Sedimentary structures I


Wave ripples in Triassic Moenkopi Fm., Wupatki National Monument

Sedimentary structures:

Macroscopic three-dimensional features of sedimentary rocks recording processes occurring during deposition or between deposition and lithification. They are probably the most critical means of interpreting sedimentary and post-depositional processes. Their recognition and application are key to defining depositional environments, geological history, and surface processes.

Sedimentary structures function as:

Types of Sedimentary Structures:

We recognize two principle types: As sedimentologists, we care about sedimentary structures because of their wealth of information about the environment of deposition. We will focus on primary sedimentary structures in this lecture; later, we'll go into depth about some chemical and biological structures.

Primary Sedimentary Structures

Plane bedding:

Laminations:


Bedforms


Stream ripples, Big Bend National Park, TX

Bedforms generated by unidirections currents

So far we have discussed the variation in bedforms strictly as a function of FR. In fact, clast size is also an issue. (Recall what the Hjulstrom diagram says about the relationship between clast size, entrainment, and deposition.) Thus, at higher FR we begin to see the transport and deposition of coarser clasts.

To review:


Aggradation:

Reactivation surfaces:

Bedforms generated by multidirections currents

Wave ripples:

Tide dominated environments:


Sand and mud proportions:

Muds (clay and silt sized sediment) are usually carried as suspended load, and are deposited only under low-flow conditions as mud drapes over coarser clastic bedforms. The alternate deposition of sand (transported as bedload) and mud (settling from suspension) during different flow conditions creates distinctive patterns in which sand and mud are segregated. These are distinguished by the relative proportions of the two clast sizes: Although these can occur in many subaqueous environments, they are particularly characteristic of tidally dominated ones, where there is daily variation in flow regime.

Secondary Sedimentary Structures

Bedding plane structures

Another class of sedimentary structures form on the interface between beds, usually on the exposed surface of a recently deposited bed before it is buried. These features are useful because they indicate current direction and post-depositional deformation of the sediment.


Tool mark caused by floating branch, Roosevelt Island, DC
Sole marks are formed by currents acting on sediment.


Mud crack in Moenkopi Fm. Barringer Crater, AZ
Mud cracks Indicate subaerial exposure. Recent


Rain drop casts in Gettysburg FM, Rocky Ridge, MD
Rain drop prints


Vug partially filled with sediment, Guadalupe Mountains National Monument, TX.

Geopetal structures:

These indicate the up direction of beds, and can be found as:

Soft Sediment Deformation:


Pseudonodules along Corridor H, WV
Soft sediment deformation structures result from movement of sediment after deposition but prior to cementation. Sometimes this is due to the application of some sort of external load (e.g. soft sediment faulting) but are usually due to a density instability between different sediments layers. The most common are load structures, irregular bulbous features formed when a denser material has sunk into a less dense material (right). In some cases, denser material pinches off to form pseudonodules (a.k.a. ball and pillow structures).

Tongue like protuberances of mud into overlying soft sediment are known as flame structures.

Finally, deformation of soft sediment leads to convolute bedding, suggesting intense structural deformation.