Sedimentary Structures, Ch 4

Front 1

Why are sedimentary structures important?

Back 1

Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terranes.

Front 2

What is imbricate bedding?

Back 2

Imbricate bedding is one of the types of primary sed. structs. Elongated grains can sometimes pile up on each other to form what is called imbricate bedding. Imbricate bedding can be a current direction indicator if some other means is present to provide top/bottom directions.

Front 3

What is graded bedding?

Back 3

Graded Bedding is another primary sed. struct. As current velocity decreases, the larger or more dense particles are deposited first, followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed. This gives us a method for determining tops and bottoms of beds, since reverse grading will not be expected unless deposition occurs under unusual circumstances.

Front 4

What are the other types of primary sedimentary structures?

Back 4

Plane Bedding; Bedforms generated by unidirectional currents (like river channels and gravity flows); Bedforms generated by multidirectional flow (like currents and waves).

Front 5

Describe unidirectional current ripples.

Back 5

They consist of fine-grain sediments and go through stages of aggradation (to raise the grade or level of (a river valley, a stream bed, etc.) by depositing detritus, sediment, or the like.)

Front 6

What are the two most important bedding structures?

Back 6

Planar and Trough.

Front 7

What are 2D bedforms?

Back 7

An expample of 2D bedforms is planar bedding, which is created by ripples and dunes migrating over uniform surfaces, giving rise to tabular (planar) crossbeds.

Front 8

What are 3D bedforms?

Back 8

An expample of 3D bedforms is trough bedding, which is created by ripples and dunes migrating over scours or troughs. The filling thereof gives rise to trough crossbeds.

Front 9

Why are some ripples asymmetrical?

Back 9

Ripple marks are characteristic of shallow water deposition. They are caused by waves or winds piling up the sediment into long ridges. Asymmetrical ripple marks can give an indication of current direction. These are often found in rivers (and deserts)

Front 10

Why are some ripples symmetrical?

Back 10

Symmetrical ripple marks occur in environments where there is a steady back and forth movement of the water. Such ripple marks can still be used as top and bottom indicators. Often found in marine environments

Front 11

What are "Allen's" 5 types of ripples?

Back 11

straight, sinuous, catenary (chains), linguoid (tongue), lunate (crescent moon). See Slide 9

Front 12

Low flow regimes create what kind of structures?

Back 12

LFR create ripples, sand waves and dunes (megaripples)

Front 13

High flow regimes create what kind of structures?

Back 13

HFR create plane beds, antidunes and chutes & pools

Front 14

What is a gravity flow?

Back 14

A gravity flow is fluid flowing due to the forces of gravity alone and not to an applied pressure head. In the Bernoulli equation, the pressure term is omitted, and the height and velocity terms are the only ones included.

Front 15

Describe wave ripples.

Back 15

Asymmetric wave ripples are common in shallow waters along sandy shores. They are produced by bottom oscillations generated by passing breaker waves, which have unequal intensity in opposite directions. Wave-formed ripples indicate an environment with weak currents where water motion is dominated by wave oscillations.

Front 16

What are sedimentary structures?

Back 16

Sedimentary structures are those structures formed during sediment deposition. Sedimentary structures typically refer to the coherent arrangement of particles composing individual sedimentation units, as well as to the patterns of sediment erosion found on the surfaces separating many sedimentation units. This organization of the grain framework is loosely referred to as stratification. The stratification observed within individual beds or between individual laminae is typically the product of different combinations of grain composition, size, shape and orientation.

Front 17

Stratification and the structures it defines are basically generated in two ways:

Back 17

1) size segregation of particles simply as a consequence of their transport as bedload and suspended load; 2) spatial segregation of particle sizes as a function of sediment transport interacting the bed topography

Front 18

How is stratification produced by bedload transport alone?

Back 18

A well mixed, but poorly sorted bed of sediment will be naturally worked into patches of like-sized sediment under the action of bedload transport. These patches develop because oversized clasts move relatively quickly over finer-grained portions of the bed and slow down they encounter and interact with similarly sized sediment.

Front 19

Commonly associated with the bedload transport of sediment is development of a grain fabric. Fabric refers...

Back 19

Fabric refers to any preferred orientation for some particular axis of the constituent clasts (e.g., alignment of the clast long axes).
The shingled arrangement of adjacent particles is called imbrication and is most easily seen in gravelly deposits. Studies of clast imbrication can be used to constrain paleocurrent directions.

Front 20

x

Back 20

x

Front 21

How is the stratification that is produced by bedload transport alone done?

Back 21

The transport of sediment by suspension segregates particles of deffering mass in two ways: 1) Coarser grains aren't moved very far before settling onto the bed; 2) Normal grading w/in a deposit develops as the less massive grains require a greater length of time to settle from a depositiong flow.

Front 22

What is normal grading?

Back 22

Normal grading refers to a systematic reduction in both the coarsest and finest particles constituting bedding from its base to its top.

Front 23

Do bedforms climb or migrate?

Back 23

A train of bedforms migrating during deposition are moving upward (climbing) with respect to the bed they are covering. Each set is made of up sediment added to the portion of the lee face of a bedform that was not re-eroded during passage of following bedform troughs.

Front 24

What is a sole mark?

Back 24

Sole marks are features that form in the early stages of deposition, usually when a coarser-grained bed, like a sandstone, overlies a finer-grained unit, like a mudstone or shale bed. These structures often form by erosional processes, followed by deposition, and thus, may show directional features that make them useful in determining paleocurrent direction, in additional to stratigraphic top.

Front 25

What are two important types of sole marks?

Back 25

Two important types of sole marks are 1) flute casts and 2) tool marks.

Front 26

What are flute casts?

Back 26

Recall that the preserved specimen is the flute cast that formed on top of the softer sediment that has since eroded away. The asymmetry of these structures allows for determination of paleocurrent direction—flows originate from the direction of the steeper side of the flute. Flute casts typically have a "pointy" end, oriented in the upstream direction, and a more gentle, flared end that merges downstream into the bed surface.

Front 27

What provides the scour that allows flute casts to form?

Back 27

Tool marks, which are Any of the wide variety of current marks, such as groove marks, prod marks, and skip marks, produced by the continuous contact or intermittent impact of solid, current-borne objects against a muddy bottom. Direction of flow is often difficult/impossible.

Front 28

How are mud cracks formed?

Back 28

Shrinkage cracks (mud cracks) are formed during desiccation of a cohesive sediment layer. Cracks are typically polygonal in plan form, but can be less regular.

Front 29

What are trace fossils?

Back 29

Trace fossils are non-body remains indicating the activity of plants or animals. Examples are: footprints, trails, rhizoliths (root-related cementation), borings, and burrows.

Front 30

What is bioturbation?

Back 30

Bioturbation is the disturbance by organism digging, burrowing or simply moving across the surface.

Front 31

What does bioturbation tell us about the environment in which it was found?

Back 31

Production of bioturbation is a function of distance from the sediment/fluid interface (as in it decays rapidly with depth). It also indicates an envr lacking in deposition/energy/activity.

Front 32

Load casts, unlike flute casts, are deformational structures, rather than erosional structures. They occur commonly along the bases of sandstone beds that overlie mudstone or shale beds. While they may sometimes resemble flute casts, load casts can be differentiated from flute casts by their greater irregularity of shape and the lack of any indication of paleoflow direction.

Back 32

Load casts range in size, and may be associated with other deformational structures like flame structures and/or ball-and-pillow structures. Load casts form in settings where water-saturated muddy sediments are buried rapidly by coarser (sandy) sediments, creating a situation where dewatering of the mud cannot take place. The weight of the overlying sand causes it to sink unevenly into the muddy substrate, creating these features.

Front 33

What are three kinds of soft-sediment deformation structures?

Back 33

1. Loading structures
2. Dish & Pillar structures (associated with fluidization)
3. Convoluted Bedding & Sand Injections (associated with liquefaction)

Front 34

How are load structures formed?

Back 34

Load structures are soft-sediment deformation structures that result from an unstable difference in the density of a deposit and its immediate substrate (top layer “sinks” into layer below).

Front 35

What is fluidization and what two types of soft-sediment deformation structures are associated with it?

Back 35

Fluidizationis the remobilization of sediment by intergranular flow. In other words, it is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the granular material; Dish & Pillar structures.

Front 36

How are dish structures formed?

Back 36

Dish structures are produced by a relatively slow upward flow of pore fluid that transports finer particles through the grain framework. This creates zones of pore space that are depleted of or clogged with the finer particles.

Front 37

How are Pillar structures formed?

Back 37

Pillar structures are fomed when escaping pore water constructs a network of vertical pipes. Fluid velocities in these pipes are sufficient to transport and remove finer grain sizes from the deposit.

Front 38

What is liquefaction and what two types of soft-sediment deformation structures are associated with it?

Back 38

Liquefaction is the transformation of water-saturated sediments from the solid to liquid state as a consequence of increased pore pressure; Convoluted Bedding & Sand Injections

Front 39

How are Convoluted Beddings formed?

Back 39

Convoluted Bedding is a structure associated with local permeability barriers that “trap” elevated pore pressures. It is formed by complex folding or intricate crumpling of beds into irregular anti/synclines.

Front 40

How does sand injection occur?

Back 40

Sand injection occurs when high pore-pressures fracture confining layers and drive flow of sand & pore water.