Lab 5: Sedimentary Rocks - Sedimentary Structures


Sedimentary Structures are various ways that sedimentary material can be bedded in layers, or shaped into forms by moving water or wind (such as ripples and dunes).


The most basic sedimentary structure is bedding. A sedimentary bed, or stratum, is a single lithology usually (rock type), such as sandstone, claystone, and limestone. Bedding planes are the contacts, the surfaces, between layers.


Here are some of the more important sedimentary structures:


Ripples and Dunes


Moving water or wind will shape sediment into bed forms ranging from small ripples up to large dunes. The bedding planes on the tops of sedimentary layers commonly show ripple shapes. Often, as with the case of river flow and wind, the ripples form from water or wind moving primarily in one direction. These ripples and dunes (current ripples and dunes) tend to show a profile that is shaped in an asymmetrical way, with the steeper side on the down-current side. Waves in a tidal zone produce a special type of ripple called wave (or oscillation) ripples that have a symmetrical profile, because the current moves back and forth, shaping the sediment one way, only to reshape it back toward the other way, during counter flow, back and forth.




Ripples and dunes migrate. They move along as the sand grains that form them get picked up individually by the current and dropped down-current. If you dig a trench across a ripple or dune, you will see internal cross-bedding (inclined internal thin beds, or laminae). The internal cross-bedding shows successive positions of the surface of the ripple or dune. For current ripples, that are asymmetrical in profile, as described above, you can tell the direction of water or wind movement.


Graded Bedding


The most common way that individual beds are graded by grain size, is to find the coarsest sediment on the bottom of the layer, grading upward into finer sediment. This more common type of graded bedding results from the settling out in a single depositional event, first of the larger grains, which settle out first onto the bottom, then of successively smaller grains, which take longer to settle out of the water column. Fine clay-size material is the slowest to settle out, because the tiny particles are flat, and tend to waffle to and fro during settling. An interesting place to find graded bedding is the foot (base) of the continental slope, where turbidite deposits accumulate. Turbidity currents are one-fell-swoop currents that come down the ramp of the continental slope, gaining speed and picking up bottom sediment. When a turbidity current reaches the flatter bottom of the deep seafloor, the current gradually slows down, and there is time for differential settling out by grain size to occur. Graded bedding of several sorts can also be found along a typical river system, but it is usually not as well-developed as with turbidite deposits.


Mud Cracks


You've seen a dried up mud puddle, with the mud in bottom cracked and curled up along the edges of the cracks. These are mud cracks, which result from shrinkage of the mud as it dries out. As the water evaporates away and the mud dries, there is a loss of volume (shrinkage) and the surface of the mud cracks, usually in a polygonal pattern. Mud cracks form anywhere wet sediment at the surface dries out. Common places where this happens are any coastal setting, where wet back-beach or bay margin muds are exposed to dry air during low tide, and along river systems, where flow is not always so steady. A sand bar can be thoroughly wetted, and then dried, and often thin mud layers can be coated onto a part of the sand bar, only to dry out and crack.