Sedimentary structures are important because they tell us about processes that were operating during the deposition of that sediment. Things like flow velocity, depth, and direction can be determined from some sorts of structures. From there, we can start to make some educated guesses about what sorts of depositional environments these processes were taking place in. But, the first step is having the vocabulary to describe what you are seeing.
Learning Objectives
Identify common sedimentary structures
Interpret appropriate sedimentary structures to determine paleoflow direction
Interpret and describe the processes responsible for and recorded by sedimentary structures
Use appropriate sedimentary structures to determine way up and top/bottom of beds
Stratification is a generic term used to describe layering in sedimentary rocks. Layers >1 cm thick are called beds and those <1 cm thick are called laminae. Some layers show internal changes in grain size. A variety of descriptive terms can be applied to the contacts between layers and their overall geometry.
Bedforms are three dimensional structures in sediment; if preserved or partially preserved they form sedimentary structures. A predictable sequence of bedforms and structures for in response to unidirectional flows as a function of grain size, flow depth and velocity.
When waves start to "feel" bottom (at a depth of 1/2 of their wavelength) they cause a back-and-forth sense of motion at the sediment surface which creates wave symmetrical wave ripples. Sloshing action caused by storm events causes wavy and complexly cross-cutting layers in hummocky cross stratification. Changes in flow strength and direction result in heterolithic bedding and/or herringbone cross-bedding.
Erosional structures are caused by turbulence in a flowing fluid or interaction of a transported particle with the substrate. Soft-sediment deformation structures are secondary features formed shortly after deposition but before the sediment becomes lithified and starts to undergo diagenesis.
