formed by moving water or
wind. It can be used to determine water depth, fluid velocity, and flow
direction.

Cross-bed Formation
Ripples are characterized internally by cross-bedding. Cross-beds
are formed in fluid flow as sediment is eroded from and transported
up the relatively gentle stoss side of a ripple and deposited as
avalanches on the steeper lee side. Cross-beds form in both
aqueous and eolian environments.

Flow Separation
In eolian deposits, as sediment reaches the dune crest, courser
grains avalanche down the lee face; whereas, finer particles blow
across to the top of the stoss slope of a leading dune.

Cross-bed Sets
Cross-bed sets form as one ripple migrates over another. A single
layer of cross-bedding is a “set.” Multiple layers are “co-sets.”

Cross-bed Set Animation

Trough Cross-bedding
Trough cross-bedding clearly flattens toward the base in longitudinal
profile and forms “trough” shapes in transverse sections. Trough
cross-stratification is produced by sinuous-crested ripples.

Trough Cross-bedding Animation 1

Trough Cross-bedding Animation 2

Planar (tabular) Cross-bedding
Planar cross-bedding flattens little or none toward the base in
longitudinal profile and forms apparent planar bedding in transverse
sections. Planar cross-stratification is produced by straight-crested
ripples.

Planar Cross-bedding Animation

Climbing Ripples
The angle of climb between ripple sets increases with the rate of
deposition. Very high depositional rates result in climbing ripples.

Climbing Ripple Animation

Herring Bone Cross-bedding

Transitional Flow-Regime
Planar Beds
At the boundary between lower and upper flow regime
(Fr = 1), “washed-out ripples” or planar beds are
produced. Sand greater than 0.7 mm in diameter can
form similar structures under lower flow regime
conditions.

Parallel Laminations in Sandstone

Swash Deposits

Antidunes
Under upper flow regime conditions
bedforms accrete on the upstream side
and erode at the downstream end,
forming antidunes.

Antidune Cross-bedding Animation

Channel Forms

Lateral Accretion Surfaces

Scroll Bars

Parallel Laminations in Mudstone

Introduction to Sedimentary Structures - Part 1

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Introduction to Sedimentary Structures - Part 1