Sedimentary Structures


STRATIFICATION refers to the way sediment layers are stacked over each other, and can occur on the scale of hundreds of meters, and down to submillimeter scale.   It is a fundamental feature of sedimentary rocks.

canyonlands.jpg (32953 bytes)
This picture from Canyonlands National Monument/Utah shows strata
exposed by the downcutting of the Green River.  Large scale
as seen here is often the result of the migration of
sedimentary environments
(see below). Let us imagine a shoreline
that has coexisting slat marsh, beach, and offshore muds.  Each
environment is characterized by a different sediment type. If this
shoreline receives more sediment than the waves can remove, it will
gradually build out (to right).  Over time the different sediment types
will be stacked on top of each other and the migration of the shoreline
will produce superimposed layers (stratification) of different types of
sedimentary rock.
striped_shale.jpg (89028 bytes)
facies_migration.jpg (35064 bytes) Above image shows small scale stratification
in a shale (image is 7 mm tall).  This kind of
stratification is due to alternately operating
depositional processes in the same
.  Dark layers are rich in organic
matter and are remains of algal mats.  Light
layers were deposited by storms or floods,
and briefly interrupted algal growth.


CROSS-BEDDING is a feature that occurs at various scales, and is observed in conglomerates and sandstones.  It reflects the transport of gravel and sand by currents that flow over the sediment surface (e.g. in a river channel).  sand in river channels or coastal environments

xbed.jpg (31524 bytes) When cross-bedding forms, sand is transported as sand-dune like bodies (sandwave), in which sediment is moved up and eroded along a gentle upcurrent slope, and redeposited (avalanching) on the downcurrent slope (see upper half of picture at left).  After several of these bedforms have migrated over an area, and if there is more sediment deposited than eroded, there will be a buildup of cross-bedded sandstone layers.  The inclination of the cross-beds indicates the transport direction and the current flow (from left to right in our diagram).  The style and size of cross bedding can be used to estimate current velocity, and orientation of cross-beds allows determination direction of paleoflow.
crossbeds.jpg (40964 bytes) Cross-bedding in a sandstone that was originally deposited by rivers.  The deposition currents were flowing from right to left.
dunes.jpg (29038 bytes) Cross-bedding can also be produced when wind blows over a sand surface and creates sand dunes.  The picture on the left shows ancient sanddunes with cross-bedding.


GRADED BEDDING means that the grain size within a bed decreases upwards. This type of bedding is commonly associated with so called turbidity currents. Turbidity currents originate on the the slope between continental shelves and deep sea basins. They are initiated by slope failure (see diagram below), after sediment buildup has steepened the slope for a while, often some high energy event (earthquake) triggers downslope movement of sediment. As this submarine landslide picks up speed the moving sediment mixes with water, and forms eventually a turbid layer of water of higher density (suspended sediment) that accelerates downslope (may pick up more sediment). When the flow reaches the deep sea basin/deep sea plain, the acceleration by gravity stops, and the flow decelerates. As it slows down the coarsest grains settle out first, then the next finer ones, etc. Finally a graded bed is formed. However, decelerating flow and graded bedding are no unique feature of deep sea sediments (fluvial sediments -- floods; storm deposits on continental shelves), but in those other instances the association of the graded beds with other sediments is markedly different (mud-cracks in fluvial sediments, wave ripples in shelf deposits).

turbidite.jpg (22975 bytes) Diagram illustrating the formation of a graded bed (turbidite).  Slope failure produces turbulent suspension that moves/accelerates downslope.  Once it reaches the flat deep sea regions, it slows down due to friction, and gradually the sediment settles out of suspension.  Larger grain sizes settle out first, and then successively smaller ones.
grading.jpg (41535 bytes) Example of a graded bed.  Largest grains occur at the base, and the grain size gradually decreases.


RIPPLE MARKS are produced by flowing water or wave action, analogous to cross-bedding (see above), only on a smaller scale (individual layers are at most a few cm thick).

currentripp.jpg (37034 bytes) Current ripples in a creek in Arlington.  Ripples are asymmetrical and have a gentle slop on the right and a steep slope on the left.  Comparing with the explanation of cross-bedding  from above, it is obvious that the currents were flowing from right to left.
crosslamin.jpg (25944 bytes) Side-view of current rippled sandstone (note coin for scale).  The cross-beds or (more accurately) cross-laminae are inclined to the right, thus the water was flowing from left to right.
waverippnew.jpg (25428 bytes) Modern wave ripples in Lake Whitney.  Note that ripples are symmetrical, and that they can branch in a "tuning-fork" fashion.  Both features are characteristic of wave ripples.
oldripples.jpg (34587 bytes) Ancient ripples on a sandstone surface.  Ripples are symmetrical and show "tuning-fork" branches.  This indicates to a geologist that the sandstones were deposited in an environment with wave action (nearshore).

MUD CRACKS form when a water rich mud dries out on the air.

mudracks.jpg (57306 bytes) You all have seen this when the mud in a puddle dries out in the days following a rainstorm.  This example is from a construction pit in Arlington.  Due to stretching in all directions, the mudcracks form a polygonal pattern.  We also see several successive generations of cracks.
oldcracks.jpg (53281 bytes) An example for ancient mudcracks from rocks that are over 1 billion years old (Snowslip Formation, Montana).  Same crack pattern as above, and also second and third generation cracks.