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.

This picture from Canyonlands National Monument/Utah shows strata exposed by the downcutting of the Green River.  Large scale stratification 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.

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 environment.  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

	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.
	Cross-bedding in a sandstone that was originally deposited by rivers.  The deposition currents were flowing from right to left.
	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).

	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.
	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).

	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.
	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.
	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.
	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.

	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.
	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.